Korte by xiangpeng

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									Utility and accuracy of geomagnetic
        repeat station surveys
     M. Korte(1), M. Mandea(1) and P. Kotze(2)
        (1) GeoForschungsZentrum Potsdam, Germany
       (2) Hermanus Magnetic Observatory, South Africa



   Acknowledgements for participation in field work and data
                          processing:
    M. Fredow(1) , A. Hemshorn(1), E. Julies(2), E. Nahayo(2),

                 B. Pretorius(2) , M. Schüler(1)
              “Traditional” applications
• Mapping of regional reference main and secular variation field
  - more accurate than IGRF in regions with sparse observatory
  coverage
  - more frequent updates compared to IGRF




     Example: Secular variation from Southern African repeat station data
     2004 and 2005 (Kotzé et al., 2007)
                 “New” applications I
• Models of lithospheric vector anomalies by combination of satellite
  (large scale, vector), aeromagnetic (small scale, F only) and repeat
  station ground (localized intermediate scale, vector) data.


     X anomaly               Y anomaly              Z anomaly




       Example: Lithospheric vector R-SCHA model for Germany
                      (Thébault and Korte, 2007)
                           “New” applications II
   •   There is increasing evidence of small-scale secular variation, which might
       be due to mantle/lithospheric conductivity (induction) e.g.
       - models from satellite data do not describe SV completely e.g. in the
         southern African region with strong gradients
       - decadal models fail to describe long-term SV at some European locations
   •   Repeat station time series are useful to investigate this
   •   Very high data accuracy necessary!




Differences between European observatory
data and the CM4 model (Sabaka et al., 2002)
show insufficient SV description at several    Detailed European SV model by E. Thébault and
locations (Verbanac et al., 2007)              the MagNetE (Magnetic Network of Europe) group
               The problem of accuracy
• Measurement errors:
  - likely to be higher than at an observatory
  - can be kept small by careful set-up and measurement
    procedure
• Main problem: elimination of external field influences
  - reduction to quiet night time or annual mean
  - variation recordings from nearest observatory or on-site variometer

    C(xi,tmean) = C(xi,ti) – C(O,ti) + C(O, tmean)


   Repeat station        Repeat station          Observatory    Observatory
   “annual mean”         measurement             recording at   annual mean
   of component C        value at time ti        time ti        of component C


                         This difference determined most robustly
                         from quiet night time values (on-site variometer)
               Example for using a local variometer

                           Differences between recordings of the LEMI variometer
                   at a German repeat station and NGK observatory recordings for 3 days

          0
          -5
         -10

30 nT    -15
         -20
         -25                                H(nT)
         -30
               0      6    12   18    24   30     36         42   48      54    60    66    72    78
                                                       [h]
                                                  EBH/6.5.-9.5.2006/0-8.29 UTC/LEMI-NGK
        -400
        -405
        -410
30 nT   -415


                                                 D(nT)
        -420
        -425
        -430
               0      6    12   18    24   30     36         42   48      54    60    66    72    78
                                                       [h]             Quiet hours used for final data
                                                                       reduction to annual means
        -355
        -360
        -365

30 nT   -370
        -375
        -380                                Z(nT)
        -385
               0      6    12   18    24   30     36         42   48      54    60    66    72    78

                                                       [h]
                  Local variometers I
• Short term variations well known
• Data reduction by means of quiet night time values, which are
  assumed to represent undisturbed core (+lithospheric) field
• Optimum: on-site variometer for several days to include truly quiet
  night time
  Compromises:
  - on-site variometer for one full night with measurements
    in the evening and morning
  - regional variometers no further than ~100 km away for
    several nights
                 Local variometers II
• Short term variations well known
• Data reduction by means of quiet night time values, which are
  assumed to represent undisturbed core (+lithospheric) field
• Optimum: on-site variometer for several days to include truly quiet
  night time
  Compromises:
  - on-site variometer for one full night with measurements
    in the evening and morning
  - regional variometers no further than ~100 km away for
    several nights
• Stability of variometer should be checked by several measurements
  (baseline)
• Temperature control for variometer is important!
        Test comparison: stable temperature

10 nT




10 nT




10 nT




          S       Stable temperature
20°
              E
        Test comparison: sensor temperature
                      change
60 nT




60 nT




60 nT




40°       S
              E
        Test comparison: electronics temperature
                        change
60 nT




60 nT




60 nT




65°               E
              S
 Improvement to results by variometer
Experience values from repeat station surveys in Germany and southern Africa

• Baseline difference between evening and morning or over up to 10 days in
  general in the order of 1 nT compared to a couple of nT if using only
  observatory recordings

• Differences to the nearest observatory often differ by several nT for individual
  measurements without variometer, but are robust for quiet night time means

• Differences to the nearest observatory can differ systematically from quiet
  night time differences

• Maximum deviation from the mean of 8 measurements mostly no larger
  than 1.5 nT

• However, the time for the variometer temperature to stabilize can vary
  significantly depending on climatic conditions:
  Germany: often up to 6 hours
  Southern Africa: mostly only 2 to 3 hours
                    Quiet night times
• How well do quiet night time values reflect undisturbed internal field?
• Is an average over one arbitrary night comparable to a quiet night
  time average?

• Studies on data from southern African observatories
  HER, HBK and TSU
  - all night annual means (6:00 pm to 6:00 am) and quiet night time
    annual means (0:00 to 4:00 am, Kp < 2) in general agree within 1 nT
  - However, the night time annual means differ from the standard
    annual means by up to 9 nT
                     Quiet night times
• Quiet night time differences between 2 observatories over a year
  (2001), averages subtracted




 - Individual minute differences lie in the order of up to 5 nT
 - Individual one night averages (6:00 pm to 6:00 am) mostly lie within
   +/- 2 nT of the monthly average of quiet night time values for quiet
   and moderately disturbed nights (Kp up to about 4)
 - Secular variation has to be taken into account in reduction to annual mean
                        Conclusions
• Repeat station surveys, particularly long time series, can be useful
  for several purposes
• New applications require high accuracy of repeat station data.
  A suitable processing to eliminate the external field influences is
  extremely important
• The use of on-site variometers is optimal, but their stability
  has to be ensured
• Variometer recordings (averages) over only one full night can be
  a good compromise under quiet to moderately disturbed conditions

								
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