A Warm-weather Observer�s Guide to the Aurora by guy21


									A Warm-weather Observer’s
   Guide to the Aurora

           Mark Haun
   U of I Astronomical Society
        24 February 2003
 Aurora basics
  Solar activity
  Earth’s magnetosphere; magnetic storms

  What to look for

 The aurora alarm
 Slide show
   What causes the aurora
(northern and southern lights)?

    The aurora is caused by
 charged particles from the sun.
Solar activity
 Follows an 11-year cycle
 Tied to the sun’s magnetic field
 Last peak in 2001
 At solar maximum, more
   Sunspots
   Energetic events (flares, prominences)

   Disturbances in the solar wind
Solar flares

      Reconfiguration of
      magnetic field lines to
      lower energy state
      Huge energy release
      X-ray classifications:
      A, B, C, M, X
      as measured by GOES
      weather satellites
(from Today’s Space Weather)
Coronal mass ejections (CMEs)
 Large puffs of solar plasma, usually
 launched by solar flares
 Creates a shock wave in the solar wind,
 often followed by a magnetic cloud
 Transit time to Earth: 1 to 3 days
 (faster is better)
 Observe with SOHO coronagraphs:
   Full halo CME: It’s headed right for us!?
   Best results from flares near the center of
    the sun’s disk
SOHO LASCO C3 coronagraph
   six-day loop, April 2001
The Waiting Game
        The perfect flare
Timing is everything!
 Flare near center of solar disk?
 High-speed, full-halo CME?
 Projected arrival time after noon but
 before midnight?
 Good characteristics behind the shock?
 Favorable moon phase?
 No clouds?
Analysis and forecasts
 NOAA Space Environment Center
 (conservative, just the facts)
 Solar-terrestrial Dispatch
 (more detail, discussion forum, targeted
 to aurora-watchers; also IRC service)
     Sample SEC forecast & report
Joint USAF/NOAA Report of Solar and Geophysical Activity
SDF Number 033 Issued at 2200Z on 02 Feb 2003

IA. Analysis of Solar Active Regions and Activity from 01/2100Z to 02/2100Z: Solar activity
was low. Region 276 (S14E66) produced numerous C-flares during the past 24 hours. The
largest of these was a C2/Sf at 0334 UTC. The region appears to be an E-type sunspot group
with an area of about 240 millionths. Region 274 (S06E05) showed some growth during the
past 24 hours, and displayed occasional brightenings but did not produce any flare level
activity. The remaining solar active regions were quiet and stable.

IB. Solar Activity Forecast: Solar activity is expected to be mostly low, but there is a fair
chance for an isolated M-class event from Region 276 sometime during the next three days

IIA. Geophysical Activity Summary 01/2100Z to 02/2100Z: The geomagnetic field ranged
from active to minor storm levels with a period of major storm levels at high latitudes from
1200-1500 UTC. The enhanced solar wind flow which was reported yesterday continued
during the past 24 hours, with speeds in the 500 to 700 km/s range and total magnetic field
around 10 to 12 nT. Occasional periods of southward turning of the interplanetary magnetic
field were associated with times of enhanced geomagnetic activity.
       Forecast & report (continued)
IIB. Geophysical Activity Forecast: The geomagnetic field is expected to be mostly unsettled
to active during the next 24 hours, but there will probably be some periods of minor storm
levels as the current disturbance persists partway into the first day. A decrease to mostly
unsettled is expected for the second day. An increase to unsettled to active is anticipated on
The third day in response to a favorably positioned coronal hole.

III. Event Probabilities 03 Feb-05 Feb
Class M 35/35/35
Class X 05/05/05
Proton 01/01/01
PCAF    green

VI. Geomagnetic Activity Probabilities 03 Feb-05 Feb
A.    Middle Latitudes
Active             35/25/30
Minor storm        20/15/20
Major-severe storm 10/05/05
ACE (Advanced Composition Explorer)
   ACE is stationed ~1 million mi. from
   Earth, toward the Sun
   Measures solar wind characteristics
   Gives ~30 minutes warning of incoming
   Instruments to watch:
     MAG (interplanetary magnetic field)
     SWEPAM (charged particles and solar
      wind parameters)
ACE data at onset of a
  magnetic storm
            Watch for:
             Sudden jump in solar
             wind speed, indicating
             shock front at leading
             edge of CME
             interplanetary magnetic
             field (negative values
             on Bz graph)
The magnetosphere
From the solar wind to the aurora
 Solar wind confines Earth’s magnetic field
 into comet-shaped region: magnetosphere
 Solar wind has its own magnetic field (IMF)
 When IMF points south (anti-parallel to
 Earth’s field), get reconnection
 This allows solar wind particles to enter the
 Complex processes accelerate particles
 (mostly electrons) into the upper
 atmosphere  aurora!
Magnetic storms
 Along with aurora, have strong electric
 currents circulating in the ionosphere, 
 fluctuations in the magnetic field
 measured on the ground
 Storm intensity; K index
 Other effects of magnetic storms:
  Geomagnetically induced currents
  Shortwave radio blackouts

  Atmospheric heating; more drag on satellites
(from Today’s Space Weather)
Observing the aurora
 Find a place dark enough to see the
 Milky Way
 If near a town, go to its north side
 Allow your eyes plenty of time to dark-
 If aurora is faint, may not perceive color
 Look for vertical ray structure, and
 changes on timescale of minutes
Auroral forms: rays
Auroral forms: rayed arc
Auroral forms: diffuse
Auroral forms: corona
 Easy, and a good way to record detail
 your eye cannot see
 Best camera: old SLR with standard
 fixed-focal-length lens (e.g. 50 mm f/2)
 and “bulb” mode on shutter
 Expose from 2 to 30 seconds at f/2 and
 400 ISO. Bracket.
 Ask around for best film (sometimes
 “fast” film isn’t, due to reciprocity failure)
Satellite observations: POLAR
The Aurora Alarm
 Two optical detectors, in Walla Walla,
 WA and Urbana, IL
 Each consists of:
  Photomultiplier tube
  Optical filter for 558 nm “green line”

  Control circuitry which talks to a PC over its
   serial port
Reading the data
              Blue line (“high voltage”)
              records voltage supplied
              to photomultiplier tube
              PMT is operated in a
              feedback loop to keep
              output level constant
              (green line)
              Thus, blue line is a
              measurement of 558 nm
System architecture
Results so far
Walla Walla, WA
   11 April 1997
                           Urbana, IL
   4 May 1998                18 February 1999
   16, 23 July 1998
   22 August 1998            24 May 2000
   25 September 1998
   10 March 1999             12 August 2000
   17 April 1999
   13, 15 September 1999     31 March 2001
   15, 22 October 1999
   7 April 2000              18 April 2001
   24 May 2000
   29 October 2000           22 October 2001
   28, 29 November 2000
   20 March 2001             6 November 2001
   31 March—1 April 2001
   18 June 2001              24 November 2001
   25 October 2002
   2 November 2002
                             4, 8 September 2002
Mailing lists
 Two lists for alerts and monthly test
 messages only (safe for cell phones
 and pagers):
   aurora-midwest
   aurora-northwest

 aurora-announce, for administrative
 Subscribe/unsubscribe via web-based
Simultaneous observations
24 May 2000
31 March 2001
6 November 2001

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