High resolution spectropolarimetric diagnostics of weak, small-scale magnetic fields in the solar photosphere

Student thesis: Doctoral ThesisDoctor of Philosophy


It has been known for some time that there exists a substantial amount of ‘hidden’ magnetic energy in the quiet solar photosphere, and unlocking an understanding of this phenomenon requires the study of magnetic activity on the smallest scales accessible to observations. With the advent of next generation high resolution telescopes, our understanding of how the magnetic field is organized in the internetwork (IN) photosphere is likely to advance significantly. Presented are high spatio-temporal resolution observations that reveal the dynamics of two disk-centre IN regions taken by the GREGOR Infrared Spectrograph Integral Field Unit (GRIS-IFU) with the highly magnetically sensitive photospheric Fe I line at 15648.52 ˚A. Inversions are applied with the Stokes inversion based on response functions (SIR) code to retrieve the parameters characterizing the atmosphere, tracking the dynamics of small-scale magnetism. Linear polarization features (LPFs) are found with magnetic flux density 130 − 150 G, appearing preferentially at granule-intergranular lane boundaries. The weak magnetic field appears to be organized in terms of complex ‘loop-like’ structures, with transverse fields often flanked by opposite polarity longitudinal fields. A snapshot produced from a high resolution three-dimensional radiative magnetohydrodynamic (MHD) simulation is used with SIR to produce synthetic observables in the same spectral window as observed by the GRIS-IFU. A parallelized wrapper to SIR is then used to perform nearly 14 million inversions of the synthetic spectra to test how well the ‘true’ MHD atmospheric parameters can be constrained statistically. Finally, the synthetic Stokes vector is degraded spectrally and spatially to GREGOR resolutions and the impact of unpolarized stray light contamination, spatial resolution and signal-to-noise is considered. A LPF exhibiting very similar magnetic flux density at its centre as those observed by the GRIS-IFU is studied. Thus, it is demonstrated that MHD simulations are capable of resembling real observations.
Date of AwardJul 2022
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsNorthern Ireland Department for the Economy & Andor Technology plc
SupervisorFrancis Keenan (Supervisor), Peter Keys (Supervisor) & Michail Mathioudakis (Supervisor)


  • Photosphere
  • infrared
  • spectropolarimetry
  • sun
  • magnetic fields

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