Physical interpretation of phase- and group-motion in auroras

Abstract:

It is well established that fine-scale features in the aurora (<100-m perpendicular to B) follow distinctly different rules of motion than the mesoscale arcs (order 10-km) within which they are embedded. These scale-dependent distinctions may be conveniently captured and quantified through the formalism of phase- and group- motion from classical wave theory. But less clear is how such empirical propagation models should be connected to the underlying physics of energy release and dissipation in the magnetosphere. This paper seeks to connect multi-scale models of auroral motion to dynamic wave-particle interactions in the magnetotail and near-Earth auroral acceleration region (AAR). Our focus is on auroras associated with the established phases of the substorm cycle (growth, onset, expansion, recovery). A unifying construct is the ‘backward wave’ (oppositely directed phase- and group- motion), which is argued to be a natural consequence of magnetic reconnection and concomitant release of magnetic stress, applicable to both the magnetotail and the AAR domains.