Observations And Study Of Small Scale Activities In The Solar Lower Atmosphere

There are a lot of small scale activities in solar lower atmosphere. With the use of high-resolution solar telescopes, various small scale solar activities have attracted more and more attention. Small scale activities include Ellerman bombs, mini fila-ments, microflares, surges and so on. The activities have relatively simple structures, and can give us a more clear physical insight. Therefore, this research will provide a new understanding of the basic physical process and give us a clue to study the more complicated eruption events.Ellerman bombs(EBs) are small short-lived brightening events mainly in the pho-tosphere.Since it was found in 1917, EBs have been widely studied.Their size can be smaller than 1", the lifetime can be as short as 10-20 minutes,and the energy is in the range of 1025?1028 ergs.The most obvious feature is the excess emission in the wings of some chromospheric lines.EBs have been deeply studied with high-resolution observations.EBs mainly occurred in the upper photosphere, around minimum temper-ature region and in the lower chromosphere.It was found that most of the EBs appear at the places where positive and negative polarities are approached together by the photospheric surface flows,and a number is also found in unipolar regions.By carefully checking the Big Bear Solar Observatory (BBSO) high-resolution Ha and 8542A spectra,which was obtained by the 1.6 m New Solar Telescope (NST) on 2013 June 6,we selected three well-observed small EBs to study their characteris-tics. It is shown that all the EBs are located near the parasitic areas in the longitudinal magnetograms and are co-spatial with mass motions of several km s-1.Our NLFFF ex-trapolation clearly shows that the EBs appear at the bald patches and the separatrices of the magnetic field, which confirms the schematic model of Pariat et al.2004.Checking the lightcurves of the EBs, the evolution of the EBs can be divided into three phases: the pre-heating, flaring, and cooling phases.All of these indicate that the EBs and solar flare are similar, except that the EBs is smaller in size.Using the Non-LTE theory, we computed the thermal semi-empirical models for the three small EBs. Our results indicate that the required extra temperature enhance-ment in the lower atmosphere is 2700-3000 K when compared with the quiet-Sun model.The temperature enhancement in our models is larger than previous values giv-en by some authors from lower resolution observations. Such a result is not surpris-ing.Another interesting thing is that compared to the plage atmospheric model, there is also a temperature enhancement in the EB upper chromosphere. It can be caused by jets or some kind of waves which are produced during the magnetic reconnection process.According to the EBs lightcurves and our estimation of the magnetic reconnection rate, we propose that the evolution of the EBs can be divided into the pre-heating, flaring, and cooling phases. All of these indicate that EBs and solar flares are more or less similar phenomena, except that the EBs are smaller in size.As one of various ejections observed in the solar atmosphere,solar surges are cool plasma ejections from the chromosphere into the corona.Generally, surges are ejected along a slightly curved trajectory with a speed of several tens of km s-1 and can be accelerated to 100-300 km s-1 or more.In recent years, high-resolution observations have explored that a surge actually consists of many fine threads, the length of which is 7-38Mm and its ejection speed is about 100 km s-1 or more.It is found that surges often occur in the regions of newly emerging flux and near the parasitic magnetic po-larities within active regions.These magnetic characteristics indicate that surges are triggered by magnetic reconnection.However, up to now, the high resolution observations of fan-shaped surges are very few. It is worthwhile to study more examples and obtain more details about their dynamic structures. We report a large fan-shaped surge, which was observed by the BBSO 1.6 m New Solar Telescope on 2013 June 5.We select three threads of the surge that are clearly observed in the Ha images. Along each thread, we make a slice and obtain the plane-of-the-sky velocity and the acceleration/deceleration.We found that the ejection starts with a velocity of several km s-1, and then is accelerated up to over 60-80 km s-1 in several minutes with an acceleration of up to 0.2-0.3 km s-2.The Ha brightening at the root of the fan-shaped surge implies that there is heating in the chromosphere. Compared with the NLFFF structure, the heating may be produced by interchange magnetic reconnection, which is the result of the interaction between emerging flux and the pre-existing magnetic field. The heating at the footpoint would generate a higher plasma pressure, which exerts an upward force on the surge.Generally,it is difficult to completely analyze the stuctures and evolution of small scale activitivies.more accurate observations are needed.Fortunately,in recent years, a number of large solar telescopes like SST, NST, GREGOR and NVST have been Suc-cessively put into observations, and provide a wealth of data for the study of small scale activities.The spatial resolution of these telescopes has reached sub-arcsecond,so the fine structures of small scale activities such as EBs, microflare, photospheric bright points etc can be observed.However, it is noticed that the FOVs of the telescopes are not more than 3', which restrict the research of some solar activities. We need observa-tions of solar activity not only covering multiple wavelengths with high temporal and spatial resolution, but also having the combination of high resolution and large field of view. Considering the fact that internationally no telescope can complete the above requirements, and there is no 10830A image observations domestically, so we decided to develope a telescope called optical and near-infrared solar eruption tracer (ONSET).After many years of efforts, ONSET was successfully developed, and since 2014 it has been formally put into scientific observations. Currently ONSET can perform routine observations to acquire 10 full-disk or part-disk images every 1 min at the Ha line center and its two wings at ±0.5A, at the He I 10830A line center and its two wings at ±0.85A, and at two white-light wavelengths. We can also carry out partial-disk observations of solar activities with high spatial (1" or better) and temporal (0.1 to 1 s) resolutions.In addition,for the first time in China, we developed the Hel 10830A filter,established a near infrared imaging system and obtained the first solar 10830A image in China.During my PhD, I attended ONSET commissioning and trial observation, and is responsible for the development of all the observation software and some data process-ing software, and established a database. ONSET was inspected by seven experts in March 2015, and was named Science and Technology Progress Award from the Min-istry of Education in February 2016.