ION HEATING ACROSS THE MAGNETIC FIELD IN THE SOLAR CORONA BY KINETIC ALFVE´N WAVES

摘要

The perpendicular heating of the ions observed by SOHO in the solar corona at 2-4 solar radii has been mainly attributed to the ion-cyclotron damping of high-frequency Alfv´en waves. We investigate an alternative mechanism of heating by low-frequency Alfv´en waves that have short wavelengths across the magnetic field - kinetic Alfv´en waves (KAWs). The energy reservoir for these kinetic waves is provided by low-frequency large-scale MHD waves that are launched in the corona by the photospheric motions or excited at the coronal base by magnetic restructuring. The short perpendicular wavelengths, developed by phase mixing, convert MHD Alfv´en waves into KAWs. KAWs can be also excited in situ by various linear and nonlinear mechanisms. We show that above a threshold value of the wave amplitude, KAWs can stochastically accelerate ions across the background magnetic field. In particular, KAWs with transversal wavelengths of the order of the ion inertial length and with a wave/background magnetic field ratio of the order 0.1, can contribute to the stochastic heating of oxygen ions O5+. We discuss advantages of this mechanism over the ion-cyclotron heating scheme for the intense transverse heating of ions observed by SOHO at 2-4 solar radii.