ICS-13 Abstract View

 

Alfvenic generation of Substorm Auroras and Triggers of Substorm Onset
Song, Yan, yan@umn.edu (1)
Lysak, L.R., lysak001@umn.edu (1)
Substorms result from the dynamical response of the M-I coupling system to external solar wind driving and to internal dynamics. Substorms have been often referred to as auroral substorms. The substorm onset and its expansion and recovery phases have mainly been described by the morphology of auroral arcs during substorms (Akasofu, 1964). In this presentation, we emphasize that understanding the physics of the generation of substorm auroral arcs is the key to understanding the triggering mechanism of substorm onset.

It is well known that a necessary condition for the occurrence of substorms is the accumulation of free energy stored in the magnetotail during the growth phase. We have suggested that another necessary condition for producing substorm expansion phase is to have a large earthward force acting in the magnetotail (Lin et al., 2009; Song and Lysak, 2012). During the growth phase, energy and momentum are transferred from the solar wind into the magnetosphere. At this stage, the M-I coupling system, which is constantly driven by the solar wind, is far from a thermal equilibrium. Thus, a decrease in momentum transfer from the solar wind into the magnetosphere due to, for example, an IMF northward turning, will start a preconditioning stage, and cause force imbalance, producing a net earthward body force acting on the magnetotail. This earthward force can cause a large scale movement of the tail towards a more dipole-like configuration.

During the preconditioning stage, the generation of parallel electric fields and the formation of substorm auroral arcs in localized regions at the equatorward boundary can redistribute perpendicular mechanical and magnetic stresses in auroral flux tubes, decoupling the magnetosphere from ionosphere drag locally. This will enhance the tail earthward shear flows and rapidly buildup stronger parallel electric fields in the auroral acceleration region, leading to a sudden and violent tail energy release.

Parallel electrostatic electric fields are effective in accelerating auroral particles to high energy, creating auroral arcs. In particular, Alfvenic double layers, produced by non-linear interaction of incident and reflected Alfven wave packets in the M-I coupling system, can act as a powerful high energy particle accelerator. The Poynting flux carried by Alfven waves can continuously supply a large amount energy from the generator region in the magnetotail to the auroral acceleration region, producing substorm auroral arcs. During the expansion phase, the formation of enhanced substorm auroral arcs is powerful means to rapidly convert the accumulated free magnetic energy stored in the magnetotail to the kinetic energy of charged particles that produce substorm auroral arcs.
(1) School of Physics and Astronomy, University of Minnesota, 116 Church Street, S.E., Minneapolis, MN 55455, USA