Whistler instability enhanced by suprathermal electrons within the Earth's foreshock

Abstract

The electron temperature‐anisotropy driven whistler instability in the presence of a suprathermal power law tail on the electron population, such as is observed in the Earth's electron foreshock, is investigated. The enhancement in energetic electrons significantly accentuates whistler growth at wavenumbers smaller than those expected for a bi‐Maxwellian electron distribution of similar temperature anisotropy and brings the unstable frequencies and wavelengths closer to those expected for the 1‐Hz (in the spacecraft frame) whistler waves observed in the electron foreshock. The presence of a power law tail, furthermore, causes a rapid increase in growth rate with increasing electron temperature anisotropy. This means that even very modest temperature anisotropies, like those that are frequently present in the magnetically connected foreshock, can strongly destabilize parallel whistlers. This instability could be responsible for the nearly field aligned so‐called “1 Hz” whistlers previously observed in the Earth's foreshock, which are more difficult to account for in models of the oblique whistler instability.