Thermalization and transport of photoelectrons: A comparison of theoretical approaches

Abstract

Three commonly cited methods to study ionospheric photoelectron thermalization and transport are compared. Transport effects are evaluated quite differently in the three approaches: one uses a diffusion equation formulation, the second proceeds from a two-stream approximation to the general particle transfer equation, and the third employs a Monte Carlo technique to simulate electron trajectories. The comparison is achieved by applying each method to identically prepared models, one representing an overhead sun, another for a sunrise situation. Photoelectron fluxes are computed as a function of altitude, energy, and pitch angle. Heating rates for the ambient thermal electrons and neutral excitation rates are also presented. The results show close agreement between the three methods for altitudes in the region of local energy loss, but higher altitudes reveal a discrepancy of as much as a factor of 2 in the net flux of photoelectrons. This discrepancy is discussed and then removed by appropriately modifying the computational techniques. The escape fluxes predicted (∼3 × 108 cm−2 sec−1) from the input data are generally lower than most measured values.