Gyrokinetic Theory of Electromagnetic Drift Instabilities Near the Center of a Harris Current Sheet with Guide Field†

Abstract:

Electromagnetic fluctuations in the lower-hybrid frequency range have been observed near the center of current sheets in space and laboratory plasmas. These fluctuations exhibit longer wavelengths, k²ρ_eρ_i~1, and stronger magnetic fluctuations than typical lower-hybrid drift instabilities(LHDIs), and several theoretical efforts have been undertaken to explain these mode properties. These works have used fluid theory or the local approximation to handle the electron response, and results have suggested the observed fluctuations may result from an electromagnetic beam mode, or lower order LHDIs. We have adopted a gyrokinetic-electron, unmagnetized kinetic-ion model to study these electromagnetic instabilities in an ion-scale Harris current sheet with a guide field. Our results include instabilities which are destabilized by finite parallel wavevector, and the electron grad-B drift, which resemble the observed fluctuations. Our three dimensional local and eigenmode stability analyses indicate that these modes belong to a group of instabilities which is distinct from the LHDI branch. Both numerical and analytical results will be presented and compared with those of direct simulations.