Conditions for auroral particle acceleration

Auroral particle acceleration is the result of the appearance of effective resistance in field-parallel electric currents and of the voltages needed to overcome them. The basic condition for that to occur is that the field-aligned current density somewhere in the system approaches the thermal flux of the background electrons. The generation of such high-current density is a result of the dynamic interaction of the magnetospheric and ionospheric plasmas. The free magnetic energy stored in the current system is converted into kinetic energy by the auroral acceleration process and, consequently, magnetic shear stresses are released. Thus auroral acceleration can be seen as the manifestation of a magnetic elasticity problem in low-beta plasmas, a problem that may arise in any cosmic system with sufficiently strong magnetic field. The lecture concentrates on mechanisms for current concentration and the dynamic consequences of energy conversion and stress release on the overall system.