Towards constraining the solar dynamo with observational studies of the Sun's magnetic field

by Ameya Prabhu
Cumulative thesis
Date of Examination:2021-05-28
Date of issue:2022-04-26
Advisor:Prof. Dr. Maarit Käpylä
Referee:Prof. Dr. Maarit Käpylä
Referee:Prof. Dr. Stefan Dreizler
Referee:Prof. Dr. Kandaswamy Subramanian
Persistent Address: http://dx.doi.org/10.53846/goediss-9200

 

 

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Abstract

English

A complete understanding of the physical mechanisms responsible for the solar magnetic field is yet to be attained. The origin of the Sun's magnetic field, with its spatio-temporal coherence, is attributed to a dynamo mechanism operating within its convection zone. The solar dynamo operates in a turbulent regime, this presents a considerable difficulty for realistic numerical modelling and makes analytical treatment intractable. A step towards gaining a better understanding of the solar dynamo is by providing observational constraints for theoretical predictions. This is the path is adopted in this thesis. The theory of turbulent dynamos predicts the presence of an alpha effect as an important ingredient of the solar dynamo. The alpha effect generates magnetic helicity of opposite signs on small and large scales, accompanied by a further sign change upon crossing the solar equator, known as the hemispheric sign rule (HSR). Providing evidence for such a HSR, lends indirect confirmation for the role of the alpha effect in generating and maintaining the solar magnetic field. However, inferring magnetic helicity from observations is a challenging task. In this thesis, we use existing methods, as well as develop newer ones, with the aim of characterising solar magnetic helicity. We find evidence for the HSR, robust against instrumental effects. Fluxes of helicity from the Sun's poles are postulated to be crucial for the survival of the solar dynamo. Thus in this thesis, we also present an observational study of the solar polar region. In the near future, with the availability of high resolution observations of the Sun's poles, the techniques developed in this thesis provide a promising framework to observationally constrain fluxes of magnetic helicity
Keywords: Solar dynamo; Magnetism; Magnetic helicity; Polarisation; Spectropolarimetry
 
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