The Formation and Dissipation of Current Sheets and Shocks due to Compressive Waves in a Stratified Atmosphere Containing a Magnetic Null

Abstract

We study the propagation and dissipation of magnetohydrodynamic waves in a set of numerical models that each include a solar-like stratified atmosphere and a magnetic field with a null point. All simulations have the same magnetic field configuration but different transition region heights. Compressive wave packets introduced in the photospheric portion of the simulations refract towards the null and collapse it into a current sheet, which then undergoes reconnection. The collapsed null forms a current sheet due to a strong magnetic pressure gradient caused by the inability of magnetic perturbations to cross the null. Although the null current sheet undergoes multiple reconnection episodes due to repeated reflections off the lower boundary, we find no evidence of oscillatory reconnection arising from the dynamics of the null itself. Wave mode conversion around the null generates a series of slow mode shocks localized near each separatrix. The shock strength is asymmetric across each separatrix, and subsequent shock damping therefore creates a tangential discontinuity across each separatrix, with long-lived current densities. A parameter study of the injected wave energy to reach the null confirms our previous WKB estimates. Finally, using current estimates of the photospheric acoustic power, we estimate that the shock and Ohmic heating we describe may account for ≈ 1 − 10% of the radiative losses from coronal bright points with similar topologies, and are similarly insufficient to account for losses from larger structures such as ephemeral regions. At the same time, the dynamics are comparable to proposed mechanisms for generating type-II spicules.

Conversion and Smoothing of MHD Shocks in Atmospheres with Open and Closed Magnetic Field and Neutral Points

“…Our simulations do not include 3D or diffusive terms, so these observations are qualitative, not quantitative, simply showing that transient sites favourable for dissipative heating are created by shocks, especially in concert with neutral points. However, in their MBP modelling, Tarr and Linton (2019) find that the attendant heating is still insufficient to supply radiative losses. Shock smoothing would further reduce heating.…”

“…Similar ray calculations were carried out byTarr and Linton (2019) in 3D, including the Alfvén wave which we have neglected.3 A stiffness-switching scheme with projection is used to maintain D = 0 to high order.…”

3D MHD wave propagation near a coronal null point: New wave mode decomposition approach

“…These new results have drawn renewed interest in wave-heating mechanisms, both observationally and theoretically. Combinations of MHD wave aspects, vortices, and null point ingredients have only been studied to a certain extent (McLaughlin et al 2011(McLaughlin et al , 2019Thurgood & McLaughlin 2013;Shelyag et al 2013;Mumford et al 2015;Tarr et al 2017;Tarr & Linton 2019;Liu et al 2019;Tziotziou et al 2020;Battaglia et al 2021). Because vortex flows, resulting torsional Alfvén waves, and localized null points are so common, their co-occurrence is quite likely, opening up new challenges in the study of wave and energy propagation.…”

Three-dimensional MHD wave propagation near a coronal null point: a new wave mode decomposition approach

“…These new results have drawn renewed interest in wave heating mechanisms, both observationally and theoretically. Combinations of MHD wave aspects, vortices and null point ingredients have only been studied to some extent (McLaughlin et al 2011;Thurgood & McLaughlin 2013;Shelyag et al 2013;Mumford et al 2015;Tarr et al 2017;Tarr & Linton 2019;McLaughlin et al 2019;Liu et al 2019;Tziotziou et al 2020;Battaglia et al 2021). Because vortex flows, resulting torsional Alfvén waves and localized null points are so common, their co-occurrence is quite likely, opening up new challenges in the study of wave and energy propagation.…”

“…The left vertical axis displays the temperature and the pressure, and the right vertical axis displays the mass density in CGS units. Gruszecki et al 2011;Fedun et al 2011;Vigeesh et al 2012;Pascoe et al 2013;Santamaria et al 2015;Tarr et al 2017;Tarr & Linton 2019;Sabri et al 2020). In particular, Sabri et al (2020) investigated the nonlinear effects associated with interaction of an Alfvén wave with a magnetic null point in a 2.5D simulation setup.…”