Using velocity dispersion to put outer limits on the source of short duration pulsating aurora

Abstract:

A recently published observation of pulsating aurora with a period of around 30ms suggests a source for the modulation low in the magnetosphere, away from the equatorial plane, due to the expected velocity dispersion of the auroral precipitation. We developed a numerical time-of-flight calculation to determine the dispersion at the ionosphere of electrons precipitated from a given location along the field line, then feed the result into an electron transport code to determine an emission intensity profile. Using source location and plasma temperature as adjustable parameters, we then step through the parameter space, building up an index of outer-bound estimates of the location for modulation, correlated to various observed auroral pulsation periods. Both weak diffusion and strong diffusion cases are considered. Preliminary results suggest that for realistic plasma temperatures these short pulsations are modulated below around 1.5 R_e above the ionosphere.