Thursday, June 24, 1999 Published at 15:11 GMT 16:11 UK
Comet's shadow captured on cosmic screen
The white glow in the left image is the "projection" of the Sun's light
By BBC News Online Science Editor David Whitehouse
The shadow of a comet projected on a "screen" of hydrogen on the other side of the solar system has been seen by scientists.
They have also found a way to peek around the edge of the Sun and predict whether solar storms on its far side will appear on the side facing the Earth.
The discovery could help give warnings when solar storms threaten the Earth.
The solar system is embedded in a huge cloud of thin hydrogen gas that blocks ultaviolet light.
Radiation from the Sun blows a bubble in this cloud. The bubble's inside surface forms a sort of screen on which light from the Sun is projected.
When radiation from solar active regions strikes the screen, the hydrogen gas begins to glow and hot spots are formed.
Because the hydrogen bubble is so large - it is bigger than Earth's orbit around the Sun - it is possible for SWAN to see hot spots caused by active regions on the far side of the sun, simply by looking at the part of the sky which the far side of the sun faces.
"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 French Service d'Aéronomie and principal investigator for SWAN.
"They move in the sky in accordance with the Sun's rotation, which takes about 28 days. So we can monitor the activity on the back side of the Sun without looking at it directly."
When comet Hale-Bopp flew near the Sun parading its 100-million-kilometre-long tail in 1997, SWAN spotted a remarkable feature, never before seen by astronomers. A shadow, more than 150 million kilometres long, of a comet projected on the interior of the hydrogen bubble.