One of the highest hopes for SOHO, the European Space Agency (ESA)-NASA spacecraft is fulfilled with the detection of sunspots on the invisible far side of the Sun. This scientific marvel promises practical benefits. It could give an extra week’s warning of possible bad weather in space, to astronauts and operators of satellites, power networks and other systems liable to be affected by eruptions on the Sun linked to sunspots.

The story is told today in the journal Science by Charles Lindsey of Tucson, Arizona, and Doug Braun of Boulder, Colorado. They realised that the analytical witchcraft called helioseismic holography might open a window right through the Sun. And the technique worked when they used it to decode waves seen on the visible surface by one of SOHO’s instruments, the Michelson Doppler Imager, or MDI.

“We’ve known for ten years that in theory we could make the Sun transparent all the way to the far side,” said Charles Lindsey. “But we needed observations of exceptional quality. In the end we got them, from MDI on SOHO.”

For more than 100 years scientists have been aware that groups of dark sunspots on the Sun’s visible face are often the scene of flares and other eruptions. Nowadays they watch the Sun more closely than ever, because modern systems are much more vulnerable to solar disturbances than old-style technology was.

The experts can still be taken by surprise, because the Sun turns on its axis. A large group of previously hidden sunspots can suddenly swing into view on the eastern (left-hand) edge of the Sun. It may already be blazing away with menacing eruptions. With a far-side preview of sunspots, nasty shocks for the space weather forecasters may now be avoidable.

Last year, French and Finnish scientists used SWAN, another instrument on SOHO, to detect activity on the far side. They saw an ultraviolet glow lighting up gas in the Solar System beyond the Sun, and moving across the sky like a lighthouse beam as the Sun rotated. The method used by Lindsey and Braun with MDI data is completely different, and it pinpoints the source of the activity on the far side.


Solar seismology chalks up another success

Detection of sound waves reverberating through the Sun opened its gassy interior for investigation, in much the same way as seismologists learned to explore the Earth’s rocky interior with earthquake waves.

Using special telescopes on the ground and in space, helioseismologists detect many different modes of vibration appearing at the Sun’s surface, all with tales to tell about how the interior is

structured and how the gas moves about.

The SOHO spacecraft is an ideal platform for helioseismology because its station 1.5 million kilometres out in space allows it to watch the Sun for 24 hours a day. Its own motions are very gentle — an important consideration when scientists are looking for subtle motions on the Sun’s surface.

Developed and operated by a Californian team, the MDI instrument is the most elaborate of three helioseismic instruments on SOHO. It measures rhythmic motions at a million points across the Sun’s visible surface.

Computers can interpret the motions in terms of sound waves travelling through the Sun. The waves are affected by the various layers and movements of gas that they encounter. MDI has already revealed many unknown features of the interior, including layers where the speed of the gas changes abruptly and hidden jet streams circling the Sun’s poles. The team is also discovering what goes on underneath sunspots on the near side of the Sun.

Philip Scherrer of Stanford University, California, leads the MDI team. He is gratified but not surprised that his instrument has chalked up another success, with the detection of sunspots on the far side.

“Up till now we’ve explored the Sun’s interior quite thoroughly from the near surface down to the core,” Scherrer commented. “Charlie Lindsey and Doug Braun told me many years ago how they hoped to use MDI on SOHO to see all the way to the far side. I was always sure they could do it.”

The technique of helioseismic holography used by Lindsey and Braun examines a wide ring of sound waves that emanate from a small region on the far side, and reach the near side by rebounding internally from the solar surface. A sunspot group reveals itself because the Sun’s surface is depressed and very strong magnetic fields speed up the sound waves.

As a result the sound waves arrive at the front side about 6 seconds earlier than equivalent waves from sunspot-free regions, in a total travel time of about 3 hours. The change in speed becomes evident when sound waves shuttling back and forth get out of step with one another.

MDI data for 28-29 March 1998 revealed, on the far side, a sunspot group that was not plainly visible on the near side until ten days later. Observations for 24 hours were more than sufficient to detect the sunspots, which means that routine monitoring is a realistic possibility.

The far-side sunspots are a good example of why this spacecraft is so exciting to work with,” said Bernhard Fleck, ESA’s project scientist for SOHO. “We can make a completely new discovery in fundamental solar physics, and immediately think of applying it to the practical task of monitoring the daily activity of the Sun and predicting its effects on the Earth.”

The SOHO project is an international cooperation between the European Space Agency (ESA) and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA. NASA launched SOHO in December 1995, and in 1998 ESA and NASA decided to extend its highly successful operations until 2003.


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