#27.     Auroral Images

    While the aurora is plainly visible to the eye, it changes constantly. A camera is therefore a useful tool in studying its features. For instance, how high up is the aurora? In principle, two observers on the ground can tell, by comparing the position of the same auroral arc against the background stars. In practice, only photographs snapped at exactly the same instant give satisfactory accuracy. The method was pioneered by Martin Brendel in 1892 and was greatly expanded by Carl Stoermer around 1910; it turned out that auroras occured most frequently around 100 km (60 miles), though much higher auroras (usually red) were also sometimes seen     During the International Geophysical Year 1957-8, "all-sky cameras" were devised to record the entire aurora, horizon to horizon, by photographing its reflection (rather distorted!) from a curved mirror. But the best view is still from space. Starting in 1968, military satellites of the DMSP series scanned the land below them, their sensor sweeping again and again from far left to far right, perpendicular to their orbit. As the minutes passed, the scanned strips added up to full images, which often included auroral arcs.

    The picture on the right is an example of such an image, showing the bright aurora of the great magnetic storm of 14 March, 1989 (for a web document containing two articles on that storm, click here). The display stretches across Canada; just below it, in the middle of the picture, are the lights of Chicago, next to Lake Michigan. Other US cities are also visible, Florida is outlined and so is Hudson Bay, near the top of the picture.

Scientific Imagers in Space

    More comprehensive observations of the aurora from above were carried out (1981 - 1987) by the Dynamics Explorer 1 (DE-1) satellite which moved in an elongated polar orbit rising to 4.65 RE. Several modes of operation were used; some pictures were taken in the greenish oxygen "line" (emitted at the precise wavelength of 5577 angstrom) which usually dominates the aurora's appearance from the ground, but many used the ultra-violet. The typical time resolution was 2-12 minutes, enough to resolve the phases of a substorm but not finer details.

    Later auroral imagers included Sweden's "Viking" (in 1986) and " Freja." Currently, NASA's Polar (1996) collects images of the polar aurora using three cameras--in visible light (sometimes as frequently as 12 seconds apart), in ultra-violet and in x-rays.

Theta Aurora

    DE also studied a special class of auroras (mapped earlier by Isis 2), aligned not with the auroral oval but sticking out from it, into the dark interior around the magnetic pole, generally aligned with the sunward direction. They occur at quieter times, away from substorms. The DE imaging camera found that at times such arcs extended completely across the oval, bridging the dark space from the night side to the day side. The global pattern then resembled the Greek letter theta, a circle with a bar across its middle, and this form was therefore named the "theta aurora." No good explanation exists as yet for either sunward arcs or the theta aurora.

The Speed of Auroral Motions

    Sensitive TV cameras can filter out that light and view the aurora in other emissions, fainter but responding much more promptly. The resulting pictures show more details and change quite rapidly.