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
Quemerais 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

“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: