Time‐dependent transport of field‐aligned bursts of electrons in flickering aurora

Abstract

We have developed a time‐dependent auroral electron transport model to study emission rates caused by field‐aligned bursts of electrons (FABs) seen in flickering aurora. We simulate flickering FABs by turning on and off a downward electron intensity distribution at a given frequency. We assume this electron beam originates and is modulated at an altitude of 4000 km. We apply collisionless transport from 4000 to 600 km and solve a time‐dependent Boltzmann equation below 600 km. Because FABs have significant flux over a large energy range, dispersion has the most important effect on the resulting emission rates. We find that for a 5 Hz flickering FAB, the column emission rate varies 93% from peak to valley, whereas for 100 Hz flickering, the variation in column emission rate is only 12% from peak to valley. This variation is dependent on the frequency and source altitude. We show that with a time‐dependent transport calculation and a filtered fast photometer or imager looking in the zenith, it is possible to obtain an upper limit on the altitude from which the optical flickering originates. We also study what electron detectors on a rocket or satellite might measure in the lower ionosphere when there exists field‐aligned bursts of electrons. Velocity dispersion calculations will give source altitudes much lower than is correct if they are derived from low energy electrons (<2 keV) measured at altitudes below 150 km. Our results agree with the interpretation that field‐aligned bursts are a temporal rather than spatial feature, and from this knowledge it should be possible to reconstruct the initial electron distribution function at the source altitude.