Fast reconnection in a coronal torn plasma sheet

Abstract

Tearing instability, also known as plasmoid instability, is an effective mechanism to speed up magnetic reconnection process, working in a wide range of magnetized plasma systems with different spatial scales, ionization degrees, and collisionality. However, due to observational limitations, observations of {plasma sheet} tearing and the resulting plasmoids are rather scarce. This scarcity significantly hinders our understanding of the role of plasmoids in the reconnection process from an observational perspective. Using high-spatiotemporal multiwavelength observations from the Solar Dynamics Observatory, we traced the entire evolution of a coronal {plasma sheet}. Its formation was driven by the emergence of photospheric magnetic flux, followed by tearing, and eventual decay. The evolution of the {plasma sheet} exhibited two distinct stages. Initially, it rose rapidly, lengthened, and underwent tearing at a low frequency. Subsequently, its ascent slowed, it began to shorten, and the tearing occurred more frequently. Detailed analysis of the reconnecting {plasma sheet} focuses on heating, plasmoid dynamics (formation and ejection), and the resulting reconnection rate change. Two key heating processes are identified: {plasma sheet} tearing and coalescence involving plasmoids and magnetic cusps. More importantly, combining observations with analytical studies suggests that plasmoids are key carriers of magnetic flux fast transferring in the observed torn {plasma sheet}, and their formation and ejection significantly enhance the reconnection rate and facilitate the onset of fast reconnection.

Figures (5)

• Figure 1: The layered atmospheric evolution during magnetic reconnection. From top to bottom are observations from SDO/HMI, SDO/AIA 171 Å, and SDO/AIA 131 Å. Time runs from left to right. The temporal sequence illustrates the photospheric magnetic flux emergence process (first row) and the synchronous evolution of coronal atmospheric reconnection activities (second and third rows), including plasma sheet formation, lengthening, tearing, and shortening. Representative structures or information are labeled in the corresponding panels, including key magnetic polarities (a; P1, P2, and N1), instantaneous magnetic flux (a-e), coronal jets (g and l), plasma sheets (g and l), lower and upper cusps (g and l), plasmoids (h), torn plasma sheets, and post-reconnection loops (o). In panel b, the SDO/AIA 171 Å image is overlaid on the SDO/HMI image from panel g to examine the connectivity between the atmospheric reconnection field lines and the photospheric polarities. An animation spanning 19:00--22:00 UT is available online, including the observations of the SDO/HMI, averaged temperature, SDO/AIA 171 Å and SDO/AIA 131 Å.
• Figure 2: The photospheric magnetic flux evolution and the development of the coronal plasma sheet. Panels (a) to (d) are slices respectively showing temporal-spatial trajectories of photospheric footpoints, plasma sheet height, plasma sheet length, and plasma sheet temperature, with their path shown in (f). Note that (f) only show the path of S3 and S4 in a fixed frame, which indeed ascends in step with the rising of the main plasma sheet in animated frames. Panel (a) includes the integrated photospheric magnetic flux curves of the source region, with the integration area shown in (f); all slices have a sampling width of 7 pixels. Panel (e) shows a low-pass filtered version of panel (c) to further shown tearing progress of the plasma sheet. Panels (g) and (h) are zoom-in versions of panels (c) and (d), respectively, with the field of view (FOV) indicated by the dashed boxes in (c) and (d); the contours from panel (g) are overlaid on panel (h) to facilitate comparison. Key observed structures are labeled in each panel, including footpoints P1, N1, and P2 in (a), the plasma sheet ascent speed in (b), the cusp structure and plasma sheet expansion speed in (c) and (d). An animation spanning 19:00--22:00 UT is available online, including the observations of the SDO/AIA 171 Å and its synchronous slice.
• Figure 3: Case-by-case analysis of heating progresses. (a) to (d) show four heating cases, namely magnetic plasmoid-plasmoid merging process, plasmoid downward escape process, plasmoid upward escape process, and plasma sheet tearing process. Each panel consists SDO/AIA extreme ultraviolet (EUV) evolutionary images and synchronal temperature maps. Key structures are annotated in each panel.
• Figure 4: The details of how the formation and ejection of plasmoids on different scales modulate the plasma sheet heating. (a) shows a fixed frame of the plasmoid formation at 20:09:09 UT. (b) presents a time slice of the plasmoid widening process, with its path indicated in panel (a); the slice has a sampling width of 7 pixels. (c, d) and (e, f) illustrate the plasma sheet heating before the formation (pre-ejection) and after the ejection of plasmoids at different scales. The left and right sides of each panel correspond to SDO/AIA 171 Å observations and temperature maps, respectively. An animation spanning 20:07--20:15 UT is available online, showing animated versions of panels (a) and (b).
• Figure 5: Graphic illustration of the observed event. The first and second rows correspond to composite images of SDO/AIA 171 Å and temperature maps with SDO/HMI images, respectively. From left to right, panels display the observational characteristics before and after the tearing of the coronal plasma sheet that was driven by photospheric magnetic flux emerging, including the main plasma sheet, secondary plasma sheets, and plasmoids. Representative magnetic field lines have been artificially added to guide eyes. (e) and (f) are DEM curves of regions indicated by black boxes in (c) and (d), respectively; the red lines stands for the best-fitted DEM curves, while the black and blue curves represent the reconstructed curves from the 50 and 100 Monte Carlo (MC) simulations, respectively.