Simulations of storm time diffuse aurora with plasmasheet electrons in strong pitch angle diffusion

Abstract

Using a guiding‐center simulation of plasmasheet electrons postulated to be in strong pitch angle diffusion, we compute drift trajectories from a Hamiltonian formulation and map phase space densities from the nightside neutral line in Dungey's model magnetosphere (dipole field plus uniform southward Bz) according to Liouville's theorem modified for exponential loss implicit in the strong diffusion hypothesis. From the resulting phase space distributions we compute the precipitating energy flux into the auroral ionosphere as functions of magnetic latitude and magnetic local time (MLT) so as to simulate numerically the spatial and spectral structure of diffuse auroral electron precipitation for comparison with observational data. Storm‐associated impulses (by which we enhance the convection electric field) can typically transport plasmasheet electrons from the nightside neutral line to the near‐midnight region of maximum precipitating energy flux (latitude ≈ 65°) in ∼20–30 min, which is roughly the strong diffusion lifetime of 4‐keV electrons at the corresponding L value (≈5.7). The maximum precipitating electron energy flux in our simulation of the model storm is thus modulated by random variations in the mean cross‐magnetospheric electric potential drop over the 20–30 min before the time of interest. Our results also show a consistent lack of precipitating electron energy flux in the afternoon quadrant, essentially because this is the last quadrant to be visited by plasmasheet electrons (and therefore features the most strongly attenuated phase space densities) as they drift through the magnetosphere on open trajectories. The result agrees qualitatively with the typically observed “darkness” of X‐ray images of the diffuse aurora in that sector (1200–1800 MLT). While our simulation results locate the region of maximum energy flux slightly postmidnight during both prestorm and stormtime, in good agreement with previously published statistical compilations of auroral electron precipitation. Our simulations do not explain other large intensifications of electron precipitation (near dawn and in the morning sector) that are also found in both statistical and storm event studies. This suggests the need for a model in which pitch angle diffusion is less than everywhere strong throughout the plasma sheet.