An Observational Study Of Magnetic Reconnection In Solar Flares

Solar flare is a sudden brightening phenomenon in the solar atmosphere,which can release an energy up to?10²⁸-10³³ erg through thermal plasma and energetic particles within a duration normally ranging from minutes to hours.The flare energy is consid-ered to originate from magnetic fields,which are proven to be the only sufficient source to supply so much energy within a short time in the solar corona.The mechanism that changes magnetic field topology and converts the pre-stored magnetic energy to flare energy is called magnetic reconnection.Magnetic reconnection plays an important role in heating plasma and accelerating energetic particles and is thought to be responsible for many flare observational features.Therefore,studying magnetic reconnection is always one of the most important topics in flare research.In past decades,the basic concept of magnetic reconnection was proposed and developed by many flare researches.By analyzing the flare observations,a standard 2D flare model called CSHKP model is proposed and used to explain many aspects of solar flares.However,the standard flare model has shown some shortages in de-scribing the detailed flare dynamics in 3D as uncovered by recent high-resolution and multi-perspective observations.On the other hand,due to the limited resolution of the previous observations,the evolution of magnetic reconnection is still unclear posing challenges toward a full understanding of flare mechanism.Therefore in this thesis,we study in detail the magnetic reconnection processes in solar flares using the observa-tions from the state of the art observatories.We first present a brief background of solar flares and magnetic reconnection the-ory in Chapter 1.The relationship between these two topics is then reviewed,especially how they promote each other.The main problems remaining in flare researches and magnetic reconnection theory are presented at the end of the chapter.In Chapter 2,we introduce the data sources and the main observing instruments.We also interpret the data analysis methods related to this thesis including the Differ-ential Emission Measure method,RHESSI reconstructions,nonlinear force-free field extrapolation of magnetic field,and 3D reconstruction of multi-perspective observa-tions.In Chapter 3,we perform Differential Emission Measure?DEM?analysis of an M7.7 flare that occurred on 2012 July 19.It is found that,during the flare,the highest emission measure?EM?region is located in the flare loop top with a value varying between?8.4×10²⁸ cm-5 and?2.5×1030 cm-5.The temperature there rises from?8 MK at about 04:40 UT?the initial rise phase?to a maximum value of?13 MK at about 05:20 UT?the hard X-ray peak?.Moreover,we find a hot region that is above the flare loop top with a temperature even up to?16 MK.We also analyze the DEM properties of the reconnection site.The temperature and density there are not as high as that in the loop top and the flux rope,indicating that the main heating may not take place inside the reconnection site.In the end,we examine the dynamic behavior of the flare loops.Along the flare loop,both the temperature and the EM are the highest in the loop top and gradually decrease towards the footpoints.In the northern footpoint,an upward force appears with a biggest value in the impulsive phase,which we conjecture originates from chromospheric evaporation.In Chapter 4,we further investigate the thermal energy release and transport in a long duration thermal flare that occurred on 2012 July 17.During the rise phase,the hard X-ray spectra of the flare are found to be well fitted by two isothermal compo-nents:a hot component?8 MK<Te<15 MK?and a super-hot component?Te>20 MK?.The super-hot component is found to appear earlier and locate higher than the hot component near the flare loop-top region and supposed to be a primary energy re?lease site from magnetic reconnection.The thermal energy of the super-hot plasma is further transported toward the footprint region via thermal conduction,producing the flare ribbons and chromospheric evaporation with temperatures of?2-5 MK and ve-locities of?20-50 km s-1.The evaporation supplies most of the plasma in the loop-top hot region that is found to be further heated to a temperature of?0-15 MK there.By quantitative analyses,we find that the thermal energy contained in the super-hot plasma is just?50%of the energy in the hot plasma.It implies that other forms of energy from the magnetic reconnection exist and heat the evaporated plasma,contributing to the accumulation of the thermal energy in the loop-top hot region.In Chapter 5,we study the magnetic topology associated with a solar reconnec-tion event in three dimensions using the combined perspectives of two spacecraft.The sequence of extreme ultraviolet images clearly shows that two groups of oppositely directed and non-coplanar magnetic loops gradually approach each other,forming a separator or quasi-separator and then reconnecting.The plasma near the reconnec-tion site is subsequently heated from?1 to ?5 MK.Shortly afterwards,warm flare loops??3 MK?appear underneath the hot plasma.Other observational signatures of reconnection,including plasma inflows and downflows,are unambiguously revealed and quantitatively measured.These observations provide direct evidence of magnetic reconnection in a three-dimensional configuration and reveal its origin.In Chapter 6,we report a rare X-shaped structure,encompassing a magnetic null,above a trans-equatorial quadrupole active region group.The observations show that this X-shaped structure is visible in all AIA EUV passbands and stably exists for days.However,possibly induced by flare activities at the northern part of the quadrupole active region group,the X-shaped structure starts to destabilize and meanwhile a jet erupted near its center at?15:05 UT on 2013 October 7.Through the non-linear force-free field modeling,we identify a magnetic null,which is above the quadrupole polarities and well corresponds to the X-shaped structure.After the jet eruption,the temperature and emission measure of the plasma near the X-shaped structure rise from?2.3 MK and?1.2×1027 cm-5 at 15:01 UT to?5.4 MK and?3.7×1027 cm-5 at 15:36 UT,respectively,indicating that magnetic reconnection most likely takes place there to heat the plasma.Moreover,the height of the null has an increase of?10 Mm,which is most likely due to the partial opening of the field lines near the fan surface that makes the null underneath rise to seek for a new equilibrium.In Chapter 7,we show the observations of magnetic reconnection around a mag-netic null associated with an eruption that resulted in an Ml.7 flare and a coronal mass ejection.The GOES X-ray profile of the flare exhibited two peaks at?02:23 UT and?02:40 UT on 2012 November 8,respectively.Based on the imaging observations,we find that the first and also primary X-ray peak was originated from magnetic reconnec-tion in the current sheet underneath the erupting magnetic flux rope.On the other hand,the second and also weaker X-ray peak was caused by magnetic reconnection around a null-point located above the pre-eruption magnetic flux rope.The interaction of the null-point and the erupting magnetic flux rope can be described as a two-step process.During the first step,the erupting and fast expanding magnetic flux rope passed through the null-point,resulting in a significant displacement of the magnetic field surrounding the null.During the second step,the displaced magnetic field started to move back,resulting in a converging inflow and subsequently the magnetic reconnection around the null.The null-point reconnection is a different process from the current sheet re-connection in this flare;the latter is the cause of the main peak of the flare,while the former is the cause of the secondary peak of the flare and the conspicuous high-lying cusp structure.In Chapter 8,we give a conclusion of this thesis and some prospects for future work.