| Solar flares are among the most magnificent activities in the solar atmosphere,and they are often accompanied by filament/prominence eruptions and coronal mass ejec-tions(CMEs).These solar eruptions are the main drivers of disastrous space weather events,impacting on the magnetosphere as well as the geospace environment.It is hence crucial to study the physical mechanism of solar flares,which on one hand,can help us to understand the solar magnetic field and its evolution,as well as the related solar activities;on the other hand,can provide theoretical bases for the space weather forecasting as an indispensable part of monitoring the geospace environment.Magnetic reconnection is considered as the main energy release mechanism in so-lar flares,which can change the magnetic topology and convert the free magnetic en-ergy into thermal and kinetic energies of flare plasmas.However,there are significant differences in both physical picture and process between the real three-dimensional re-connection in the solar corona and the classic two-dimensional magnetic reconnection that accounts for the standard flare model,i.e.,CSHKP model.Thus,observations are urgently needed to provide constrains for flare models and simulations.In this the-sis,we study magnetic reconnection and the associated energy release process in solar flares by using the high-resolution and multi-wavelength data from the recent satellites and instruments,such as the Solar Dynamics Observatory(SDO),the Solar Terrestrial Relations Observatory(STEREO),and the Reuven Ramaty High Energy Solar Spec-troscopic Imager(RHESSI).By using the state-of-the-art methods such as the differen-tial emission measure(DEM)inversion,photospheric magnetic field extrapolation and magnetic squashing factor calculation,we study in detail the physical properties of flare plasmas,magnetic configuration of active regions,and the associated solar phenomena.The main contents and results are as follows.1.Flare Plasma Heating by Magnetic ReconnectionBased on the high-resolution and multi-wavelength observation by the Atmospheric Imaging Assembly(AIA)onboard SDO,we performed a DEM analysis of typical candle-flame-shaped flares.The results show that the cusp structure above has systematically higher temperatures(>10 MK)than the rounded flare arcade underneath.The majority of the flares studied,including a confined flare with a double candle-flame shape shar-ing the same cusp-shaped structure,has the highest temperature around the tip of the cusp,characterized by two high-temperature ridges.Our analysis hence corroborates the classic reconnection model,in which the slow-mode shocks attached to the recon-nection site play a significant role in heating the plasma.However,the M7.7 flare on 19 July 2012 poses a very intriguing violation of this paradigm:the temperature increases with altitude from the tip of the cusp toward the top of the arcade.This signifies that a different heating mechanism from the slow-mode shocks operates in the cusp region during the flare decay phase.The DEM profile of flaring plasmas generally exhibits a double peak distribution in temperature,with a cold component around log T?6.2 and a hot component around log T?7.0.Attributing the cold component mainly to the coronal background,we propose a "corrected" mean temperature weighted by the hot DEM(?(T))component(T?4 MK,log T?6.6):<T>h=fr?4MK?(T)×TdT/fT?4MK?(T)dT,other than the conventionally defined mean temperature(T)w,which is weighted by the whole DEM profile.With the presence of a strong coronal background,<T>w could significantly underestimate the temperature of the flaring plasma.Hence,<T>h is a better representation of flaring plasma than<T>w.2.Direct Observation of Magnetic ReconnectionMagnetic reconnection plays an important role in solar flares.However,its direct observation is still rare.Here we observed an unusual two-step reconnection in a solar flare,and also performed the first stereoscopic observation of slipping reconnection by two satellites.The eruptive X2.8 flare on 2013 May 13 shows two distinct episodes of energy release in its impulsive phase.The first episode is characterized by the eruption of a magnetic flux rope,similar to the standard model.The second stronger episode of en-ergy release is closely associated with magnetic reconnection of a large-scale loop in the aftermath of the eruption.We observ ed the cool,horizontal inflow and hot,perpen-dicular outflow,with speeds of about 130 km/s and 740 km/s,respectively.In addition,RHESSI detects a strong burst of hard X-ray(HXR)and y-ray emissions with hard electron spectra of ??3,exhibiting a soft-hard-harder behavior.A distinct altitude decrease of the HXR loop-top source coincides with the sudden inward swing of the loop leg in the AIA 304 A passband,which is suggested to be related to the coronal implosion.This fast loop-leg inflow of the second-step reconnection greatly enhances the reconnection rate(MA?0.18)and results in very efficient particle acceleration as well as plasma heating,which helps to achieve an episode of higher-energy emissions(>300 keV)and a second higher temperature peak(up to T ? 30 MK)of the flare.The 2011 January 28 M1.4 flare exhibits two side-by-side candle-flame-shaped flare loop systems underneath a larger cusp-shaped structure,and the southern loop sys-tem hosts the main flaring region.Besides SDO's "face-on" view,the "Ahead" satellite of STEREO provides a top view,in which the post-flare loops apparently slip eastward.By performing stereoscopic reconstruction of the post-flare loops in EUV and calcu-lating the squashing factor Q to map out magnetic connectivities,we found that the footpoints of the post-flare loops are slipping along the footprint of a hyperbolic flux tube(HFT)separating the two loop systems.These results argue strongly in favor of slipping magnetic reconnection at the HFT.The slipping reconnection may contribute to the late-phase peak in Fe XVI 33.5 nm,and may also play a role in the density and temperature asymmetry observed in the northern loop system through heat conduction.3.The Origin of Magnetic Flux RopeMagnetic flux rope is widely considered as a fundamental structure in solar erup-tions.However,it remains elusive how the flux rope forms and evolves toward erup-tion.We present imaging observations of a stellar-sized CME bubble evolving from plasmoids,mini flux ropes that are barely resolved,in the early stage of the 2013 May 13 X2.8 flare.The eruption initiates when plasmoids springing from a vertical current sheet merge into the leading plasmoid,i.e.,the 'seed' flux rope,at the upper tip of the current sheet.Rising at increasing speeds to stretch the overlying loops,the flux rope expands impulsively into the CME bubble because of the established positive feedback mechanism,which is also in tandem with strong hard X-ray bursts.Our results substan-tiate the connection between macro-scale activities and micro-scale dynamics in current sheets,therefore providing a unified view for a broad spectrum of plasma phenomena.4.Flare-CME CouplingDuring the long gradual phase of the 2013 May 13 X2.8 flare,we observe fast supra-arcade downflowing loops(SADLs)in the high-altitude current sheet region,and continuous contraction of low-altitude flare loops,as well as the growth of the post-flare loop system.All these features suggest that there is still ongoing magnetic reconnec-tion during the gradual phase.In addition,both RHESSI and Fermi GBM detect distinct hard X-ray bursts>100 keV,indicative of significant energy release by magnetic re-connection beyond the impulsive phase.This flare produces a fast halo CME,which drives an interplanetary shock.The acceleration of the associated CME lasts for a long period of about 40 minutes,which covers not only the flare impulsive phase,but also the gradual phase.This unusual late-phase acceleration of CME is well in line with the flare energy release during the gradual phase,suggesting a close coupling between the flare and CME by magnetic reconnection.That is to say,the ongoing magnetic reconnection beyond the main flare phase still facilitates the acceleration of the fast CME,probably by providing additional magnetic flux via the large-scale current sheet.It is also for the first time that we found the close flare-CME coupling during the flare gradual phase. |
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