In near-Earth orbit, contributions from aurorae can obscure the Thomson-scattering brightness. One of the most notable serendipitous discoveries using the SMEI data was the measurement of high altitude aurora (Mizuno et al., 2005).
Before SMEI, aurorae above 1000 km were expected to be a rare phenomenon, and since SMEI viewed only above its 840 km altitude, aurorae were expected to be an insignificant contaminant. This assumption proved to be false, with these often saturating the cameras, most significantly during times of enhanced geomagnetic activity.
The SMEI aurorae are seen in two general forms: a flash, or a streamer. A flash is a bright region in which all the cameras are illuminated simultaneously, typically for ~ 2 minutes. Streamer events consist of one or more filaments curving asymptotically with increasing time toward the rearward direction of the satellite, these appear primarily in the antisolar hemisphere. These phenomena show much variation from orbit to orbit, typically appearing in a single isolated orbit or otherwise not identifiable as a consistent feature from one orbit to the next.
During SMEI's first year, a total of 823 SMEI orbits with flashes and 219 with streamers were observed: an average of one auroral signature every five days. A good correlation with geomagnetic activity indices was found. Flashes occurred as the spacecraft passed directly through a region brightly illuminated by the aurora, and the streamers as SMEI viewed columns of luminous material some distance away from the spacecraft.