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Spectroscopic studies of coronal structures using ground and space based data a thesis submitted for the award of the degree of doctor of philosophy in physics, Mangalore university

Krishna Prasad, S

Spectroscopic studies of coronal structures using ground and space based data a thesis submitted for the award of the degree of doctor of philosophy in physics, Mangalore university [Ph.D Thesis] S. Krishna Prasad - Bangalore Indian Institute of Astrophysics 2013 - [various pagings.]

Solar corona, the tenuous outer atmosphere of the Sun, is very hot with temperatures of the order of a few million kelvin. The main energy source for heating the corona and the physical mechanism behind the acceleration of fast solar wind are not clearly
understood yet. These are major long-standing problems in solar physics and serve as a common motivation for most of the coronal studies. Our knowledge on these issues is advancing with the recent advent of instruments with high spatial, temporal, and
spectral resolutions, which at the same time brought many new problems into light. A few such problems motivated the studies presented in this thesis. Broadly, two di_erent issues are addressed, one connected with the variation of spectral line parameters and the other related to the ambiguity on the nature of propagating intensity disturbances. Data from a ground-based coronagraph, a space-based Extreme Ultraviolet (EUV) imager and a space-based EUV spectrometer were used for this purpose. The thermal structure of a coronal loop is crucial for determining the plasma heating mechanism. Temperature variation along 18 di_erent loop structures was studied us-ing high resolution spectroscopic data from Norikura coronagraph. Observations were made simultaneously in multiple iron emission lines. Using the temperature sensitive emission line ratios [Fe xiv] 5303 _A/[Fe xiii] 10747 _A, and [Fe xi] 7892 _A/[Fe x]6374 _A the temperature and its gradients were computed for all the structures. It was found that the temperature gradients are negative for most of the structures observed in hotter lines and positive for those observed in colder lines. This indicates that the loop tops in general appear cooler or hotter depending on the line pair chosen. To explain this complex behaviour, a gradual interaction between di_erent temperature plasma was proposed. Interestingly, extensive analysis in the past, on line width vari-ation with altitude, also indicated a similar behaviour, in agreement with this mixing scenario. This behaviour is now veri_ed for polar regions. It was found that the FWHM of red emission line ([Fe x] 6374 _A) increase with height whereas that of green emission line ([Fe xiv] 5303 _A) decrease with height consistent with the results from equatorial regions. An interesting behaviour of green line was observed when latitudinal comparison was made. Line widths are higher in polar regions for all the
emission lines studied except for the green line. The higher line widths in polar regions are often associated with the existence of a non-thermal source powering the fast solar wind. But, the observed behaviour of green line seem to disagree with this. Some of the recent observations question the uniqueness of the interpretation of prop- agating disturbances (PDs) as slow magneto-acoustic waves. It is argued that the
high-speed quasi-periodic upows produce similar signatures and it is di_cult to dis-tinguish them. In an attempt to resolve this ambiguity, these PDs were studied indi_erent open structures. In polar regions, space-time maps were constructed for sev-eral plume and interplume structures using arti_cial slits that are wider than usual.
This is to suppress the e_ect of fainter jets, if any present, causing the upows. Stronger jets were identi_ed from movies. Despite these measures, PDs were seen in all the structures studied. Moreover, they were found to be insensitive to changes in slit width. This implies the coherent nature of these disturbances. The average propagation speeds were found to be dependent on the temperature. These properties
favour the interpretation of PDs as due to slow waves rather than high speed upows. Same technique cannot be applied for AR fan loops since they are relatively thin. To compensate for this, simultaneous spectroscopic and imaging data were used to study PDs in active region fan loops. Two di_erent periodicities short (<3 min) and long (_9 min) were observed with di_erent nature. The shorter periodicities showed oscil- lations in intensity, Doppler shift and line width whereas the longer ones did not show any line width oscillations. The line pro_les are symmetric with no visible blue-shifted
component. The apparent propagation speeds were computed for the longer periods from the co-temporal imaging data which show the temperature dependence. Theobserved properties suggest the longer periods are due to slow MHD waves and the shorter ones could be due to the simultaneous presence of more than one MHD modes. To further supplement the previous study, spatial damping was studied in di_erent openstructures both on-disk and o_-limb. Powermaps constructed at three di_erent period ranges indicate that the PDs with long periods travel farther distances before getting
damped. Similar behaviour is observed in all the structures. Spatial damping was also studied in di_erent temperature channels. Results indicate lower amplitudes and
faster damping in hotter channels. All these observed properties were explained using a simple slow wave model considering thermal conduction as the damping mechanism.


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