Fast plasmoid-mediated reconnection in a solar flare

Abstract

Magnetic reconnection is a multi-faceted process of energy conversion in astrophysical, space and laboratory plasmas that operates at microscopic scales but has macroscopic drivers and consequences. Solar flares present a key laboratory for its study, leaving imprints of the microscopic physics in radiation spectra and allowing the macroscopic evolution to be imaged, yet a full observational characterization remains elusive. Here we combine high resolution imaging and spectral observations of a confined solar flare at multiple wavelengths with data-constrained magnetohydrodynamic modeling to study the dynamics of the flare plasma from the current sheet to the plasmoid scale. The analysis suggests that the flare resulted from the interaction of a twisted magnetic flux rope surrounding a filament with nearby magnetic loops whose feet are anchored in chromospheric fibrils. Bright cusp-shaped structures represent the region around a reconnecting separator or quasi-separator (hyperbolic flux tube). The fast reconnection, which is relevant for other astrophysical environments, revealed plasmoids in the current sheet and separatrices and associated unresolved turbulent motions.

Fig. 1. Involvement of the filament in magnetic reconnection during the confined flare, as observed by the NVST in the center of the Hα line.

a Hα image at 08:04:54 UT, superimposed by the line-of-sight magnetogram from SDO/HMI at 08:04:55 UT, with positive (red) and negative (blue) contours drawn at ±500 G. The dotted yellow and pink lines mark the filament and relevant chromospheric fibrils, respectively. b SDO/HMI line-of-sight magnetogram with field lines from the NLFFF extrapolation superimposed. The selected yellow lines trace the twisted flux of the filament. The pink lines show the relevant loops rooted in chromospheric fibrils. c–h Reconnection of a filament thread in the current sheet. White arrows indicate the thread, first in the inflow region (c–d), then in the outflow region (g–h). Blue arrows in e–h indicate the newly formed connection of the thread with the loops rooted in the chromospheric fibrils on the other side of the current sheet. Note that the east and west directions described in the text correspond to the left and the right sides of the Figures. Source data are provided as a Source Data file.

Fig. 2. Images of the reconnection process in the center of the Hα line and in characteristic EUV lines.

The Hα images in the first column (a, f, k, p) show the reconnecting filament (dotted yellow line), magnetic loops rooted in chromospheric fibrils (dotted pink line), the current sheet region (blue arrows), and the cusp-shaped magnetic separatrices (blue dotted lines). White (black) arrows mark the reconnection inflow (outflow/cusp) regions. The lines AB, CD, EF, and GH in panel p are used to measure the reconnection flows. The bright structures in panel p, mostly flare ribbons, are marked with blue contours that are overlaid in panels q–t for comparison. Columns 2–5 display nearly simultaneous AIA images at 304 (b, g, l, q), 171 (c, h, m, r), 131 (d, i, n, s), and 1600 Å (e, j, o, t). Contours at ±500 G of the magnetogram at 08:08:40 UT are superimposed in panels g–j. The yellow arrow in panel l denotes the bright structure. Note that the east and west directions described in the text correspond to the left and the right sides of the figures. Source data are provided as a Source Data file.

Fig. 3. Profiles of GOES X-ray flux, inflows, and outflows during reconnection.

a The profile of GOES X-rays in 1–8 Å (black line) and 0.5–4 Å (blue line). b The profile of the RHESSI X-ray counts in the 3–6, 6–12, 12–25, and 25–50 keV bands. c Inflow (solid curve) along the A-B slit inferred from time-slices of the intensity. d Outflow along the C-D slit. e Outflow along the curved line E–F. f Outflow along the curved line G–H. All these lines are marked in Fig. 2p. Note that the y-axis denotes the direction information in the time–distance maps of Figure 3c–f. Note that the east and west directions of Figure 3c-f described in the text correspond to the left and the right sides of the figures in Fig. 2. Source data are provided as a Source Data file.

Fig. 4. Hinode/EIS spectroscopic observation at Fe XVII 255 Å during magnetic reconnection.

a–c Evolution of the cusp-shaped structure in the intensity images. d–f Doppler velocity and g–i non-thermal velocity obtained from single Gaussian fitting. Source data are provided as a Source Data file.

Fig. 5. Plasmoids in the SDO/AIA 211 Å images revealed by the unsharp masking technique.

Yellow arrows mark the different bright blobs moving from the reconnection region (a–f). The cyan-dotted line in a indicates the trajectory I-J used to make the time-distance diagram of g. The projected velocities of the blobs lie in the range from 79 km s⁻¹ to 208 km s⁻¹. Note that the y-axis denotes the direction information in the time-distance maps of g. Source data are provided as a Source Data file.

Fig. 6. Emission measure and temperature maps and Hα images with RHESSI X-ray sources superimposed.

a–d DEM maps in four temperature ranges. e Total emission measure. f Temperature of the reconnection region derived from six AIA wavelengths. g–i Hα images with RHESSI hard X-ray sources superimposed. The levels of the contours are at 50% and 80% of the peak values in the 3–6 (blue), 6–12 (cyan), 12–25 (pink), and 25–50 (white) keV bands. Source data are provided as a Source Data file.

Fig. 7. RHESSI photon spectra accumulated around 08:12 UT and 08:19 UT during the quoted 1-min intervals on 2014 February 2.

a 08:12–08:12 UT. b 08:18–08:19 UT. The spectra are fitted by combining the vth and thick2 functions (see Methods, subsection RHESSI spectrum). The black line shows the photon spectrum with the background subtracted. The pink line shows the background. The green and yellow lines display the fitting with the single-temperature thermal model and the thick-target nonthermal model, respectively. vth is used to fit the thermal portion of the spectra, generally from 4 keV to between 20 and 25 keV depending on the attenuator state. thick2 is used to fit Thick-Target Bremsstrahlung X-ray/gamma-ray spectrum from an isotropic electron distribution. Bk indicates the background photon flux. Data-Bk indicates the observational photon flux with the background flux subtracted. Detectors indicate the detectors that used to obtain the photon flux. The error is the square root of the counts used in the fitting. When combining time bins, these errors are averaged. Source data are provided as a Source Data file.

Fig. 8. Pre-reconnection structure of the magnetic field lines, current sheet, and separatrices or QSLs, as reproduced by the data-constrained MHD model, and their relation with the observed structures.

a Selected magnetic field lines showing the magnetic structure at the side of the filament (green) and the magnetic loops rooted in the chromospheric fibrils (pink) overlaid on the longitudinal magnetogram at 08:00 UT observed by SDO/HMI. Note that the background image of panels b, c, and d is the same as panel a. b, c Two views of the same field lines and the current sheet between them, displayed as an iso-surface of the current density color coded with the height information. d The X-type structure of the separatrices or QSLs, displayed as iso-surface ${\log }_{10}Q=3$ of the squashing degree Q, also color coded with height. e Horizontal map of the separatrices or QSLs (here ${\log }_{10}Q>2.5$) at height z = 2.5 Mm superimposed on the Hα image at 08:09:21 UT. f Same separatrix or QSL map superimposed on an enhanced 211 Å image at 08:05:35 UT showing the plasmoids. Note that the east and west directions described in the text correspond to the left and the right sides of the Figures. Source data are provided as a Source Data file.

Fig. 9. Evolution of magnetic field, current sheet, and plasmoids (mini flux ropes) during magnetic reconnection in the data-constrained MHD simulation.

a Evolution of the overall magnetic structure involved in the modeled reconnection event. Note that the field lines are randomly colored from column to column, and thus the same color does not indicate the same field line. The background image is the longitudinal magnetic field at 08:00 UT observed by SDO/HMI. b Distribution of electric current density at three selected heights and times in the current sheet. The heights are chosen at which the mini flux rope is sliced at its middle, i.e., the plasmoid is seen most clearly. The white arrows indicate the biggest plasmoid.

Fig. 10. Magnetic structure of the current sheet.

a The corresponding magnetic structure of the current sheet. The black arrows indicate the magnetic structure of the biggest plasmoid. b 3D magnetic structure of the biggest plasmoid in a side view at three selected times. The arrows mark the heights given in panel b of Fig. 9. Note that the magnetic field lines are shown by the thick colored lines, and the colors are used for a better visualization of the different lines.