Counterstreaming Beams and Flat-Top Electron Distributions Observed with Langmuir, Whistler, and Compressional Alfvén Waves in Earth's Magnetic Tail

Abstract

Relevant new clues to wave-particle interactions have been obtained in Earth’s plasma sheet (PS). The plasma measurements made on Cluster spacecraft show that broadband (∼2-6kHz) electrostatic emissions, in the PS boundary layer, are associated with cold counterstreaming electrons flowing at 5-12×10³kms^-1 through hot Maxwellian plasma. In the current sheet (CS), electromagnetic whistler mode waves (∼10-80Hz) and compressional Alfvén waves (<2Hz) are detected with flat-topped electron distributions whose cutoff speeds are ∼15-17×10³kms^-1. These waves are damped in the central CS where |B|≤1.5nT, plasma β∼100, and electron distributions isotropic. Three mechanisms are at work: the β-dependent lower hybrid drift instability (LHDI), acceleration of electrons along the B field by the LHD waves and whistler mode emissions triggered by the cyclotron resonance instability.