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Register a new userDynamics of Electric Currents, Magnetic Field Topology and Helioseismic Response of a Solar Flare
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The solar flare on July 30, 2011 was of a modest X-ray class (M9.3), but it made a strong photospheric impact and produced a sunquake, observed with the Helioseismic and Magnetic Imager (HMI) on NASAs Solar Dynamics Observatory (SDO). In addition to the helioseismic waves (also observed with the SDO/AIA instrument), the flare caused a large expanding area of white-light emission and was accompanied by substantial restructuring of magnetic fields, leading to the rapid formation of a sunspot structure in the flare region. The flare produced no significant hard X-ray emission and no coronal mass ejection. This indicates that the flare energy release was mostly confined to the lower atmosphere. The absence of significant coronal mass ejection rules out magnetic rope eruption as a mechanism of helioseismic waves. We discuss the connectivity of the flare energy release with the electric currents dynamics and show the potential importance of high-speed plasma flows in the lower solar atmosphere during the flare energy release.
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The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) provides a new tool for the systematic observation of white-light flares, including Doppler and magnetic information as well as continuum. In our initial analysis of the highly impulsive gamma-ray flare SOL2010-06-12T00:57 (Mart{i}nez Oliveros et al., Solar Phys., 269, 269, 2011), we reported the signature of a strong blueshift in the two footpoint sources. Concerned that this might be an artifact due to aliasing peculiar to the HMI instrument, we undertook a comparative analysis of Global Oscillations Network Group (GONG++) observations of the same flare, using the PArametric Smearing Correction ALgorithm (PASCAL) algorithm to correct for artifacts caused by variations in atmospheric smearing. This analysis confirms the artifactual nature of the apparent blueshift in the HMI observations, finding weak redshifts at the footpoints instead. We describe the use of PASCAL with GONG++ observations as a complement to the SDO observations and discuss constraints imposed by the use of HMI far from its design conditions. With proper precautions, these data provide rich information on flares and transients.
Chromospheric evaporation is observed as Doppler blueshift during solar flares. It plays one of key roles in dynamics and energetics of solar flares, however, its mechanism is still unknown. In this paper we present a detailed analysis of spatially-resolved multi-wavelength observations of chromospheric evaporation during an M 1.0 class solar flare (SOL2014-06-12T21:12) using data from the NASAs IRIS (Interface Region Imaging Spectrograph) and HMI/SDO (Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory) telescopes, and VIS/NST (Visible Imaging Spectrometer at New Solar Telescope) high-resolution observations, covering the temperature range from 10^4 K to 10^7 K. The results show that the averaged over the region Fe XXI blueshift of the hot evaporating plasma is delayed relative to the C II redshift of the relatively cold chromospheric plasma by about 1 min. The spatial distribution of the delays is not uniform across the region and can be as long as 2 min in several zones. Using vector magnetograms from HMI we reconstruct the magnetic field topology and the quasi-separatrix layer (QSL) and find that the blueshift delay regions as well as the H-alpha flare ribbons are connected to the region of magnetic polarity inversion line (PIL) and an expanding flux rope via a system of low-lying loop arcades with height < ~4.5 Mm. This allows us to propose an interpretation of the chromospheric evaporation based on the geometry of local magnetic fields, and the primary energy source associated with the PIL.
The standard model for eruptive flares has in the past few years been extended to 3D. It predicts typical J-shaped photospheric footprints of the coronal current layer, forming at similar locations as the Quasi-Separatrix Layers (QSLs). Such a morphology is also found for flare ribbons observed in the EUV band, as well as in non-linear force-free field (NLFFF) magnetic field extrapolations and models. We study the evolution of the photospheric traces of the current density and flare ribbons, both obtained with the SDO instruments. We investigate the photospheric current evolution during the 6 September 2011 X-class flare (SOL2011-09-06T22:20) from observational data of the magnetic field obtained with HMI. This evolution is compared with that of the flare ribbons observed in the EUV filters of the AIA. We also compare the observed electric current density and the flare ribbon morphology with that of the QSLs computed from the flux rope insertion method/NLFFF model. The NLFFF model shows the presence of a fan-spine configuration of overlying field lines, due to the presence of a parasitic polarity, embedding an elongated flux rope that appears in the observations as two parts of a filament. The QSLs, evolved via a magnetofrictional method, also show similar morphology and evolution as both the current ribbons and the EUV flare ribbons obtained at several times during the flare. For the first time, we propose a combined analysis of the photospheric traces of an eruptive flare, in a complex topology, with direct measurements of electric currents and QSLs from observational data and a magnetic field model. The results, obtained by two different and independent approaches, 1) confirm previous results of current increase during the impulsive phase of the flare, 2) show how NLFFF models can capture the essential physical signatures of flares even in a complex magnetic field topology.
In the standard model of solar flares, a large-scale reconnection current sheet is postulated as the central engine for powering the flare energy release and accelerating particles. However, where and how the energy release and particle acceleration occur remain unclear due to the lack of measurements for the magnetic properties of the current sheet. Here we report the measurement of spatially-resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a magnetic bottle. This spatial structure agrees with theoretical predictions and numerical modeling results. A strong reconnection electric field of ~4000 V/m is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons concentrate instead at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is likely the primary site for accelerating and/or confining the relativistic electrons.
Multiple-ribbon flares are usually complex in their magnetic topologies and eruption mechanisms. In this paper, we investigate an X2.1 flare (SOL2015-03-11T16:22) that occurred in active region 12297 near the center of the solar disk by both potential and nonlinear force-free field models extrapolated with the data observed by the Helioseismic and Magnetic Imager (HMI) on board Solar Dynamics Observatory (SDO). We calculate the three-dimensional squashing degree distribution. The results reveal that there are two flux ropes in this active region, covered by a large scale hyperbolic flux tube (HFT), which is the intersection of quasi-separatrix layers with a null point embedded in it. When the background magnetic field diminishes due to the separation of the northwest dipole and the flux cancellation, the central flux rope rises up forming the two brightest central ribbons. It then squeezes the upper lying HFT structure to generate further brightenings. This very energetic flare with a complex shape is accompanied by a coronal mass ejection (CME). We adopt the simplified line-tied force-balance equation of the current ring model and assign the observed value of the decay index to the equation to simulate the acceleration profile of the CME in the early stage. It is found that the path with an inclination of $45^circ$ from radial best fits the profile of the actual acceleration.
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