Wave generation via oscillatory reconnection at a three-dimensional magnetic null point

TL;DR

This work shows that oscillatory reconnection at a 3D magnetic null point, when driven by a non-periodic perturbation, generates self-sustained, period-$P$ dynamics that excite distinct MHD waves. Using three wave proxies and SPOD, the study reveals a dominant slow magnetoacoustic wave radiating outward along the spine and fan plane, and an Alfvén wave propagating along the fan-plane $y$-direction, both tied to the null’s $j_y(0,0,0,t)$ oscillations. The SPOD analysis identifies a leading mode accounting for about 73% of the energy and a small set of higher-order modes that capture the remaining dynamics, including a $P/2$ component associated with cavity-trapped standing waves. The results illuminate how 3D nulls can serve as persistent wave sources in coronal plasmas, offering a potential avenue for coronal seismology to infer local $v_A$ and $c_s$ from OR-driven wave signatures.

Abstract

This work conducts a three-dimensional (3D), nonlinear magnetohydrodynamic (MHD) simulation to investigate wave generating, time-dependent reconnection around a magnetic null point. A non-periodic perturbation (in the $xz$-plane) triggers oscillatory reconnection (OR) at the 3D null, resulting in a self-sustained oscillation with a constant period $P$. We investigate the response of the system using three distinct wave proxies (compressible parallel, compressible transverse and incompressible parallel) as well as Spectral Proper Orthogonal Decomposition for decoupling and analyzing the resultant MHD wave behavior. We find that OR generates a slow magnetoacoustic wave of period $P$ that propagates outwards in all directions along the spine and fan plane of the 3D null point. We also find the generation of a propagating Alfvén wave of period $P$, exclusively along the $y$-axis in the fan plane, i.e. in the direction perpendicular to the spine motion. These findings provide new insights into waves generated from a 3D null point and their implications for coronal seismology.

Figures (15)

• Figure 1: Traced magnetic field lines for the initial condition, panels show different views of the initial condition: (a) a $xz-$view, (b) a $xy-$view, (c) 3D view. The green line represents the null point spine, while the blue and red lines represent the fan plane traced from the upper and lower boundaries, respectively.
• Figure 2: (a) Current density oscillations at the null point, $j_y(0,0,0,t)$, showing a signature of oscillatory reconnection, where the blue curve represents the simulated $j_y(0,0,0,t)$ and orange dots indicate the simulation times displayed in panels (b)-(c) and Figures \ref{['fig:slices']}, \ref{['fig:slices-fan']} and \ref{['fig:fan-MHD-alfven']}. (b) Spine evolution during reconnection cycles, with blue and red curves corresponding to the $j_y(0,0,0,t)$ orange dots in panel (a). (c) Fan plane evolution at $y=0$, using the same color scheme from panel (b) to denote the oscillation phases. Field lines from panels (b)-(c) were traced from the null point.
• Figure 3: SPOD energy spectra, where mode energy, $\lambda_i$, is normalized by the total perturbation energy, $\sum_i^N \lambda_i$.
• Figure 4: Comparison between simulation and SPOD for the current density measured at the null point.
• Figure 5: Isosurfaces of SPOD dominant spatial mode $\mathbf{\phi(\mathbf{x})}$ for the MHD wave proxies: (a) $\xi_A$, (b) $\xi_\|$ and (c) $\xi_\bot$.