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.

BASED ON A EUROPEAN SPACE AGENCY PRESS RELEASE

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."

Spacecraft in orbit around the Earth are blinded to this effect 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.

MPG video sequence of SWAN hot spots (1.2 MB)

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

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 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.

MPG video sequence of Hale-Bopp's UV shadow (0.48 MB)

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."

Web Links

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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