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Bertaux will present an amazing video sequence from images gathered by SWAN at a SOHO science workshop held in Paris this week from June 22 to 25. 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. Below: Depicted here is a set of
full-sky images taken by SOHO's SWAN instrument 10 days apart
- on 20 July and 30 July, 1996. July 20, 1996:
The right disk in the July 20th image shows the celestial hemisphere
which is illuminated by the side of the Sun facing the Earth.
The left disk shows the celestial hemisphere beyond the Sun as
it is illuminated by the ultraviolet light emitted by the Sun's
far side. In this image from 20 July, a lighter shade is visible
on the right portion of the left disk. This indicates the presence
of an active region on the far side of the Sun as it emits strong
beams of ultraviolet light. July 30, 1996: In the
30 July image, the lighter area that was on the left disk has
spread to the right disk, i.e. to the opposite celestial hemisphere.
This shift was concurrent with an active region appearing on
the side of the Sun facing the Earth. On the right of each picture
is a green EIT (Extreme ultraviolet Imaging Telescope) photograph
with no hot spots visible on July 20, and an active region visible
on July 30. Click for a larger two
frame animation of the images below.
"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 spaceAlthough 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." |
Radiation from
the Sun blows a bubble in this cloud about one astronomical unit
across. The bubble's inside surface forms a sort of theatre screen.
When radiation from solar active regions hit the screen, the
hydrogen gas begins to glow and UV "hot spots" are
formed. Because the hydrogen bubble is so large -- it's bigger
than Earth's orbit in some directions -- it's 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. 
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.
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ESA Press Release - June 22, 1999 SOHO-8 Workshop -- June 22 - 25, 1999 SOHO - home page The Sun - from the SEDS Nine Planets web site More images -- from SWAN Related
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