Aurorae

Sky Map with Aurorae

Sample all-sky orbit difference image, in a Hammer-Aitoff projection, showing a number of examples of SMEI aurora events and other features typically seen in the difference images. The data are from the period of intense geomagnetic activity in late October 2003; the start of the orbit is at 1137 UT 30 October. The numbers show the satellite’s geographic latitude over the Earth at the time of the observation. The Sun is at the center, indicated by a cross. Ecliptic north is up, east is to the left, and the antisunward direction is toward the extremes at either side. Streamers and flashes are diffuse features, easily distinguished from particle strikes that affect only one or a few pixels on a given CCD.

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.