SOHO Spies the Far Side of the Sun
SOHO spies the far side of the Sun
A new technique for viewing the previously hidden
side of the Sun could improve space weather forecasting.
June 23, 1999: Scientists 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, short for Solar Wind Anisotropies, is a telescope on board
SOHO that can map the whole sky in ultraviolet light. This kind
of observation is impossible from Earth because the atmosphere
completely filters short-wavelength UV rays. A European team
of scientists headed by Jean Loup Bertaux, of the CNRS Service
d'Aronomie in France, have discovered an ingenious way to use
SWAN's UV mapping capabilities to infer what's on the far side
of the Sun.
Above: A white light picture of the
sun as seen from the Big
Bear Solar Observatory at at 14:37:11 (UT) on June 22, 1999.
The sunspot group near the left hand side of the image has just
rotated into the field of view on June 21. Using the new technique
described in this story, scientists could have detected the active
region while it was still invisible to Earth on the other side
of the sun.
The solar system is embedded in a huge cloud of interstellar
hydrogen gas (see the figure below). The cloud is relatively
tenuous - about 100 atoms per litre - yet it is thick enough
to be partially opaque to ultaviolet light. Just as visible light
cannot pass through a block of wood, short wavelength UV light
does not pass easily through neutral (un-ionized) hydrogen gas.
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.
"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'Aéronomie
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."
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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.
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"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! 
Right: SOHO's SWAN instrument sees
the shadow of comet Hale-Bopp, 7 March 1997
"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."
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
Stories:
June 2: Solar
Flares show their true colors
- New research points to a common mechanism for spectral behavior
in Solar Flares
May 31: "Cool" microflares
could be solar hot spots - One longstanding mystery of
the sun is why its outer atmosphere - the corona - is 200 times
hotter than its surface. Now, a trio of scientists says it's
because the corona is heated by incessant mini-explosions, called
microflares.
March
9: Finding
the 'smoking gun' before it fires. Physicists discover a new
tool for predicting solar eruptions.
March 3: Future telescope could shatter solar
high-resolution barrier
Oct. 19,
1998: Sunspot cycle closely following
prediction.
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For more information, please contact: Dr. John M. Horack , Director of Science Communications |
Author: Dr.
Tony Phillips Curator: Bryan Walls NASA Official: John M. Horack |