Large scale current sheets are important structures in solar eruptions. Models of coronal mass ejection (CME)/flare include the large scale current sheet connecting the ejected flux rope to the post-flare loop. Studies of physical properties in current sheets could improve our understanding on rapid magnetic reconnection during a solar eruption, instabilities and turbulence and high-energy particle acceleration taking place inside the current sheet. This thesis focuses on the physical properties, internal structures, and the features of emission lines of current sheets that form during solar eruptions by performing numerical experiments, and giving predictions from CME/flare models to be compared with observations. We perform resistive magnetohydrodynamic simulations to study the internal structure of current sheets that form during solar eruptions. The simulations start with a vertical current sheet in mechanical and thermal equilibria, which separates two regions of the magnetic fields of opposite polarity that are line-tied to the lower boundary representing the photosphere. Reconnection commences gradually due to an initially perturbation to the system, but becomes faster when plasmoids appear and produce small-scale structures inside the current sheet. These structures include magnetic islands or plasma blobs flowing in both directions along the sheet, and X-points between pairs of adjacent islands. We show detailed evolutions of the current sheet and look into dynamical features of magnetic islands or plasmoids. Then we examine the statistical properties of the fine structure and the dependence of the energy spectra on these properties. The spectral profiles of magnetic and kinetic energy inside the current sheet are both of the power law. The corresponding spectral indices are found to vary with the magnetic Reynolds number $R_m$ of the system, but tend to approach to a constant for large ($R_m$ $> 10^5$). The motion and growth of blobs change the spectral index. The growth of new islands causes the power spectrum to steepen, but spectrum becomes shallower when old and large plasmoids leave the computational domain. To understand observational features and physical properties of CME/flare current sheets, we deduced some important results and the corresponding observational consequences from flare/CME models in order to compare with observations, such as those from Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory and Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory. We used a simulation of a large-scale CME current sheet previously reported by Reeves et al., and performed time-dependent ionization calculations of the plasma flow in a CME/flare current sheet. The results show differences in the emission line intensities between equilibrium and non-equilibrium ionizations. The current sheet plasma is found under-ionized at low altitude and over-ionized at high altitude. The assumption of ionization equilibrium would lead to a significant underestimate of the temperature low in the current sheet and overestimate at larger heights. By assuming intensities of emission line, we compute the count rates for each of the AIA bands and emission features, and compare the results with observations from UVCS, including a low intensity region around the current sheet corresponding to this model.
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