Spatial and temporal changes of the photospheric magnetic field in solar flares

Alanazi, Nawal Abed M (2026) Spatial and temporal changes of the photospheric magnetic field in solar flares. PhD thesis, University of Glasgow.

Abstract

Solar flares are understood to be driven by the rapid release of energy in the solar corona through magnetic reconnection. However, the response of the photospheric magnetic field to this process remains not fully clear, particularly in terms of its temporal and spatial relationship with chromospheric emission and signatures of coronal energy release. This thesis investigates the evolution of the photospheric magnetic field during solar flares, with a focus on its association with flare ribbon emission and coronal energy-release signatures.
Using observations from the Solar Dynamics Observatory (SDO), including the Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly (AIA), we analyse multiple flare events through high-cadence magnetograms and ultraviolet imaging. In the first study (Chapter 3), we examine the spatially and temporally varying line-of-sight magnetic field within flare ribbon regions for six M-class flares. These regions trace the chromospheric footpoints of reconnecting coronal magnetic field lines. We find that in four out of six events, the changes in the line-of-sight magnetic field precede the UV emission by several minutes on average. In addition to previously reported monotonic changes, oscillatory variations are detected, suggesting the presence of non-flare-related signals. In addition, a moderate correlation is identified between magnetic field strength and its rate of change, indicating that stronger fields evolve more rapidly.
To establish more general behaviour, and motivated by the need to characterise the full magnetic vector, in Chapter 4, we extend the analysis to the horizontal magnetic field component (Bh), which is expected to respond strongly to flare-related magnetic restructuring. A sample of 35 flares spanning C-, M-, and X-class events is analysed using unsupervised machine learning (k-shape clustering) to identify regions exhibiting step-like changes in Bh. We find that in 14 events, stepwise increases in Bh are concentrated near polarity inversion lines and are closely associated in space and time with the onset of flare ribbon emission. The peak rate of increase of Bh generally lags the ultraviolet brightening by several minutes, with no statistically significant dependence on flare class. Furthermore, the time lag increases systematically with distance from the polarity inversion line, while decreases in Bh are more spatially dispersed. The outward progression of Bh enhancements provides strong observational evidence for a photospheric response to the sequential relaxation and downward contraction of coronal magnetic fields during energy release, consistent with the coronal implosion scenario.
Finally, the relationship between photospheric magnetic evolution and coronal energy release is examined using the time derivative of the GOES soft X-ray flux as a proxy (Chapter 5). The timing of magnetic field changes is found to closely coincide with impulsive energy release, with delays around 2 minutes, indicating near-simultaneous evolution. Spatially, these changes also exhibit an outward progression from the polarity inversion line.
Overall, these results show that photospheric magnetic changes are closely linked to flare ribbon emission and impulsive coronal energy release. Increases in the horizontal magnetic field near polarity inversion lines are consistent with coronal field contraction, supporting the coronal implosion scenario and improving our understanding of how energy is transferred through the solar atmosphere.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QB Astronomy
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Fletcher, Professor Lyndsay and Hannah, Dr. Iain
• Date of Award: 2026
• Depositing User: Theses Team
• Unique ID: glathesis:2026-86039
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Jun 2026 14:51
• Last Modified: 19 Jun 2026 15:01
• Thesis DOI: 10.5525/gla.thesis.86039
• URI: https://theses.gla.ac.uk/id/eprint/86039

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