Photospheric Vortices and Coronal Energy Storage And Release

The dynamics of a closed corona where photospheric vortices twist the coronal magnetic field lines is investigated by means of a Cartesian reduced magnetohydrodynamic (MHD) model. We consider first two corotating or counter-rotating vortices localized at the center of a photospheric plate, and additionally more corotating vortices that fill the plate entirely. The other plate is line-tied. After an initial linear transient stage, during which the vortices create laminar and smoothly twisting flux tubes, the system relaxes to a fully non-linear state that approaches a form of statistical stationarity: the main goal of this investigation is to understand this state, i.e. a vortex-forced coronal magnetic configuration permeated by finite amplitude broadband fluctuations. We find that depending on the system parameters and the arrangement and handedness of the photospheric vortices an inverse cascade storing a significant amount of magnetic energy may or may not occur. In the first case a reservoir of magnetic energy available to large events such as destabilization of a pre-CME configuration develops, while in the second case the outcome is a turbulent heated corona. Although our geometry is simplified our simulations are shown to have relevant implications for coronal dynamics and CME initiation.