SOHO ready for next cosmic events
17 May 1999
Three months after a challenging, long-distance space rescue, the European Space Agency's Solar and Heliospheric Observatory (SOHO) is preparing for two important cosmic events: the 11 August solar eclipse in Europe and Asia - the last of this millennium - and an increase in solar activity on the Sun itself, called a solar maximum - thefirst of the next millennium.The two-tonne spacecraft is the largest and most sophisticated solar observatory ever launched. It is virtually suspended between the Earth and the Sun, in a spot where the gravity of the two celestial bodies cancel each other out.
SOHO observes the Sun's outermost layer - called the corona - using five instruments:
- the Extreme Ultraviolet Imaging Telescope (EIT)
- the Large Angle Spectroscopic Coronagraph (LASCO)
- the Ultraviolet Coronagraph Spectrometer (UVCS)
- the Solar Ultraviolet Measurement of Emitted Radiation (SUMER)
- the Coronal Diagnostic Spectrometer (CDS).
Scientists using ground telescopes to observe details of the corona during the 2=-minute total eclipse will have the opportunity to compare their pictures with those taken by SOHO.
Despite the eclipse, the spacecraft will have a direct view of the Sun's surface. In short, it will see no eclipse. That's possible because SOHO is four times farther from Earth than the distance between the Earth and the moon, explains Pel Brekke, SOHO's deputy project scientist.(see diagram) 
The spacecraft will gather images of both the corona and the Sun surface before, during and after the eclipse. That will add valuable information to the interpretation of the pictures gathered by ground-based instruments. The satellite will be watching for other phenomena that may occur on the surface of the Sun, such as coronal mass ejections - enormous gas explosions that send shock waves across the solar disk and even can cause magnetic storms on Earth.
Currently, scientists are detecting up to five coronal mass ejections per day. "The eclipse gives us a unique opportunity to observe the corona from the ground and to perform all of the standard ground-based coronal observations. But it's very important to see what happens on the Sun with SOHO," Brekke says.
"If a coronal mass ejection happens during the eclipse, it may be observed from the ground. At the same time, data from SOHO will provide information on the events on the Sun surface that triggered the ejection."
Solar Maximum Watch While the Sun is approaching one of its 11-year peaks of activity, now expected in 2000, the spacecraft is also keeping a watchful eye on the sunspots. The sunspot number - an index used by scientists to determine the level of solar activity - is the result of the actual sum of individual sunspots and groups of sunspots multiplied by a constant value related to the type of telescope used for the observations.
Scientists currently see an average sunspot number of 65, whereas the predicted number by now should have reached 130. The assumption is based on some of the most recent cycles, which were all quite strong, Brekke says. This could be an indication of a solar maximum much weaker than expected. If that's true, he says, it could also mean a lower chance of electrical problems at northern latitudes on our planet, and fewer disruptions for satellites in space, SOHO included.
Despite the apparent weak start of the maximum - which could still escalate to a full-blown peak - Brekke says, "SOHO is already seeing a lot more activity than last year. And the corona looks very different. It's very stunning to see how the corona has changed over the last two to three years."
