A mechanism for generation of Ultra Low-frequency waves in the polar cusp Region

Abstract

Ultra low-frequency (ULF) electromagnetic waves, in the frequency range of ~1 mHz to 3 Hz, have been observed in the polar cusp and other regions of the Earth's magnetosphere by several spacecrafts, e.g., Geotail, Polar and CLUSTER. There are strong indications that these waves are generated locally by the energetic ion beams injected during magnetic reconnection taking place at the magnetopause. Ion beams observed in the polar cusp, plasma sheet boundary layer (PSBL), and on the auroral zone field lines are expected to have spatial gradients in their drift velocity. A generation mechanism for the ULF waves is proposed in terms of kinetic Alfvén wave instability driven by velocity shear of the ion beams. The noise due to velocity shear driven Alfvén modes is electromagnetic in nature, and also has a finite parallel electric field component.