Spectral Analyses And Radiative Hydrodynamic Simulations Of Solar Activities In The Lower Atmosphere

The lower atmosphere of the Sun is partially ionized with a low temperature and a large density,which is much more complicated compared with the corona.Many activ-ities in small scales occur in the lower atmosphere,whose mechanisms remain unclear.Recent high-resolution observations provide us with a wealth of information about the solar lower atmosphere.However,as the spectral lines that form in the lower atmo-sphere are normally optically thick,it is very difficult to infer the physical parameters of the atmosphere from the spectral features.On the other hand,radiative hydrody-namic simulations with different energy inputs can help us to get a clear picture of the heating and dynamic processes of the solar lower atmosphere,and to understand how the spectral features originate.This thesis aims to study the response of the solar lower atmosphere to solar activities based on spectral analyses and radiative hydrodynamic simulations.The main content of the thesis is as follows.In Chapter 1,we briefly introduce the history of spectral observations of the Sun,in particular those to the solar lower atmosphere.By taking Ellerman bombs(EBs)and microflares as examples,we then address the spectral features and triggering mecha-nisms of solar activities in the lower atmosphere.We also mention the influence of solar activities on photospheric lines.In Chapter 2,we present the instruments used in our study and the data reduc-tion methods,including the co-alignment of images from different instruments and the extrapolation of vector magnetic fields.We discuss the basic method of radiative hy-drodynamic simulation in Chapter 3,and the calculation of spectral lines under partial frequency redistribution in Chapter 4.In Chapter 5,we study the spectral features of chromospheric lines in EBs.We find that it is very difficult to obtain the Doppler velocity from the shift or asymmetry of an optically thick line.The velocity derived from the bisector method varies in a wide range.Considering that EBs mainly occur in a restricted region in the lower atmosphere,and taking into account the particular spectral features of EBs,we propose a two-cloud model to fit the EB line profiles,in which the lower cloud corresponds to the emission wings,while the upper cloud is responsible for the absorption core.After carefully determining the parameter ranges,we get satisfactory fitting results.As expected,the source function of the lower cloud shows an increase compared with the quiet region,corresponding to a temperature enhancement of 400-1000 K.This result agrees with previous calculations using semi-empirical models,and confirms the local heating in the lower atmosphere when EBs occur.We also find that the optical depths of both clouds are increasing,either from direct heating(lower cloud)or illumination of enhanced radiation from below(upper cloud).However,the velocities obtained from the cloud model are averaged ones,which are different from the values derived with the bisector method.Generally speaking,the two-cloud model can serve as an effective method to deduce the basic physical parameters of EBs.In Chapter 6,we check the response of ultraviolet spectral lines in EBs.Through a spectral analysis of EBs simultaneously observed with FISS and IRIS,we find emis-sions in the line wings of the Ha,Ca ? 8542 A and Mg II triplet lines,and brightenings in the 1700 A and 2832 A ultraviolet continuum images,which confirm a local heating in the lower atmosphere.We also find that when an EB occurs,the intensity of the Mg II triplet lines is correlated with the intensity of the Ha line.Thus,the former lines can serve as an alternative to identify EBs.However,we do not find any response in the hotter lines of IRIS(C ? and Si ?).Spectral fitting of both chromospheric lines(Ha and Ca ? 8542 A)using the two-cloud model shows a temperature enhancement of 2300 K,which is quite large among previous results,yet still not able to generate UV burst features.In Chapter 7,we explore the evolution of chromospheric lines in EBs with radiative hydrodynamic simulations.We consider two different heating methods,thermal(direct plasma heating)and non-thermal(heating by an electron beam).Generally speaking,the line profiles from simulations are comparable with observations.In non-thermal models,we find a dimming in the Ha line wing and continuum when heating begins;while in thermal models,the dimming only occurs in the Ha line center with a longer lifetime.Such a difference in the line profiles can be used to determine whether an EB is subject to non-thermal heating or thermal heating.In our simulations,if a higher heating rate is adopted,the Ha line would be unrealistically enhanced,while there are still no clear UV burst features.In Chapter 8,we describe the observational evidence of magnetic reconnection,bidirectional flows,in the chromosphere during a microflare.At the flare peak time,the chromospheric lines show apparent blueshifted and redshifted components in the two sides of the flaring site,corresponding to an upflow and a downflow with veloc-ities of ±(70-80)km s-1,similar to the local Alfven speed in the chromosphere.By reconstructing the three-dimensional non-linear force-free field,we further disclose the twisted magnetic field lines(a flux rope)in the lower atmosphere,co-spatial with the dark threads observed in the He I 10830 A image.The instability of the flux rope might trigger the magnetic reconnection leading to the microflare.Our observations provide clear evidence for magnetic reconnection in the chromosphere,and show that the mechanism for microflares is similar to that for major flares.In Chapter 9,we investigate the response of the Fe I 6173 A line in a flaring atmo-sphere by radiative hydrodynamic simulations.We use both a quiet-Sun and a penum-bral atmosphere as the initial atmosphere.We find that for the quiet-Sun atmosphere,the Fe I 6173 A line center is largely enhanced when an intermediate flare occurs.The enhancement of the line center comes from radiative backwarming in the photosphere,as well as electron beam heating in the lower chromosphere.The line profile also shows a blue asymmetry due to the upward mass motion in the lower chromosphere.If we use a penumbral atmosphere,this line shows a more significant response.There appears an emission in the line center and the line asymmetry is more obvious.In real obser-vations,the low spectral resolution of HMI may lose some information of this line,but the enhancement at the line center and the line asymmetry remain visible.We also cal-culate the polarized line profiles and find that the Stokes I and V profiles can be altered when there is a flare.Thus the distortion of this line may have a crucial influence on the inverted magnetic field,and it provides a plausible interpretation of the magnetic transients frequently observed in solar flares.In the end,we summarize our work and make a expectation for future research in Chapter 10.