Turbulent generation of electrostatic fields in the magnetosphere

Abstract

Large-scale E × B turbulence can generate large potential differences across magnetic field lines in the magnetosphere. Resulting potential differences between the ionosphere and the turbulent eddies in the magnetosphere can accelerate auroral electrons. In a two-dimensional guiding center plasma, energy flows from smaller- to larger-scale sizes, so that the system tends toward a state with most energy in the longer wavelengths. Application of a numerical model demonstrates the transfer of turbulent energy from scale sizes of the order of the ion gyroradius to scale sizes where a macroscopic fluid description is valid. By taking the gyrational energy in the earthward streaming plasma sheet ions as the energy source for the turbulence it is found that some proton velocity distributions will be unstable to electrostatic flute mode instability with growth rates of the order of the ion cyclotron frequency. Large-scale turbulence is fed by nonlinear coupling of energy from the instability. Wave normal directions will be primarily perpendicular to magnetic L shells, and turbulent eddies generated by the waves may be greatly elongated in the east-west direction. The theory given here is consistent with the observed formation of multiple and irregular auroral arcs with a preference for east-west alignment.