Nonresonant instability of kinetic Alfvén waves with κ-electrons

Abstract

A nonresonant instability of kinetic Alfvén waves (KAWs) is studied in a three-component plasma system consisting of background cold ions, an ion beam, and hot electrons with a κ-distribution. The nonresonant KAW instability is produced by the combined sources of ion beam and velocity shear. It is found that the wave excitation by velocity shear alone will give rise to purely growing KAWs, whereas the ion beam velocity alone as a source cannot excite the waves for the considered plasma parameters. It is also observed that the combined sources of ion beam and velocity shear can excite the KAWs in nonresonant instability with finite wave frequency (the mode is not a purely growing mode). Also note that κ-electrons restrict the wave propagation very close to 90°, whereas the Maxwellian electrons permit the wave to propagate a few degrees away from 90°. It is inferred that the presence of κ-electrons shrinks the wave-unstable region of a KAW’s nonresonant instability. The coupling between KAWs and ion-acoustic waves occurs at a lower value of βi for Maxwellian electrons as compared to κ-electrons.