Instability and energy transport of kinetic Alfvén waves in the solar corona

Abstract

Kinetic Alfvén waves become unstable when the field-aligned drift velocity is larger than the phase velocity of the waves. The unstable wave can grow and play a very important role in the energy transport in solar coronal plasma. In this paper, we consider field-aligned drift and temperature anisotropic velocity distribution function to study the energy transport mechanism. We numerically solve the dispersion relation and find two different modes: the usual kinetic Alfvén wave and the modified kinetic Alfvén wave. The ratios of the electric field to the magnetic field of both modes are shown to decrease when the field-aligned drift velocity gets large. These results show that for identifying the kinetic Alfvén waves in solar coronal plasma, the field-aligned current must be considered. On further analysis, we find that the Poynting flux associated with the waves either enhances or reduces depending upon the ratio of the drift to Alfvén speed. In the case of the kinetic Alfvén waves, when the drift velocity is less than the Alfvén speed, the Poynting flux is significantly reduced and the wave gets damped as it travels forward. However, when the drift velocity is larger than the Alfvén speed, the wave grows, i.e., the Poynting flux becomes large, as the wave moves. The Poynting flux of the modified kinetic Alfvén wave sensitively depends on the drift velocity. Even for very small variations, the wave becomes significantly unstable and can carry a large amount of energy. The electromagnetic energies of the damped/unstable waves are converted into heat energy as the waves travel across.