The Solar Mass Ejection Imager (SMEI) was speciﬁcally designed to detect and forecast the arrival of solar mass ejections and other heliospheric structures which are moving towards the Earth. Such events may cause geomagnetic storms, with resulting radiation hazards and disruption to military and commercial communications; damage to Earth-orbiting spacecraft; and also terrestrial effects such as surges in transcontinental power transmission lines. The detectors are sensitive over the optical wave-band, which is measured using CCD cameras.
The instrument contains three identical cameras, each with a ﬁeld of view of 60◦ × 3◦, which are mounted onto the spacecraft such that they scan most of the sky every 102-min orbit. The sensitivity is such that changes in sky brightness equivalent to a tenth magnitude star in one square degree (one S10 unit) of sky may be detected. Each camera records an image every 4 seconds.
In order to extract the Thomson-scattering emission from a typical large coronal mass ejection, stellar images and the signal from the zodiacal dust cloud must be subtracted. This requires accurate relative photometry to 0.1%. As a by-product, images of stars and the zodiacal cloud are also measured to this photometric accuracy once per orbit. This enables studies of transient zodiacal cloud phenomena, ﬂare stars, supernovae, comets, and other varying point-like objects.
The Solar Mass Ejection Imager (SMEI) is currently ﬂying on the US Department of Defence Space Test Program ‘Coriolis’ mission. It was launched on 6 January 2003 from Vandenberg Air Force Base in California. Coriolis was inserted into an 840-km circular, Sun-synchronous polar orbit along the Earth’s terminator. After having delivered 8.6 years of nearly-full-sky photometric maps, SMEI was deactivated on September 28th, 2011.