The Sun in 3D

The STEREO (Solar Terrestrial RElations Observatory) mission is an ambitious attempt to make three-dimensional observations of the Sun. To do this, two near-identical NASA spacecraft have recently been launched into orbits that carry them away from the Earth in opposite directions at around a million kilometres a year. The two spacecraft (named Ahead or Behind, depending on their position relative to the Earth) will be far enough apart this coming April to begin their joint stereoscopic observation of the Sun.

While most of the telescopes onboard the spacecraft look towards the Sun, a British-built instrument, called the Heliospheric Imager, constantly monitors the space between the Sun and the Earth, watching for any CMEs heading our way. These storms contain a billion tons of electrified gas traveling at a million miles an hour, and this gas, known as a plasma, is at temperatures around one hundred thousand degrees. When such a cloud reaches the Earth, it can cause spectacular space weather effects. It can also damage spacecraft, and even black out electrical grids on the ground.

The UK contribution to STEREO are the twin Heliospheric Imagers (HIs), developed by a UK-led consortium from the Rutherford Appleton Laboratory and the University of Birmingham, in conjunction with the Centre Spatial de Liege, Belgium, and the Naval Research Laboratory, USA.

Each instrument contains two wide field cameras (HI-1 and HI-2) mounted inside a series of precision baffles which remove nearly all scattered sunlight, allowing the sensors to detect features that are one hundred million, million times fainter than the Sun, despite the fact that the Sun is only a few degrees outside the field of view. HI-1 has a 20˚ field of view, while HI-2 encompasses a 70˚ field. In this way, each instrument array is capable of imaging a 90˚ section of the sky, allowing CMEs to be followed from the Sun all the way out to the Earth.

STEREO observed its first Coronal mass Ejection (CME) on December 9 2006. A ‘coronagraph’ is essentially an opaque mask that blocks the bright disk of the Sun, allowing scientists to see its faint outer atmosphere, the corona. The white circle shows the location of the solar disk, and the CME can be seen on the right hand side of the image as an outward-directed streak ending in a faint ring.