Dispersive Alfven waves: nonlinear and kinetic effects

Abstract

Summary form only given, as follows. The auroral accelerator contains localized field-aligned potential drops capable of energizing electrons to many keV. One possible explanation involves excitation of dispersive Alfven field line resonances (FLRs) that lead to inverted-V electron precipitation and density cavities depleted of current carrying electrons. We present results of two-fluid modeling of nightside FLRs on stretched geomagnetic field lines, and discuss the various saturation mechanisms affecting the scale and spatial structure of the excited waves. We classify various nonlinearities affecting the evolution of FLRs, and discuss the need for a kinetic treatment of parallel electron dynamics: The bounce time of electrons on geomagnetic field lines is much smaller than the wave period of observed mHz FLRs, implying that the electron dynamics is very non-local along the field line. Accounting for the mirror force, and solving the Vlasov equation for parallel electron motion, we show that in the auroral accelerator, the Alfven wave conductivity is much smaller than predicted by two-fluid theory. The large parallel Alfven wave current and low conductivity lead to very enhanced parallel electric fields, on the order of mV/m. Accounting for hot magnetospheric and cold ionospheric plasma populations, we show that the characteristic electron energy is comparable to the quasistatic potential associated with the density and temperature gradient along the field line, Finally, we compare the results of our model to observations, and indicate how auroral MPA data can be used to infer the stretching of nightside field lines that is necessary to explain the low frequencies of observed FLRs.