Author ORCID Identifier
Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Petrus C. Martens
While it is believed magnetic spots on solar-like stars originate in their convection zones, constraining the exact location of their generation has not been hitherto possible. Based on theoretical magnetohydrodynamic considerations and an analysis of helioseismic data on solar torsional oscillation, here we demonstrate that dynamic Lorentz forces and an equivalent Lenz’s law for magnetic induction conspire together to reveal where sunspots - strong magnetic field concentrations observed on the Sun’s surface - originate. Our results illuminate physical processes at the heart of solar-stellar magnetic cycles and indicate that stellar magnetic spots are generated in the lower half of their convection zones.
The Sun’s axisymmetric flows - differential rotation and meridional flow, govern the dynamics of the solar magnetic cycle and serve as vital inputs for numerical models of the solar cycle. A variety of methods are used to measure these flows, each with its own strengths and weaknesses. Measurements based on cross-correlating images of the surface magnetic field (magnetograms) have been made since the 1970s. Measuring these flows precisely with
this method requires advanced numerical techniques which are capable of detecting movements of less than the pixel size in images of the Sun. We have identified several systematic errors which influence previous measurements of these flows and propose numerical techniques which can minimize these errors. Our analysis of magnetograms from the Michelson Doppler Imager (MDI) on the ESA/NASA Solar and Heliospheric Observatory (SOHO) and Helioseismic and Magnetic Imager (HMI) on the NASA Solar Dynamics Observatory (SDO) shows long-term variations in the meridional flow and differential rotation from 1996 to 2019 which has implications for solar cycle prediction.
We also introduce and make openly accessible a comprehensive, multivariate time series (MVTS) dataset extracted from solar photospheric vector magnetograms in the Spaceweather HMI Active Region Patch (SHARP) data series obtained from HMI onboard SDO. Our dataset includes a cross-checked NOAA solar flare catalog which immediately facilitates solar flare prediction efforts, for the first time using time series in a detailed, quantitative manner. We discuss methods used for data collection, cleaning and pre-processing of the active region and flare data; and we further describe a novel data integration and sampling methodology. This dataset covers 4,075 MVTS of active regions between May 2010 and August 2018 matched to over 10,000 flare reports. Potential directions toward expansion of the time series, either "horizontally" - by adding more prediction specific parameters, or "vertically" - by generalizing it in order to predict other solar eruptions, are also indicated. The purpose of this dataset is two fold. First, it serves as a mine for precursors to solar flares and it also serves as a benchmark which facilitates the comparison of different solar flare prediction algorithms with both operational (research-to-operations), and basic research (operations-to-research), benefits potentially following in the future.
Mahajan, Sushant Sushil, "Observational Constraints on the Solar Dynamo and the Hunt for Precursors to Solar Flares." Dissertation, Georgia State University, 2019.
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