Numerical simulation of asymmetric particle precipitation by pitch angle diffusion

Abstract

A numerical simulation code is developed to evaluate the loss rate of particles trapped in a mirror magnetic field geometry with asymmetric loss cones. The one‐dimensional model can accommodate particle diffusion at any prescribed rate and loss cones of any prescribed sizes, and it incorporates the important effect of atmospheric backscattering. Numerical solutions for the loss cone particle distribution function calculated for the case of equal loss cones provide an acceptable simulation of the well‐known modified Bessel function solution. The code provides the first quantitative solutions for any specified rate of pitch angle scattering for the general case of arbitrary asymmetry in loss cone size. In the case of weak or moderate diffusion the ratio of particle precipitation fluxes into the two loss cones can provide a sensitive measurement of the rate of particle scattering, but to utilize this important diagnostic property, one must also have information on the fraction of particles that are backscattered from the atmosphere.

Modeling energetic electron precipitation near the South Atlantic anomaly

“…The relative contribution of the precipitating energetic electrons to atmospheric ion found to be strongly correlated with variations in the size of the loss cone boundary with pronounced localized maxima in the region of the South Atlantic anomaly. This spatial pattern of electron precipitation is consistent with the trend of satellite observations which have primarily been made at higher L shells[ Vampola and Gorney, 1983;Sheldon, 1991.…”

“…The effect of pitch angle scattering in a mirror magnetic field geometry is simulated by means of a simple one-dimensional model that specifies the particle phase space density f(c•) as a function of the distribution in pitch angle c• at the magnetic field minimum. Details of this model are given by Thorne et al [1996] and a brief summary is outlined in this section. For a quasi-dipolar planetary field, the function f(c•) is specified at the magnetic equatorial plane, where the field strength is B0.…”

Energetic electron precipitation from the inner zone

“…All other longitude regions should experience precipitation at levels well below the average values predicted by our calculations. We plan to study this longitudinal variability and North/South asymmetry in the precipitation using a recently developed asymmetric loss cone code (Thorne et al, 1996). This will allow us to predict the global pattern of electron precipitation from the inner radiation zone.…”