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Session 7 - Corona II.
Oral session, Saturday, June 28
Ballroom A, Chair: Fran Bagenal
Coronal loops in solar active regions are likely to be heated by Joule dissipation of highly structured electric currents. The development of small scales in the spatial distribution of electric currents is necessary to enhance magnetic energy dissipation and therefore provide sufficient heating to the plasma confined in these loops. Since the kinetic and magnetic Reynolds numbers in coronal active regions are quite large, we expect footpoint motions to drive the loop into a strongly turbulent MHD regime. We perform direct numerical simulations of an externally driven two-dimensional magnetohydrodynamic system over extended periods of time to simulate the dynamics of a transverse section of a loop. A stationary and large-scale magnetic forcing was imposed, to model the photospheric motions at the magnetic loop footpoints. A turbulent stationary regime is reached, and energy dissipation rates consistent with the heating requirements of coronal loops are obtained.
The temporal behavior of the energy dissipation rate shows clear indications of intermittency, which is a direct consequence of the strong nonlinearity of the system. We associate these impulsive dissipation events, whose energies range from 5 \times 10^24 erg to 10^26 erg, to the so-called nanoflares. A statistical analysis of these events yields a power law distribution like dnøverdE\propto E^-1.5, which is consistent with those obtained for flare energy distributions reported from X-ray observations. Also, turbulent hydrodynamic velocities of 30-50\ km.s^-1 are obtained, which might explain the line broadening observed in several coronal spectral lines.
Program listing for Saturday