Observations and modeling of wave‐induced microburst electron precipitation

Abstract

Energy‐time features of X ray microbursts are examined and compared with the predictions of a test particle simulation model of wave‐induced electron precipitation resulting from gyroresonant wave‐particle interactions in the magnetosphere. The observations were obtained on a balloon flight at Siple station, Antarctica (L ≈ 4.2) on December 30, 1980. The energy and time evolution of the microbursts were studied for X rays in the energy range from 25 to 175 keV. An algorithm designed to search the E > 25 keV counting rate data for single isolated microbursts identified 651 events in a 3‐hr interval. The distribution of burst durations ranged from 0.2 to 1.2 s. Approximately two‐thirds of the distribution were narrow bursts (0.2–0.6 s), the rest wide (0.6–1.2 s), with the average burst durations equal to ∼0.4 s and ∼0.7 s, respectively, for the two classes. The precipitation was characterized by exponential electron spectra with e‐folding energies Eo of 25‐50 keV. Individual and superposed microburst profiles show that the X ray energy spectrum is softest (equivalent to a reduction in electron Eo of ≤ 5 keV) near the peak of the energy influx. Computer simulations of the flux‐ and energy‐time profiles of direct and mirrored electron precipitation induced by a whistler‐mode wave pulse of 0.2‐s duration and linear frequency increase from 2 to 4 kHz were performed for plasma, energetic particle and wave parameters appropriate for the location and geophysical conditions of the observations. The wave pulse is representative of the VLF chorus elements that were observed at Siple station and at the conjugate location of Roberval, Quebec during the microburst activity on this day. In both the direct and mirrored cases the combined effect of latitude dependence of the energy of maximum gyroresonant scattering and energy dispersion leads to energy spectrum variations qualitatively consistent with the microburst observations. The average burst durations in the two cases were ∼0.3 s (direct) and ∼0.7 s (mirrored). On this basis we identify the narrow microbursts with electrons which are precipitated directly after undergoing pitch‐angle scattering interactions with the wave pulse; the wide microbursts are identified with electrons which, after interacting with the wave, mirror in the conjugate ionosphere before precipitating into the southern ionosphere. In general, the results provide further support for the gyroresonant test particle simulation model, and for the belief that the observed type of microbursts originates in the vicinity of the magnetic equator in a gyroresonant process involving discrete chorus emissions.