STEREO (Solar TErrestrial RElations Observatory) is a solar observation and space weather detection mission launched in October 2006 comprising two nearly identical spacecraft, one orbiting ahead of Earth (STEREO A), and one behind (STEREO B.) This enables stereoscopic imaging of the Sun and solar phenomena such as coronal mass ejections (CME’s or solar storms). These are violent eruptions of matter from the sun, a billion tons of material travelling at a million miles an hour, that can disrupt satellites and power grids and put astronauts on the International Space Station at risk. Images are returned to Earth from a range of cameras aboard the twin spacecraft which when analysed not only help us understand why solar storms happen but also enable the speed and direction of storms to be calculated providing an early warning system of Earth bound storms. That’s where Solar Stormwatch comes in. The mission has produced over 25 terabytes of data – more than 100,000 images – which are made into short videos for Stormwatchers to analyse.
But solar storms are not the only thing the cameras have captured. Dust, comets and planets make regular appearances in the videos. The STEREO spacecraft are now further away from the Earth than they are from the Sun but at the start of the mission the Earth and Moon were up close in the field of view. Because the cameras were designed to detect the tenuous and faint light scattered by the solar wind (100 million million times fainter than the Sun) the bright Earth-Moon system caused all sorts of odd reflections in the camera optics.
The Solar Stormwatch picture of the year is, in fact, a picture of an optical effect! The Earth is just out of view on the right. The intensely bright sunlight reflecting off the Earth produced internal reflections in the telescope attached to the STEREO B camera causing a bright flare with a “ghost” ring to appear on the image. Fondly known as the “White Doughnut” is has made an appearance in several Solar Stormwatch videos but Stormwatchers thought that this appearance was a particularly beautiful one.
[Many thank to The Solar Stormwatch Forum moderator Jules for putting this post together and organising the vote for the Image of the Year]