Gyrofluid-kinetic electron modeling of dispersive scale Alfven waves associated with broadband aurora

Abstract:

The formation of the broadband aurora (such as seen at substorm onset) is well correlated with Poynting flux associated with dispersive scale Alfven waves. Ion temperature effects are important in this context as the ion gyroradius is a fundamental scale-length for the transfer of global scale substorm energy to particle energization and since these effects modify the phase speed of the wave (which has implications for both substorm onset timing and electron acceleration). With these motivations in mind, we present simulations of dispersive scale Alfven wave pulses using a new gyrofluid ion-kinetic electron model in dipolar coordinates. This model is an extension of an established hybrid MHD-klnetic electron model that has been generalized to include ion gyroradius effects based on a kinetic-fluid model where the ion pressure tensor is computed using a solution of the linear gyrokinetic equation. It is found that consideration of a realistic ion to electron temperature ratio significantly reduces the propagation time of the wave from the plasma sheet to the ionosphere (relative to the case that neglects ion temperature effects) and leads to an increased dispersion of wave energy perpendicular to the ambient magnetic field. Additionally, as the ion gyroradius is increased, we observe a reduction in the parallel current carried by the wave and hence a reduction in the electron energization.