The effects of driving time scales on heating in a coronal arcade

Abstract

Context. The relative importance of alternating current (AC) and direct current (DC) heating mechanisms in maintaining the temperature of the solar corona is not well constrained. Aims. We aim to investigate the effects of the characteristic time scales of photospheric driving on the injection and dissipation of magnetic and kinetic energy within a coronal arcade. Methods. We conducted three-dimensional magnetohydrodynamic simulations of complex foot point driving imposed on a potential coronal arcade. We modified the typical time scales associated with the velocity driver to understand the efficiency of heating obtained using AC and DC drivers. We considered the implications for the injected Poynting flux and the spatial and temporal nature of the energy release in dissipative regimes. Results. For the same driver amplitude and complexity, long time scale velocity motions are able to inject a much greater Poynting flux of energy into the corona. Consequently, in non-ideal regimes, slow stressing motions result in a greater increase in plasma temperature than for wave-like driving. In dissipative simulations, Ohmic heating is found to be much more significant than viscous heating. For all drivers in our parameter space, energy dissipation is greatest close to the base of the arcade, where the magnetic field strength is strongest, and at separatrix surfaces, where the field connectivity changes. Across all simulations, the background field is stressed with random foot point motions (in a manner more typical of DC heating studies), and, even for short time scale driving, the injected Poynting flux is large given the small amplitude flows considered. For long time scale driving, the rate of energy injection was comparable to the expected requirements in active regions. The heating rates were found to scale with the perturbed magnetic field strength and not the total field strength. Conclusions. Alongside recent studies that show that power within the corona is dominated by low frequency motions, our results suggest that, in the closed corona, DC heating is more significant than AC heating.

Can Multi-threaded Flux Tubes in Coronal Arcades Support a Magnetohydrodynamic Avalanche?

“…In the light of our intention to drive the field at the base, two features in particular are desirable: firstly, a vertical photospheric field that reverses sign at the centre of the domain and, secondly, sufficiently large regions of (near-)uniform field strength on either side of a polarity inversion line (PIL). To that end, we modify a hyperbolic trigonometric field structure (as used by Howson, De Moortel, and Fyfe, 2020), adding further Fourier modes to create wide regions of near-uniform photospheric B z at the edges of the arcade, while smoothly reversing sign at the domain centre. The general form of the field is:…”

Investigating coronal wave energy estimates using synthetic non-thermal line widths

“…The last of our three numerical models considers a potential coronal arcade where a complex velocity driver is implemented. This simulation was studied by Howson et al (2020b) and hence we direct the reader to this article for further information. The authors considered several numerical simulations (with different characteristic driving timescales) and they present results for ideal, resistive, and viscous regimes.…”

“…The authors considered several numerical simulations (with different characteristic driving timescales) and they present results for ideal, resistive, and viscous regimes. Howson et al (2020b) constructed a numerical arcade within an initially homogeneous plasma with a temperature and density of approximately 1 MK and 1.67 × 10 −12 kg m −3 , respectively. The arcade magnetic field has the form B(x, z) = (B x , 0, B z ) where…”

“…Finally, the viscous simulations implement a uniform viscosity which produces a fluid Reynolds number of 10 3 . Howson et al (2020b) implemented a boundary driver which mimics this chaotic nature of photospheric motions by varying the driver in time and space. The velocity driver on z = 0 Mm was created using the summation of 2D Gaussians and takes the Notes.…”

How Turbulent is the Magnetically Closed Corona?

“…Some studies intentionally launch waves (e.g.,van Ballegooijen, et al, 2011;Howson, et al, 2020), but those are not considered here.2 We use full MHD and impose a driving pattern in which vorticity is non-constant on streamlines-a condition said to be necessary to activate the nonlinear terms in the equations(Rappazzo et al, 2008). Despite this, no "nonlinear dynamics" occurs until kinking sets in.Frontiers in Astronomy and Space Sciences | www.frontiersin.org May 2021 | Volume 8 | Article 662861…”