Plasma waves associated with density holes upstream of Earth's bow shock

Abstract

The regions upstream of Earth's bow shock provide a natural laboratory for a variety of fundamental plasma processes, which include the interaction between particle populations that stream away from the shock with the solar wind and low frequency waves. Recent observations from Cluster and Double Star have exhibited novel short-duration (~4-20 s) density holes. These density holes are accompanied by a variety of plasma waves at ~ 0.01 Hz to several tens kHz. Preliminary analysis shows that the gross-scale structures appear fast magnetosonic-like, while in the interior ion cyclotron waves, whistler mode waves, and strong electrostatic waves are observed. A broad spectrum of electrostatic turbulence (a few kHz to ~50 kHz)is observed at the upstream edges of density holes.These waves are likely to play important roles in determining the holes' properties, including heating of the suprathermal particles seen. Some of the wave activity, especially at the upstream edges of density holes, are similar to those observed at the Earth's bow shock. We have studied the wave characteristics in a number of density holes, and will discuss their possible mechanisms, influences on the plasma properties, and role in the structures' formation