SOHO glimpses far side of the Sun, looks at a comet's shadow
22 June 1999Scientists have found that they can peek around the Sun and predict whether solar storms on its far side will shortly appear on the side facing the Earth. This surprising discovery by SOHO's SWAN instrument could help to predict the solar storms that sometimes threaten the Earth. SWAN has seen something else extraordinary - the biggest shadow ever observed in our solar system, that of a comet projected on the sky behind it.
Using the ESA/NASA Solar and Heliospheric Observatory (SOHO), the researchers have seen hot-spots as they light up a cloud of hydrogen atoms in space with strong beams of ultraviolet light. This moving glow in the sky (invisible from Earth) could give several days' warning of an active region on the Sun's surface that will come into view as the Sun rotates.
SWAN, short for Solar Wind Anisotropies, is used to map the whole sky in ultraviolet light. It sees a huge cloud of interstellar hydrogen that bathes the entire Solar System and interacts with the solar wind. The cloud is relatively tenuous - about 100 atoms per litre - yet it is thick enough to shine when illuminated by the Sun's ultraviolet light.
This kind of observation is impossible from Earth because the atmosphere completely filters the short-wavelength ultraviolet light. Even spacecraft in orbit around the Earth are blinded to the hydrogen haze of the Solar System by a large swarm of hydrogen atoms that surrounds our planet.
But that's not the case for SWAN - one of 12 instruments aboard SOHO, which operates from a special vantage point 1.5 million kilometres out in space, on the sunward side of Earth. The SWAN scientists detect hot-spots when the hydrogen cloud beyond the Sun glows more strongly than would be expected if the Sun were uniformly bright on its far side.
"Strong ultraviolet emissions from active regions on the back of the Sun behave like beams from a lighthouse on the landscape," says Jean-Loup Bertaux, of the CNRS Service d'Aironomie in France, and principal investigator for SWAN. "They move in the sky in accordance with the Sun's rotation," which takes about 28 days. "We can monitor the activity on the back side of the Sun without looking at it directly. This method could be used in future studies on space weather, which can seriously affect orbiting satellites and other technological systems on Earth." Bertaux reports his team's discovery on 22-25 June at a scientific meeting where more than 200 scientists from all over the world are gathered to discuss their findings on the dynamics and diagnostics of the solar transition region and corona. This meeting, which is the eighth in a series of SOHO workshops, takes place in Paris (CAP 15, 1-13, Quai de Grenelle, 75015 Paris).
Bertaux will present an amazing video sequence from images gathered by SWAN. These show the reflection of big solar spots imprinted in the sky and rotating with the Sun. Eventually, they come around the limb, on the eastern (left-hand) side of its visible surface.
"With SOHO we have studied the Sun from the inside out, as well as its surroundings. It is fascinating to think that now we could foresee even what's in store for us on the other side of the Sun," says Martin Huber, Head of ESA's Space Science Department.
A Comet's Shadow in Space
Although most of the hydrogen atoms in the Solar System blow in from interstellar space, comets are surrounded by large hydrogen clouds of their own. When comet Hale-Bopp flew near the Sun parading its 100-million-kilometre-long tail in 1997, SOHO was already in orbit. In SWAN observations from that time, the scientists have now spotted a remarkable feature - never before seen by astronomers - the elongated shadow, more than 150 million kilometres long, of a comet projected on the sky beyond the comet.
Says Bernhard Fleck, SOHO Project Scientist for ESA: "The nice thing about this discovery is that with SOHO we're not just confined to studying the Sun. Here we are contributing to a different and intriguing field. We're learning more about comets and their physics."
As Hale-Bopp neared the Sun - at a distance of about 150 million kilometres - water-ice in the comet's nucleus began to vaporize. As expected, the Sun's ultraviolet radiation split the water molecules, liberating a cloud of hydrogen atoms, which glowed in the ultraviolet light.
With the distance between the comet and the Sun quickly decreasing, the amount of solar radiation hitting the comet increased, as did the release of vapour from the nucleus and the consequent production of hydrogen. As a result, in a huge, 10 million kilometre-wide region around the nucleus, the comet absorbed most of the ultraviolet light it received from the Sun.
In ultraviolet light, the comet projected a distinct shadow on the hydrogen haze of the Solar System. For an imaginary ultraviolet-eyed onlooker situated on the side of the comet opposite the Sun, it would have been a perfect opportunity to observe a total solar eclipse by a comet!
"This phenomenon provides an absolute determination of the amount of hydrogen and water released by the comet - about 300 tonnes per second," says Bertaux.
Roger Bonnet, ESA's Director of the Scientific Programme, expressed his appreciation for the SOHO results:
"After the dramatic loss in space last year and a miraculous recovery in the following months, SOHO is back at work and fully operational. As in the case of the comet's shadow, it keeps making discoveries and amazing observations."
Note for editors:SOHO is a project of international cooperation between ESA and NASA. The spacecraft was launched by an Atlas rocket from Cape Canaveral on 2 December 1995.