Fluid closures for the modelling of reconnection and instabilities in magnetotail current sheets
2019
The integration of kinetic effects in fluid models is important for global simulations
of the Earth’s magnetosphere. Although magnetohydrodynamics (MHD), which is
currently used for large scale modelling in space weather research, is a useful tool,
there are other physical effects not included in the MHD model which are important
at smaller scales such as the Hall effect, the pressure tensor and electron inertia.
In this thesis we use a two-fluid ten-moment model, which includes the pressure
tensor of both species, to study some of the phenomena associated with magnetotail
dynamics – magnetic reconnection, current sheet instabilities and ballooning instabilities,
followed by global simulations in which these processes interact. We first
introduce an extension of existing ten-moment models by using a nonlocal heat flux,
which approximates Landau damping in the fluid framework, followed by a study of
magnetic reconnection during the merging of two flux tubes or magnetic islands. We
then perform linear calculations and simulations of the drift-kink and lower hybrid
drift instabilities in thin current sheets, and simulations of the ballooning instability
in current sheets with curved mangetic geometries. Comparisons to kinetic simulations
show the improvements compared to MHD and standard two-fluid models as
well as the limitations of the ten-moment model. We then perform simulations which
include a dipole field and self-consistent formation of the magnetotail current sheet
and show that the stretching and disruption of the sheet, which is expected to occur
during magnetic substorms, can be captured by the model.
Finally, in an exploration of how the moment equations can be extended and what
additional insight is gained from higher moment models, we use the maximum entropy
method to reconstruct particle distributions in reconnection regions. The results
show that without information other than the moments, the model can reproduce the
general structure of the distributions but not all of the finer details.
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