Two-dimensional simulations of coronal rain dynamics. I. Model consisting of a vertical magnetic field and an unbounded atmosphere

Abstract

<p><em>[eng] Context. Coronal rain often comes about as the final product of evaporation and condensation cycles that occur in active regions.</em></p><p><em>Observations show that the condensed plasma falls with an acceleration that is less than that of free fall.</em></p><p><em>Aims. We aim to improve the understanding of the physical mechanisms behind the slower than free-fall motion and the two-stage</em></p><p><em>evolution (an initial phase of acceleration followed by an almost constant velocity phase) detected in coronal rain events.</em></p><p><em>Methods. Using the Mancha3D code, we solve the 2D ideal magnetohydrodynamic equations. We represent the solar corona as</em></p><p><em>an isothermal vertically stratified atmosphere with a uniform vertical magnetic field. We represent the plasma condensation as a</em></p><p><em>density enhancement described by a 2D Gaussian profile. We analyse the temporal evolution of the descending plasma and study its</em></p><p><em>dependence on such parameters as density and magnetic field strength.</em></p><p><em>Results. We confirm previous findings that indicate that the pressure gradient is the main force that opposes the action of gravity</em></p><p><em>and slows down the blob descent, and that larger densities require larger pressure gradients to reach the constant speed phase. We</em></p><p><em>find that the shape of a condensation with a horizontal variation of density is distorted during its fall because the denser parts of the</em></p><p><em>blob fall faster than the lighter ones. This is explained by the fact that the duration of the initial acceleration phase and, therefore,</em></p><p><em>the maximum falling speed attained by the plasma, increases with the ratio of blob to coronal density. We also find that the magnetic</em></p><p><em>field plays a fundamental role in the evolution of the descending condensations. A strong enough magnetic field (greater than 10 G in</em></p><p><em>our simulations) forces each plasma element to follow the path given by a particular field line, which allows for the description of the</em></p><p><em>evolution of each vertical slice of the blob in terms of 1D dynamics, without the influence of the adjacent slices. In addition, under</em></p><p><em>the typical conditions of the coronal rain events, the magnetic field prevents the development of Kelvin-Helmholtz instability.</em></p>