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

Abstract

Context. Ubiquitous vortex flows at the solar surface excite magnetohydrodynamic (MHD) waves that propagate to higher layers of the solar atmosphere. In the solar corona, these waves frequently encounter magnetic null points. The interaction of MHD waves with a coronal magnetic null in realistic 3D setups requires an appropriate wave identification method. Aims. We present a new MHD wave decomposition method that overcomes the limitations of existing wave identification methods. Our method allows for an investigation of the energy fluxes in different MHD modes at different locations of the solar atmosphere as waves generated by vortex flows travel through the solar atmosphere and pass near the magnetic null. Methods. We used the open-source MPI-AMRVAC code to simulate wave dynamics through a coronal null configuration. We applied a rotational wave driver at our bottom photospheric boundary to mimic vortex flows at the solar surface. To identify the wave energy fluxes associated with different MHD wave modes, we employed a wave decomposition method that is able to uniquely distinguish different MHD modes. Our proposed method utilizes the geometry of an individual magnetic field-line in the 3D space to separate the velocity perturbations associated with the three fundamental MHD waves. We compared our method with an existing wave decomposition method that uses magnetic flux surfaces instead. Over the selected flux surfaces, we calculated and analyzed the temporally averaged wave energy fluxes, as well as the acoustic and magnetic energy fluxes. Our wave decomposition method allowed us to estimate the relative strengths of individual MHD wave energy fluxes. Results. Our method for wave identification is consistent with previous flux-surface-based methods and provides the expected results in terms of the wave energy fluxes at various locations of the null configuration. We show that ubiquitous vortex flows excite MHD waves that contribute significantly to the Poynting flux in the solar corona. Alfvén wave energy flux accumulates on the fan surface and fast wave energy flux accumulates near the null point. There is a strong current density buildup at the spine and fan surface. Conclusions. The proposed method has advantages over previously utilized wave decomposition methods, since it may be employed in realistic simulations or magnetic extrapolations, as well as in real solar observations whenever the 3D fieldline shape is known. The essential characteristics of MHD wave propagation near a null – such as wave energy flux accumulation and current buildup at specific locations – translate to the more realistic setup presented here. The enhancement in energy flux associated with magneto-acoustic waves near nulls may have important implications in the formation of jets and impulsive plasma flows.

MPI-AMRVAC 3.0: updates to an open-source simulation framework

“…We use the latter test to evaluate the behavior of the various super-time-stepping strategies available in MPI-AMRVAC for handling specific parabolic source additions. This test also employs the more generic splitting strategy usable in gravitationally stratified settings, also adopted recently in Yadav et al (2022). We note that the mhd module offers many more possibilities than showcased here: we can e.g.…”

MPI-AMRVAC 3.0: Updates to an open-source simulation framework

“…We used this test to evaluate the behavior of the various super-timestepping (STS) strategies available in MPI-AMRVAC for handling specific parabolic source additions. This test also employs the more generic splitting strategy usable in gravitationally stratified settings, also adopted recently in Yadav et al (2022). We note that the mhd module offers many more possibilities than showcased here: we can, for example, drop the energy evolution equation in favor of an isothermal or polytropic closure, can ask to solve for internal energy density instead of the full (magnetic plus kinetic plus thermal) energy density, and have switches to activate anisotropic thermal conduction, optically thin radiative losses, viscosity, external gravity, as well as Hall and/or ambipolar effects.…”

“…The setup is similar to that employed for a previous study of the ambipolar effect on MHD waves in a 2D setup in Popescu Braileanu & . The MHD equations solved are for (up to nonlinear) perturbations only, where the variables distinguish between equilibrium (a hydrostatically stratified atmosphere with fixed pressure p 0 (z) and density ρ 0 (z)) and perturbed variables, as described in Yadav et al (2022) (see Eqs. ( 4)-( 9)).…”

“…Thus far, all these aspects are only handled approximately, with, for example, the recently added plasma-neutral src/twofl module (Popescu Braileanu & Keppens 2022) as an example where the intrinsically varying degree of ionization throughout the atmosphere can already be incorporated. To deal with large variations in plasma beta, we provided options to split off a time-independent (not necessarily potential) magnetic field B 0 in up to resistive MHD settings (Xia et al 2018), meanwhile generalized (Yadav et al 2022) to split off entire 3D magneto-static force-balanced states −∇p 0 + ρ 0 g + J 0 × B 0 = 0. For MHD and two-fluid modules, we add an option to solve internal energy equation instead of total energy equation to avoid negative pressure when plasma beta is extremely small.…”

How Nonlinearity Changes Different Parameters in the Solar Corona

“…In the solar atmosphere, waves often have significant amplitudes that can be assumed to be nonlinear. The investigation of nonlinear waves in the solar atmosphere has been carried out especially in the context of chromospheric and coronal heating (Ruderman 2006;McLaughlin et al 2009McLaughlin et al , 2011Magyar & Van Doorselaere 2016;Sabri et al 2018Sabri et al , 2019Karampelas et al 2021;Sabri et al 2022;Yadav et al 2022). It must be noted that the main effect of the nonlinearity could be the development of the Kelvin-Helmholtz instability (KHI).…”