Anisotropy of solar wind turbulence

Abstract

As well as being of fundamental interest, understanding plasma turbulence is important for many areas of astrophysics and space physics that remain to be fully understood, such as accretion disk dynamics, the origin of cosmic rays and coronal heating. The anisotropy with respect to the magnetic fi eld is central to understanding plasma turbulence, but this has only recently started to be measured in detail. The solar wind provides a unique opportunity to study this anisotropy due to the range of high precision in situ measurements available. In this thesis, the anisotropy of solar wind turbulence is measured using data from the multi-spacecraft Cluster mission. At all scales measured, the fluctuations are found to be spatially anisotropic: elongated along the direction of the magnetic field. The scaling of the turbulence is also anisotropic, with a steeper spectral index in the direction parallel to the local magnetic fi eld. This is consistent with the fluctuations being in critical balance: having approximately equal linear wave timescales and nonlinear eddy decay timescales. At large scales, the anisotropy of the density and parallel magnetic fi eld fluctuations follows that of the perpendicular Alfvénic turbulence, in agreement with passive scalar theory. One puzzling result, however, is the scaling of the parallel magnetic field at small scales, which does not follow theoretical expectations. For the fi rst time, the technique used to measure the anisotropy of solar wind turbulence is applied to turbulence in reduced magnetohydrodynamic simulations. Again, the anisotropic scaling is seen, which is in agreement with critical balance predictions. It is also shown that when measuring the anisotropy with respect to the global, rather than local magnetic field, the anisotropic scaling cannot always be properly measured, which explains the previous apparently contradictory measurements in the literature.EThOS - Electronic Theses Online ServiceSTFC (plus travel to conferences funded by many other bodies)GBUnited Kingdo