Kinetic Alfvén waves and electron physics. I. Generation from ion-ion streaming

Abstract

Short-wavelength kinetic Alfvén waves (KAWs) that propagate at large angles with respect to the magnetic field and interact with both electrons and ions have broad application to laboratory and space plasmas. Using linear Vlasov theory and particle-in-cell (PIC) simulations, the generation of KAWs by ion-ion streaming along the magnetic field at relatively low (2.5× the Alfvén speed) and low plasma beta (ratio of plasma thermal pressure to magnetic pressure ⩽0.1) are investigated. The instability has been examined previously using linear theory and a hybrid simulation method in which the ions are treated kinetically and the electrons as an adiabatic fluid. In this work it is found that when the electron Landau resonance factor or equivalently the ratio of the Alfvén speed to the electron thermal speed is large enough, the interaction of KAWs with the electrons produces strong parallel electron heating. The electron parallel heating in turn increases both the wave growth rates and the range of unstable modes of the instability, leading to enhanced saturated wave levels, increased perpendicular ion heating and larger spatial fluctuations in the drift speeds of the ions that appear as greater reductions of relative ion streaming in localized regions, compared to results from hybrid simulations. The interaction of KAWs with the electrons shifts from the bulk to the tail of the parallel velocity distribution at larger values of the resonant factor, corresponding to situations at lower electron beta or more obliquely propagating waves. Ion-to-electron mass ratio effects are considered together with the scaling to electron beta and the resonant factor in order to apply the results to systems of interest. In particular, KAWs and electron kinetics are discussed in the context of oblique slow shock formation and structure [Yin et al., Phys. Plasmas 14, 062105 (2007)].