Thermodynamical Analyses Of Flare Supra-arcade Downflows And Prominence Plumes

The large-scale eruptions in solar atmosphere include flares,coronal mass ejections(CMEs),and prominence eruptions.They are thought to be different aspects of magnetic energy release,and are critical to space weather forecast.Solar flares are phenomena of sudden brightening from radio to y-ray.Prominences are structures suspended in corona,whose material is about 100-fold cooler than the corona;eruption is one of ways of their disappearance.CMEs refer to that coronal mass is ejected into heliosphere,and coronagraphs are important for studying the structures and transports of CMEs.The Advanced Space-based Solar Observatory(ASO-S)is the first Chinese satel-lite that focus on solar physics,whose scientific objectives can be summarized as"1M2B",i.e.,to simultaneously observe the solar magnetic field and the two most sig-nificant eruptions on Sun:flares and CMEs,and to study their formation mechanisms and relationships.To achieve the scientific objectives,there are 3 payloads onboard the ASO-S:Full-disk vector MagnetoGraph(FMG),Lyman-alpha Solar Telescope(LST),and Hard X-ray Imager(HXI).All the contents of this thesis is associated with the ASO-S,and mainly 3 works are included.The first one is about the thermodynami-cal evolution of flare supra-arcade downflows(SAD)(Chapter 2);the study of SAD is helpful for understanding the mechanisms of energy release and atmospheric heating of flares.The second one is about the formation mechanisms of prominence plumes(Chapter 3);prominences are one of scientific objectives of LST,and prominence erup-tions are closely related to flares and CMEs.In the third work,we simulated the stray light of LST/SCI coronagraph(Solar Corona Imager)(Chapter 4);this work not only helps us to know about the production and strength of stray light,but also is a part of technological accumulation of Chinese solar physics community.SADs are dark,teardrop-shaped features descending upon flare arcades.They are generally thought to be the results of magnetic reconnection,and possibly to be re-lated to energy release and flare plasma heating.SADs are believed to be low—density structures,however,their formation mechanisms are under debate.Using differential emission measure method,we analyzed the thermodynamical evolution of the SADs in a flare on 2011 October 22,and found 3 heating events around SADs.The first heating event starts with the increase of the emission measure,about 2.8 minutes before the arrival of the first studied SAD.The propagation speed of the plasma heating is about 140km s-1,a little faster than the speed of the SAD.However,the other two heating events have fast propagation speeds more than 700 km s-1.We suspect that the first heating event can be explained by adiabatic compression due to the dropping of the SAD,but the others may have different causes.Besides,we observed that SADs can push away their surrounding spikes.On the basis of our observations and SAD models of predecessors,we proposed that SADs are outflows of patchy and bursty magnetic re-connection,they are low-density structures due to atmospheric stratification,they push away surrounding high-temperature and high-density plasma and leave dark lanes be-hind.The reliability of the DEM results,heating and cooling mechanisms,and other SAD explanations are discussed.There are sometimes cavities below quiescent prominences,named as "bubbles".The bubble—prominence boundaries sometimes arch upwards,and rising dark flows are formed,which are called prominence plumes.Using the New Vacuum Solar Tele-scope(NVST),which is nearby the Fuxian Lake,Yunnan,we observed a limb promi-nence on 2018 November 11,and more than ten prominence plumes are recorded in Ha 3-band filtergrams.Some plumes split into small ones during their evolutions,and finger-shaped structures occur between them.Blue-shifted flows along the bubble-prominence interface are found before and during the plume formation.Smaller and denser fingers occur where the flows are strong at the late phase of the plumes.Using spectral analyses,we found the enhancements of brightening,blue shifts,and nonther-mal velocities.The splittings of plumes and fingers are signatures of Rayleigh—Taylor instabilities,and the flows along the bubble—prominence interface could increase the growth rate of Kelvin-Helmholtz/Rayleigh-Taylor instabilities.However,the turbu-lence at plume fronts suggests that extra mechanisms are necessary to trigger and drive the rising plumes,for instance,the upward gradient force of magnetic pressure.SCI is one of the three instruments of ASO-S/LST;it can image solar corona with field of view(FoV)of 1.1-2.5 R? in Lya and white-light(WL)wavebands simultane-ously.Because the radiation from solar corona is much fainter than that from solar disk,stray light suppression is a critical subject during the design and manufacture of coro-nagraphs.SCI is a reflective and internally occulted coronagraph,and the main source of SCI stray light is the scattering of solar disk radiation by the primary mirror,thus decreasing the surface roughness of the primary mirror is an important way to suppress the stray light.Using Zemax OpticStudio,we simulated SCI stray light with 3 kinds of scattering models on its primary mirror,and stray light distributions with different surface parameters were obtained.The results suggest that the ratios of signal to stray light decrease along distance from solar disk center in both channels.Generally,the stray light in Lya channel is lower than the signal,but the stray light in WL channel at 2.5 R? is about one order of magnitude higher than the signal.Through optimization,parameter assemblies of primary mirror surface that meet stray light requirements were also obtained.Through studying the thermodynamical properties of SADs and prominence plumes,we tried to explain the two phenomena,and clarify their relationships to flare energy release and the formation mechanisms of prominences,respectively.SADs provide evidence of patchy and bursty magnetic reconnection,and they also suggest that compression plays a role in flare atmosphere heating.In our work on prominence plumes,we found the turbulence at plume fronts using spectroscopical analyses for the first time.We also emphasized that extra mechanisms are necessary to drive the ris-ing plumes,which explains partly why prominence mass doesn't drop down during the occurrence of instabilities.The ASO-S has some advantages on studying SADs and prominence plumes.By HXI,we can study the relationship between SADs and energy release;what's more,SCI may provide SAD observations in white light.LST will pro-vide full-disk and long-duration observations in Lya wavebands,which is a new window for studies of prominences,including prominence plumes.The work of SCI stray light simulation provides some constraints on the manufacture of SCI primary mirror,and the work also provides theoretical basis on stray light analyses after the SCI starts to work.