Statistical Survey of Electron Pitch Angle Distributions in the Nightside CentralPlasma Sheet

摘要

The diffuse aurora is produced by precipitating electrons whose origin isbelieved to be the central plasma sheet. The process by which these electrons are continually scattered into the atmospheric loss cone is unresolved. Arguments included processes which cause pitch angle scattering at the strong diffussion limit. A survey of equatorial pitch angle distributions of about 0.2-20KeV electrons has been conducted to determine the extent of pitch angle diffusion. Pitch angle distributions have been binned by location and geomagnetic activity for the nightside near earth plasma sheet. The average pitch angle anistropy was determined for each bin. Representative samples were selected from geomagnetically quite (Kp < or =) distributions. Of the three energies examined (1.09, 4.52, 19.keV), only distributions consistent with moderate to weak diffusion were observed. Even test cases were consistent with only moderate diffusion. These results conclude that strong diffusion is usually not in effect in the quiet near earth plasma sheet. Electron Cyclotron Harmonic (ECH) waves at the 3/2 harmonic are suggested as the scattering mechanism responsible for maintaining the diffuse aurora. The necessary ECH wave amplitudes required to recreate average distributions were calculated. Derived amplitudes were one to three orders of magnitude greater than those amplitudes observed. Thus, it is concluded ECH do not occur with the frequency and amplitude necessary to sustain the continuous diffuse aurora. Theses. (jhd)