PROPERTIES OF SPIKE EVENTS IN AURORAL RADIO ABSORPTION

The spatial extent and the spectral index of spike events in auroral radio absorption observed with riometers is considered. On the evidence of narrow- and wide-beam observations at more than one radio frequency and satellite particle data, it is shown that the typical spike event extends for only tens of kilometers in the ionosphere, compared with several hundred kilometers for auroral absorption in general. The spectral index is found to be close to 2. 0, as for the general absorption, implying that the spike absorption is not produced at an unusually low altitude. It is concluded that for spike events the electron precipitation is very intense, is not unusually energetic, and is spatially restricted.