Abstract
Context Coronal rain composed of cool plasma condensations falling from coronal heights is a phenomenon occurring in footpoint-heated coronal loops as a result of thermal instability. High-resolution coronal rain observations suggest that condensations move with less than free-fall speed and can sometimes undergo longitudinal oscillations. Aims We investigate the evolution and dynamics of plasma condensations in a gravitationally stratified coronal loop. Methods We carried out 2.5 dimensional magnetohydrodynamic simulations of a cool plasma condensation in a gravitationally stratified coronal loop and analysed its evolution, kinematics, and the evolution of the forces acting on the condensation. We further propose a one-dimensional analytical model of the condensation dynamics. Results The motion of plasma condensations is found to be strongly affected by the pressure of the coronal loop plasma. Maximum downward velocities are in agreement with recent coronal rain observations. A high coronal magnetic field or low condensation mass can lead to damped oscillatory motion of the condensations that are caused by the pressure gradient force and magnetic tension force that results from bending of the magnetic field in the lower part of the coronal loop. Period and damping scaling time of the oscillatory motion seen in the simulations are consistent with values predicted by the model. Conclusions The combined effect of pressure gradients in the coronal loop plasma and magnetic tension force that results from changes in magnetic field geometry can explain observed sub-ballistic motion and longitudinal oscillations of coronal rain
Highlights
Coronal loop plasma can be thermally unstable and subject to formation of cool and dense condensations (Field 1965)
We carried out 2.5 MHD simulations of the dynamics of cool plasma condensations in a gravitationally stratified coronal loop
The motion and evolution of plasma condensations were found to be strongly affected by the pressure of the coronal loop plasma, and the pressure gradients can be high enough to account for the lower-than-free-fall speed of the coronal rain even in finite magnetic field cases
Summary
Coronal loop plasma can be thermally unstable and subject to formation of cool and dense condensations (Field 1965). The thermal instability onset and coronal rain formation in a coronal loop with footpoint-concentrated heating typically depends on the spatial distribution of the heating input and often occurs in periodically repeating limit cycles (Müller et al 2003, 2004). The effect of the finite magnetic field on the coronal rain evolution is only properly accounted for when using multidimensional magnetohydrodynamical (MHD) models. The density enhancement was found to rebound multiple times in this setup, which was explained as a two-step process: a deceleration phase caused by the pressure build-up below the enhancement, and a rebound phase caused by the restoring action of the Lorentz force, stressing the importance of the effect a finite magnetic field can have on the evolution of plasma condensations. We choose our problem setup to be representative for small coronal rain condensations that are typically formed in thermally unstable loops as a result of catastrophic cooling. We propose an analytical model for the condensation dynamics in order to explain the oscillatory behaviour of the plasma condensations and compare it with the numerical findings
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