Short-burst auroral radiations in Alfvenic acceleration regions: FAST observations

We examined particle and field data during the first 3 years (1997-1999) of the FAST mission searching for short-burst auroral kilometric radio (AKR) emissions in Alfvenic auroral acceleration regions. Eight such events were found at altitudes between 2500 and 3600 km in the midnight local time sector during winter months when the AE index was observed to be greater than 150. The emission source regions are estimated to be similar to 300-900 km below the satellite. The frequency of observed short-bursts is in the range between 400 and 600 kHz. The average bandwidth of the burst, Delta f/f, is approximately 2 x 10(-2). The reoccurrence frequency of discrete short bursts is estimated to be between 7 and 18 Hz, which is similar to that of Jovian S-bursts. The negative drift of each discrete burst may be associated with upward moving electrons. As in the case for ordinary AKR, short-bursts can be explained by the electron cyclotron maser instability due to the positive gradient in the phase space of an unstable electron distribution. Shell, conic, and ring distributions, and a combination of these three, were observed in each of the eight events. Snapshots of observed multiple shells and rings are suggested to be the results of the temporal effect, where low energy electrons are accelerated by the Alfvenic perturbation at an earlier time, while high energy electrons are newly accelerated by later Alfvenic pulses. Results presented within this paper suggest multiple Alfvenic disturbances in the magnetotail as the source of the multiple shell ( and ring) distributions. Multiple Alfvenic disturbances may also explain the observed reoccurrence pattern of short-bursts.