A s t r o  A l e r t
                               Sun-Earth Alert

                          Solar Terrestrial Dispatch
                            http://www.spacew.com
                            
                         Valid: Immediately

            * Middle Latitude Auroral Activity Warning Cancelled *

     Conditions in the solar wind are no longer strongly favorable for
producing visible levels of auroral activity over the middle latitudes.
Although there may be a period or two of weaker substorming during the local
evening hours of 24/25 May, the most intense phase of activity has ended.

     The source of this disturbance has not been precisely determined. It may
be related to a coronal mass ejection, or to the leading edge of a high speed
coronal hole stream. The unusually geoeffective nature of this disturbance is
attributed in-part to the close proximity of the heliospheric current sheet,
which was swept up in the disturbance as it passed the Earth, resulting in
periods of strongly enhanced southwardly directed magnetic fields imbedded
within the high velocity solar wind that resulted in periods of strong
auroral storm activity.

     The solar wind was also blowing at an unusually strong level near 600
kilometers per SECOND (2.16 million kilometers per hour, or about 1.35
million kilometers per hour) when the heliospheric current sheet (HCS) was
encountered. Compressed plasma in the vicinity of the HCS produced very
strong solar wind pressures against the Earth's magnetosphere, peaking at
near 34 nP at 03:35 UTC on 24 May which is more than 17 times stronger than
normal levels. This drove the sunward side of the Earth's magnetosphere
inward toward the Earth, possibly at times inside the location where
geosynchronous satellites operate. Although there were not any reported
magnetopause crossings by the GOES spacecraft (which were not ideally located
for observing magnetopause crossings at the time of highest magnetospheric
compression), other geosynchronously orbiting spacecraft crossing the noon
sector between about 00:30 and 04:00 UTC may have briefly crossed the
magnetopause boundary into the harsher conditions within the magnetosheath.

     It is clear this disturbance became intense enough to become visible
over many middle and some lower latitude regions. Observations as far south
as Oklahoma and Missouri were reported. Refer to:
http://www.spacew.com/www/auroras.html for a list of observed sightings.

     We do not expect any additional significant sightings of auroral
activity from the middle latitudes. The Earth will, over the next week, begin
entering the regime of one or possibly two additional coronal holes. Each may
be associated with minor increases in auroral activity, but models suggest
there will probably not be any significantly geoeffective interaction with
the heliospheric current sheet as occurred over the last 24 hours. The
heliospheric current sheet is now expected to dip well to the south of the
Earth's ecliptical plane and should therefore not interact with these
anticipated coronal hole streams.


** End of AstroAlert **