Abstract
AbstractMagnetohydrodynamic turbulence has been proposed as a mechanism for the heating of coronal active regions, and has therefore been actively investigated in recent years. According to this scenario, a turbulent regime is driven by footpoint motions. The energy being pumped this way into active region loops, is efficiently transferred to small scales due to a direct energy cascade. The ensuing generation of fine scale structures, which is a natural outcome of turbulent regimes, helps to enhance the dissipation of either waves or DC currents.We present an updated overview of recent results on turbulent coronal heating. To illustrate this theoretical scenario, we simulate the internal dynamics of a coronal loop within the reduced MHD approximation. The application of a stationary velocity field at the photospheric boundary leads to a turbulent stationary regime after several photospheric turnover times. This regime is characterized by a broadband power spectrum and energy dissipation rate levels compatible with the heating requirements of active region loops. Also, the energy dissipation rate displays a complex superposition of impulsive events, which we associate to the so-called nanoflares. A statistical analysis yields a power law distribution as a function of their energies, which is consistent with those obtained from observations. We also study the distributions of peak dissipation rate and duration of these events.
Highlights
Models of coronal heating in loops have been traditionally classified into two broad categories, according to the time scales involved in the driving at the loop bases: (a) AC or wave models, for which the energy is provided by waves at the Sun’s photosphere, with timescales much faster than the time it takes an Alfven wave to cross the loop; (b) DC or stress models, which assume that energy dissipation takes place by magnetic stresses driven by slow footpoint motions at the Sun’s photosphere
Two common factors prevail: (i) the source for the heating is the kinetic energy of the photospheric velocity field, (ii) the existence of fine scale structure is essential to speed up the dissipation mechanisms invoked
We discuss the role of MHD turbulence in developing small scales and in speeding up energy dissipation by the ensuing direct energy cascade
Summary
Models of coronal heating in loops have been traditionally classified into two broad categories, according to the time scales involved in the driving at the loop bases: (a) AC or wave models, for which the energy is provided by waves at the Sun’s photosphere, with timescales much faster than the time it takes an Alfven wave to cross the loop; (b) DC or stress models, which assume that energy dissipation takes place by magnetic stresses driven by slow footpoint motions (compared to the Alfven wave crossing time) at the Sun’s photosphere. Most of the theories of coronal heating invoke different mechanisms to speed up the energy dissipation (Parker (1972), Parker (1988), Heyvaerts & Priest (1983), van Ballegooijen (1996), Mikic et al (1989), Longcope & Sudan (1994), Hendrix & van Hoven (1996), Galsgaard & Nordlund (1996), Gudiksen & Nordlund (2002))
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