Kinetic Alfvén wave instability driven by a field-aligned current in high-βplasmas

Abstract

Including the ion-gyroradius effect, a general low-frequency kinetic dispersion equation is presented, which simultaneously takes account of a field-aligned current and temperature anisotropy in plasmas. Based on this dispersion equation, kinetic Alfvén wave (KAW) instability driven by the field-aligned current, which is carried by the field-aligned drift of electrons relative to ions at a drift velocity V(D), is investigated in a high-β plasma, where β is the kinetic-to-magnetic pressure ratio in the plasma. The numerical results show that the KAW instability driven by the field-aligned current has a nonzero growth rate in the parallel wave-number range 0<k(z)<k(z)(u), where the growth rate reaches a maximum at k(z)∼k(z)(u)/2. On the other hand, in the perpendicular wave-number range 0<k(⊥)<k(⊥)(u) the growth rate monotonously decreases with the wave number k(⊥) from a maximum at k(⊥)=0 to zero at k(⊥)=k(⊥)(u). In particular, the upper boundaries of the growing ranges in the wave-number space, k(z)(u) and k(⊥)(u), as well as the growth rate increase with V(D), imply that there are wider growing ranges and larger growth rate for larger drift velocities. Including the temperature anisotropy of plasma particles, the result shows that the instability conditions for these two driven mechanisms are both modified considerably, in which the growing parametric ranges of KAWs widen. The results have potential importance in understanding the physics of space and astrophysical plasma active phenomena since the field-aligned current, also called the Birkeland current, is one of the most active factors in plasma phenomena.