Leonardo Milano

QUASI-SEPARATRIX LAYERS IN A REDUCED MAGNETOHYDRODYNAMIC MODEL OF A CORONAL LOOP

We run a pseudospectral magnetohydrodynamic code to simulate reconnection between two —ux tubes inside a solar coronal loop. We apply a stationary velocity —eld at one of the footpoints consisting of two vortices in such a way as to induce the development of a current layer and force the —eld lines to reconnect. During the process we —nd a remarkable coincidence between the location of the current layer and the location of quasi-separatrix layers, which are thin magnetic volumes where the —eld line connec- tivity changes abruptly. This result lends support to a scenario in which quasi-separatrix layers are the most likely locations for impulsive energy release in the solar corona. Another important result of this simulation is the observed transient of strong magnetohydrodynamic turbulence characterized by a k~3@2 energy spectrum. This transient reaches its peak activity in coincidence with a maximum in the energy dissipation rate, thus suggesting that the direct energy cascade associated with this turbulent transient plays a key role in enhancing energy dissipation in magnetic reconnection processes. Subject headings: MHDSun: coronaSun: —aresSun: magnetic —elds

Solar Coronal Heating: AC versus DC

The heating of the plasma confined in active regions of the solar corona is caused by the dissipation of magnetic stresses induced by the photospheric motions of the loop footpoints. The aim of the present paper is to analyze whether solar coronal heating is dominated by slow (DC) or rapid (AC) photospheric driving motions. We describe the dynamics of a coronal loop through the reduced magnetohydrodynamic equations and assume a fully turbulent state in the coronal plasma. The boundary condition for these equations is the subphotospheric velocity field that stresses the magnetic field lines, thus replenishing the magnetic energy that is continuously being dissipated inside the corona. In a turbulent scenario, energy is efficiently transferred by a direct cascade to the microscale, where viscous and Joule dissipation take place. Therefore, for the macroscopic dynamics of the fields, the net effect of turbulence is to produce a dramatic enhancement of the dissipation rate. This effect of the microscale on the macroscale is modeled through effective dissipation coefficients much larger than the molecular ones. We consistently integrate the large-scale evolution of a coronal loop and compute the effective dissipation coefficients by applying a closure model (the eddy-damped, quasi-normal Markovian approximation). For broadband power-law photospheric power spectra, the heating of coronal loops is DC dominated. Nonetheless, a better knowledge of the photospheric power spectrum as a function of both frequency and wavenumber will allow for more accurate predictions of the heating rate from this simple model.

Recent Theoretical Results on Coronal Heating

The scenario of magnetohydrodynamic turbulence in connection with coronal active regions has been actively investigated in recent years. According to this viewpoint, a turbulent regime is driven by footpoint motions and the incoming energy is efficiently transferred to small scales due to a direct energy cascade. The development of fine scales to enhance the dissipation of either waves or DC currents is therefore a natural outcome of turbulent models. Numerical integrations of the reduced magnetohydrodynamic equations are performed to simulate the dynamics of coronal loops driven at their bases by footpoint motions. These simulations show that a stationary turbulent regime is reached after a few photospheric times, displaying a broadband power spectrum and a dissipation rate consistent with the energy loss rates of the plasma confined in these loops. Also, the functional dependence of the stationary heating rate with the physical parameters of the problem is obtained, which might be useful for an observational test of this theoretical framework.