Dynamics of plasma condensations in a gravitationally stratified coronal loop

摘要

Context. Coronal rain composed of cool plasma condensations falling from coronal heights is a phenomenon occurring in footpointheatedudcoronal loops as a result of thermal instability. High resolution coronal rain observations suggest that condensations move withudless than free fall speed and can sometimes undergo longitudinal oscillations.udAims. We investigate the evolution and dynamics of plasma condensations in a gravitationally stratified coronal loop.udMethods. We carry out 2.5 dimensional magnetohydrodynamic simulations of a cool plasma condensation in a gravitationally strati-udfied coronal loop and analyse its evolution, kinematics and the evolution of the forces acting on the condensation. We further proposeudone-dimensional analytical model of the condensation dynamics.udResults. Motion of plasma condensations is found to be strongly affected by the pressure of the coronal loop plasma. Maximumuddownward velocities are in agreement with recent coronal rain observations. High coronal magnetic field or low condensation massudcan lead to damped oscillatory motion of the condensations caused by the pressure gradient force and magnetic tension force resultingudfrom bending of the magnetic field in the lower part of the coronal loop. Period and damping scaling time of the oscillatory motionudseen in the simulations are consistent with values predicted by the model.udConclusions. The combined effect of pressure gradients in the coronal loop plasma and magnetic tension force resulting from changesudin magnetic field geometry can explain observed sub-ballistic motion and longitudinal oscillations of coronal rain