Interaction of Alfven waves with a turbulent layer

Abstract

Abstract We consider the interaction of Alfven waves with a resistive turbulent layer with anomalous conductivity. High-frequency turbulence causes the occurrence of both field-aligned and transverse resistivity. The correct dispersion relationship for Alfven waves in a turbulent medium with anisotropic conductivities has been derived. Alfven waves may partially reflect from a resistive layer, be absorbed in it, or be transmitted through it. When field-aligned resistivity dominates, the relative effectiveness of these processes critically depends on the wave transverse scale. For a thin layer as compared with the wave field-aligned length, the characteristic parameter that controls the effectiveness of the wave interaction with a layer is the resistive Alfven scale λA, determined by the field-aligned resistance and Alfven velocity above the layer. Comparison of energy losses estimated from analytical relationships for a “thin” layer and from numerical calculations for a finite width layer shows that the thin layer approximation provides a reasonable estimate over a wide range of wave scales, not only very small. Estimation of the effective damping scale of the Pc1 waves in a turbulent cusp shows that the cusp proper cannot be a conduit of Pc1 wave energy from the magnetosheath to the ground. The “thin” layer model has been applied to the interpretation of the results of early studies of transient ULF wave (Pi2 pulsations) damping during substorm onset, which showed that the damping rate increased for accompanying magnetic bays stronger than ∼100 nT. Our estimates confirm that this additional damping can be caused by the occurrence of anomalous transverse resistance when magnetospheric current exceeds the threshold necessary for the excitation of highfrequency plasma turbulence.