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A composite of the Sun s disk viewed from Proba-2 and the corona imaged in July 2010. Credit: ESA/Proba-2 consortium/SWAP team/Institut d’Astrophysique de Paris A composite of the Sun's disk viewed from Proba-2 and the corona imaged in July 2010. Credit: ESA/Proba-2 consortium/SWAP team/Institut d’Astrophysique de Paris

Proba-2 will observe solar eclipse from orbit

Sen— Thousands of astronomers have descended on Australia for a total eclipse of the Sun that passes directly over the city of Cairns in Queensland on Wednesday morning, local time.

Many have crossed the world to get there and will be praying for clear skies so that they can witness a spectacle that will climax with two minutes of totality when the Moon completely blots out the Sun.

In space, a European solar satellite called Proba-2 will have an ideal vantage point to view the celestial conjunction without any clouds to worry about.

It won't catch totality because, unlike north-eastern Australia, it will not be directly in line with the eclipse's primary players. But instead it will experience three partial eclipses during its orbit around the Earth.

And at the very time when astronomers Downunder are experiencing the total phase, Proba-2 will have a clear view of the solar disk. This will allow scientists to compare what is happening on the Sun at that time with observations of the solar atmosphere, or corona, from the ground.

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How Proba-2 appears as it orbits the Earth. Credit: ESA

By an extraordinary chance of nature, the Moon and the Sun appear the same size in the sky, despite the Sun being 400 times further away. This allows observers during a total eclipse to view the solar atmosphere right up to the Sun's edge.

With the Sun blotted from view the ground-based observers are expected to see fiery prominences leaping from the Sun's circumference plus the corona that would usually be far too pale and ghostly to be seen from the ground. Its streaming shape will be determined by the magnetic field lines along which are channelled hot plasma.

The image, above, from the European Space Agency is a composite with the view of the solar disk from Proba-2 superimposed on a photograph of the majestic corona as imaged from the ground during the total eclipse of July 2010. In May this year, when observers in the Far East watched an annular eclipse of the Sun, Proba-2 recorded four partial eclipses from space.

May's solar eclipse captured by Proba-2 from orbit. Credit: ESA

Proba-2 is so-named because it is the second in the series of small satellites in the PRoject for OnBoard Autonomy project. It has two experiments for observing the Sun. The Large Yield Radiometer (LYRA) monitors four bands in a very wide ultraviolet spectrum, and an extreme-ultraviolet telescope (SWAP) makes measurements of the solar corona in a very narrow band.

The satellite also carries two experiments, both Czech-built, to monitor space weather. Its Dual Segmented Langmuir Probes (DSLP) measure electron density and temperature in the background plasma of the Earth’s magnetosphere, and a thermal plasma measurement unit (TPMU) checks ion densities and composition.

• Two NASA probes launched in August to investigate the effects of space weather on the magnetic field surrounding the Earth have been renamed. The twin Radiation Belt Storm Probes are now known as the Van Allen Probes in honour of physicist James Van Allen who, in 1958, discovered the radiation belts encircling our planet.

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