Simulations of diffuse aurora with plasma sheet electrons in pitch angle diffusion less than everywhere strong

• Published in: Journal of Geophysical Research: Space Physics 2001-12, Vol.106 (A12), p.28949-28966
• Main Authors: Chen, Margaret W., Schulz, Michael
• Format: Article
• Language: English
• Subjects: Airglow. Aurora; Earth, ocean, space; Exact sciences and technology; External geophysics; Particle precipitation; Physics of the high neutral atmosphere; Physics of the ionosphere
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/2001JA000138

To investigate the spatial and spectral structure of the diffuse aurora during a model geomagnetic storm characterized by random impulses in the cross‐magnetospheric convection electric field, we simulate the bounce‐averaged drift motion and precipitation of plasma sheet electrons. Bounce‐averaged drift trajectories are computed from a Hamiltonian formulation in which we have treated the plasma sheet electrons as though they were undergoing strong pitch angle diffusion in Dungey's model magnetosphere (dipole field plus uniform southward Bz). Using the simulation results, we map phase space densities from near the nightside neutral line according to Liouville's theorem, modified to account for particle losses consistent with the postulated pitch angle scattering. We consider three different idealized scattering rate models for the plasma sheet electrons: (1) strong diffusion everywhere, (2) an MLT‐independent model for diffusion that is less than everywhere strong, and (3) an MLT‐dependent scattering rate model with the same azimuthal average as the MLT‐independent model. We evaluate the precipitating energy flux and mean energy at ionospheric altitude h = 127.4 km, as well as the ionospheric Hall and Pedersen conductances, for the three different scattering rate models considered, and we compare the results with each other and with available observations. The limit of everywhere strong pitch angle diffusion yields a simulated diffuse aurora that seems too intense at its maximum (storm time energy flux of ∼5–10 erg cm−2 s−1) and seems too concentrated near midnight in magnetic local time (MLT). The distribution of energy flux above half maximum extends from ∼2000–0700 MLT throughout the model storm. Corresponding maxima in simulated Hall (∼18–34 mhos) and Pedersen (∼9–11 mhos) conductances also seem too large. Our model for pitch angle diffusion that is less than everywhere strong yields a more realistic maximum energy flux (∼2 erg cm−2 s−1) and yields a realistically broader distribution in MLT, essentially because the plasma sheet electrons of interest live longer before precipitating. An even more realistic MLT distribution of precipitating energy flux is obtained by modulating the latter scattering rate model with an MLT dependence based on the observed statistical distribution of waves with respect to MLT. With such a model we can account for intensifications of diffuse auroral electron precipitation found near dawn and late in the morning quadrant in