Auroral electrodynamics with current and voltage generators

Abstract

The electrodynamic structure of auroral currents is studied in the steady state by coupling Maxwell's equations with effective Ohm's laws for ionospheric Pedersen currents, field‐aligned currents and currents in the generator region. An effective generator conductivity is introduced which allows consideration of a variety of states from pure current (ΣG = 0) to pure voltage (ΣG → ∞) generators. On short time scales when Alfvén waves are important, this conductivity mapped to the ionosphere is lower than the ionospheric conductivity, implying a currentlike generator. This is in contrast with previous electrodynamic models of the aurora, which have assumed voltage‐fixed generators. Current generators differ from the voltage generators in that the natural scale size is smaller for current generators, indicating that the narrow discrete arcs may be current driven, while the overall large‐scale auroral currents are voltage driven. In addition, reversals of the electric field occur naturally in the current‐driven case, leading to V‐shaped potential structures. Examination of the effects of ionospheric conductivity gradients and comparison with a time‐dependent MHD model of auroral currents indicate that either regions of nonlinear resistivity or generator motions can decouple the ionospheric currents from the generator. In this case the effective conductivity becomes the Alfvén wave conductivity, which, being small compared to the ionospheric conductivity, leads to a structure typical of a current‐driven system. This implies that the current structure is not affected by ionospheric conductivity gradients, while the ionospheric electric field is enhanced on the low‐conductivity side.

Development of parallel electric fields at the plasma sheet boundary layer

“…The ionosphere is treated as a conducting boundary as described by Lysak [2004] and Lysak and Song [2006] with Pedersen conductance of 3 mho, and Hall conductance is set to 0 to isolate the role of the density gradients at coupling the fast and shear modes. The system is driven at the top boundary by imposing an electric field; in addition, the ratio of the electric and magnetic fields at this boundary is set to the local Alfvén speed in order to absorb wave energy incident on this boundary [Lysak, 1985].…”

Particle simulation of the auroral zone showing parallel electric fields, waves, and plasma acceleration

“…Other approaches assume that finite electron inertia causes a parallel electric field to develop when magnetospherically generated kinetic Alfv6n waves reflect off the ionosphere. On electron timescales this parallel field is quasi-static and causes electron acceleration and precipitation [Hasegawa, 1976;Mallinckrodt and Carlson, 1978;Goertz and Boswell, 1979;Lysak, 1985Lysak, , 1991Lysak, , 1993. A similar mechanism that involves lower hybrid waves has also been proposed [Bingham et al, 1984].…”

“…Observations have indicated, however, that microscale wave-particle interactions play an active role in dissipating and transferring energy in the auroral zone from beam kinetic energy to thermal energy via plasma waves [Reiff et al, 1986[Reiff et al, , 1988 The study here examines both the formation of semiglobal quasi-static parallel electric fields and microscale wave particle interactions in a self-consistent particle-in-cell simulation. This approach differs from static equilibria models constructed for adiabatic conditions [Chiu and Cornwall, 1980;Lyons, 1981;Newman et al, 1986;Cornwall, 1990], kinetic Alfv6n wave models [Lysak and Dum, 1983;Lysak, 1985Lysak, , 1993Kletzing, 1994] [Yamamoto and Kan, 1985], and the investigation of anisotropic magnetotail distributions as a driver for auroral processes [Winglee et al, 1988]. With the exception of the last study, these simulations initially assumed the existence of parallel electric fields or particle beams, and their formation was not addressed.…”

Auroral ion outflow: low altitude energization

“…Treating the ionosphere as a discrete boundary, the reflection coefficient is given by (Lysak, 1985;Knudsen, 1990) …”