Magnetic Shocks and Substructures Excited by Torsional Alfvén Wave Interactions in Merging Expanding Flux Tubes

Abstract

Vortex motions are frequently observed on the solar photosphere. These motions may play a key role in the transport of energy and momentum from the lower atmosphere into the upper solar atmosphere, contributing to coronal heating. The lower solar atmosphere also consists of complex networks of flux tubes that expand and merge throughout the chromosphere and upper atmosphere. We perform numerical simulations to investigate the behaviour of vortex driven waves propagating in a pair of such flux tubes in a non-force-free equilibrium with a realistically modelled solar atmosphere. The two flux tubes are independently perturbed at their footpoints by counter-rotating vortex motions. When the flux tubes merge, the vortex motions interact both linearly and nonlinearly. The linear interactions generate many small-scale transient magnetic substructures due to the magnetic stress imposed by the vortex motions. Thus, an initially monolithic tube is separated into a complex multi-threaded tube due to the photospheric vortex motions. The wave interactions also drive a superposition that increases in amplitude until it exceeds the local Mach number and produces shocks that propagate upwards with speeds of approximately 50 km s −1 . The shocks act as conduits transporting momentum and energy upwards, and heating the local plasma by more than an order of magnitude, with peak temperature approximately 60, 000 K. Therefore, we present a new mechanism for the generation of magnetic waveguides from the lower solar atmosphere to the solar corona. This wave guide appears as the result of interacting perturbations in neighbouring flux tubes. Thus, the interactions of photospheric vortex motions is a potentially significant mechanism for energy transfer from the lower to upper solar atmosphere.

Chromospheric Heating by Magnetohydrodynamic Waves and Instabilities

“…Hence, non-linear Alfvén waves are able to steepen into shocks (Cohen and Kulsrud 1974;Murawski et al 2015;Shestov et al 2017;Snow et al 2018), with such steepening being expedited when the Alfvén speed is approximately equal to the local sound speed (i.e., the plasma-β ∼ 1; Montgomery 1959).Of course, it is important at this stage to highlight the differences between the non-linear evolution of plane shear Alfvén waves and torsional oscillations in magnetic flux tubes. For both cases, Vasheghani Farahani et al (2012) found that a non-linear self-interaction resulted in the steepening of the waves.…”

Modelling 3D magnetic networks in a realistic solar atmosphere

“…The stability of the solution can also be confirmed by numerical simulation for each configuration. This was carried out for the flux-tube pair solution used in Snow et al (2018), by treating the solution as the MHD perturbations, and the system remained stationary to within machine accuracy.…”

“…Gent et al (2014, hereafter Paper II) generalised the background configuration to 3D, multiple, non-identical flux tubes, extending into the lower corona. This was successfully applied to a 3D model of a flux-tube pair by Snow et al (2018), who showed that chromospheric shocks at the intersections between the tubes are capable of driving supersonic jets.…”

Influence of ambipolar and Hall effects on vorticity in three-dimensional simulations of magneto-convection

“…Significant modelling efforts have been made in an attempt to understand the physics of swirling motions in the solar atmosphere. They range from numerical experiments where vortices are generated in idealized conditions, such as isolated flux tubes, controlled driver properties [13][14][15][16][17][18], to more realistic models where the vortical motions are naturally produced by magnetoconvection [8,[19][20][21][22][23][24][25][26][27][28][29]. It has been shown that the nature of magnetic and non-magnetic vortices is intrinsically different, see Shelyag et al [28].…”