Nonlinear Dynamics of the Parker Scenario for Coronal Heating

Abstract

The Parker or field line tangling model of coronal heating is studied comprehensively via long-time high-resolution simulations of the dynamics of a coronal loop in cartesian geometry within the framework of reduced magnetohydrodynamics (RMHD). Slow photospheric motions induce a Poynting flux which saturates by driving an anisotropic turbulent cascade dominated by magnetic energy. In physical space this corresponds to a magnetic topology where magnetic field lines are barely entangled, nevertheless current sheets (corresponding to the original tangential discontinuities hypothesized by Parker) are continuously formed and dissipated. Current sheets are the result of the nonlinear cascade that transfers energy from the scale of convective motions ($\sim 1,000 km$) down to the dissipative scales, where it is finally converted to heat and/or particle acceleration. Current sheets constitute the dissipative structure of the system, and the associated magnetic reconnection gives rise to impulsive ``bursty'' heating events at the small scales. This picture is consistent with the slender loops observed by state-of-the-art (E)UV and X-ray imagers which, although apparently quiescent, shine bright in these wavelengths with little evidence of entangled features. The different regimes of weak and strong MHD turbulence that develop, and their influence on coronal heating scalings, are shown to depend on the loop parameters, and this dependence is quantitatively characterized: weak turbulence regimes and steeper spectra occur in {\it stronger loop fields} and lead to {\it larger heating rates} than in weak field regions.Comment: 22 pages, 18 figures, uses emulateapj, for mpeg file associated to Figure 17e see (temporarily) http://www.df.unipi.it/~rappazzo/arxiv/jfl.mpg, ApJ, in pres

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

“…1 of [90]). In that original work, the magnetic field is (roughly) constant in space, which is similar to three-dimensional experiments without gravity [15]. Therefore, also the currents and thus the Ohmic heating will be (roughly) constant in space.…”

“…In particular, these models will also describe the variability of the heating in the direction perpendicular to the fieldlines. Solving a simplified problem, it has been shown that current sheets are roughly aligned with the magnetic field and result in a heat input intermittent in space and time [13,15]. These current sheets are also seen in three-dimensional models in a more realistic atmospheric set-up (albeit at a reduced resolution) as narrow fingers of enhanced heat input; see figs.…”

The magnetic field in the solar atmosphere

“…Such braiding is a measure of how often different field lines cross each other in space Berger and Asgari-Targhi 2009 and is supported by MHD models (Gudiksen and Nordlund 2005;Rappazzo et al 2008;van Ballegooijen et al 2011). These braided small-scale field structures efficiently heat the coronal environment by the dissipation of electric currents, where oppositely directed magnetic fields are found in close vicinity to each other (Parker 1983, see also Cirtain et al 2013;Thalmann et al 2014).…”

How skeletons turn into quasi-separatrix layers in source models

“…The first family of these models involve transient and alternative currents (AC), either created by MHD turbulence (e.g. Buchlin et al 2003;Gudiksen & Nordlund 2005;Rappazzo et al 2008) or by the resistive damping of nonlinear magnetoacoustic waves (e.g. Heyvaerts & Priest 1983;Einaudi & Mok 1987;Parker 1991;Ofman & Davila 1995).…”