AbstractThis thesis presents results detailing the study of magnetoacoustic shocks and more elusive forms of magnetohydrodynamic shocks in the solar atmosphere. Simultaneous slit-based spectro-polarimetry and spectral imaging observations of the chromospheric He I 10830 A and Ca ˚ II 8542 A lines is employed to examine fluctuations in the ˚ umbral magnetic field caused by the steepening of magneto-acoustic waves into umbral flashes. Following the application of modern inversion routines, evidence that umbral shock events cause expansion of the embedded magnetic field lines due to the increased adiabatic pressure is found. An adiabatic index, γ = 1.12 ± 0.01 is calculated, for chromospheric umbral locations. Examination of the vector magnetic field fluctuations perpendicular to the solar normal revealed changes up to ∼ 200 G at the locations of umbral flashes. Such transversal magnetic field fluctuations have not been described before. Through comparisons with non-linear force-free field extrapolations, we find that the perturbations of the transverse field components are oriented in the same direction as the quiescent field geometries. This implies that magnetic field enhancements produced by umbral flashes are directed along the motion path of the developing shock, hence producing relatively small changes, up to a maximum of ∼ 8 degrees, in the inclination and/or azimuthal directions of the magnetic field, highlighting that umbral flashes are able to modify the full vector magnetic field.
Chromospheric Ca II 8542 A spectropolarimetric data reveals elusive magnetohydro-dynamic shocks in sunspot umbrae. Inversion techniques are employed to uncover perturbations in the temperatures, line-of-sight velocities, and vector magnetic fields occurring across a range of optical depths synonymous with the shock formation. Classification of these non-linear signatures is carried out by comparing the observationallyderived slow, fast, and Alfven shock solutions to the theoretical Rankine-Hugoniot relations. When employing over 200 000 independent measurements, the Alfven (intermediate) shock solution provides the closest match between theory and observations at optical depths of log10 τ = −4, consistent with a geometric height at the boundary between the upper photosphere and lower chromosphere. This work uncovers first-time evidence of the manifestation of chromospheric intermediate shocks in sunspot umbrae, providing a new method for the potential thermalization of wave energy in a range of magnetic structures, including pores, magnetic flux ropes, and magnetic bright points.
|Date of Award||Jul 2020|
|Supervisor||Michail Mathioudakis (Supervisor) & David Jess (Supervisor)|