SOHO Tracks Recent Stormy Regions On Sun's Far Side As They Turn For Second Shot At Earth

http://www.gsfc.nasa.gov/news-releas...2003/03-98.htm

SOHO TRACKS RECENT STORMY REGIONS ON SUN'S FAR SIDE AS
THEY TURN FOR SECOND SHOT AT EARTH

Contact: Bill Steigerwald
NASA Goddard Space Flight Center
301-286-5017

November 21, 2003 - Release 03-98

Scientists using the Solar and Heliospheric
Observatory (SOHO) spacecraft have been able to
monitor the activity of the recent powerful
solar magnetic active regions that were hidden
on the far side of the Sun as they rotated with
the Sun to face the Earth again.

Principal Investigator Dr. Jean-Loup Bertaux and
colleague Dr. Eric Quémerais of the Service
d'Aeronomie in the Paris suburb of Verrieres le
Buisson have found that the activity of the
sunspot regions numbered 10486 and 10484 by the
National Oceanic and Atmospheric Administration
Space Environment Center (NOAA SEC) has
decreased dramatically in recent days. However,
even more recently (18 and 19 November), the
activity of these active regions has increased
again.

These giant active regions were on the side of
the Sun facing the Earth during the period
October 26 - November 4, when a series of
intense flares and coronal mass ejections
produced dramatic space weather effects. (There
were eight X-class flares, the most intense
classification of soft X-ray events measured by
NOAA's GOES spacecraft, from the two giant
regions.) On October 28, the shock wave driven
by a very fast coronal mass ejection (CME)
associated with an X28 flare accelerated
electrically charged particles that affected
spacecraft throughout the solar system.

The Sun rotates once every roughly 27 days at
its equator: would these active regions appear
again this week and create more problems for
satellite operators? Until recently, the problem
of knowing what was happening on the far side of
the Sun appeared intractable. In 2000, however,
researchers using both the Michelson Doppler
Imager (MDI) and Solar Wind ANisotropies (SWAN)
instruments on SOHO began producing data on
farside activity. MDI uses a holographic
reconstruction technique to map the presence of
sunspot groups that modify the transmission of
acoustic waves beneath the solar surface; SWAN
is able to determine how "active" the regions
are in the ultraviolet portion of the spectrum.

"Any hope of improving longer term forecasts of
space weather requires an ability to monitor
active regions as they transit the far side of
the Sun," said Dr. Joseph Kunches of NOAA SEC.
"SOHO instruments MDI, SWAN, and LASCO have
demonstrated the ability to track active regions
across the invisible disk in new ways,
benefiting forecasters and users of space
weather information."

"When we first proposed the SWAN instrument for
SOHO in 1989,we wanted to study the science of
the interaction of the solar wind with
interstellar gas. Now, we are delighted to see
that it could actually be useful outside the
field of pure science, by improving the
prediction of solar activity, which may impact
many sectors of technology like spacecraft
operations", said Bertaux.

SWAN can indirectly monitor the activity on the
far side of the Sun as it maps the whole sky in
ultraviolet light. A huge cloud of interstellar
hydrogen that bathes the entire Solar System
interacts with the solar wind, and lights up in
the Lyman-alpha spectral line when it is hit by
UV radiation from the Sun. Since active regions
on the Sun are brighter in Lyman-alpha light,
the part of the sky facing an active region is
brighter. Just as a rotating lighthouse beam
will illuminate different patches of fog, the
Sun's rotation produces a changing pattern of
Lyman-alpha illumination on the sky behind the
Sun's far side. Any change in the solar activity
is in this way directly reflected in the amount
of Lyman-alpha emission that is observed by
SWAN.

After October 28, the SWAN team started
observing farside activity more intensively.
They found that the Lyman-alpha sky brightness
could be correlated with the "Mg II index," a
measure of the integrated brightness of a strong
spectral line of ionized magnesium in the Sun's
outer atmosphere. The Mg II index is measured
from earth-orbiting spacecraft such as the UARS
and SORCE missions of NASA's Earth Science
enterprise, and is a good measure of total solar
activity.

The SWAN team used their data to estimate what
the MgII index would be for an observer rotating
with the Sun, and always facing a given active
region during the solar rotation. The MgII index
estimated from SWAN data increased up to
November 7, but then began rapidly decreasing.
The corresponding decrease of the solar
Lyman-alpha brightness found by SWAN was 20%, an
indication that the activity of the two active
regions decreased significantly since their
stunning performance on the near side of the
Sun. This method should prove of value to space
weather forecasters, who are just as interested
in predicting "clear" days as they are in
forecasting storms from the Sun. For images,
refer to:

http://soho.nascom.nasa.gov/hotshots/2003_11_20/