Phase-space density mappings for diffuse auroral electrons under strong pitch-angle diffusion in Dungey's model magnetosphere

Abstract

Strong pitch-angle diffusion (an idealization) so thoroughly randomizes the first two adiabatic invariants ( M and J) of charged-particle motion that it conserves the phase-space volume Λ ≡ p 3Ψ, where p is the particles' scalar momentum and Ψ is the occupied flux-tube volume per unit magnetic flux. Assuming strong pitch-angle diffusion, we trace drift shells and thus simulate guiding-center motions of plasmasheet electrons with representative values of Λ in a magnetic field model consisting of a dipole plus a uniform southward field (Dungey's model magnetosphere) with a time-independent Stern-Volland electric field superimposed. Our goal here is to account for typically observed characteristics of the diffuse aurora under geomagnetically quiet conditions. Using our simulation results, we map phase-space densities from the neutral line according to Liouville's theorem (modified by exponential attenuation to account for particle precipitation under the postulated strong pitch-angle diffusion) and compute corresponding rates of energy deposition into the auroral ionosphere as functions of magnetic latitude and magnetic local time (MLT) along the trajectories and at the energies corresponding to conservation of Λ. The present study thus numerically simulates kinematical aspects of the diffuse aurora for comparison with quiet-time observational data. Our results show a remakable paucity of precipitating electron energy flux in the afternoon quadrant, essentially because this is the last quadrant to be visited by plasmasheet electrons as they drift through the magnetosphere on open trajectories. It is therefore the quadrant subjected to the most strongly attenuated phase-space densities. The relative lack of electron energy flux in the afternoon quadrant agrees qualitatively with the typically observed “darkness” of X-ray images of the diffuse aurora in that sector (1200–1800 MLT).