Jiong Qiu

A solar flare releases magnetic energy by reconnecting field lines across a current sheet, thereby allowing their relaxation to a lower-energy state. The maximum possible energy is released if all field lines relax to a current-free (potential) state. The progress of a flare’s reconnection is often measured as the angle-complement between the observed post-reconnection flare loops and the polarity inversion line of the photospheric magnetic field: shear angle. Many observations have shown strong-to-weak shear evolution over the course of a flare. A field line’s shear angle is, however, an imperfect measure of its relaxation. We develop a new technique for observationally inferring the 3D structure of post-reconnection field lines, including their local twist, α , which will vanish for potential fields. Our method fits loops in extreme-ultraviolet (EUV) images to extrapolations subject to constraints such as matching the feet of model field lines to observed flare ribbons. We apply the new method to an eruptive two-ribbon flare (SOL2014-12-18T22), which exhibits strong-to-weak shear-angle evolution. We find that, as the flare progresses, α decreases in post reconnection loops anchored to newly brightened ribbons. Our study demonstrates that post-reconnection magnetic field is neither potential nor linear force-free. The method quantifies, for the first time, the time-history of a flare’s energetic relaxation. It also quantifies the increasing height of the subsequently reconnected field, and the time delay between reconnection forming a flare loop and its appearance in EUV passbands. These results promise to enable improvements in both magnetic modeling and hydrodynamic modeling of flares.

Solar flares are major space weather events that result from the explosive conversion of stored magnetic energy into bulk motion, plasma heating, and particle acceleration. While the standard flare model has proven highly successful in explaining key morphological features of flare observations, many aspects of the energy release are not yet understood. In particular, the turbulent three-dimensional structure of the flare current sheet is thought to play an important role in fast reconnection, particle acceleration, and bursty dynamics. Although direct diagnosis of the magnetic field dynamics in the corona remains highly challenging, rich information may be gleaned from flare ribbons, which represent the chromospheric imprints of reconnection in the corona. Intriguingly, recent solar imaging observations have revealed a diversity of fine structure in flare ribbons that hints at corresponding complexity in the reconnection region. We present high-resolution three-dimensional magnetohydrodynamics simulations of an eruptive flare and describe our efforts to interpret fine-scale ribbon features in terms of the current sheet dynamics. In our model, the current sheet is characterized by many coherent magnetic structures known as plasmoids. We derive a model analog for ribbons by generating a time series of field-line length maps ( L -maps) and identifying abrupt shortenings as flare reconnection events. We thereby demonstrate that plasmoids imprint transient “spirals” along the analog of the ribbon front, with a morphology consistent with observed fine structure. We discuss the implications of these results for interpreting SolO, Interface Region Imaging Spectrograph, and Daniel K. Inouye Solar Telescope observations of explosive flare energy release.

Both eruptive and confined flares—those having and lacking, respectively, an associated large-scale coronal mass ejection—are frequently observed in solar active regions (ARs). The physical mechanisms controlling the nature of these flares are poorly understood, requiring a detailed investigation of both the magnetic environment and the dynamic flare production process within the source region. Here we study a confined X6.4 flare with a failed filament eruption in AR 13590, observed by the SDO spacecraft on 2024 February 22. AR 13590 exhibited a quadrupolar magnetic configuration consisting of one central and two side arcades of low-lying loops, overlain by a fourth arcade of high-lying loops. A potential-field extrapolation contains a low-altitude null point at a height of approximately 5 Mm. This is a classic magnetic-breakout topology, except that the rising filament lay under one of the side arcades. The confined flare generated four ribbons located in all four of the major polarities, and proceeded in two phases, characterized by two successive stages of ribbon brightenings accompanied by forward and then reverse motion of the brightenings along the interior ribbons. We propose that the two phases represent breakout reconnection in the null region, which removed the flux ahead of the rising filament, followed by “anti-breakout” reconnection, corresponding to a reversed process as those flux systems evolved back toward the pre-eruption configuration. We discuss the implications of these observations for understanding how intense flares can occur without the expulsion of material from the solar corona.

Coronal dimmings associated with coronal mass ejections (CMEs) from the Sun have gained much attention since the late 1990s when they were first observed in high-cadence imagery of the SOHO/EIT and Yohkoh/SXT instruments. They appear as localized sudden decreases of the coronal emission at extreme ultraviolet (EUV) and soft X-ray (SXR) wavelengths, that evolve impulsively during the lift-off and early expansion phase of a CME. Coronal dimmings have been interpreted as “footprints” of the erupting flux rope and also as indicators of the coronal mass loss by CMEs. However, these are only some aspects of coronal dimmings and how they relate to the overall CME/flare process. The goal of this review is to summarize our current understanding and observational findings on coronal dimmings, how they relate to CME simulations, and to discuss how they can be used to provide us with a deeper insight and diagnostics of the triggering of CMEs, the magnetic connectivities and coronal reconfigurations due to the CME as well as the replenishment of the corona after an eruption. In addition, we go beyond a pure review by introducing a new, physics-driven categorization of coronal dimmings based on the magnetic flux systems involved in the eruption process. Finally, we discuss the recent progress in studying coronal dimmings on solar-like and late-type stars, and how to use them as a diagnostics for stellar coronal mass ejections and their properties.

We present a unique observation of the X6.4-class flare SOL2024-02-22T22:34 using the Mid-InfraRed Imager (MIRI) at the Goode Solar Telescope. Three ribbon-like flare sources and one unidentified source were detected in MIRI’s two mid-infrared (mid-IR) bands at 5.2 and 8.2 μm. The two stronger ribbons displayed maximum mid-IR enhancements of 21% and 18% above quiet-Sun levels and 10% in Helioseismic and Magnetic Imager (HMI) continuum intensity (I _c ). The weak ribbon and the unidentified source had maximum mid-IR enhancements of 7% but showed HMI/I _c dimmings, instead of excess emissions. Our result suggests that mid-IR emission forms in a higher layer during the flare and is more sensitive to flare heating than HMI/I _c emission. The MIRI observations have high temporal resolution (2.6 s cadence in these observations) and show apparent source motions. One flare ribbon extends along weak vertical magnetic-field channels in the sunspot umbra, light bridge, and penumbra, with an approximately 30 s delay between HMI/I _c and 8.2 μm emissions. Meanwhile, the unidentified source moved at an apparent speed of 130 km s⁻¹ from a mixed-polarity area to one flare ribbon with a strong HMI/I _c enhancement. We studied available hard X-ray/microwave imaging spectroscopy and used nonlinear force-free field extrapolation modeling to identify flare structures. The observational evidence strongly favors the chromospheric origin of the unidentified mid-IR source. Comparison with the X1.0 flare SOL2022-10-02T20:25 indicates that the total amount of high-energy electron (>60 keV) flux density is a key factor in determining the total brightening area and the maximum intensity enhancement in HMI/I _c emissions.

We generalize a magnetogram-matching Biot–Savart law (BSl) from planar to spherical geometry. For a given coronal current density J , this law determines the magnetic field B ˜ whose radial component vanishes at the surface. The superposition of B ˜ with a potential field defined by a given surface radial field, B _r , provides the entire configuration where B _r remains unchanged by the currents. Using this approach, we (1) upgrade our regularized BSls for constructing coronal magnetic flux ropes (MFRs) and (2) propose a new method for decomposing a measured photospheric magnetic field as B = B pot + B T + B S ˜ , where the potential, B _pot, toroidal, B _T , and poloidal, B S ˜ , fields are determined by B _r , J _r , and the surface divergence of B – B _pot, respectively, all derived from magnetic data. Our B _T is identical to the one in the alternative Gaussian decomposition by P. W. Schuck et al., while B _pot and B S ˜ are different from their poloidal fields B P < and B P > , which are potential in the infinitesimal proximity to the upper and lower side of the surface, respectively. In contrast, our B S ˜ has no such constraints and, as B _pot and B _T , refers to the same upper side of the surface. In spite of these differences, for a continuous J distribution across the surface, B _pot and B S ˜ are linear combinations of B P < and B P > . We demonstrate that, similar to the Gaussian method, our decomposition allows one to identify the footprints and projected surface-location of MFRs in the solar corona, as well as the direction and connectivity of their currents.

The buildup of the preeruptive magnetic field configuration and the eruption onset mechanism are critical yet poorly understood in solar eruptions. Coronal structures like sigmoids and filaments have been identified as preeruptive magnetic structures; their associated preflare motions as well as preflare brightenings have been identified as precursor signatures, yet none of these definitively lead to eruptions, and the cause and effect is contentious. Most importantly, how the preeruptive coronal structure evolves with the photospheric magnetic field is elusive. Here we report the development of the photospheric nonneutralized electric current associated with the buildup of a preeruptive coronal structure observed as a bundle of hot low-lying coronal loops collectively taking a sigmoidal shape. The significant nonneutralized electric current appeared several hours ahead of the formation of the preeruptive structure. The buildup of the preeruptive structure in the corona was simultaneous with the gradual extension of the nonneutralized electric current in the photosphere. The synchronous evolution seemed to stop when intermittent brightening occurred along the preeruptive structure in the corona. The preflare brightening lasted for about 4 hr, with two ribbon-like structures matching the nonneutralized electric current. Eventually, the preeruptive structure evolved into a magnetic flux rope (MFR) and erupted. Quantitative measurements indicate that the significant nonneutralized electric current also flows through the footpoints of the erupting MFR, which are well identified by a pair of conjugate dimmings. The evolution of the photospheric nonneutralized electric current is demonstrated to signal the buildup of the preeruptive structure and the imminent eruption.

We analyze high-resolution observations of an X-1.0 white-light flare, triggered by a filament eruption, on 2022 October 2. The full process of filament formation and subsequent eruption was captured in the Hα passband by the Visible Imaging Spectrograph (VIS) on board the Goode Solar Telescope (GST) within its center field of view. White-light emissions appear in flare ribbons following the filament eruption and Hα ribbon brightening. GST Broadband Filter Imager data show that the continuum intensity, as compared to the nearby quiet-Sun area, has increased by up to 20% in the photospheric TiO band around 7057 Å. The Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory reported 10% contrast enhancement in the continuum near Fe i 6173 Å line. The separation motion of two white-light kernels is recorded by the high-cadence GST/TiO images and is well accompanied by the motion of the VIS Hα flare ribbon leading edge. One kernel, located in a 150 Gauss field within a granulation area, exhibited an average apparent motion speed of 55 km s⁻¹, which is the highest average speed ever reported. The other kernel drifted at 9 km s⁻¹ in an 800 Gauss magnetic field area. Hard X-ray (HXR) emissions reaching up to 300 keV have been observed for this flare. The simultaneous occurrence of high-cadence HXR, microwave, and white-light emissions strongly suggests that the energetic particles from the flare directly contribute to the heating. The inverted HXR energy flux density corresponding to 10% TiO brightening is 2.07 ± 0.23 × 10¹¹ erg cm⁻² s⁻¹ during the flare peak.

The efficient release of magnetic energy in astrophysical plasmas, such as during solar flares, can in principle be achieved through magnetic diffusion, at a rate determined by the associated electric field. However, attempts at measuring electric fields in the solar atmosphere are scarce, and none exist for sites where the magnetic energy is presumably released. Here, we present observations of an energetic event using the National Science Foundation’s Daniel K. Inouye Solar Telescope, where we detect the polarization signature of electric fields associated with magnetic diffusion. We measure the linear and circular polarization across the hydrogen Hε Balmer line at 397 nm at the site of a brightening event in the solar chromosphere. Our spectro-polarimetric modeling demonstrates that the observed polarization signals can only be explained by the presence of electric fields, providing conclusive evidence of magnetic diffusion, and opening a new window for the quantitative study of this mechanism in space plasmas.

The explosive release of energy in the solar atmosphere is driven magnetically, but the mechanisms that trigger the onset of the eruption remain controversial. In the case of flares and coronal mass ejections (CMEs), ideal or non-ideal instabilities usually occur in the corona, but it is difficult to obtain direct observations and diagnostics there. To overcome this difficulty, we analyze observational signatures in the upper chromosphere or transition region, particularly brightening and dimming at the base of coronal magnetic structures. In this paper, we examine the time evolution of spatially resolved light curves in two eruptive flares and identify a variety of tempo-spatial sequences of brightening and dimming, such as dimming followed by brightening and dimming preceded by brightening. These brightening–dimming sequences are indicative of the configuration of energy release in the form of plasma heating or bulk motion. We demonstrate the potential of using these analyses to diagnose the properties of magnetic reconnection and plasma expansion in the corona during the early stages of the eruption.