Excitation of high frequency Alfvén waves by plasma outflows from coronal reconnection events

Abstract

We study a kinetic excitation mechanism for high-frequency dispersive Alfven waves in the solar corona by magnetic reconnection events. The ion-cyclotron and Cerenkov kinetic effects are important for these waves which we call the ion-cyclotron kinetic Alfven waves (IC KAWs). The plasma outflowing from the reconnection site sets up a neutralized proton beam in the surrounding plasma, providing free energy for the excitation of waves. The dependence of the phase velocity of the IC KAW on the parallel wavenumber is different from that on the perpendicular wavenumber. The phase velocity is an increasing function of the perpendicular wavenumber and overtakes the Alfven velocity for sufficiently large values of k ⊥. However, the phase velocity is a decreasing function of k ∥, and sufficiently large values of k ∥ result in a phase velocity below the Alfven velocity. As a result, the IC KAWs can undergo the Cerenkov resonance with both super- and sub-Alfvenic particles, and for the waves to be excited the outflow velocity does not need to be super-Alfvenic, as for KAWs, but the beam/Alfven velocity ratio can span a wide range of values. High growth rates of the order of γ∼104 s−1 are found for the values of the plasma parameters typical for the low solar corona. The waves excited by (sub-)Alfvenic beams are damped mainly due to kinetic wave-particle interactions with ions at the cyclotron resonance (ion-cyclotron damping), and with ions and electrons at the Cerenkov resonance (Landau damping). Therefore, IC KAWs can heat the plasma species of the corona in both the parallel and perpendicular direction, giving rise to an anisotropic heating of the ions. The observational consequences of the processes under study are discussed.