Ion gyroradius effects on Alfvenic field aligned currents and electron energization in planetary magnetospheres.

Abstract:

Alfvenic current systems are a ubiquitous feature of planetary magnetospheres that can be generated by several mechanisms including the braking of flows (e.g. associated with reconnection at substorm onset) and via moon-planet interactions. The energetic electrons needed to carry the field-aligned currents are generally thought to be accelerated on either electron inertial or ion acoustic gyroradius scale lengths in the limit of inertial and kinetic Alfven waves respectively. Recent 2D dipolar hybrid gyrofluid-kinetic electron simulations of kinetic Alfven waves (Damiano et al., JGR, 2015), associated with the braking of fast flows in the terrestrial magnetotail, have illustrated that hot ion effects can act to limit the extent of the parallel current (all along the field line) from what would be expected in the cold ion limit. This correspondingly affects the characteristics of the electron energization, reducing both the parallel elongation in the electron distribution function associated with electron trapping in the kinetic Alfven wave regime and the extent of high energy tails evident in the inertial Alfven wave region above the ionosphere. In this presentation, we build on these initial simulation results analyzing the characteristics of the parallel current system and electron acceleration (associated with both inertial and kinetic Alfven waves) for a range of wave amplitudes and ratios of the electron to ion temperature. One finding is that for a given ion temperature, increasing wave amplitude recovers some of the features of the electron energization evident in the cold ion limit, but this is modulated by the effect of wave energy dispersion perpendicular to the ambient magnetic field. These results will be summarized and the relevance and extension of this work to consider Alfvenic aurora in the Jupiter magnetosphere (e.g. via either interchange motion or the Io-Jupiter interaction) will also be discussed.