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 $\approx 1-10\%$ 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.

24 pages, 20 figures. Accepted for publication in ApJ

References in corpus (10)

• On the generation of solar spicules and Alfvénic waves
• Nonlinear Fast Magnetoacoustic Wave Propagation in the Neighbourhood of a 2D magnetic X-point: Oscillatory Reconnection
• Three-dimensional oscillatory magnetic reconnection
• Generation of quasi-periodic waves and flows in the solar atmosphere by oscillatory reconnection
• On the Periodicity of Oscillatory Reconnection
• MHD mode coupling in the neighbourhood of a 2D null point
• MHD wave propagation from the sub-photosphere to the corona in an arcade-shaped magnetic field with a null point
• Internal Gravity Waves in the Magnetized Solar Atmosphere. I. Magnetic Field Effects
• Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point
• Magnetic topological analysis of coronal bright points

Cited by in corpus (4)

• Measurements of Photospheric and Chromospheric Magnetic Field Structures Associated with Chromospheric Heating over a Solar Plage Region
• Three-dimensional MHD wave propagation near a coronal null point: a new wave mode decomposition approach
• Direct Imaging of Magnetohydrodynamic Wave Mode Conversion Near a 3D Null Point on the Sun
• Conversion and Smoothing of MHD Shocks in Atmospheres with Open and Closed Magnetic Field and Neutral Points