Other
Scientific paper
Mar 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011acasn..52..171c&link_type=abstract
Acta Astronomica Sinica, vol. 52, no. 2, p.171-173
Other
Sun: Flares, Sun: Atmosphere, Sun: Chromosphere
Scientific paper
Solar flares are one of the most significant active phenomena in the solar atmosphere. It is involved in very complicated physical processes, including energy release, plasma instability, acceleration and propagation of energetic particles, radiation and dynamics of the flaring atmosphere, mass motions and ejections, and so on. Enhanced radiation during flares spans virtually the entire electromagnetic spectrum originating from different layers of the solar atmosphere. High energetic particles and strong radiations that are produced during the flare eruptions play a major role in space weather. Therefore, it is very important and necessary to study the mechanisms of solar flares. In this thesis, combined with ground and space observations, the theoretical calculations are used to study the spectral features and radiation mechanisms of solar flares. In particular, our research is concentrated on the diagnostics of non-thermal processes and origin of the white-light flares. The main contents are described as follows: (1) Different chromospheric lines are used to diagnose the heating mechanisms in flares. We calculate the Hα and Ca II 8542 Å line profiles based on four different atmospheric models, including the effects of non-thermal electron beams with various energy fluxes. These two lines have different responses to the thermal and non-thermal effects, and can be used to diagnose the thermal and non-thermal heating processes. We apply our method to an X-class flare occurred on 2001 October 19 and find that the non-thermal effects at the outer edge of the flare ribbon are more notable than that at the inner edge, while the temperature at the inner edge seems higher. On the other hand, the results show that non-thermal effects increase rapidly in the rise phase and decrease quickly in the decay phase, but the atmospheric temperature can still keep relatively high for some time after getting to its maximum. For the two kernels that we analyze, the maximum energy fluxes of the electron beams are approximately 1010 erg cm-2 s-1 and 1011 erg cm-2 s-1, respectively. However, the atmospheric temperatures are not so high, i.e., lower than or slightly higher than that of the weak flare model F1 at the two kernels. We discuss the implications of the results for the two-ribbon flare models. (2) The white-light emission in solar flares is studied by radiative hydrodynamic simulations. It is believed that solar white-light flares (WLFs) originate from the lower chromosphere and upper photosphere. In particular, some recently observed WLFs show a very large continuum enhancement at 1.56 μm where the opacity reaches its minimum. Therefore, it is important to make clear how the energy is transferred to the lower layers responsible for the production of WLFs. Based on radiative hydrodynamic simulations, we study the role of non-thermal electron beams in increasing the continuum emission. We vary the parameters of the electron beams and disk positions and compare the results with observations. The electron beam heated model can explain most of the observational white-light enhancements. (3) The effect of periodic non-thermal electron beam on chromospheric lines is studied. Heated by the periodic non-thermal electrons, the Hα line center and wings show the same periodicity as the injected electrons. The line center and wings have different time delays compared to the bombarded electron beam. The line center has a relatively small phase difference. The red and blue wings also show different time delays. The blue wing shows a smaller phase difference compared to the red wing. The phase differences between the line center and wings can be explained by their different formation layers. However, the phase difference between the red and blue wings can not be fully explained in this manner. A possible explanation is that the macroscopic velocity field changes the emission and absorption features at the red and blue wings. The above results provide useful information for diagnosing the heating processes by using the fine time structures observed in chromospheric lines. (4) A statistical study of RHESSI hard X-ray spikes is made. The spikes refer to fine time structures on time scales of seconds to milliseconds in hard X-ray time profiles during solar flares. We get a preliminary statistical result of temporal and spectral properties of hard X-ray spikes. About one fifth of the spikes can be detected in photon energies higher than 100 keV. Some main properties of the spikes are as follows: (i) Spikes are produced in both impulsive flares and long-duration flares with nearly the same occurrence rates. 90% of the spikes occur during the rise phase of the flares, and about 70% occur around the peaks of the flares. (ii) The durations of the spikes vary from 0.2 s to 2 s, with an average being 1.3 s, which is independent of photon energies. The spikes exhibit symmetric time profiles with no significant difference between the rise and decay phases. (iii) Among the most energetic spikes, about two thirds of them have harder count spectra than their underlying slow-varying components. There is also a weak indication that spikes exhibiting time lags in high-energy emissions tend to have harder spectra than spikes with time lags in low-energy emissions.
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