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Developing An Ionospheric Map for South Africa

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					Ann. Geophys., 28, 1431–1439, 2010
www.ann-geophys.net/28/1431/2010/                                                                        Annales
doi:10.5194/angeo-28-1431-2010                                                                       Geophysicae
© Author(s) 2010. CC Attribution 3.0 License.




Developing an ionospheric map for South Africa
D. I. Okoh1,2 , L. A. McKinnell1,2 , and P. J. Cilliers1
1 Hermanus   Magnetic Observatory, P.O. Box 32, Hermanus, South Africa
2 Department  of Physics and Electronics, Rhodes University, P.O. Box 24, Grahamstown, South Africa
Received: 12 August 2009 – Revised: 19 June 2010 – Accepted: 5 July 2010 – Published: 12 July 2010


Abstract. The development of a map of the ionosphere over       it contains a reasonable distribution of free electrons. This
South Africa is presented in this paper. The International      paper presents the development of a suitable method for de-
Reference Ionosphere (IRI) model, South African Bottom-         scribing the behaviour of the ionosphere given the current
side Ionospheric Model (SABIM), and measurements from           available sources.
ionosondes in the South African Ionosonde Network, were            Plasma electrons in the ionosphere are known to oscil-
combined within their own limitations to develop an accurate    late with a characteristic frequency known as the plasma fre-
representation of the South African ionosphere. The map is      quency. Chen (1984) showed that the plasma frequency (in
essentially in the form of a computer program that shows spa-   units of radians per second) at any point in plasma is given
tial and temporal representations of the South African iono-    by
sphere for a given set of geophysical parameters. A vali-
dation of the map is attempted using a comparison of Total
Electron Content (TEC) values derived from the map, from                       ne2
the IRI model, and from Global Positioning System (GPS)         ωp = 2πf =          .                                      (1)
                                                                               ε0 m
measurements. It is foreseen that the final South African
ionospheric map will be implemented as a Space Weather
product of the African Space Weather Regional Warning           where f is the corresponding Plasma frequency (in Hertz),
Centre.                                                         n is the number density of the electrons (in electrons
                                                                per cubic meter), e=1.60×10−19 C is the electron charge,
Keywords. Ionosphere (Mid-latitude ionosphere; Modeling
                                                                ε0 =8.85×10−12 Fm−1 is the permittivity of free space, and
and forecasting) – Radio science (Ionospheric propagation)
                                                                m=9.11×10−31 kg is the electron mass.
                                                                   Equation (1) shows a square-law relation between the
                                                                plasma frequency, f , and the electron density, n. Points of
1   Introduction
                                                                peak electron densities are, therefore, points of peak plasma
The ionosphere (that region of our atmosphere extending         frequencies. The peak plasma frequency in ionospheric stud-
from an altitude of about 50 km to about 1000 km) affects       ies (also called the critical plasma frequency) is denoted
lives in diverse ways, ranging from its usefulness in High      foF2, after the F2 region of the ionosphere. In the absence of
Frequency (HF) radio propagation, to its attenuation of ra-     occurrences such as sporadic E (Es), the F2 region contains
dio signals that have to pass through it. By increasing gen-    the maximum concentration of electrons in the ionosphere.
eral ionospheric knowledge above lesser known areas, use of     Our map does not take into account sporadic E Layer effects.
the ionosphere can be greatly enhanced, and significant al-      In relation to HF radio propagation, the critical plasma fre-
lowance can be made for the effects ionospheric behaviour       quency for any location on Earth represents the highest radio
can have on radio signals. What distinguishes the iono-         frequency that the ionosphere above that location is able to
spheric region from other regions of the atmosphere is that     reflect if the radio waves are at normal incidence. A map of
                                                                foF2 values for the South African region was developed in
                                                                this work, and then used to adapt the International Reference
                    Correspondence to: D. I. Okoh               Ionosphere (IRI) model to generate electron density profiles
                    (okodan2003@gmail.com)                      for the region.

Published by Copernicus Publications on behalf of the European Geosciences Union.
1432                                                             D. I. Okoh et al.: Developing an ionospheric map for South Africa

           -12                                                        work done by McKinnell (2002) showed that SABIM predic-
           -14                                                        tions were more accurate than IRI predictions over Graham-
                                                                      stown when compared to measured data from the Graham-
           -16
                                                                      stown ionosonde. Section 4.5 also illustrates an improvement
           -18                                                        in performance of SABIM over the IRI at the ionosonde lo-
           -20
                                                                      cations. The latest version of SABIM (version 3) was imple-
                                                                      mented as an executable produced from a code written in the
           -22                                        Madimbo         C programming language for use in our map.
Latitude




           -24                                                           The South African Ionosonde Network currently consists
           -26
                                                                      of 4 ionosonde stations as shown in Fig. 1; the 3 already men-
                                                                      tioned above and used in the development of SABIM plus the
           -28
                               Louisvale                              latest addition located at Hermanus (Western Cape, 34.4◦ S,
           -30                                                        19.2◦ E). Since ionosonde measurements are the most accu-
                                                                      rate recordings of ionospheric behaviour and reflect the true
           -32
                                            Grahamstown
                                                                      state of the ionosphere at a given time and location, real-time
           -34            Hermanus                                    data from the 4 ionosondes were given first priority in the
                 15       20          25         30         35        development of this map.
                                     Longitude
                                                                         In summary, the IRI model was incorporated into our
Fig. 1. Locations of the South African ionosonde stations. SABIM      map since it is an international standard for the ionosphere,
is used inside the marked triangular region.                          SABIM was used since it is more adapted for the South
                                                                      African region, and the South African ionosonde network
                                                                      provides the most accurate real-time ionospheric information
2          Available data sources                                     for the region.

The sources of ionospheric information used for developing
the map include the IRI model, the South African Bottom-              3   Developing the ionospheric map
side Ionospheric Model (SABIM), and measurements from
ionosondes in the South African Ionosonde Network.                    A major product of this work is software (written in the MAT-
   The IRI model is an empirical model developed using                LAB programming language) that generates maps illustrat-
available data from all around the world. This model was              ing spatial and temporal representations of the South African
considered as a data source for the map since it is the in-           ionosphere. The software (having a Graphical User Inter-
ternational standard for the specification of ionospheric den-         face (GUI) for more user-friendliness) produces maps of the
sities and temperatures (Bilitza, 2001) and the most com-             foF2 values, and the electron density profiles, over the South
monly used global model. According to Bilitza and Reinisch            African region for a given set of input parameters. Figure 2
(2008), the IRI model has become so widely accepted that a            illustrates the user interface presented by the software.
comparison with IRI is often one of the first science tasks by            To produce the foF2 map, SABIM was used inside of the
an ionospheric, satellite or rocket team. The latest version of       triangular region illustrated in Fig. 1. The vertices of that
the IRI model (IRI-2007) was implemented in our map as an             triangle are at the 3 ionosonde stations which were used to
executable produced from the IRI FORTRAN source code so               develop SABIM. The IRI model was used outside of the tri-
as to allow for user defined automatic operations.                     angular region, and a smoothing function was introduced at
   SABIM is a model of South Africa’s bottomside iono-                the boundary between where these 2 models were used such
sphere developed using the technique of training Neural               that SABIM still contributed outside of the triangular region,
Networks (NNs) with an archive of data from 3 of the                  but that its contribution decreases with increasing distance
South African ionosonde stations located at Grahamstown               from the closest edge of the triangle. The smoothing func-
(33.3◦ S, 26.5◦ E), Madimbo (22.4◦ S, 30.9◦ E) and Louis-             tion used is
vale (28.5◦ S, 21.2◦ E) as shown in Fig. 1. SABIM was uti-            f = fs cos2 θ + fi sin2 θ.                                 (2)
lized as an available data resource since it is a national model
developed particularly for the South African region due to the        where fs is SABIM’s foF2 value for a location outside of the
insufficiencies observed in using global ionospheric models            triangular region, fi is IRI’s corresponding foF2 value for
for the South African region. McKinnell and Poole (2004)              that location, and f is the resulting foF2 value for the loca-
noted that the IRI model was inaccurate in the South African          tion. The parameter θ is a measure of the location’s distance
region due to an historical paucity of available data for the re-     from the closest edge of the triangle, and was arbitrarily de-
gion. In their words, comparisons with the IRI global model           fined to have a value of 0◦ anywhere on the triangular edge,
showed that, for Grahamstown, a NN-based model predicted              and linearly increases to 90◦ at a distance of 10 longitudi-
the noon value of foF2 more realistically than the IRI. Earlier       nal degrees (≈ 1117 km) from the closest triangular edge. In

Ann. Geophys., 28, 1431–1439, 2010                                                             www.ann-geophys.net/28/1431/2010/
D. I. Okoh et al.: Developing an ionospheric map for South Africa                                                                 1433




Fig. 2. The Graphical User Interface of the software developed in this work.


essence, IRI’s contribution is zero for points on the triangular        the topside ionosphere. In addition, significant time would
edge, increases with distance from the triangle, and is 100%            also be required by the computer program to combine elec-
at a distance of 1117 km and beyond from the closest trian-             tron densities from the IRI model and SABIM at each of the
gular edge.                                                             altitudes involved in the construction of a single vertical den-
   The rationale behind this combination is that SABIM is               sity profile. Note that the electron density profiles generated
known to perform more accurately than the IRI model inside              by adapting the IRI model with foF2 and hmF2 values from
of the triangular region bounded by the 3 ionosonde stations            the map are contributions from all 3 sources used in the de-
that were used to develop the model (Poole and McKinnell,               velopment of the map since the foF2 and hmF2 values come
2000), but that its performance outside of that region has not          from a combination of data from all 3 sources.
been proven.
   Measurements from the ionosondes were further used to
                                                                        4      Results and discussions
adapt the produced foF2 map to fit the ionosonde measure-
ments; for each ionosonde location, the program calculates              4.1     Variations over a typical day
the difference between the measured foF2 and the foF2 value
calculated by the existing modeled map at the ionosonde lo-             Figure 3a to d illustrates how the foF2 values vary over a
cation, and then fits a best plane, by the method of least               typical day in South Africa as determined by our map. The
squares, of these differences over the whole map. This plane            figures show foF2 maps of the South African ionosphere
is added to, or subtracted from, the existing modeled map in            for 08:00 UT (a South African morning), 12:00 UT (a South
order to adapt it to the ionosonde measurements.                        African afternoon), 16:00 UT (a South African evening), and
   To generate the electron density profile for a given loca-            22:00 UT (the South African midnight) respectively for day
tion, the program uses the IRI model, but augments it with              number 13 (a summer day) of year 2009.
the location’s foF2 and hmF2 values as obtained from the                   The maps show that the foF2 values are larger in the after-
map. The hmF2 values are obtained by exactly the same pro-              noon than for other times of the day. This is expected since
cess used for obtaining the foF2 values. SABIM also pro-                in the afternoon the sun is closest to zenith. The amount of
duces electron density profiles for the South African region,            solar radiation reaching the ionosphere during this time of
but was not used in the generation of electron density pro-             the day is also generally greater than during other times of
files for this work since the model, being a bottomside iono-            the day, giving rise to greater ionospheric electron densities
spheric model, does not produce electron density profiles for            (and hence greater foF2 values). In the early hours of the day

www.ann-geophys.net/28/1431/2010/                                                               Ann. Geophys., 28, 1431–1439, 2010
                                      foF2 map for Year Day Day Nunmber 13, UT fof2 fof2
                                 foF2 map for Year 2009,2009,Nunmber 13, 08:0008:00 UT (MHz)(MHz)               foF2 foF2 map for Year 2009, Nunmber 13, 12:00 UT fof2 (MHz)
                                                                                                                      map for Year 2009, Day Day Nunmber 13, 12:00 UT fof2 (MHz)
                                                                                        12 12                                                                           12 12

                                                                                                                  -15 -15
1434 -15 -15                                                                                              D. I. Okoh et al.: Developing an ionospheric map for South Africa
                                                                                                     10 10                                                                  10 10

                     -20 foF2 map for Year 2009, Day Nunmber 13, 08:00 UT fof2 (MHz)
                          -20                                                                                      -20 map for
                                                                                                                       -20                                        fof2 (MHz)
                                                                                                                 foF2foF2 mapYearYear 2009, Day Nunmber 13, 12:00 UT fof2 (MHz)
                               foF2 map for Year 2009, Day Nunmber 13, 08:00 UT fof2 (MHz)                                     for 2009, Day Nunmber 13, 12:00 UT
                                                                                12 8
                                                                                8 12                                                                                  812 12
                                                                                                                                                                           8




                                                                                                               Latitude
                                                                                                               Latitude
         L a titu d e
         L a titu d e                                        Madimbo
                                                                   Madimbo                                                                          Madimbo
                                                                                                                                                         Madimbo

                -15 -25
                 -25 -15                                                                                          -15 -25
                                                                                                                  -25 -15
                                                                                                     10 6
                                                                                                     6 10                                                                   610 10
                                                                                                                                                                                6
                                                        Louisvale
                                                                Louisvale                                                              Louisvale
                                                                                                                                              Louisvale
                -20 -30
                 -30 -20                                                                                          -20 -30
                                                                                                                  -30 -20
                                                                                                     8
                                                                                                     4                                                                      48 8
                                                                                                                                                                               4




                                                                                                               Latitude
                                                                                                          4
      L a titu d e




                                                                               MadimboMadimbo             8                                                  Madimbo




                                                                                                               Latitude
                        L a titu d e




                                                                                                                                                                 Madimbo
                                                                      Grahamstown
                                                                            Grahamstown                                                             Grahamstown
                                                                                                                                                         Grahamstown
                                                  Hermanus
                                                        Hermanus                                                                    Hermanus
                                                                                                                                         Hermanus
                 -35 -25
                -25 -35                                                                                           -25 -35
                                                                                                                  -35 -25
                                                                       6
                                                                       2                                  2
                                                                                                          6                                                                 26 6
                                                                                                                                                                               2
                        10 10 15 15 20 Louisvale25 25 30 30 35 35 40 40
                                         20 Louisvale                                                                               20 25
                                                                                                                    10 10 15 15 20 Louisvale 25 30 30 35 35 40 40
                                                                                                                                         Louisvale
                -30               -30                   Longitude
                                                           Longitude                                              -30 -30         Longitude
                                                                                                                                        Longitude
                                                                                                     4    4                                                                 4 4
                                                                     Grahamstown
                                                                            Grahamstown                                                             Grahamstown
                                                                                                                                                         Grahamstown
                 (a) -35
                -35 (a)
                                                  Hermanus
                                                         Hermanus                                                   (b) (b)
                                                                                                                  -35 -35
                                                                                                                                    Hermanus
                                                                                                                                         Hermanus


                                                                                                      2   2                                                                 2 2
                      10               10 15   15 20      20 25        25 30     30 35      35 40   40                10 10 15 15 20 20 25 25 30 30 35 35 40 40
(a)                                                    Longitude
                                                           Longitude                                            (b)                    Longitude
                                                                                                                                          Longitude
                    map for Year 2009, Day Day Nunmber 13, 16:00 UT fof2 (MHz)
               foF2foF2 map for Year 2009, Nunmber 13, 16:00 UT fof2 (MHz)                                            map for Year 2009, Day Day Nunmber 13, 22:00 UT fof2 (MHz)
                                                                                                                foF2 foF2 map for Year 2009, Nunmber 13, 22:00 UT fof2 (MHz)
               (a) (a)                                                12 12                                        (b) (b)                                              12 12

                     -15 -15                                                                                      -15 -15
                                                                                                     10 10                                                                  10 10

              -20 -20                                                                                              -20 -20
                  foF2 map for Year Day Day Nunmber 13, 16:00 UT fof2 (MHz)
            foF2 map for Year 2009,2009,Nunmber 13, 16:00 UT fof2 (MHz)                                                map for for 2009, Day Nunmber 13, 22:00 UT fof2 (MHz)
                                                                                                                 foF2foF2 mapYearYear 2009, Day Nunmber 13, 22:00 UT fof2 (MHz)
                                                                   8 12
                                                                   12 8                                                                                               812 12
                                                                                                                                                                           8
                                                                                                               Latitude
      Latitude




                                                                                                               Latitude
      Latitude




                                               Madimbo
                                                    Madimbo                                                                                         Madimbo
                                                                                                                                                         Madimbo

                -25 -15
               -15 -25                                                                                            -25 -15
                                                                                                                  -15 -25
                                                                                                     6 10
                                                                                                     10 6                                                                   610 10
                                                                                                                                                                                6
                                                       Louisvale
                                                              Louisvale                                                                Louisvale
                                                                                                                                              Louisvale
                -30 -20
               -20 -30                                                                                            -30 -20
                                                                                                                  -20 -30
                                                                                                     4    4
                                                                                                          8                                                                 48 8
                                                                                                                                                                               4
                                                                                                                Latitude
                      Latitude




                                                                                    Madimbo          8                                                            Madimbo
                                                                                                               Latitude
  Latitude




                                                                             Madimbo
                                                                     Grahamstown
                                                                           Grahamstown                                                                        Madimbo
                                                                                                                                                    Grahamstown
                                                                                                                                                          Grahamstown
                                                  Hermanus
                                                       Hermanus                                                                     Hermanus
                                                                                                                                         Hermanus
                -35 -25
               -25 -35                                                                                            -35 -25
                                                                                                                  -25 -35
                                                               2                                          6
                                                                                                          2                          2 6 2
                  10 10 15 15 20 Louisvale 25 30 30 35 35 40 406
                                   20 25 Louisvale                             10 10 15 15 20 Louisvale 25 30 30 35 35 40 40 6
                                                                                                   20 25Louisvale

(c)            -30 -30           Longitude
                                       Longitude                           (d) -30
                                                                             -30                 Longitude
                                                                                                       Longitude
                                             Grahamstown     4 4                                                  Grahamstown         4 4
                                                                                                    Hermanus midnight
Fig. 3. Variations in foF2 over South Africa for (a) a morning, (b) an afternoon, (c) an evening and (d) a Grahamstown hour of a summer day in
                                       Grahamstown
       (c) -35
2009.-35 (c)
                               Hermanus
                         Hermanus                                             (d) (d)          Hermanus
                                                                             -35 -35
                                                             2    2                                                                   2 2
        10 1015 1520 20 25 25 30 30 35 35 40 40                                10 10 15 15 20 20 25 25 30 30 35 35 40 40
                                                           Longitude
                                                       Longitude                                                                          Longitude
                                                                                                                                       Longitude
(Fig. 3a), the sun is still rising and the foF2 values are there-                                              4.2      Seasonal variations
fore relatively lower. During the evening hours, the sun is
      (c) (c)
setting and the foF2 values are therefore gradually decreas-                                                   The(d) (d)
                                                                                                                     intensity of solar radiation reaching the ionosphere
ing. Nights have the lowest foF2 values due to the absence                                                     changes with the seasons of a year. Figure 4a to d illus-
of solar radiation.                                                                                            trate variations in the South African ionosphere during the
   The maps also show that during the early hours of the day                                                   4 seasons of year 2008. These figures are foF2 maps for day
(Fig. 3a) the foF2 values are greater at the north-eastern parts,                                              numbers 13, 104, 195, and 285 of year 2008, respectively
during the afternoon (Fig. 3b) the region of greatest foF2                                                     chosen to represent a day in summer, a day in autumn, a day
values shifts towards the north-central parts, and during the                                                  in winter, and a day in spring. The maps are for 10:00 UT
evening (Fig. 3c) the north-western parts have the greatest                                                    (midday local time) for each of the chosen days.
foF2 values, following the sun’s motion from the northeast                                                        As expected, the foF2 values are greater for the summer
direction of the country through to the northwest direction of                                                 day than for the winter day. The tilt of the Earth’s rotational
the country.                                                                                                   axis relative to the orbital plane coupled with the Earth’s rev-
                                                                                                               olution round the sun gives rise to seasonal modulation of
                                                                                                               the solar flux. In summer, a given hemisphere is tilted to-
                                                                                                               wards the sun and so experiences greater solar intensity than

Ann. Geophys., 28, 1431–1439, 2010                                                                                                       www.ann-geophys.net/28/1431/2010/
           map for for 2008, Day Nunmber 13, 13, 10:00 fof2 (MHz)                                                          (MHz)
      foF2foF2 mapYearYear 2008, Day Nunmber 10:00 UT UT fof2 (MHz) foF2 map for Year 2008, Day Nunmber 104, 10:00 UT fof2fof2 (MHz)
                                                            12 12       foF2 map for Year 2008, Day Nunmber 104, 10:00 UT 12
                                                                                                                               12
        -15 -15                                                                                 -15 -15
D. I. Okoh et al.: Developing an ionospheric map for South Africa
                                                      10                                                                                                                101435
                                                                              10                                                                                           10
        -20 -20                                                fof2 (MHz)    -20 -20
      foF2foF2 map for Year 2008, Nunmber 13, 10:0010:00 fof2 (MHz)
           map for Year 2008, Day Day Nunmber 13, UT UT 8                      foF2 map for Year Day Nunmber 104, 10:00 10:00 UT fof2 (MHz)
                                                                          foF2 map for Year 2008, 2008, Day Nunmber 104, UT fof2 (MHz)
                                                                                                                                    8 12
   Latitude




                                                                                         Latitude
  Latitude                                Madimbo
                                                              12 8  12                                           Madimbo
                                                                                                                                 12    8




                                                                                        Latitude
                                               Madimbo                                                                Madimbo

        -25 -25                                                                                  -25 -25
         -15 -15                                                                               -15 -15
                                                                        6
                                                                       10     10
                                                                              6                                                                                    10 6 10
                                                                                                                                                                         6
                              Louisvale                                                                                      Louisvale
                                    Louisvale                                                                                      Louisvale
        -30 -30
         -20 -20                                                                                 -30 -30
                                                                                               -20 -20
                                                                       84     8                                                                                    8 4 8
      Latitude




                                                                                                 Latitude
                                                         Madimbo              4                                                               Madimbo Madimbo           4
      Latitude




                                                                                    Latitude
                                                   Madimbo
                                           Grahamstown                                                                                   Grahamstown
                           Hermanus             Grahamstown                                                               Hermanus            Grahamstown
                                Hermanus                                                                                      Hermanus
        -35 -35
         -25 -25                                                                                 -35 -35
                                                                                               -25 -25
                                                        62                    6
                                                                              2                                                              6 2 62
                           20 25
           10 10 15 15 20 Louisvale 25 30 30 35 35 40 40
                                 Louisvale                                                                     Louisvale 25 25
                                                                                                  10 10 15 15 20 20 Louisvale 30 30 35 35 40 40
         -30 -30         Longitude
                               Longitude                                                       -30 -30           Longitude
                                                                                                                       Longitude
                                                                       4      4                                                              4   4
                                             Grahamstown
                                       Grahamstown                                                                                       Grahamstown
                                                                                                                                  Grahamstown
         (a) (a)
         -35 -35
                                Hermanus
                           Hermanus
                                                                                               -35(b)(b)
                                                                                                    -35
                                                                                                                            Hermanus
                                                                                                                     Hermanus


                                                                        2     2                                                                                    2      2
            10 10 15 15 20 20 25 25 30               30 35     3540   40                         10         1015   1520      20
                                                                                                                              25         25
                                                                                                                                          30       30
                                                                                                                                                    35       35
                                                                                                                                                              40   40
(a)                          Longitude
                                Longitude                                                 (b)                             Longitude
                                                                                                                              Longitude
                                                           fof2 (MHz)
       foF2foF2 map for Year 2008, Nunmber 195,195, 10:00 UT fof2 (MHz)                        map for               Nunmber 287, 10:00 UT fof2 (MHz)
                                                                                          foF2foF2 mapYear 2008, Day Day Nunmber 287, 10:00 UT fof2 (MHz)
            map
         (a) (a)for Year 2008, Day Day Nunmber 10:00 UT          12 12                      (b) (b) for Year 2008,                               12 12

         -15 -15                                                                                -15 -15
                                                                            10 10                                                                                       10 10

        foF2 foF2 map for Year 2008,Nunmber 195, 10:0010:00 UT (MHz) (MHz) foF2-20 -20 Year 2008, 2008, Day Nunmber 287, UT fof2 (MHz) (MHz)
         -20 map for Year 2008, Day Day Nunmber 195, UT fof2 fof2
              -20                                                                map for for Year Day Nunmber 287, 10:00 10:00 UT fof2
                                                                                  foF2 map
                                                                    12 12                                                                12 8 12
                                                                                        Latitude


                                                                      8 8                                                   Madimbo            8
      Latitude




                                                                                        Latitude
                                                       Madimbo                                                                   Madimbo
      Latitude




                                                            Madimbo
          -15 -15                                                             -15 -15
                                                                                -25 -25
         -25 -25
                                                                    10 10                                                                10
                                                                      6 6                                                                   6 10
                                                                                                                                               6
                                Louisvale                                                            Louisvale
                                                                                                            Louisvale
          -20 -20                      Louisvale                              -20 -20
         -30 -30                                                                -30 -30
                                                                                                                                         8
                                                                                    Latitude




                                                                    8
                                                                                                                                            4 84
                                                                                                                        Madimbo Madimbo
       Latitude




                                                      Madimbo
                                                                                                 Latitude




                                                                        8
      Latitude




                                                            Madimbo
                                                                      4 4                                          Grahamstown
                                              Grahamstown                                                               Grahamstown
          -25 -25           Hermanus
                                                   Grahamstown                -25 -25            Hermanus
                                                                                                       Hermanus
         -35 -35
                                  Hermanus                                      -35 -35
                                                                    6                                                                    6
                                Louisvale
                                                                        6
                                                                      2 2                         Louisvale                                 2 62
                                        Louisvale
            10 10 15 15 20 20 25 25 30 30 35 35 40 40                              10 10 15 15 20 20 Louisvale 30 30 35 35 40 40
                                                                                                             25 25
          -30 -30                                                             -30 -30
(c)                            Longitude
                                     Longitude                               (d)                    Longitude
                                                                                                           Longitude                     4
                                                                    4   4                                     Grahamstown                     4
                                           Grahamstown                                                               Hermanus                  Grahamstown
                       Hermanus                  Grahamstown                                Hermanus
Fig. 4.-35                                                            -35 -35
        Variations
                                                                         (d) day,
      (c) (c) in foF2 over South Africa for (a) a summer day, (b) an autumn (d) (c) a winter day and (d) a spring day.
           -35
                             Hermanus

                                                         2                                                                                                         2
                                                             2          10      15    20      25      30     35     40                                                    2
         10      15    20       25       30    35    40                       10    15      20       25    30      35                                              40
              10    15      20        25    30    35    40                               Longitude
                             Longitude
                          Longitude                                                                                      Longitude
in winter when the same hemisphere will be tilted away from                             4.4            Latitudinal and longitudinal variations
the sun.
         (c) (c)                                                                                (d)
                                                                                                (d)
                                                                                        The sun is mostly overhead at locations on, or close to, the
4.3      Variations over a solar cycle                                                  equator, than at higher latitudes. For this reason the sun’s
                                                                                        radiation is more intense at lower latitudes than at higher lat-
The map exhibits the expected change in ionospheric re-                                 itudes. The overall effect is a gradual degradation in iono-
sponse to varying degrees of solar activity. Figure 5a to c                             spheric photo-ionization, and hence foF2 values, as we move
shows the foF2 maps for identical periods of year 2000 (a                               from the equatorial regions to the polar regions. This degra-
year of solar maximum), year 2003 (a year of moderate solar                             dation is evident in each of the foF2 maps illustrated. Lon-
activity), and year 2006 (a year of minimum solar activity)                             gitudinal variations are related to local-time variations; for a
respectively. The maps are for 08:00 UT and day number 50                               given locality, the intensity of solar radiation increases as the
(early autumn) of each of the chosen years.                                             sun rises, peaks at about local midday, and decreases as the
   As expected, the maps show large foF2 values for the year                            sun sets.
of maximum solar activity, moderate foF2 values for the year                               Movies produced using these maps (available from the
of moderate solar activity, and lower foF2 values for the year                          first author) more clearly demonstrate the group of varia-
of minimum solar activity.                                                              tions described above. The movies show, graphically and

www.ann-geophys.net/28/1431/2010/                                                                                             Ann. Geophys., 28, 1431–1439, 2010
                 1436                                                                                               D. I. Okoh et al.: Developing an ionospheric map for South Africa

                                                                                                                             dynamically, the spatial and temporal variations in the foF2
                            foF2 map for Year 2000, Day Nunmber 50, 08:00 UT fof2 (MHz)                                      values over the South African region. 08:00 UT fof2 (MHz)
                                                                                                                              foF2 map for Year 2003, Day Nunmber 50,
                                                                                   12                                                                                                12

                                      -15                                                                                    4.5 Performance of the map at storm times
                                                                                                                                -15
                                                                                                             10                                                                      10
                                                                                                           A comparison is made here between the MAP-foF2 val-
                                      -20                                                                     -20
                                                                                                           ues, the IRI-foF2 values, and the ionosonde foF2 values
                                                                                                 8                                                                8
                                                                                                           from the 3 ionosonde stations (Grahamstown, Louisvale, and
                           Latitude




                                                                                                                             Latitude
                                                                               Madimbo                                                           Madimbo
                                                                                                           Madimbo) at storm times. The storm periods considered
                                      -25                                                                     -25
                                                                                                           were taken from times of year 2000 to year 2006 when the
                                                                                                 6                                                                6
                                                          Louisvale                                        geomagnetic storm-time index, Dst, was less than −100 nT,
                                                                                                                                  Louisvale
                  -30                                                                                      and when ionosonde data was available. Table 2 illustrates
                                                                                                              -30
2000, Day Nunmber 50, 08:00 UT fof2 (MHz)                                 foF2 map for Year 2003, Day Nunmber 50, 08:00 UT fof2 (MHz)
                                     12                                                          4         the average of the absolute percentage deviations of the Map-
                                                                                                                                 12                               4
                                                                       Grahamstown
                                                       Hermanus                                                                 IRI-foF2 Grahamstown
                                                                                                           foF2 values and theHermanus      values from the ionosonde-foF2
                                      -35                                   -15                               -35
                                                                                                           values, calculated using the formula given in Eq. (3).
                                                        10                                                   2                                         10                            2
                                        10        15   20         25           30               35     40                               10       15   20      25   30    35    40
                                                                                                                             Average of absolute percentage deviations
                 (a)                                     Longitude -20                                                                       n
                                                                                                                                                       Longitude
 fof2 (MHz)           foF2 map for Year 2003, Day Nunmber 50, 08:00 UT fof2 (MHz)
                                          8                                                                     100 × xi8 i xi
                                                                                                                        −¯
                                                                        Latitude




                        Madimbo                                                                     Madimbo
        12                                                                         12                                   x
                                                                                                          i=1
                         (a)                                -25                                       (b)
                                                                                                      =                                                       (3)
                         -15
                                                                                                                    n
                                          6                                                                             6
        10
      Louisvale                                                                    10
                                                                                  Louisvale      Where the xi s represent the ionosonde-foF2 values, the xi s ¯
                                                            -30                                  represent the MAP-foF2 values or the IRI-foF2 values as the
                         -20
                                          4                                                                             4
                                                                                                 case may be, and n represents the sample size.
        8       Grahamstown                                  foF2 map for YearHermanus Day Nunmber 50, 08:00 UT fof2 (MHz)
                                                                                   8
                                                                               2006,        Grahamstown
                           Latitude




                                                               Madimbo
  Hermanus                                                                                          The Map-foF2 values used here are without incorporating
                                                                                                                            12
                                                            -35
                         -25                                                                     the ionosonde data into the map. Incorporating the ionosonde
                                          2                    -15                                                      2
 20 6 25            30       35    40                          10      15     20 6 25            data 35          40
                                                                                                30 would mean a zero deviation of the Map-foF2 from the
                                             Louisvale                                                                      10
    Longitude                                                                    Longitude ionosonde foF2.
                         -30
                                                               -20                                  To graphically present the results of the comparison, a sub-
        4                                                                          4
                                                              (b)
                                                       Grahamstown                               set of storm times were selected from the periods when Dst
                                                                                                                            8
                                                                               Latitude




                                                                                                        Madimbo
                                         Hermanus
                         -35                                                                     was less than −100 nT, such that the periods under consid-
                                                               -25                               eration were representative of all seasons of a year, and all
        2                                                                          2
0                           10   15      20        25      30       35     40                                               6
                                                                                                 times of a day. The results are illustrated in Fig. 7a–c and re-
                                                                                                                 Louisvale
                 (b)                                     Longitude                                                     veal that both models provide good estimates of foF2 during
                                                                  -30
                              foF2 map for Year 2006, Day Nunmber 50, 08:00 UT fof2 (MHz)                              storm times, with Map-foF2 being slightly closer to the ob-
                                                                                                                                             4
                                                                                     12
                                        (b)                                                                 Hermanus
                                                                                                                       servations than IRI-foF2 where the two models deviated sig-
                                                                                                                      Grahamstown

                                      -15                                                 -35                          nificantly. In general, the overall improved performance of
                                                                                             10       15     10 25 Map-foF2 over 40
                                                                                                            20             30     35
                                                                                                                                       IRI-foF2 is in line with the idea that the IRI
                                                                                                                                             2
                                                                                                                       model performs better when adapted with regional data. Ear-
                   -20                                                                                         Longitude work by McKinnell (2002) also showed that SABIM per-
                                                                                                                       lier
ear 2006, Day Nunmber 50, 08:00 UT fof2 (MHz)                                                                8         formed better than IRI over Grahamstown when compared to
                       Latitude




                                                                                          Madimbo
                                         12
                   -25
                                                                                                (c)                    data recorded with the Grahamstown digisonde.
                                                                                                                           The storm times in Fig. 7a–c are shown in the format
                                                                                                             6
                                                           10
                                                          Louisvale                                                    “YYYY-DDD-HH” where YYYY is year, DDD is day of
                                      -30                                                                              the year, and HH is hour of the day. Dst indices were ob-
                                                                                                             4         tained from the website of the World Data Centre for Geo-
                                        Madimbo              8         Grahamstown
                                                       Hermanus                                                        magnetism (http://wdc.kugi.kyoto-u.ac.jp/dstdir/).
                                      -35
                                                                                                             2
                                        10        15   20 6 25                 30               35     40
         Louisvale
                                                         Longitude                                                           5          Comparison with GPS-derived TEC
                     (c)
                                              4                                                                              This section illustrates a comparison between vertical Total
                 Fig. 5. Variations in foF2 over South Africa for (a) a year of max-
                   Grahamstown
     Hermanus               (c)
                 imum solar activity, (b) a year of moderate solar activity and (c) a                                        Electron Content (TEC) values derived from Global Posi-
                 year of minimum solar activity.                                                                             tioning System (GPS) receivers and vertical TEC values ob-
                                              2
    20          25      30      35     40                                                                                    tained as follows:
         Longitude
                 Ann. Geophys., 28, 1431–1439, 2010                                                                                                         www.ann-geophys.net/28/1431/2010/
D. I. Okoh et al.: Developing an ionospheric map for South Africa                                                                     1437

                      -12

                      -14

                      -16

                      -18
                                                                                                        Ionosonde
                                                                                                        Stations
                      -20                                                                               GPS
                                                                                                        Receiver
                      -22                                                   Madimbo
                                                                                                        Stations
                      -24                                             Pietersburg

                      -26

                      -28
                                                  Louisvale
                                                               Bloemfontein
                      -30
                                              Calvinia
                      -32
                                                               Grahamstown
                      -34                    Hermanus
                                15           20           25          30            35

Fig. 6. Location of the ionosonde stations and the GPS receiver stations used for the validation of the TEC values obtained from the map.


Table 1. Correlation coefficients between GPS-TEC values, IRI-TEC values and MAP-TEC values.

                                     Correlation coefficient between        Correlation coefficient between
                    Station                                                                                 Sample size
                                        GPS-TEC and IRI-TEC                  GPS-TEC and MAP-TEC
                    Bloemfontein                   0.69                                  0.83                   201
                    Calvinia                       0.73                                  0.85                   189
                    Pietersburg                    0.72                                  0.83                   135




  1. Vertical TEC values were derived from the map using                      1. The Bloemfontein station (26.3◦ E, 29.1◦ S) was chosen
     the IRI model; the IRI model was adapted with foF2 and                      to be representative of places inside the quadrilateral re-
     hmF2 values from the map so as to output the TEC val-                       gion bounded by the 4 ionosonde stations in the South
     ues. For simplicity and quick identification, TEC values                     African ionosonde network. Bloemfontein is in the cen-
     derived using this method will be referred to as MAP-                       tral part of the country.
     TEC.
                                                                              2. The Calvinia station (19.8◦ E, 31.5◦ S) was chosen to
  2. Vertical TEC values were also obtained from the IRI                         be representative of places outside the quadrilateral re-
     model using the standard IRI options. For simplicity                        gion bounded by the 4 ionosonde stations in the South
     and quick identification, TEC values derived using this                      African ionosonde network. Calvinia lies in the south-
     method will be referred to as IRI-TEC.                                      western part of the country.
                                                                              3. The Pietersburg station (29.5◦ E, 23.9◦ S) was chosen to
Vertical TEC values were also derived from GPS data using
                                                                                 be representative of places close to the ionosonde sta-
the method described by Opperman et al. (2007). This will
                                                                                 tions. Pietersburg is close to Madimbo, in the north-
be referred to as GPS-TEC.
                                                                                 eastern part of the country.
   To obtain the MAP-TEC and the IRI-TEC, the upper in-
tegration limit of the IRI model was set to 20 200 km (the                 Available GPS-TEC values for 10:00 UT of each day were
altitude of the GPS satellites), and the NeQuick option of the             used. IRI-TEC values and MAP-TEC values corresponding
IRI was used for the topside of the ionosphere.                            to these times were obtained. Figure 8a to c illustrates how
   GPS data from the following three dual-frequency GPS re-                the IRI-TEC and MAP-TEC values compare with the GPS-
ceiver stations within South Africa were used. The stations                TEC values at the three selected GPS receiver stations. We
were chosen to cover certain regions of interest and are indi-             note here that since the GPS-TEC values are measured val-
cated in Fig. 6.                                                           ues, they show much more day-to-day variability than either

www.ann-geophys.net/28/1431/2010/                                                                   Ann. Geophys., 28, 1431–1439, 2010
1438                                                                                    D. I. Okoh et al.: Developing an ionospheric map for South Africa


Table 2. Deviation of the MAP-foF2 values and the IRI-foF2 values from ionosonde-foF2 values, as calculated from Eq. (3).

                                                 Average of the absolute percentage                 Average of the absolute percentage
                        Station                                                                                                                                                                Sample size
                                                     deviations of the MAP-foF2                      deviations of the IRI-foF2 values
                                             values from the ionosonde foF2 values (%)             from the ionosonde foF2 values (%)
                        Grahamstown                            18.14                                                                         70
                                                                                                                                                            22.24                                    488
                        Louisvale                              12.58                                                                                        13.25                                    273
                                                                                                                                             60
                        Madimbo                                16.57                                                                                        20.10                                    188
                                                                                                                                             50




                                                                                                       TEC (TEC ) TEC (TEC ) TEC (TEC )
                                                                                                                                     U
                                                                                                                                             70
                                                                                                                                             40                                                                   GPS-TEC
                                                                                                                                             60
                                                                                                                                             30                                                                   IRI-TEC
                                                                                                                                             50                                                                   MAP-TEC
                                                                                                                                             20




                                                                                                                          U
                                                                                                                                             70
                                                                                                                                             40
                                                                                                                                             10                                                                   GPS-TEC
                                                                                                                                             60
                                                                                                                                             30                                                                   IRI-TEC
                                                                                                                                              0
                                                                                                                                                                                                                  MAP-TEC
                   14                                                                                                                        50 40
                                                                                                                                             20        90          140       190       240     290         340




                                                                                                               U
                                                                                                                                                                         Day of Year
                   12                                                                                                                        40
                                                                                                                                             10                                                                   GPS-TEC
                                                                                           (a)
  foF2 (MHz)




                                                                                                                                             30                                                                   IRI-TEC
                   10                                                                                                                         0
                                                                                                                                                                                                                  MAP-TEC
                                                                                                                                             20 40     90          140       190       240     290          340
                   8
                                                                                                                                                                         Day of Year
                                                                                                                                             10
                   6                                                   Map-foF2            (a)                                               90
                                                                                                                                              0
                                                                                                                                             80
                   4                                                   IRI-foF2                                                                   40   90          140       190       240     290         340
                                                                                                                                             70
                   2                                                   Ionosonde-foF2            (a)                                                                     Day of Year
                                                                                                                                             60


                                                                                                          TEC (TECU) TEC (TECU) TEC (TECU)
                   0
                                                                                           (a)                                               90
                                                                                                                                             50
                                                                                                                                                                                                                  GPS-TEC
                                                                                                                                             80                                                                   IRI-TEC
                        2000-312-01
                        2003-150-02
                        2004-205-03
                        2006-349-04
                        2002-108-05
                        2000-198-06
                        2005-135-08
                        2001-091-10
                        2001-310-12
                        2001-079-13
                        2002-274-15
                        2003-325-17
                        2000-043-19
                        2003-230-20
                        2002-131-21




                                                                                                                                             40
                                                                                                                                             70                                                                   MAP-TEC
                                                                                                                                             30
                                                                                                                                             60
                                                                                                                                             20
                                                                                                                                             90                                                                   GPS-TEC
                                                                                                                                             50
                                                                                                                                             10
                                                                                                                                             80                                                                   IRI-TEC
                                                                                                                                             40
(a)                     Storm times in format (YYYY-DDD-HH)                                                                                   0
                                                                                                                                             70                                                                   MAP-TEC
                                                                                                                                             30
                                                                                                                                             60 50                150              250               350
                                                                                                                                             20                                                                   GPS-TEC
                   14                                                                                                                        50                          Day of Year
                                                                                                                                             10                                                                   IRI-TEC
                                                                                           (b)                                               40
                   12                                                                                                                         0                                                                   MAP-TEC
      foF2 (MHz)




                                                                                                                                             30
                                                                                                                                                50                150              250               350
                   10                                                                                                                        20
                                                                                                 (b)                                                                     Day of Year
                    8                                                                                                                        10
                                                                                                                                             60
                                                                                           (b)
                                                                                                                                              0
                    6                                                  Map-foF2                                                              50 50                150              250               350
                    4                                                  IRI-foF2                                                                                          Day of Year
                                                                                                                                             40
                                                                                                       TEC (TECU) TEC (TECU) TEC (TECU)




                    2                                                  Ionosonde-foF2      (b)                                               60                                                                   GPS-TEC
                                                                                                                                             30                                                                   IRI-TEC
                    0                                                                                                                        50
                                                                                                                                                                                                                  MAP-TEC
                                                                                                                                             20
                        2000-279-01
                        2001-329-02
                        2000-262-03
                        2001-296-04
                        2004-207-05
                        2002-214-06
                        2005-135-08
                        2002-251-09
                        2003-169-10
                        2001-301-12
                        2002-110-13
                        2002-108-17
                        2001-310-18
                        2001-101-20
                        2001-079-22




                                                                                                                                             40
                                                                                                                                             60                                                                   GPS-TEC
                                                                                                                                             10
                                                                                                                                             30                                                                   IRI-TEC
                                                                                                                                             50
                                                                                                                                              0                                                                   MAP-TEC
                                                                                                                                             20
(b)                      Storm times in format (YYYY-DDD-HH)                                                                                 40 60          110            160           210          260
                                                                                                                                                                                                                  GPS-TEC
                                                                                                 (c)                                         10
                                                                                                                                             30
                                                                                                                                                                         Day of Year
                                                                                                                                                                                                                  IRI-TEC
                14                                                                         (c)
                                                                                                                                                                                                                  MAP-TEC
                                                                                                                                              0
                12                                                                            Fig. 8. Comparison of the 10:00 UT TEC values for (a) year 2003
                                                                                                       60      110      160       210
                                                                                                                                             20
                                                                                                                                           260
                                                                                              at Bloemfontein, (b) year 2002 at Calvinia and (c) year 2003 at
                                                                                                    10                Day of Year
   foF2 (MHz)




                10
                                                                                           (c)Pietersburg.
                                                                                                     0
                   8
                                                                                                                                                  60        110            160           210         260
                   6                                                   Map-foF2                                                                                          Day of Year
                   4                                                   IRI-foF2            (c)
                   2                                                   Ionosonde-foF2
                   0
                                                                                             of the modeled values, and as such we do not expect a perfect
                        2000-279-02
                        2006-349-03
                        2003-150-04
                        2004-207-05
                        2005-022-06
                        2002-251-07
                        2001-102-09
                        2006-104-10
                        2001-079-12
                        2001-274-14
                        2001-276-16
                        2004-042-17
                        2000-278-19
                        2001-078-22
                        2001-301-23




                                                                                             systematic relation between them. Table 1 shows the corre-
                                                                                             sponding correlation coefficients. Generally, both the IRI-
(c)                      Storm times in format (YYYY-DDD-HH)                                 TEC and the MAP-TEC correlate well with the GPS-TEC
                                                                                             values, but the results show a better correlation between the
Fig. 7. Comparison of storm time foF2 values for (a) Grahamstown,                            MAP-TEC and the GPS-TEC than the correlation between
(b) Louisvale and (c) Madimbo.                                                               the IRI-TEC and the GPS-TEC. This validates the improve-
                                                                                             ment obtained by using our foF2 map to adapt the IRI model
                                                                                             for the South African region.

Ann. Geophys., 28, 1431–1439, 2010                                                                                                                                       www.ann-geophys.net/28/1431/2010/
D. I. Okoh et al.: Developing an ionospheric map for South Africa                                                             1439

6   Conclusion                                                    Acknowledgements. Special thanks to the National Astrophysics
                                                                  and Space Science Programme (NASSP) for their financial support.
Most industries that make use of Earth-Space systems require      We appreciate the IRI team for making the IRI model available, and
ionospheric maps to adequately correct for the effects of the     Ben Opperman for helping with the GPS-derived TEC data.
ionosphere on their systems. This paper presented the de-             Topical Editor M. Pinnock thanks D. Bilitza and A. Belehaki for
velopment of an ionospheric map that can be used for this         their help in evaluating this paper.
purpose.
   A major limitation of the map is that it is spatially con-     References
strained to the South African region, a great concern for users
who will be interested in using the map beyond these bound-       Bilitza, D.: International Reference Ionosphere 2000, Radio Sci.,
aries. The techniques presented here can be applied to any           36, 261–275, 2001.
region with an adequate number of ionosondes. The possibil-       Bilitza, D. and Reinisch, B. W.: International Reference Ionosphere
ity of expanding the map to include other African countries          2007: Improvements and new parameters, Adv. Space Res., 42,
is proposed for future work. A major deterrent to achiev-            599–609, 2008.
ing this is the paucity of ionospheric information available      Chen, F. F.: Introduction to plasma physics and controlled fusion,
for the African continent, especially from ionosondes. How-          Springer, 1984.
                                                                  McKinnell, L. A.: A neural network based ionospheric model for
ever, given the increased availability of GPS receivers, GPS
                                                                     the bottomside electron density profile over Grahamstown, South
data may be used in conjunction with the available models.
                                                                     Africa, PhD Thesis, Rhodes University, 2002.
It is hoped that the map will subsequently be developed to        McKinnell, L. A. and Poole, A. W. V.: Neural network-based iono-
suit the needs of various users. It is also planned that this        spheric modelling over the South African region, South African
map, and subsequently a map covering Africa, will become             Journal of Science, 100, 519–523, 2004.
a regular real-time feature of the African Space Weather Re-      Opperman, B. D. L., Cilliers, P. J., McKinnell, L. A., and Hag-
gional Warning Centre website (http://spaceweather.hmo.ac.           gard, R.: Development of a regional GPS-based ionospheric TEC
za). Since the accuracy of the map greatly depends on that           model for South Africa, Adv. Space Res., 39, 808–815, 2007.
of the models used, and since both models are regularly up-       Poole, A. W. V. and McKinnell, L. A.: On the predictability of foF2
dated, the plan is to regularly update the map in line with the      using neural networks, Radio Sci., 35, 225–234, 2000.
models so as to maintain desirable accuracies.                    World Data Centre for Geomagnetism: Geomagnetic Dst Index
                                                                     Home Page, http://wdc.kugi.kyoto-u.ac.jp/dstdir/, 2009.




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