Major solar activities such as major flares and associated coronal mass ejections (CMEs), have a great impact on space weather, which requires us to develop the ability to predict them. Despite the extensive studies already made, however, many basic processes, such as energy accumulation processes in solar active regions and the triggering mechanism of solar activities, are still not well understood.
One reason why the basic processes are hard to be understood is that the large-scale activities are complicated, involving many small-scale energy releases, making it hard to understand the basic picture of the events. Small-scale solar activities, such as chromospheric jets, small filaments, and surges, are generally accepted as the epitome of major solar activities, coupled with less physical processes. Studying these small-scale events will help us understand the basic physical process in large-scale solar activities, and further benefit space weather predictions. With the advent of large aperture solar telescopes, numerous well-sampled high resolution observational data enable us to study the small-scale solar activities, which are distributed at the lower end of the power-law spectrum. The New solar telescope (NST), among the new generation of large aperture, high resolution ground-based solar telescopes, is now the largest optical solar telescope in the world, which provides the major data source for this dissertation.
Another major reason why the basic processes are hard to be understood is that most of previous observations did not provide information of the solar interface region, which is essential for understanding how the convective motions and flows in the photosphere activate and drive solar activity in the corona. He I 10830 Å imaging has proven to be an excellent choice for observing the interface region. First, the formation of 10830 Å triplet requires extreme conditions which are mainly present in the upper solar chromosphere or lower corona, providing the information in the interface layer. Second, the triplet is optically thinner than other chromospheric lines, such as Hα 6563 Å or Ca II 8542 Å, making the photosphere visible as a background. In this way, the solar activity could be traced from the photosphere, the chromosphere to the transition region. The purpose of the present work has been to investigate the small-scale activity using observations with NST in He I 10830 Å, based on which the goal of this dissertation is to further the understanding of the driving mechanism and energy release process of solar activities. Meanwhile, we believe that this kind of research will also be very helpful for learning the formation mechanism of the He I 10830 Å triplet.
Besides 10830 Å observations with NST, the research presented in this dissertation benefits significantly from multiwavelength observations, including photo-spheric images in TiO and chromospheric images in Hα from NST, (extreme) ultraviolet data and full-disk magnetograms from Solar Dynamic Observatory (SDO), X-ray data from RHESSI, and X-ray images from Hinode. In addition, several advanced data analysis tools are utilized such as Kiepenheuer-Institute Speckle Interferometry Package speckle reconstruction code, Fourier Local Correlation Tracking, Regularised Inversion to Infer Differential Emission Measure from SDO and Zero-dimensional Enthalpy-based Thermal Evolution of Loops Model. Studies are carried out for one surge on July 22 2011. The associated photospheric motions are also investigated. A small filament eruption on June 17 2012 is analyzed quantitatively. The fan-spine structure of a limb jet on July 8 2012 is al so investigated.
The main findings in this dissertation are as follows: (1) the study of the surge on July 22 2011 shows that it is produced by magnetic flux cancellation which is triggered by the advection in a rapidly developing large granule at the base of the surge; (2) both clear fan-spine topology and bi-directional flows of chromospheric jet are observed, which are signatures of the magnetic reconnection; (3) footpoint emission of the filament eruption is analyzed and the results indicate that photoionization of chromospheric plasma followed by radiative recombination is essential for populating the triplet states of the 10830 Å during the decay phase of a flare.