The dependence of global super-rotation on planetary rotation rate

An atmosphere may be described as globally super-rotating if its total zonal angular momentum exceeds that associated with solid-body co-rotation with the underlying planet. In this paper, we discuss the dependence of global super-rotation in terrestrial atmospheres on planetary rotation rate. This dependence is revealed through analysis of global super-rotation in idealised General Circulation Model experiments with time-independent axisymmetric forcing, compared with estimates for global super-rotation in Solar System atmospheres. Axisymmetric and three-dimensional experiments are conducted. We find that the degree of global super-rotation in the three-dimensional experiments is closely related to that of the axisymmetric experiments, with some differences in detail. A scaling theory for global super-rotation in an axisymmetric atmosphere is derived from the Held-Hou model. At high rotation rate, our numerical experiments inhabit a regime where global super-rotation scales geostrophically, and we suggest that the Earth and Mars occupy this regime. At low rotation rate, our experiments occupy a regime determined by angular momentum conservation, where global super-rotation is independent of rotation rate. Global super-rotation in our experiments saturates at a value significantly lower than that achieved in the atmospheres of Venus and Titan, which instead occupy a regime where global super-rotation scales cyclostrophically. This regime can only be accessed when eddy induced up-gradient angular momentum transport is sufficiently large, which is not the case in our idealised numerical experiments. We suggest that the 'default' regime for a slowly rotating planet is the angular momentum conserving regime, characterised by mild global (and local) superrotation.Comment: Submitted to Journal of the Atmospheric Sciences. Comments welcome. This manuscript has not yet been peer reviewe

Gyrokinetic turbulence and transport in the Mega Ampere Spherical Tokamak

In this thesis we consider energy transport due to turbulence in the Mega Ampere Spherical Tokamak (MAST), using a multi-scale framework based on local δf gyrokinetics. Transport is modelled globally by means of local flux-tube simulations at each magnetic flux surface. In view of the evidence indicating that turbulent ion transport can be substantially suppressed by rotation shear, our flux tubes span only electron-gyroradius scales. This both simplifies our physics model and reduces the computational resource requirements to a manageable level. Our electrostatic simulations of a MAST H-mode discharge exhibit turbulent electron heat transport comparable with experiment in the outer core region, and are broadly consistent with the paradigm of an electron temperature profile governed by pedestal height and critical electron temperature gradient. Neoclassical transport dominates the ions, and is also comparable with experiment. The two species are decoupled in the sense that the collisional equilibration between ions and electrons is small compared to the sources across most of the plasma. Focusing on a single flux surface in this region, still using both kinetic electrons and kinetic ions, we find that at early simulation times the heat flux "quasi-saturates" with the turbulence dominated by streamer-like radially elongated structures. However, the zonal fluctuation component continues to grow slowly until much later times, eventually leading to a new saturated state dominated by the zonal component. Simplifying further to an adiabatic ion model (which shows the same slow evolution behaviour), we find that in the final saturated state (which determines the macroscopic energy transport) the electron heat flux is approximately proportional to the collision rate. We outline an explanation of this effect based on zonalnonzonal interactions and a scaling of the zonal damping rate with electron-ion collisionality. Improved energy confinement with decreasing collisionality has previously been observed experimentally in STs, and is favourable towards the performance of future devices, which are expected to be hotter and thus less collisional.</p

Gyrokinetic turbulence and transport in the Mega Ampere Spherical Tokamak

In this thesis we consider energy transport due to turbulence in the Mega Ampere Spherical Tokamak (MAST), using a multi-scale framework based on local δf gyrokinetics. Transport is modelled globally by means of local flux-tube simulations at each magnetic flux surface. In view of the evidence indicating that turbulent ion transport can be substantially suppressed by rotation shear, our flux tubes span only electron-gyroradius scales. This both simplifies our physics model and reduces the computational resource requirements to a manageable level. Our electrostatic simulations of a MAST H-mode discharge exhibit turbulent electron heat transport comparable with experiment in the outer core region, and are broadly consistent with the paradigm of an electron temperature profile governed by pedestal height and critical electron temperature gradient. Neoclassical transport dominates the ions, and is also comparable with experiment. The two species are decoupled in the sense that the collisional equilibration between ions and electrons is small compared to the sources across most of the plasma. Focusing on a single flux surface in this region, still using both kinetic electrons and kinetic ions, we find that at early simulation times the heat flux "quasi-saturates" with the turbulence dominated by streamer-like radially elongated structures. However, the zonal fluctuation component continues to grow slowly until much later times, eventually leading to a new saturated state dominated by the zonal component. Simplifying further to an adiabatic ion model (which shows the same slow evolution behaviour), we find that in the final saturated state (which determines the macroscopic energy transport) the electron heat flux is approximately proportional to the collision rate. We outline an explanation of this effect based on zonalnonzonal interactions and a scaling of the zonal damping rate with electron-ion collisionality. Improved energy confinement with decreasing collisionality has previously been observed experimentally in STs, and is favourable towards the performance of future devices, which are expected to be hotter and thus less collisional