Phenomena in the Solar Corona: characteristic structure and dynamic evolution

Alexander, Caroline (2012) Phenomena in the Solar Corona: characteristic structure and dynamic evolution. Doctoral thesis, University of Central Lancashire.

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This thesis presents a study of multiple phenomena that exist within the solar corona. The structures explored in this work cover a range of sizes from a small-scale X-ray bright point (<10 Mm), to medium-scale coronal loops(10–100 Mm), and finally to a large-scale prominence (>100 Mm). Observational data and numerical simulations were utilised in order to investigate the structure and evolution of each type of feature.

A small-scale X-ray bright point (XBP) was investigated using complete Hinode observations in order to examine it over its entire lifetime (∼12 hours). The XBP was found to be formed directly above an area of cancelling magnetic flux on the photosphere. A good correlation between the rate of X-ray emission and decrease in total magnetic flux was found. The magnetic fragments of the XBP were found to vary on very short timescales (minutes), however the global quasi-bipolar structure remained throughout the lifetime of the XBP. Electron density measurements were obtained using a line ratio of Fe XII and the average density was found to be 5±1x109 cm−3 with the volumetric plasma filling factor calculated to have an average value of 0.04±15%. Emission measure loci plots were then used to infer a steady temperature of log Te [K] ∼ 6.1±0.1. The calculated Fe XII Doppler shifts show velocity changes in and around the bright point of ±15 kms−1 which are observed to change on a timescale of less than 30 minutes. The results indicate that higher cadence spectroscopic measurements are required if the velocity flows are to be related to corresponding changes in the magnetic field.

The next feature investigated was a 100 Mm multistranded coronal loop that was simulated in order to investigate how changing the various model parameters would affect the resulting differential emission measure DEM distributions and intensity values. Once the model was fully understood, it was used to test a DEM solver and quantify the ‘goodness-of-fit’ that could be achieved. This allowed the imitations of the DEM method to be understood. As the model parameter space was altered, a number of changes in the resulting synthetic DEMs were observed. In most cases these changes were subtle and could be explained by the changing physics of the system. The cooling simulation showed the most unique changes where the total energy of the system could be identified by examining the evolution of the intensity values and DEM shape. The iterative solver solution XRT DEM iterative2 did an excellent job of reconstructing the original model intensity values and DEM distributions in the majority of cases. The only instance where the solver did not do well was when the synthetic DEM was very narrow i.e., only covering a few temperature bins. This highlights the under-constrained problem of using DEM solvers and shows that this particular solver works best when the original DEM being reconstructed is smoother and more multithermal.

Finally, a large-scale prominence eruption was investigated using observations from two points of view. The structure and evolution of the prominence material and cavity were examined over the eruption process. Many possible initiation methods were investigated to see if the cause of the prominence eruption could be pinned down. It was found that the polar-crown cavity could be defined as a density depletion sitting above denser polar-crown filament plasma which has drained down from the cavity due to gravity. The eruption of the polar crown cavity as a solid body can be decomposed into two phases: a slow rise at a speed of 2 ±0.2 km s−1 , and an acceleration phase at a mean speed of 15–25 ±0.6 km s−1 . The initiation of the prominence was concluded to be caused by a combination of mass un-loading and a type of kink instability.

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