QFP Waves Driven by the Tuning-Fork Effect during Magnetic Reconnecion

TL;DR

This study addresses how quasi-periodic fast-propagating (QFP) coronal waves are generated during solar eruptive magnetic reconnection. Using high-resolution 3D MHD simulations with gravity and anisotropic conduction, the authors model a vertical current sheet formed during eruption and identify termination shocks at the sheet ends that reflect fast reconnection outflows. They find two synchronized fast wave trains, W1 and W2, propagating at about $1400$ km s$^{-1}$ with a $2$-s period, excited by a tuning-fork mechanism at the sheet ends and associated Separatrix forks. The work provides a cohesive 3D mechanism linking reconnection topology to QFP wave generation and offers predictions for observations near CMEs and post-flare loops, while noting limitations related to 3D effects and parameter exploration.

Abstract

Through three-dimensional MHD simulations, we have uncovered a kind of fast coronal wave originating from both ends of a current sheet (CS) during a solar eruption. These waves are observed to appear near the top and bottom ends of the reconnection-related CS. The simulations demonstrate the presence of termination shock regions above the two ends of the CS. As the reconnection outflows escape from the vertical CS and encounter these termination shocks, they undergo partial reflection, redirecting towards the CS terminal fork walls. The identified waves propagate rapidly at a speed of approximately 1400 km/s with a period of just 2 s. Concurrently, the time-evolution of intensity within a small region of the CS terminal fork structures, exhibits a similar oscillation period of 2 s. All these evidence supports the notion that these QFP (Quasi-periodic Fast-Propagating) waves were excited by tuning fork effects within the CS system. Essentially, the rapid reconnection outflows are reflected by the terminal shocks, striking the fork walls at the CS ends. Moreover, parts of the oscillations along the tuning fork handle are transformed into thermal energy, accumulating in the CS center and elevating the temperature. This is the first time to report such QFP waves resulting from tuning fork effects within the CS during a solar eruption. These waves are anticipated to manifest closely following the propagation of CMEs and adjacent to the related post-flare loops in observations, with partial confirmation in current observations.

Figures (5)

• Figure 1: The distributions of initial magnetic field at t = 0 s and multiple physical parameters at t = 133.09 s. (a)-(b) shows the initial magnetic field configuration from two perspectives. This model incorporates a flux rope with axial current $I$, a subphotospheric charge pair ($\pm q_{1}$, separation L, depth $d_{1}$) and a buried magnetic dipole ($q_{2}$, depth $d_{2}$). This configuration reproduces characteristic active region topology with $B \propto z^{-2}$ field decay. The bottom shows spatial distributions of multiple physical parameters in the solar atmoshpere at t = 133.09 s, including (c) logarithmic density, (d) temperature , (e) current density and (f) velocity divergence. The current sheet (CS) is indicated, and "Separatrix" denotes the boundary between the CS and the ambient coronal plasma. All 2D spatial snapshots are taken at a fixed cut at y=0.
• Figure 2: Ripple-like waves shown by velocity divergence distribution as displayed in Panel (a). Panel (b) is the close-up of Region 2 of Panel (a) to show the group wave "W2" (green arrows) near the top end of the CS, propagating outside near the CME. Panel (c) is the close-up of Region 1 to display the group wave "W1" (black arrows) near the bottom end of the CS. "S1" in panel c and "S2" in panel b are used to indicate the positions for the wave periodicity analysis in Fig. \ref{['fig:fig3']}. An animated version from t = 132.31s to 239.71 s of this figure is available.
• Figure 3: Feature analysis of two group waves ("W1" and "W2"). Panel a-c presents the period analysis for "W1": (a) spatiotemporal distribution of slice "S1"; (b) horizontal dashed line within (a) ; (c) wavelet analysis plot of (a2). Panel (d-f) presents the corresponding analysis for "W2".
• Figure 4: Distributions of density and velocities revealing the locations of fast wave excitations. b-c display specific regions within the upper and lower dashed boxes in Panel a, where velocity fields are illustrated with white arrows, indicating the directions and magnitudes of plasma flow. "USP" and "LSP" denote the upper and lower sections of the "Separatrix" structure, respectively. The circled region region "K" is situated within the lower "Separatrix" and exhibits a density oscillation of 2 s, as depicted in Panel (d).
• Figure 5: Schematic diagram illustrating the generation of QFP waves. During a solar eruption, magnetic reconnection takes place within a vertical CS, propelling the erupting structure as shown by the pink twisted topology. The vertical CS is highlighted by the thick yellow lines and exhibits two fork structures at its opposing ends. Inflow during reconnection is indicated by the bold grey arrows, while reconnection outflows are represented by two thick vertical arrows at each ends. These outflows strike the termination shock structures, as indicated by the navy blue ellipses, leading to reflection. The sky blue ripple structures denote "W1" and "W2".