Observational Study On Extreme-ultraviolet Late Phase Of Homologous Solar Flares From A Complex Active Region

Solar flares are usually manifested as fast transformation of coronal magnetic topological structure through reconnection and transient release of electromagnetic radiation and plasma,whose energy is up to 1028-1033 erg within a duration of several minutes to hours.It is a key research subject in space weather.Flux profiles of flares in most passbands exhibit similar tendency,which show significant impulsive phase and gradual phase.However,a new late phase in some solar flares is revealed in recent observations by Extreme-ultraviolet Variability Experiment(EVE)onboard Solar Dynamics Observatory(SDO).It demonstrates as a second peak in warm coronal emissions(～3 MK)several tens of minutes to a few hours after the peak of soft X-ray(SXR).Besides,there is no distinct enhancement of GOES X-ray or other hot coronal emissions during this second peak.The second enhancement is regarded as a special stage and termed as EUV late phase(ELP).In the past ten years,detailed analysis on origin,mechanism,thermodynamics process and magnetic structure of ELP flares has been carried out since ELP concept was proposed.It is found that ELP usually occurs in large flares,and it originates from several longer and higher loops which are magnetic connected but spatially separated from main flare loops.Therefore,ELP is explained by either a long-lasting cooling process in long ELP loops,or a delayed energy ej ection into the ELP loops well after the main flare heating.The former view suggests that ELP loops and main flare loops are reconnected and heated up to high temperature(>10 MK)almost simultaneously at the time of eruption.ELP loops are much longer and take more time to cool down.Because cooling rate of coronal loops is low when the temperature drops to～3 MK,a large number of ELP loops are observed at same time,leading to the second enhancement.The alternative view is secondary heating mechanism.A rising flux rope(FR)stretches out the overlying field lines to drive analogous standard two-ribbon flare reconnection,and dissipation at current sheet provides weak energy to slowly-heated coronal loops due to the low plasma density and weak magnetic strength at a high altitude.It energizes the long ELP loops from low temperature to around 3 MK,forming a second peak.In previous papers,results of numerical simulation confirmed that both two mechanisms could effectively generate second peaks in warm passbands,but the energy source of ELP might be different between eruptive and the confined(non-CME,Coronal Mass Ejection)flares in observations.Moreover,the statistical results show that ELP flares tend to cluster in some complex active regions,and secondary heating during the ELP is more likely existing in two-ribbon flares than in circular-ribbon flares.Focused on the clustering effect and debates on ELP production mechanisms,we select six ELP flares(F1-F6)of M-and-above class that occurred between 2011 September 6 and September 10 in active region(AR)11283 as samples,and carry out a comprehensive study on them from the perspective of radiation and magnetic field.In Chapter 1,we will briefly introduce the observation history of solar flares,relevant flare eruption models and research history of ELP.The purpose and innovation of this paper is presented at the end of this chapter as well.In Chapter 2,we will introduce the parameters and characteristics of the observation instruments and data.We also interpret the main methods of data processing used in this work.In Chapter 3,we will use the high-resolution imaging data of the Atmospheric Imaging Assembly(AIA)on the SDO satellite and the EVE curves to analyze the evolution of the active region and six individual flares.First of all,it is found that the AIA 1600 ? flare ribbon morphology and photosphere magnetogram of these flares are very similar to each other.Considering magnetic field extrapolation results of these flares in previous papers,we infer that they are all involved in a fan-spine structure.FR lying under the fan dome drives multi-stage reconnection of each event.In the flare F2,combined with Differential Emission Measure(DEM)results and AIA light curves,it is found that both secondary heating and long-lasting cooling scenario contribute to its production of ELP.ELP of flare F1 is found to share a very similar evolution process with F2,whose ELP is produced by secondary heating.Subsequently,no significant heating evidence is found in flare F3-F6,hence long-lasting cooling mechanism must be dominant.This means the ELP mechanism gradually switches from secondary heating to long-lasting cooling as AR evolves.We will briefly analyze and discuss the correlation of flare F1-F6 in the later part of this chapter.First,we calculate delay time between main phase peak and ELP peak,the transition is evidenced by an abrupt decrease of the time lag of the ELP peak.To further validate the long-lasting cooling scenario for flares F3-F6,we also pick up one representative ELP loop from each event,and carry out DEM analysis and cooling time estimation of the loops as done in flare F2.It is found that the theoretically estimated cooling time of the loops is in good agreement with observation.Then,we distinguished the main phase and late phase region of each flare by difference maps,and calculated the emission of AIA 335 ? in ELP region at the ELP peak moment.A good positive correlation between AIA 1600 ? fluence and AIA 335 ? peak intensity is found,which is analogous to the well-known Neupert effect.It reveals that ELP loops are energized at flaring moment,and long-lasting cooling mechanism is preferable.Moreover,the last and only confined flare F6 shows an extremely large ELP,and its additional emission reveals a different energy partitioning in the ELP loops from other flares.Finally,in Chapter 4,we will summarize and discuss the work mentioned in this paper and give some prospects for future work.