Abstract
The processes that heat the solar corona and accelerate the solar wind are not well understood, despite decades of research. The dissipation of turbulence within these environments, driven by the motions of plasma at the Sun's surface, contributes to this heating; characterising the properties of this turbulence is essential for a complete understanding of the coronal heating problem. This turbulence is highly imbalanced, with more fluctuations launched and travelling away from the Sun than towards it. Ions in this turbulent environment are observed to be continuously heated, with more energisation perpendicular to the local magnetic field than parallel and heavy ions heated more than protons; any model that aims to comprehensibly describe the heating of the corona and acceleration of the solar wind must also explain these observations. In this thesis, simplified models are used to understand the key mechanisms that heat ions in the near-Sun environment, with the eventual goal being to use these insights to help build and inform more complex theories of ion heating by turbulence and its connection to coronal heating and solar-wind acceleration.
Two commonly studied mechanisms of ion heating are stochastic heating, where ions are heated by kicks from uncorrelated turbulent fluctuations, and cyclotron-resonant heating, where strong ion-wave interactions lead to efficient heating. The key idea proposed in this thesis is that stochastic heating and cyclotron-resonant heating, generally considered separate mechanisms, are in fact related and should be considered as two limits of a continuum controlled by the wavevector-frequency spectrum of turbulent fluctuations. This wavevector-frequency spectrum is itself controlled by the imbalance of the turbulence, allowing for a transition from a stochastic-heating-like mechanism in balanced turbulence to a cyclotron-resonant-heating-like mechanism when it is imbalanced.
To investigate this idea, a model of the turbulence with a dependence on imbalance is developed, which is then used with analytic quasi-linear theory to predict the dependence of ion heating on turbulence amplitude and imbalance.
These are then tested with high-resolution direct numerical simulations of a newly developed module of test particles interacting with gyrokinetic turbulence.
A unified phenomenological model is then developed to describe ion heating in a variety of turbulent regimes: balanced, imbalanced, and with and without the recently discovered helicity barrier.
The ideas and results presented in this thesis help to clarify the signatures of ion heating in an idealised setting, and may be useful to building models of coronal heating and solar-wind acceleration.