Magnetoacoustic wave dynamics in the atmosphere of solar active regions

Abstract

The work presented in this thesis concerns the observation and study of Magnetohydrodynamic (MHD) waves in the photosphere and chromosphere of solar active regions. Spectropolarimetric Si I 10827 Å Facility Infrared Spectropolarimeter (FIRS) observations were used to study propagating MHD wave activity in a group of five photospheric solar pores. Oscillations with periods on the order of 5 minutes were detected at varying atmospheric heights by examining bisector velocities. SIR inversions, coupled with spatially resolved route mean square (RMS) bisector velocities, allow the wave energy fluxes to be estimated as a function of atmospheric height for each pore. We find propagating sausage mode waves with energy fluxes on the order of 30 kW/m² at an atmospheric height of 100 km, dropping to approximately 2 kW/m² at an atmospheric height of around 500 km. The cross-sectional structuring of the energy fluxes reveals the presence of both body- and surface-mode sausage waves. Examination of the energy flux decay with atmospheric height provides an estimate of the damping length, found to have an average value across all 5 pores of L_d ≈ 268 km, similar to the photospheric density scale height. This verifies the suitability of solar pores to act as efficient conduits when guiding MHD wave energy upwards to be deposited in the outer solar atmosphere. 2D PLUTO simulations find the observed decrease in energy flux to be best replicated by a localised driver, with the majority of the damping occurring as a result of geometric spreading and lateral wave leakage.

High resolution dual-line Hα 6563 Å and Ca II 8542 Å IBIS observations encompassing an isolated sunspot were used to evaluate the differences between these two chromospheric lines and their limitations for the study of sunspot oscillations. We calculate intensity, line-of-sight velocity and line width for both lines and employ wavelet analysis techniques to look at the phase relationships present in the sunspot umbra. Dominant wave frequencies of 5–8 mHz are observed in the sunspot umbra, with peak spectral energy densities at a frequency of 6.02 mHz. Contradictory line-of-sight velocity and intensity phase relationships of φ_{V −I} ≈ 90° in the Hα line and φ_{V −I} ≈ 0° in the Ca II line are observed, indicating that the lines may be observing different wave modes. Potential explanations for this are put forward including the sensitivity of the Ca II line to umbral shock phenomena, a sensitivity bias of the Ca II line to fast mode waves and the possibility of ion-neutral inconsistencies and interactions. The presence of a propagating wave is revealed in the cross-line intensity phase relationship, where a gradient effect is observed, confirming a difference in formation height of the two lines. Phase analysis and direct comparison between the Hα full-width at half-maximum (FWHM) and Ca II intensity suggests a common sensitivity between the two parameters, supporting the hypothesis that Hα FWHM may be used as an alternative diagnostic of chromospheric temperature.

Date of Award	Dec 2022
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Invest Northern Ireland & Randox Laboratories Ltd
Supervisor	David Jess (Supervisor), Samuel Grant (Supervisor) & Michail Mathioudakis (Supervisor)