The far side of the Sun torments space weather forecasters, hiding its powerful and destructive eruptions from Earth until it's too late.
Now, two new instruments are on line to help by providing an extra week's early warning.
The Solar and Heliospheric Observatory (SOHO) spacecraft's Solar Wind Anisotropies (SWAN) instrument sees ultraviolet rays sweeping like a lighthouse beam across interplanetary gas beyond the Sun, while the Michelson Doppler Imager (MDI) instrument peers right through the Sun to locate hidden sunspots and their active regions.
Data from both instruments are now available routinely to everyone, including the forecasters of space weather. U.S. and European scientific teams contributed the instruments to this international project.
"What started as unusual research has become an everyday tool," said Jean-Loup Bertaux of the CNRS Service d'Aéronomie near Paris, who leads the French-Finnish team responsible for the SWAN instrument. "We should no longer be taken by surprise by highly active regions that suddenly come into view as the Sun rotates."
Solar storms, sparked by sunspots and coronal mass ejections on the Sun, can send torrents of charged particles at Earth within a half-hour to three days of their onset, disrupting satellite function and electrical systems on Earth, as well as posing a threat to space travelers in their path.
Seeing through the Sun
The Sun takes roughly four weeks to turn completely around on its axis, but active regions can appear and grow in only a few days. So until two years ago, no one had any way of telling when an active region might come "around the corner" -- perhaps blazing away with eruptions as soon as it appeared.
If an active region can be detected in the middle of the far side it will appear on the eastern (left-hand) side of the visible disk about seven days later. The SWAN team announced such ultraviolet observations in June 1999.
In March 2000, Charles Lindsey of the Solar Physics Research Corp. in Tucson and Doug Braun of NorthWest Research Associates in Boulder reported that they had detected, with SOHO's MDI, sound waves reflected from far-side sunspots.
Speeded by the intense magnetic fields associated with sunspot regions, the sound waves arrived a few seconds early at the Sun's near-side face, compared with sound waves from sunspot-free regions. Decoding MDI data from a million points on the Sun's near side to obtain an impression of the far side uses a technique called helioseismic holography and requires a powerful computer.
Both discoveries were made retrospectively from SOHO's archives. Since then, teams have streamlined their data gathering and analyses to the point where they can offer routine long-range forecasts of intense solar activity based on far-side foresight. The techniques are complementary, with MDI seeing the sunspot regions and SWAN reporting how active they are.
"When we started work with SOHO five years ago, most experts thought it would be impossible to see right through the Sun," said solar scientist Philip Scherrer of Stanford University, principal investigator for the MDI instrument. "Now we do it regularly in real time. For practical purposes we've made the Sun transparent."
Although conceived for scientific research, SOHO has proved invaluable as a watchdog for spotting sunstorms. Forecasters already rely heavily on SOHO's round-the-clock observations of flares and coronal mass ejections that can affect satellites, power lines and other technological systems.
Next page: SOHO's revelations
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