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 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.