Theory of the Generation of Field-Aligned Currents and Displacement Currents

Abstract:

In previous theories of field-aligned current (FAC) generation (e.g., Hasegawa and Sato, 1979; Sato and Iijima, 1979; Vasyliunas, 1984), the field-aligned gradient of the FAC is determined by using the current continuity approximation, where the perpendicular current is derived from the MHD momentum equation. These theories describe only the force balance and do not describe the generation of the FAC.

 We have derived the dynamical relationship between the generation of the total current J_||total=J_||+J_||D=J_||+(c/4π)(∂E_||/∂t) and the temporal changes and spatial gradients of magnetic and velocity shears in, for example, the auroral flux tube (e. g. Song and Lysak, 2001, 2006). Our results show that the generation of the total current is a dynamo process associated with the increase of the azimuthal magnetic flux, which is caused by the axial torque. The axial torque is produced by the Alfvenic interaction in the solar wind-magnetosphere and ionosphere coupling system. 

The parallel displacement current (J_||D= (c/4π)(∂E_||/∂t)) describes the generation of the electric field, E_||. It is E_||, not J_||, which accelerates charged particles. Thus it is crucial to understand the conditions, under which the displacement current J_||D and its generation become significant. We show that the J_||D, i.e., the E_||-generation, is closely related to low plasma density and the enhanced magnetic stress.

Continuous generation of displacement current over a fairly long time can effectively accelerate charge particles to high energy. The nonlinear interaction of incident and reflected Alfven wave packets in inhomogeneous auroral acceleration region can produce quasi-stationary non-propagating electromagnetic plasma structures, such as Alfvenic double layers and charge holes. These structures will sustain the displacement current and can constitute powerful high energy particle accelerators, where electromagnetic energy can be efficiently converted to the particle energy.