Deciphering the Solar Wind Interaction with the Terrestrial and Saturnian Magnetospheres

Gould, Wayne and Ray, Licia and Arridge, Chris (2024) Deciphering the Solar Wind Interaction with the Terrestrial and Saturnian Magnetospheres. PhD thesis, UNSPECIFIED.

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Abstract

Due to a lack of upstream monitors, the effects of the solar wind on Saturn’s magnetosphere are not well understood. Consequently, previous investigations of this relationship have been restricted to time limited case studies. However, it is known that the solar wind plays an important role in magnetosphere dynamics. With the conclusion of the Cassini mission in 2017, continuous data, spanning 13 years is available to conduct an investigation. In this thesis, we present three studies. The first exploring the relationship of directly measured solar wind and IMF data to the magnetic field perturbations detected via the geomagnetic indices, AL and SYM/H. The second then continues this work with a comparative study between coupling functions and their utility versus individual solar wind and IMF parameters to detection of the geomagnetic indices, AL and SYM/H. This research then culminates in a large-scale statistical study of Saturn kilometric radiation as a proxy for the solar wind. Due to the lack of direct solar wind monitoring at Saturn, we use a solar wind propagation model to explore several solar wind and IMF parameters and their relationship with Saturn kilometric radiation. To assess the strength of the relationships between the solar wind parameters and both the geomagnetic indices, AL and SYM/H, and Saturn kilometric radiation, we use mutual information (chapter 5). Mutual information is a measurement of how strong the relationship is between two variables, and by extension, how much we can infer from one about the other. In the latter study, that is how confident we can be of Saturn kilometric radiation’s strength when measuring a given solar wind and IMF parameter value from the propagation model. It has significant advantages over traditional metrics such as correlation, which can only consider linear relationships. Mutual information is able to explore and reveal non-linear relationships in addition. This thesis first outlines the theory behind and a means of applying mutual information to the data considered (chapter 6). Additionally, a means of calculating uncertainties for singular data sets is adapted and tested on predictable Gaussian data to ascertain the utility of this technique. In our first study (chapter 7) we analyse geomagnetic indices with directly measured solar wind data. Known relationships are drawn out in expectations with previous studies, confirming the application of the methods in this thesis. Our second study (chapter 8) investigates the utility of coupling functions as a means of describing the solar wind and IMF coupling with the terrestrial magnetosphere, relating them to the enhancement of geomagnetic indices. We find that even under general conditions, coupling functions are a much stronger indicator of the solar wind and IMF relationship to geomagnetic indices. Finally, we apply the methods of this thesis to propagated solar wind and IMF data with Saturnian kilometric radiation (chapter 9). We find the IMF parameter, By, to present a statistically significant relationship with Saturn kilometric radiation, acting as a proxy for the IMF strength and rate of magnetic reconnection. Unlike previous case studies that predict the dynamic pressure, pdyn, as the strongest relationship, this is found to be the weakest relationship of all the parameters considered. However, this is related to the uncertainty in propagation arrival times associated with the Sun-Earth-Saturn angle and therefore conclude that mutual information values for this chapter should be considered a lower limit. Further work is then suggested to improve upon these results by relating the Sun-Earth-Saturn angle to the time offset described in section 6.4.