Formation And Evolution Of Hot Channel Systems In Solar Eruptions

The magnetic flux rope(MFR)is the core structure of various eruptive phenomena in the solar atmosphere,and the study of MFR is important for understanding the physical mechanism of solar eruptions and the prediction of solar activity.To understand the formation and evolution of the MFR,the three-dimensional structure of the corona magnetic field is required.At present,the coronal magnetic field cannot be directly measured,so some models need to be used to extrapolate the three-dimensional structure of the magnetic field.Based on this,two methods are mainly used to study the MFR,one is to study the structure of MFR by non-linear force free field(NLFFF)extrapolation using the observed photospheric magnetic field.And the other is to infer the evolution of the MFR by studying the observed evidence of the MFR on the Sun,including the filaments/filament channels,coronal cavities,sigmoids,etc.The hot channel is a new observational evidence of the MFR discovered by SDO,which appears as a hot blob in the high-temperature passbands and as a dark cavity in the low-temperature passbands.This thesis focuses on the study of hot channels in solar eruptions and involves three studies.At first,we investigate the writhing and footpoints motion of two hot channels during an solar eruption(Chapter 3).In the second study,we then investigate the formation,evolution and eruption of the first kinking hot channel in the first study(Chapter 4).At last,we perform a a statistical study of hot channels in M-class and X-class solar flares from 2011 to 2014(Chapter 5).The detailed investigations are listed as follows.1.During the M6.5 class solar flare on 2015 June 22,two kinking hot channels are observed in the high-temperature passbands of SDO/AIA.The first hot channel gradually kinks during its rise.And then the brightening of both footpoints of this hot channel begins to drift clockwise at the same time,which we call the apparent rotation of the footpoints.A combination of the observed filament chirality and theory suggests that the kinking structure of the first hot channel likely corresponds to a negative writhe.For a MFR with negative writhe,the clockwise rotation of its legs corresponds to the unwrithing of the MFR.Therefore,we consider that the observed apparent footpoints rotation of the first kinking hot channel indicates that the hot channel is unwrithing.During the apparent footpoints rotation of the first kinking hot channel,the second hot channel with negative crossing appears near the first kinking hot channel’s right leg.Subsequently,with the footpoints drifting of the second kinking hot channel(especially the right footpoint moving westward),the hot channel gradually unwrithe.A comparison of the AIA observations with the distribution of photospheric squashing factor at the footpoints of the two hot channels shows that the apparent rotation trajectory of the two footpoints of the first kinking hot channel and the drifting trajectory of the right footpoint of the second kinking hot channel are consistent with the distribution of photospheric high-Q region calculated based on the potential field model.Therefore,both the apparent rotation and drifting of the footpoints of the two kinking hot channels may be caused by magnetic reconnection between the hot channel and the surrounding magnetic fields at the high-Q region during the unwrithing process.2.We conducted a detailed study of the formation and eruption of the first kinking hot channel during the M6.5-class flare using multi-wavelength observations from SDO/AIA and high-resolution observations from GST,as well as the results of NLFFF extrapolation.There are two flare precursors before the flare main phase,with the beginning of the first precursor,a few seed hot channels appear above the polarity inversion line(PIL)in the form of a group of hot loops,and brightenings are observed at the footpoints on both sides of the PIL.The formation of the seed hot channel is associated with the parallel motion of the footpoint brightenings along the PIL,and the motion proceeds into the early stage of the flare main phase.During this process,seed hot channels build up and rise slowly,accelerating at the peak of the second precursor and the magnetic reconnection during the second precursor plays a major role in accelerating the hot channel.Different seed hot channels merge during the process of acceleration,forming the larger and more twisted first kinking hot channel.The propagation of the footpoint brightenings along the PIL suggests that the first kinking hot channel may be formed due to zipper reconnection and converging reconnection.The strongly twisted magnetic field lines in the NLFFF extrapolations also show a tendency to approach each other,thus supporting the occurrence of merging reconnection between the seed hot channels observed by AIA.By calculating the magnetic flux and total current at the footpoint of the magnetic flux rope corresponding to the seed hot channel over time,we hypothesize that the writhing motion of the first kinking hot channel may be driven by a combination of the Lorentz force due to the external shear field and the magnetic tension release of the twisted field.3.At last,we perform a statistical analysis of M-class and X-class solar flares from 2011 to 2014.Firstly,we observe and analyze eight complex hot channels near the limb that exhibit changes in their structure or footpoint location during the eruption process,show multiple layer structures,or have multiple high-temperature structures at the same time.Then,the statistical results of M-class and X-class solar flares on the full disk during this period show that the probability of events with hot channels is 29%.Based on the observed shape of the hot channels,we classify these events into four types:blob/cusp shape,S-shape,twisted/writhed,and loop shape.The proportions of different shapes of hot channels are roughly the same.We further study the eruption of hot channels and the probability of observing filaments and their relationship with hot channel eruptions in these events.The percentages of successful,partial and failed hot channel eruption are 55%,4%and 41%,respectively.74%of solar flares with hot channels are associated with filaments.Among them,28%filaments do not erupt,2%filaments erupt before the hot channels,6%filaments erupt after the hot channel,and 38%erupt with the hot channels simultaneously.In addition,we find that some filaments are located near a single footpoint of the hot channel and produce a jet during the eruption.