University of Leicester
2021BradleyTJPHD.pdf (19.94 MB)

Planetary Period Modulations of Dynamics in Saturn’s Nightside Magnetosphere

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posted on 2021-11-11, 12:37 authored by Thomas J. Bradley
This thesis principally studies the effects of magnetic field perturbations, and their associated large scale current systems, that modulate Saturn’s magnetosphere near to the planet’s rotation period. These planetary period oscillations (PPOs) consist of two current systems, one associated with the northern hemisphere, and one associated with the southern hemisphere. Within the thesis are three detailed studies pertaining to the PPOs. Firstly, we newly examine Cassini magnetic field data from highly inclined orbits in 2012/2013 for signatures of field-aligned currents during Saturn’s northern spring, and compare these with a similar prior study of data from 2008 during late southern summer conditions to investigate for seasonal modulation. This study confirms a north/south seasonal asymmetry of the subcorotation currents across the polar regions, and newly reveals dual-modulation of the PPO current systems in both hemispheres. The second study consists of a statistical analysis of reconnection events in Saturn’s magnetotail observed by Smith et al. (2016), which are organised here by three different PPO phase systems. Clear modulation is found by all phase systems, however, best organisation is found for a phase system that considers the local time of the observations, indicating that the events are localised in azimuth rather than simultaneously affecting much of the tail width. Finally, in the third study reactions to solar wind compressions are investigated in Saturn’s magnetotail, for which clear responses are found using a variety of Cassini instrument observations. The response to compressions is newly found to be modulated by the concurrent relative phasing of the PPO systems, with evidence for the closure of magnetic flux being favoured when the two PPO systems act together to thin and thicken the tail plasma sheet during each PPO cycle. Overall, these studies emphasize how strongly activity in Saturn’s magnetosphere is modulated by the PPOs and heliospheric conditions.



Stanley Cowley; Emma Bunce

Date of award


Author affiliation

Department of Physics and Astronomy

Awarding institution

University of Leicester

Qualification level

  • Doctoral

Qualification name

  • PhD



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