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Plasma and magnetic configurations during the reconnection process. (a) The AIA 171 Å images at 02:14 UT (left), 04:14 UT (middle) and 08:14 UT (right) displaying the side view of the evolution of two sets of coronal loops. The cyan and green dashed curves show selected coronal loops representing two magnetic field lines involved in the process. (b) The EUVI 171 Å images showing the top view of the reconnection. The cyan and green dashed curves give another view of the same loops as in panel a. (c) The reconstructed 3D magnetic topology (cyan and green curves) and heated regions (cloud-like structures) before, during and after the reconnection. The bottom boundaries are the projected EUVI 304 Å images showing the footpoints of the flare and the separation of two flare ribbons.
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Magnetic reconnection, a change of magnetic field connectivity, is a fundamental physical process in which magnetic energy is released explosively, and it is responsible for various eruptive phenomena in the universe. However, this process is difficult to observe directly. Here, the magnetic topology associated with a solar reconnection event is st...
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... topology and origin of magnetic reconnection. Observations from SDO (the AIA 171 Å passband; Fig. 2a), in combination with STEREO-A observations (the EUVI 171 Å passband; Fig. 2b), enable us to reconstruct the 3D topology of the reconnection and its evolution. Owing to the high magnetic Reynolds number of the ionized corona, the plasma is frozen to the magnetic field; and so the loop-like plasma emission is reasonably assumed to ...
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... topology and origin of magnetic reconnection. Observations from SDO (the AIA 171 Å passband; Fig. 2a), in combination with STEREO-A observations (the EUVI 171 Å passband; Fig. 2b), enable us to reconstruct the 3D topology of the reconnection and its evolution. Owing to the high magnetic Reynolds number of the ionized corona, the plasma is frozen to the magnetic field; and so the loop-like plasma emission is reasonably assumed to outline the geometry of the magnetic field 33 . We select two magnetic loops (cyan ...
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... reconstruct the 3D topology of the reconnection and its evolution. Owing to the high magnetic Reynolds number of the ionized corona, the plasma is frozen to the magnetic field; and so the loop-like plasma emission is reasonably assumed to outline the geometry of the magnetic field 33 . We select two magnetic loops (cyan and green dashed lines in Fig. 2a,b) that can most clearly exhibit the reconnection process. With images from two perspective angles, the 3D structure of the loops is reconstructed ( Fig. 2c and Supplementary Movie 3). The results display a clear picture of how the connectivity of the loops changes as the reconnection proceeds. Before reconnection, two nearly oppositely ...
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... the magnetic field; and so the loop-like plasma emission is reasonably assumed to outline the geometry of the magnetic field 33 . We select two magnetic loops (cyan and green dashed lines in Fig. 2a,b) that can most clearly exhibit the reconnection process. With images from two perspective angles, the 3D structure of the loops is reconstructed ( Fig. 2c and Supplementary Movie 3). The results display a clear picture of how the connectivity of the loops changes as the reconnection proceeds. Before reconnection, two nearly oppositely directed loops are anchored, respectively, at each side of the filament in the active region (left panel of Fig. 2b). The plasma between their legs has ...
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... the 3D structure of the loops is reconstructed ( Fig. 2c and Supplementary Movie 3). The results display a clear picture of how the connectivity of the loops changes as the reconnection proceeds. Before reconnection, two nearly oppositely directed loops are anchored, respectively, at each side of the filament in the active region (left panel of Fig. 2b). The plasma between their legs has been heated to a moderate temperature (left panel of Fig. ...
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... display a clear picture of how the connectivity of the loops changes as the reconnection proceeds. Before reconnection, two nearly oppositely directed loops are anchored, respectively, at each side of the filament in the active region (left panel of Fig. 2b). The plasma between their legs has been heated to a moderate temperature (left panel of Fig. ...
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... the rise of the cavity, the underlying loops of opposite polarities gradually approach each other. As the inward move- ments of the loops are not coplanar, an apparent separator or quasi-separator appears at B04:14 UT (middle panel of Fig. 2b). We calculate the 3D global magnetic field on January 26 using the potential field assumption 34 and find an absence of pre-existing null points and separators in the reconnection region. However, ...
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... the simple magnetic field in the original bipolar source region is strongly sheared from January 21 as shown by the long-existing filament/prominence at the bottom of the cavity (Fig. 2b). It suggests that a new separator or quasi-separator is formed with the prominence taking off (middle panel of Fig. 2c). As the reconnection initiates, free magnetic energy starts to be released, the most obvious consequence of which is to form a hotter region underneath the reconnection site. Topologically, the reconnection between ...
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... the simple magnetic field in the original bipolar source region is strongly sheared from January 21 as shown by the long-existing filament/prominence at the bottom of the cavity (Fig. 2b). It suggests that a new separator or quasi-separator is formed with the prominence taking off (middle panel of Fig. 2c). As the reconnection initiates, free magnetic energy starts to be released, the most obvious consequence of which is to form a hotter region underneath the reconnection site. Topologically, the reconnection between the two groups of loops forms poloidal field lines above the reconnection site, increasing the twist of the erupted flux ...
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... underneath the reconnection site. Topologically, the reconnection between the two groups of loops forms poloidal field lines above the reconnection site, increasing the twist of the erupted flux rope. At the same time, a cusp-shaped field below the reconnection site quickly shrinks into a semicircular shape to form flare loops 35 (right panel of Fig. 2c). With the acceleration of the CME, more plasma is heated to temperatures up to B5 MK, suggesting an enhanced reconnection. However, the heated region is still confined between the reconnection site and the flare loop top but with a spatial ...
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Citations
... After 14:09 UT, the legs of the large loop approached each other and formed an elongated vertical structure similar to the current sheet in the "standard flare model." It is possible that there is reconnection between oppositely directed fields in the legs, similar to the process described by Sun et al. (2015). This is revealed by the appearance of horizontal threads going up from the upper edge of the supposed current sheet (14:20-14:30 UT). ...
Rather unique observations of a high-altitude spider-type prominence in 2023 February are presented. The prominence or corresponding filament on the disk was not visible all the time but could appear and disappear in the course of a particular day. However, it persisted during the whole half of a solar rotation, being observable from day to day starting from the east limb of the Sun to the west limb. We show that the prominence was located in sagged coronal field lines just above a coronal magnetic null point. The presence of the null point and magnetic dips above it is confirmed by calculations of the potential magnetic field. The mass of the prominence apparently was appearing due to the condensation of hot coronal plasma after several eruptions that occurred in an active-region complex where the prominence was located. The prominence material flowed down along widely spread large coronal loops as coronal rain and was sometimes swept away by subsequent eruptions.
... Such a picture has been extensively studied in past decades primarily through remote sensing spectroscopic and imaging observations. Some significant and critical features predicted by the model were identified observationally, including reconnection inflows and downflows [10][11][12] , outflow-driven termination shock 13 , and rapid change of magnetic flux connectivity 14,15 . ...
Magnetic reconnection is a key mechanism involved in solar eruptions and is also a prime possibility to heat the low corona to millions of degrees. Here, we present ultra-high-resolution extreme ultraviolet observations of persistent null-point reconnection in the corona at a scale of about 390 km over one hour observations of the Extreme-Ultraviolet Imager on board Solar Orbiter spacecraft. The observations show formation of a null-point configuration above a minor positive polarity embedded within a region of dominant negative polarity near a sunspot. The gentle phase of the persistent null-point reconnection is evidenced by sustained point-like high-temperature plasma (about 10 MK) near the null-point and constant outflow blobs not only along the outer spine but also along the fan surface. The blobs appear at a higher frequency than previously observed with an average velocity of about 80 km/s and life-times of about 40 s. The null-point reconnection also occurs explosively but only for 4 minutes, its coupling with a mini-filament eruption generates a spiral jet. These results suggest that magnetic reconnection, at previously unresolved scales, proceeds continually in a gentle and/or explosive way to persistently transfer mass and energy to the overlying corona.
... Such a picture has been extensively studied in past decades primarily through remote sensing spectroscopic and imaging observations. Some significant and critical features predicted by the model were identified observationally, including reconnection inflows and downflows [10][11][12] , outflow-driven termination shock 13 , and rapid change of magnetic flux connectivity 14,15 . ...
Magnetic reconnection is a key mechanism involved in solar eruptions and is also a prime possibility to heat the low corona to millions of degrees. Here, we present ultra-high-resolution extreme ultraviolet observations of persistent null-point reconnection in the corona at a scale of about 390 km over one hour observations of the Extreme-Ultraviolet Imager on board Solar Orbiter spacecraft. The observations show formation of a null-point configuration above a minor positive polarity embedded within a region of dominant negative polarity near a sunspot. The gentle phase of the persistent null-point reconnection is evidenced by sustained point-like high-temperature plasma (about 10 MK) near the null-point and constant outflow blobs not only along the outer spine but also along the fan surface. The blobs appear at a higher frequency than previously observed with an average velocity of about 80 km s⁻¹ and life-times of about 40 s. The null-point reconnection also occurs explosively but only for 4 minutes, its coupling with a mini-filament eruption generates a spiral jet. These results suggest that magnetic reconnection, at previously unresolved scales, proceeds continually in a gentle and/or explosive way to persistently transfer mass and energy to the overlying corona.
... These relatively weak microwave sources closely resemble the observations previously reported in the much stronger 2017 September 10 X8.2 flare [34] and are most likely caused by non-thermal electrons distributed along a long reconnection CS stretched by the erupting filament as predicted in the standard model (Fig. 2d). This is further supported by the presence of reconnection downflows along the CS, which appear intermittently above and quickly move toward the top of flare loops after the filament eruption ( Supplementary Fig. 1), consistent with previous argumentation [41][42][43]. Considering that the average magnetic field of the CS decreases with height above the primary X point [34], the frequency-dependent distribution of the weak microwave sources observed here complies with the picture that the peak frequency of the gyrosynchrotron (GS) emission decreases with a decreasing magnetic field strength [44,45]. ...
Quasi-periodic pulsations (QPPs) are frequently detected in solar and stellar flares, but the underlying physical mechanisms are still to be ascertained. Here, we show microwave QPPs during a solar flare originating from quasi-periodic magnetic reconnection at the flare current sheet. They appear as two vertically detached but closely related sources with the brighter ones located at flare loops and the weaker ones along the stretched current sheet. Although the brightness temperatures of the two microwave sources differ greatly, they vary in phase with periods of about 10--20 s and 30--60 s. The gyrosynchrotron-dominated microwave spectra also present a quasi-periodic soft-hard-soft evolution. These results suggest that relevant high-energy electrons are accelerated by quasi-periodic reconnection, likely arising from the modulation of magnetic islands within the current sheet as validated by a 2.5-dimensional magnetohydrodynamic simulation.
... These relatively weak microwave sources closely resemble the observations previously reported in the much stronger 2017 September 10 X8.2 flare 34 and are most likely caused by non-thermal electrons distributed along a long reconnection CS stretched by the erupting filament as predicted in the standard model (Fig. 2d). This is further supported by the presence of reconnection downflows along the CS, which appear intermittently above and quickly move toward the top of flare loops after the filament eruption ( Supplementary Fig. 1), consistent with previous argumentation [41][42][43] . Considering that the average magnetic field of the CS decreases with height above the primary X point 34 , the frequency-dependent distribution of the weak microwave sources observed here complies with the picture that the peak frequency of the gyrosynchrotron (GS) emission decreases with a decreasing magnetic field strength 44,45 . ...
Quasi-periodic pulsations (QPPs) are frequently detected in solar and stellar flares, but the underlying physical mechanisms are still to be ascertained. Here, we show microwave QPPs during a solar flare originating from quasi-periodic magnetic reconnection at the flare current sheet. They appear as two vertically detached but closely related sources with the brighter ones located at flare loops and the weaker ones along the stretched current sheet. Although the brightness temperatures of the two microwave sources differ greatly, they vary in phase with periods of about 10–20 s and 30–60 s. The gyrosynchrotron-dominated microwave spectra also present a quasi-periodic soft-hard-soft evolution. These results suggest that relevant high-energy electrons are accelerated by quasi-periodic reconnection, likely arising from the modulation of magnetic islands within the current sheet as validated by a 2.5-dimensional magnetohydrodynamic simulation.
... After carefully inspecting the magnetic topology of the three microflares, we do not find any signatures of null point and separator. We thus suggest that the quasi-separator reconnection, appearing as tether-cutting (as shown in Sun et al. 2015) or fan-spine-like type, could be more common for small-scale events, at least for the microflares we study. No matter which configuration, it is not always the inverse Y-shaped configuration formed by the reconnection between emerging flux and oblique open flux in the microflare models of Shibata et al. (1992) and Moore et al. (2010). ...
Microflares, one of small-scale solar activities, are believed to be caused by magnetic reconnection. Nevertheless, their three-dimensional (3D) magnetic structures, thermodynamic structures, and physical links to the reconnection have been unclear. In this Letter, based on high-resolution 3D radiative magnetohydrodynamic simulation of the quiet Sun spanning from the upper convection zone to the corona, we investigate 3D magnetic and thermodynamic structures of three homologous microflares. It is found that they originate from localized hot plasma embedded in the chromospheric environment at the height of 2--10 Mm above the photosphere and last for 3--10 minutes with released magnetic energy in the range of $10^{27}-10^{28}$ erg. The heated plasma is almost co-spatial with the regions where the heating rate per particle is maximal. The 3D velocity field reveals a pair of converging flows with velocities of tens of km s$^{-1}$ toward and outflows with velocities of about 100 km s$^{-1}$ moving away from the hot plasma. These features support that magnetic reconnection plays a critical role in heating the localized chromospheric plasma to coronal temperature, giving rise to observed microflares. The magnetic topology analysis further discloses that the reconnection region is located near quasi-separators where both current density and squashing factors are maximal although the specific topology may vary from tether-cutting to fan-spine-like structure.
... High-resolution imaging observations provide rich information about details of the reconnection process which result from its internal dynamics, especially the formation of plasmoids 21,22 and turbulent structures 23 , and from the complexity and threedimensional (3D) nature of the solar magnetic field 24,25 . Spectral and multi-wavelength data reveal aspects of the microscopic processes, e.g., particle acceleration 26,27 , the nonthermal-thermal energy partition 28 , the plasmoid instability 29,30 , and bursty 31 or turbulent reconnection 32,33 , in addition to flows 34 . ...
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. Solar flares provide wide range of observational details about fundamental processes involved. Here, the authors show evidence for magnetic reconnection in a strong confined solar flare displaying all four reconnection flows with plasmoids in the current sheet and the separatrices.
... This reconnection rate is one to two orders of magnitude lower than those inferred from many other reconnection observations in the solar atmosphere (e.g., Su et al. 2013;Tian et al. 2014;Xue et al. 2016). However, it still lies at the lower end of the possible range of reconnection rates estimated by Sun et al. (2015). ...
Coronal loops are the building blocks of solar active regions. However, their formation mechanism remains poorly understood. Here we present direct observational evidence for the formation of coronal loops through magnetic reconnection as new magnetic fluxes emerge into the solar atmosphere. Extreme-ultraviolet observations by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) clearly show the newly formed loops following magnetic reconnection within a plasma sheet. Formation of the loops is also seen in the H α line-core images taken by the New Vacuum Solar Telescope. Observations from the Helioseismic and Magnetic Imager on board SDO show that a positive-polarity flux concentration moves toward a negative-polarity one with a speed of ∼0.4 km s ⁻¹ before the formation of coronal loops. During the loop formation process, we found signatures of flux cancellation and subsequent enhancement of the transverse field between the two polarities. The three-dimensional magnetic field structure reconstructed through a magnetohydrostatic model shows field lines consistent with the loops in AIA images. Numerous bright blobs with an average width of 1.37 Mm appear intermittently in the plasma sheet and move upward with a projected velocity of ∼114 km s ⁻¹ . The temperature, emission measure, and density of these blobs are about 3 MK, 2.0 × 10 ²⁸ cm ⁻⁵ , and 1.2 × 10 ¹⁰ cm ⁻³ , respectively. A power spectral analysis of these blobs indicates that the observed reconnection is likely not dominated by a turbulent process. We have also identified flows with a velocity of 20–50 km s ⁻¹ toward the footpoints of the newly formed coronal loops.
... This reconnection rate is one to two orders of magnitude lower than those inferred from many other reconnection observations in the solar atmosphere (e.g., Su et al. 2013;Tian et al. 2014;Xue et al. 2016). However, it still lies at the lower end of the possible range of reconnection rate estimated by Sun et al. (2015). ...
Coronal loops are building blocks of solar active regions. However, their formation mechanism is still not well understood. Here we present direct observational evidence for the formation of coronal loops through magnetic reconnection as new magnetic fluxes emerge into the solar atmosphere. Extreme-ultraviolet observations of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) clearly show the newly formed loops following magnetic reconnection within a plasma sheet. Formation of the loops is also seen in the h{\alpha} line-core images taken by the New Vacuum Solar Telescope. Observations from the Helioseismic and Magnetic Imager onboard SDO show that a positive-polarity flux concentration moves towards a negative-polarity one with a speed of ~0.4 km/s, before the formation of coronal loops. During the loop formation process, we found signatures of flux cancellation and subsequent enhancement of the transverse field between the two polarities. The three-dimensional magnetic field structure reconstructed through a magnetohydrostatic model shows field lines consistent with the loops in AIA images. Numerous bright blobs with an average width of 1.37 Mm appear intermittently in the plasma sheet and move upward with a projected velocity of ~114 km/s. The temperature, emission measure and density of these blobs are about 3 MK, 2.0x10^(28) cm^(-5) and 1.2x10^(10) cm^(-3), respectively. A power spectral analysis of these blobs indicates that the observed reconnection is likely not dominated by a turbulent process. We have also identified flows with a velocity of 20 to 50 km/s towards the footpoints of the newly formed coronal loops.
... (a) reconnection inflows and outflows [22][23][24], (b) supra-arcade downflows [25,26] (c) cusp-shaped flare loops [27], (d) X-ray sources in the current sheet [28,29], at loop tops and at the footpoints of flaring loops [27,30]. ...
Magnetic reconnection is a fundamental process in a laboratory, magnetospheric, solar and astrophysical plasma, whereby magnetic energy is converted into heat, bulk kinetic energy and fast particle energy. Its nature in two dimensions is much better understood than in three dimensions (3D), where its character is completely different and has many diverse aspects that are currently being explored. Here we focus on the magnetohydrodynamics of 3D reconnection in the plasma environment of the solar system, especially solar flares. The theory of reconnection at null points, separators and quasi-separators is described, together with accounts of numerical simulations and observations of these three types of reconnection. The distinction between separator and quasi-separator reconnection is a theoretical one that is unimportant for the observations of energy release. A new paradigm for solar flares, in which 3D reconnection plays a central role, is proposed.
