Abstract
Launched in 2018, the Parker Solar Probe (PSP) is on a mission to study the dynamic environments of the solar corona and solar wind. A key topic of interest is the mechanism of solar-coronal heating: the atmosphere of the Sun is somehow thousands of times hotter than its surface, and why this happens is currently unknown. A surprising discovery made by PSP is the observation of abrupt reversals of the radial magnetic field lines within the solar wind, known as switchbacks. Although they had been observed previously at larger distances from the Sun, their extreme features and prevalence in PSP data has lead to significant interest in understanding their origin, evolution, and relevance to solar-coronal heating and other solar wind physics. Previous work by Squire et al. (2020) used simulations to mimic the expansion of the solar wind, suggesting that switchbacks form in-situ arising naturally as Alfvén waves. These waves are important for the generation of turbulence within the solar wind.
In this thesis, we further study the relationship between Alfvén waves and switchback formation within the expanding solar wind. The aim of this study is to better understand the predictions of this Alfvén wave scenario, so that it can be compared to observations and to help understand the questions of switchback formation and solar-coronal heating posed above. To investigate this, numerical simulations utilising a model of solar wind expansion are performed using the magnetohydrodynamics code Athena++, using a new solver developed as part of this thesis. These simulations span a variety of parameters mimicking those seen in the solar wind. We also explore the effect of the non-radial magnetic field seen within the solar wind, known as the Parker spiral; previous studies of this Alfvén wave scenario have only assumed a radial mean magnetic field.
We find that the evolution of Alfvén waves in this expanding solar wind model naturally generates magnetic field reversals characteristic of switchbacks, with simulations also showing robust turbulent behaviour. Magnetic fields inside generated switchbacks are found to preferentially deflect in one direction in the presence of a Parker spiral; these deflections cannot be seen when a purely radial field is used. Dropouts in magnetic field strength across switchback boundaries, a feature of switchbacks observed by PSP, are also observed in the simulated switchbacks. Correlations between fluctuations in magnetic pressure and density within switchbacks are found to vary with simulation parameters, in agreement with predictions made by an analytical theory of this Alfvén wave scenario. Overall, we see reasonable agreement with observations from PSP, although there remain significant uncertainties about some features.