Velocity‐space distributions of wave‐accelerated auroral electrons

Abstract

The possibility that auroral electrons might be accelerated by electrostatic wave turbulence is further explored. Electron velocity distributions resulting from lower hybrid wave turbulence are modeled as functions of velocities parallel and perpendicular to the magnetic vector. The model, which is highly stylized with focus on the underlying physics, is found to account readily for the wide range of experimentally observed features such as "beams," "conics," and "widened loss cones." Consideration of the energetics of the process leads to the suggestion that the acceleration may be essentially a plasma boundary phenomenon. for a wave interpretation of the phenomenon have developed over a number of years in parallel with an emerging wave theory [Bryant, 1981[Bryant, , 1983[Bryant, , 1990; Bingham et al., 1984, 1988; Bryant et al., 1991a, hi. The possibility of the process being universal throughout space platohas has also been discussed [Bryant, 1987, 1990]. A commonality with acceleration in laboratory plasmas has clearly emerged, as has the dynamic equivalence of acceleration by Landau damping of electrostatic wave packets and Fermi's [1949] mechanism of cosmic ray acceleration by momentum exchange with moving magnetic barriers [Bryant, 1992, 1993]. An earlier modelling of the acceleration of auroral electrons by lower hybrid turbulence [Bryant et al., 1991b] showed that the velocity distribution parallel to the magnetic vector, f(Vl[ , V_L --0), could be reproduced satisfactorily using a stylized Monte Carlo model of the stochastic process. The characteristic spectral peak, the high-energy tail, and the invariance at low energies were all found to arise naturally from momentum exchange between electrons and lower hybrid wave packets. By adding the effects of the magnetic mirror force and a nominal atmosphere we now extend the model to two velocity dimensions, parallel and perpendicular to the magnetic vector B, in order to show that the model also satisfactorily produces the many wellestablished features of the two-dimensional (2-d) distri-Now at Goring-on-Thames, England, United Kingdom Paper nmnber 95JA00991. 0148-0227/95/95J A-00991 $05.00 button f(vll , V_L). Beams, conics, an extended loss cone, and other features are found to anticipate quite closely the experimental observations. We begin by recalling the properties of lower hybrid waves. We then outline our stylized model of the interaction with electrons, pointing out the two basic types of interaction that can be expected to occur: one being an "electrostatic Fermi acceleration," the other being a general heating. A series of examples then reveals the properties of such an accelerator and the sensitivity of the resultant electron distribution to wave properties and other controlling factors. The effects of different combinations of the governing parameters are then summarized. We then suggest how the various experimentally observed features might be interpreted in terms of the wave theory. The required electric field strengths are estimated and the e...

New model for auroral acceleration: O-shaped potential structure cooperating with waves

“…Here s is the ®eld-aligned coordinate in units of Earth radius. The waveparticle interaction part of the program is somewhat similar but unrelated to those presented earlier by Bryant and Perry (1995).…”

“…12. The result in this case is actually not too far from real data (waveparticle interactions alone have also been suggested to explain auroral acceleration, Bryant and Perry, 1995), but anyway di erent enough from both SMU/randboost and SMO/randboost to show that the electric ®eld structure is an important ingredient here. We will discuss this more thoroughly after presenting the runs for the multiple-Maxwellian case later.…”

New model for auroral acceleration: O-shaped potential structure cooperating with waves

“…12. The result in this case is actually not too far from real data (waveparticle interactions alone have also been suggested to explain auroral acceleration, Bryant and Perry, 1995), but anyway dierent enough from both SMU/randboost and SMO/randboost to show that the electric ®eld structure is an important ingredient here. We will discuss this more thoroughly after presenting the runs for the multiple-Maxwellian case later.…”

A study of inverted‐V auroral acceleration mechanisms using Polar/Fast Auroral Snapshot conjunctions

“…We think it more likely that the waves cause the anisotropy, because it is difficult to explain the energy-selective parallel acceleration and the anisotropy by something else than waves. Especially, lower hybrid waves have been suggested as being responsible for parallel acceleration [Bryant and Perry, 1995;Bryant, 1999] • …”