| Explosive events (EEs) are small-scale dynamic phenomena often observed in the solar transition region (TR). They have been suggested to be a consequence of small-scale fast magnetic reconnections. Since the launch of SOHO spacecraft, the SUMER spectrograph has been widely used to study EEs. With high spatial and spectral resolution, and wide spectral coverage, SUMER was a powerful tool of ultraviolet spectroscopy and has greatly increased our knowledge of EEs and other small-scale TR phenomena. The physical mechanism of these small-scale TR events and their relation with each other has been extensively discussed, but remain unclear. Investigation of EEs and other TR events, such as blinkers and EUV spicules is importance to understand the structures and dynamics prevailing in the TR and to further improve the modeling and theoretical description of this region. Study of these dynamic phenomena is also important to.understand the energy transportation in TR and the process of coronal heating, origin and acceleration of the nascent solar wind. On the other hand, Lyp is one of the most important spectral line of hydrogen Lyman series. Important information on the highly dynamic TR structures may be carried by the profiles of this line. As affected by radiative transfer process, Lyβprofiles are often characterized by non-Gaussian (self-reversed at the centers, two peaks aside). Studies on the sharp of Lyβprofile are helpful to understand the structures and flows in different regions of the solar atmosphere and the formation and absorption mechanism of the line itself. In the thesis, our studies will be focused on EEs and their impacts on the Lyβprofile, based on detailed analysis and interpretation of the data observed by SUMER in the ultraviolet wavelengths. Our major findings are reported as follows.In the past, EEs were mainly studied with the data observed on the disk of quiet-Sun (QS) region, while their properties in coronal holes (CHs) remain unclear. Here, we study EEs and search for their signatures in the lower corona. Our statistical study shows that the EEs have an occurrence rate of about 10-21～10-20 cm-2 s-1 and an average lifetime of around 2min, which are consistent with previous results obtained on the disk in QS regions. However, EEs in polar CHs generally have a larger spatial size (～4 arcsec) on average. We also find that EEs are rarely observed near the limb. These two findings imply that the jet structure of EEs is likely perpendicular to the solar surface, so that they are not easy to form the broad line width and their size also becomes larger due to the line-of-sight effect when observed in the polar CH. Meanwhile, the NeⅧline formed in the high TR/lower corona responds to the EEs observed with the NⅣline formed in middle TR, during which an increased blue-shift or broadening line width is usually present. When fitting all EE profiles with three-Gaussians, we find that the average velocity of the blue jet component deduced from the NeⅧline is higher than that obtained from the NⅣline. This indicates that the plasma has been both accelerated and heated in this process. Our results are an important complement to the pervious studies of EEs on the disk.Although the connection between the disk and limb small-scale events has been mentioned in some literatures, there was no firm conclusion. In this study, we start with the velocity maps deduced by TR lines including the OⅣ, NⅣand NeⅧlines, where structures with velocities rapidly changing between red and blue shift (referred to as "transient velocity events") are often observed. It is interesting to know how the line intensity and width change during these events. We find that the maximum of blue/red-shifted velocities of such events can reach several tens of kilometers per second in both the QS and polar CHs. The velocity, spatial size and time duration are consistent with those of EUV spicules observed above the limb. At the initial stage of the transient velocity events, EEs are often observed at the locations with very strong blue shift. In this case, line profiles of NⅣand OⅣbecome non-Gaussian, which may last for several minutes and can also be seen at the NeⅧline. Sometime, bi-directional jets also present. Some EEs have a sudden enhancement of the intensity, which can obviously be identified as blinker-like events. In the polar CH, some transient velocity events are obviously repetitive with a period of～16 min. Blinkers are also identified in these events according to their intensity oscillation. Therefore, we can infer that the transient velocity events found in the polar CHs are closely connected with EEs, blinkers and EUV spicules.The line shape of Lyβis affected by the opacity and flows in the solar chromosphere and TR. We statistically analyze the Lyβprofile in the QS, equatorial CHs and polar CHs. By analyzing Lyβprofiles classified by its intensity, we find that the Lyβreversal deepens with increasing line intensity in CH regions. The profile becomes nearly Gaussian (without reversals) in cell regions. The red peak of Lyβprofile is stronger than the blue peak in QS regions, becomes basically the same in equatorial CHs, and is obviously weaker than the blue peak in polar CHs. We also find the Lyβprofile of each region varies gradually with its intensity. The statistical analysis of the relationship between the sharpness of Lyβpeaks and the velocity of CⅡI and OⅥline indicates the larger of the Doppler velocity of C IⅡ, the more obvious of the asymmetry. It seems that the wavelength position of Lyβcentral reversal varies with the center position of OⅥ. We also find that the intensity of Lyβin QS regions decreases with decreasing velocity of OⅥ. The complicated variations of Lyp are likely to indicate that this line contains abundant information on dynamic process in the solar chromosphere and TR.The variations of the shape of the Lyβline may be affected by transient flow fields in the dynamic solar TR. For the first time, we search for signatures of EEs in this line. EEs are identified from profiles of the CⅡand OⅥlines, respectively. We compare Lyβprofiles during EEs with those averaged in the entire QS and CH regions. We find that the central part of Lyβprofiles becomes more reversed and the distance of the two peaks becomes larger during EEs, both in the CH and QS region. The average Lyβprofile of the CⅡEEs has an obvious stronger blue peak. During EEs, there is a clear correlation between the increased peak emission of Lyβprofiles and the enhanced wing emission of the CⅡand OⅥlines. The correlation is more pronounced for the Lyβpeaks and CⅡwings, and less significant for the Lyβblue peak and OⅥblue wing. We also find that the Lyβprofiles are more reversed in the CH than in the QS. The finding that transient flows produced by EEs modify the Lyβline profile in QS and CHs implies that one should be careful in the modeling and interpretation of relevant observational data. When the underlying dynamic process of the solar atmosphere is analyzed by using Lyβand other Lyman lines, one should consider not only the line source function and opacity, but also the flow field in the transition region, including both the quasi-steady and transient flows. The research of Lyβline also has an implication to the Lya line in response to such dynamic phenomenon. Furthermore, it would be a new idea to study other dramatic eruption events (e.g., flares and CMEs) of the Sun through investigation of the shape of Lya line. |
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