Magnetohydrodynamic oscillations in solar coronal rain

Abstract

Coronal rain composed of cool plasma condensations falling from coronal heights along magnetic field lines is a phenomenon occurring in active region coronal loops. This work combines high-resolution observations and numerical simulations to understand the interplay between coronal rain and MHD oscillations. We analyse oscillations and kinematics of the coronal rain using high resolution observations. Two different regimes of transverse oscillations traced by the rain are detected: smallscale persistent oscillations driven by a continuously operating process and localized large-scale oscillations excited by a transient mechanism. The plasma condensations are found to move with accelerations largely below the free-fall rate. The observed evolution of the emission of the plasma at the loop top is found to exhibit clear signatures of a gradual cooling consistent with the limit cycle model and suggests the loop is going through a sequence of periodically repeating heating-condensation cycles. We further investigate the evolution and dynamics of coronal rain using 2.5D MHD simulations. We model the evolution of a cool plasma condensation in a gravitationally stratified coronal loop. The motion of plasma condensations is found to be strongly affected by the pressure of the coronal loop plasma. High coronal magnetic field or low condensation mass are found to lead to damped oscillatory motion of the condensations. The combined effect of plasma pressure gradients and magnetic tension force can therefore explain observed sub-ballistic motion and longitudinal oscillations of coronal rain. We finally address the possibility of excitation of loop oscillations by coronal rain. We carry out MHD simulations of a coronal loop containing a cool and dense condensation region near the loop apex. This is found to excite fundamental harmonic of a vertically polarised kink mode. As the condensations fall towards the loop footpoints, the fundamental mode period is found to decrease as a result of the change in distribution of mass along the loop. We also carry out simulations of a coronal loop with a siphon ow between the footpoints which is likely to arise in asymmetrically heated loops. The action of the centrifugal force associated with plasma moving along the curved axis of the loop is found to excite vertically polarised loop oscillations. We find that flows with realistic speeds are sufficient to excite oscillations with observable amplitudes. We therefore propose coronal rain as a possible excitation mechanism for transverse loop oscillations