September 24-??, 1997
The official NASA/NOAA Press Release is available here
Solar Observations from SOHO
A partial-halo event was observed by the SOHO instruments EIT and LASCO
beginning around 03:00 UT on Sept 24th. The event was associated with active region
number 8088 and a coronal shock wave was observed propagating across the
solar disk in EIT. LASCO observed a CME that started to the SE and then swept
around to the west limb. The event encompassed about 180 degrees including
both limbs, but was not observed to the north. This event may be related
to the M-class flare observed at about the same time. A large CME was also
observed which was related to the long duration C-class event on Sep 23/2136-2350UT.
The image taken at 22:02 UT with the LASCO C2 telescope did not show the event
but the following image at 23:48 showed that the event was already formed.
The WIND/WAVES instrument observed clear Type III radio bursts at around 0130
and 0250 UT on Sept 24, 1997.
The GEOTAIL Wave experiment (PWI) detected type III radio bursts at
0130 and 0250 Sept 24, the latter of which agrees with the 0248 time of
type II and IV events reported by NOAA.
The SEC at NOAA are predicting a geomagnetic disturbance based upon
several factors. The M5 had a strong type II and IV. A very weak enhancement
in the GOES >10 MeV proton flux was observed about 3 hours
after the flare. The location of the source region was approaching
central meridian. Coronal Moreton waves were observed with the M5 and an
earlier long duration C-class event between 23/2136-2350UT. The
longitudinal extent of the CME with the M5 was large.
Based upon classical predictors and the SOHO observations, a disturbance
is expected to arrive late on 26 Sept. Active to Major geomagnetic storm
conditions are predicted with isolated periods at severe storm possible
during 26-27 Sept.
From Jean-Louis Bougeret - A strong type III storm re-appeared on
September 25. WIND/WAVES observed the first long lasting storm of
Type IIIs from September 9-18, when the storm obviously went over
the West limb of the Sun. These storms are very closely associated
to the development of complex active regions on the Sun. They
are typically SOLAR MAXIMUM phenomena. The appearance of a new storm
on Sept 25 means that the second complex sunspot group that has
appeared on the Sun also follows the solar maximum scenario. Ground
based radioastronomy shows that a long-lasting noise storm has also
developped at higher frequencies (200 MHz - 300 MHz). This means that
we have a well identified helmet streamer type of structure, which is
opened into the interplanetary space and releases quasi-permanent
fluxes of energetic particles along an interplanetary structure that
appears to corrotate with the sun.
From Mike Desch - A solar Type III storm occurred on the 25th
that continues on into the 26th. This may be evidence of the
emergence of a new active region since 8088 is supposed to be winding
down. There was no evidence of any type II emission on the 25th or
26th. There was a only moderate increase in AKR toward the end of
(Courtesy of Gary Heckman, NOAA)
The predicted geomagnetic storm intensity:
The daily A-Index is predicted to be about 40 on a scale of zero to 400. The most intense storm of the last 10 years was in March, 1989, when the index was 246. There has been one geomagnetic storm a year for the last three years where the A-Index reach
ed 50 or greater. When the new solar cycle gets to high levels beginning probably early in 1999, such conditions may occur several times per month during the most active months.
The predicted K-index for every three hours through the day is for several values of 6 with a chance of a 7 on a scale from zero to 9. The event in March of 1989 had several values of 8 and 9.
At a K of 6, satellites begin to experience a noticeable
increase in atmospheric drag resulting in changes to their orbits. At
K of 6 or 7, electric power systems begin to notice the effects. At K
of 6, the aurora borealis is frequently visible over the northern
United States. At K of 7 it may be visible to about the middle of the
US. In March of 1989, it was reported from many locations around the
Gulf of Mexico. It has been reported that in some of the largest events
of the last 100 years, the aurora from the north and south converged at
The solar flares that have been observed were one Class
C and two Class M. The flare scale is based on the intensity of x-rays
from the sun. Class C is a small event, Class M is 10 times stronger,
and Class X is 100 times stronger than Class C. There were 10 Class X
events between March 6 and 18 of 1989.
The solar proton radiation event as measured in space
near the Earth was almost undetectable. The event of September 1989 was
about 100,000 times larger.
The value of these events and these observations,
especially from the NASA satellites SOHO, WIND, the Advanced Composition
Explorer (ACE) and the NOAA instrument Solar X-ray Imager scheduled to
fly on the GOES weather satellites beginning 1999 is that they provide
an opportunity to calibrate all the new instrumentation that has become
available since 1989 and will provide forecasters with far better tools
to predict these events. Using the old systems that were available in
1989, there was a 50 percent false alarm rate in the forecasts and many
events that occurred were not forecast.
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Last Updated: 11/19/97