Quasi-Periodic Fast-Mode Wave Trains Associated with the 2015-Jun-22 M6.5 Flare in AR~12371: Observations and 3D MHD Modeling

Abstract

Quasi-periodic fast-propagating (QFP) wave trains are a distinctive form of magnetohydrodynamic disturbance frequently observed in the solar corona. Yet their excitation mechanism and propagation characteristics are not well understood. In this study, we investigate a well-observed QFP wave event associated with an M6.5-class flare and coronal mass ejection that occurred in active region (AR) 12371 on 2015 June 22 by combining multi-wavelength observations from SDO/AIA and HMI with data-inspired 3D MHD simulations. The QFP wave trains propagating at high speeds of 1140$-$1760 km~s$^{-1}$ are detected in the AIA 171 Å channel, following global EUV wave fronts visible at 171 and 193~Å traveling at considerably lower speeds of 300$-$510 km~s$^{-1}$. Wavelet analysis reveals consistent 2--4 minutes periodicities in both the QFPs and flare quasi-periodic pulsations (QPPs) observed in UV/EUV and hard X-ray emissions, suggesting a common origin likely linked to intermittent magnetic reconnection. Guided by these observations, we construct realistic 3D MHD models incorporating dense fan-loop structures and periodic drivers applied at different locations. The simulations reproduce the key characteristics of the observed wave trains. Comparison between cases with and without a coronal background (non-loop plasma emission) indicates that coronal density structuring significantly modifies the detected wave amplitudes and propagation patterns. Our results highlight the importance of realistic coronal magnetic configurations in modeling QFP dynamics and suggest that their observed association with fan loops in AIA 171 Å may represent a temperature-dependent visibility effect rather than a genuine confinement of the waves.

Figures (12)

• Figure 1: (a) GOES soft X-ray fluxes and (b) their time derivatives (smoothed with a 1-minute boxcar) in comparison with the Fermi/GBM 26–50 keV X-ray flux (smoothed with a 12-second boxcar), together with light curves in SDO/AIA 1600 and 304 Å.
• Figure 2: Upper panels: (A) SDO/HMI vector magnetogram of AR 12371 observed at 17:58:25 UT on 2015 June 22. The background shows the longitudinal magnetic field component scaled between $\pm$500 G, with positive polarity shown in white and negative in black. (B) SDO/AIA 171 Å image showing flaring loops near the magnetic neutral line. The overlaid contours represent the vertical ($B_z$) component of the magnetic field at levels of $\pm$500 and $\pm$100 G, with red indicating positive and cyan negative polarities. The same arrows in (A) and (B) denote the transverse magnetic field vectors, with strengths ranging from 100 to 2100 G. (C) AIA 193 Å image taken at 18:07:06 UT. Middle and bottom panels: AIA 171 Å direct (middle) and running difference (bottom) images showing the flare-generated QFP wave trains. The red curve outlines the expanding flux rope, while the cyan curves trace the propagating wave fronts. The four numbered slices, shown in white in (C) and (a1), are used to produce the time--distance plots shown in Figure \ref{['fig:tdismap']}. An animation corresponding to the middle and bottom panels of this figure is available. The accelerated animation (12 s duration) presents the time sequence of AIA 171 Å images (top panel) and their 24 s running-difference (bottom panel) covering the interval from 18:00:47 to 18:29:59 UT on 2015 June 22. (An animation of this figure is available in the online article.)
• Figure 3: Top two rows: (a1)-(d1) Time--distance maps of the AIA 171 Å relative intensity, defined as $(I(t)-I_0)/I_0$, where $I_0$ denotes the average intensity over time at each position along the slice. (a2)-(d2) Same as the upper panels, but for the running-difference images. The cross symbols and slanted lines mark the identified wave features and their linear fits used to measure the propagation speeds. Bottom two rows: (a3)-(d3) Time--distance maps of the AIA 193 Å emission in base-difference form. (a4)-(d4) Same as (a3)-(d3), but for the running-difference images. The alternating vertical dark and bright strips are artifacts resulting from variations in the image exposure times due to AIA's automatic exposure control (AEC) during flares.
• Figure 4: (a) Time--distance map of AIA 171 Å intensity along Slice 1 (in the logarithmic scale). (b) Time profiles of AIA 171 Å intensity (purple solid line), shown relative to the averaged pre-event background (purple horizontal line) and its detrended signal (black solid line) along a spatial cut indicated by the black line in panel (a). The black dashed line shows the background trend. (c) Comparison of background-removed Fermi/GBM 26--50 keV X-ray flux (amplitudes reduced by a factor of 7) with background-removed relative intensities of AIA 1600 and 304 Å. Those detrended light curves shown in (b) and (c) are calculated as $(I(t)-I_{\rm bg}(t))/I_{\rm bg}(t)$, where, $I_{\rm bg}(t)$ represents a slowly varying background. (d) Wavelet power spectra of the intensity fluctuations in AIA 171 Å, 304 Å, 1600 Å, and Fermi/GBM X-rays (from top to bottom). Dark colors indicate regions of high power, and the dotted contours enclose areas exceeding the 99% confidence level. The black cross-hatched region marks the cone of influence where period estimates become unreliable. (e) Corresponding global wavelet power spectra (solid lines). Peaks above the 99% confidence level (dashed line) are statistically significant.
• Figure 5: Top panels: (a) Initial 3D potential field model of AR 12371 including three dense loops constructed by tracing magnetic field lines (yellow) to simulate the AIA fan loops. Note that the $xyz$ coordinates are in dimensionless unit. The background plane at $z=z_{\rm min}$ shows the radial magnetic field component at a height of 17 Mm above the photosphere, scaled between $\pm100$ G, with positive and negative polarities represented in white and black, respectively. Cyan lines denote additional field lines extrapolated from the bottom boundary. The yellow lines correspond to field lines defining the three loop models, whose footpoints at $z=z_{\rm min}$ are marked by small red circles and endpoints at $z=z_{\rm max}$ by pink circles. (b) 3D view of the initial magnetic configuration showing the three loops with enhanced density having a sharp Gaussian cross-sectional profile. The isosurface represents a density contrast of 1.01. The cross-section at $z=1.7$ illustrates the density contrast distribution $\chi_\rho$ ranging from 1 to 2. The contours show the $z$-component of the magnetic field at $z=z_{\rm min}$ with levels of $\pm12$, $\pm25$, $\pm50$, and $\pm100$ G, where red and blue denote the positive and negative polarities, respectively. The cyan lines have the same meaning as in (a). Bottom panels: Squashing factor ($Q$) maps calculated from the potential field model. (c) Logarithmic distribution of the squashing factor ($\mathrm{sign}(B_z)\log_{10}(Q\ge2)$) in the $xy$-plane at $z=z_{\rm min}$. Positive and negative magnetic polarities are shown in red and blue, respectively, with bright regions indicating high $Q$ values. Solid and dashed contours denote the $z$-component of the magnetic field with levels of $\pm12$, $\pm25$, $\pm50$, and $\pm100$ G. The two thick blue circles mark the locations of the wave drivers in Model 1 (smaller circle) and Model 2 (larger circle). Other lines are as described in (a). (d) Logarithmic squashing factor ($\log_{10}(Q\ge2)$) in the $yz$-plane at $x=0.53$ (indicated by the vertical line in (c)). High-$Q$ regions are shown in dark shading.