Energy Conversion and Particle Acceleration at Turbulent Plasma Jet Fronts

Sammanfattning: High speed plasma flows (jets) are ubiquitous phenomena in the universe. For example, they carry energy from the most powerful sources (e.g active galactic nucleï AGN) to the medium at rest surrounding them. When the plasma at rest encounters the front of the fast flow, it gains energy via conversion from magnetic field energy to particle heating and acceleration. High speed plasma flows are also common in planetary magnetospheres including the Earth's magnetotail. In particular the fast Earthward magnetic reconnection outflows in the Earth's magnetotail provide a laboratory to address some of the open questions related to plasma jet fronts and the associated energy conversion. In this thesis, we use the four Magnetospheric Multiscale (MMS) spacecraft to investigate current sheet flapping, particle heating and particle acceleration associated with the fast magnetotail flows.In paper I, we investigate a short-period kink-like flapping motion of an ion scale current sheet in the wake of a plasma jet front. We show that the kink-like motion propagates along the current direction toward the flank of the magnetosphere, and that the prediction of the wavelength of the drift-kink instability of a thin current sheet agrees with our estimation of the wavelength of the observed kink-like current sheet.In paper II, we investigate particle acceleration at turbulent Earthward jet fronts during a moderately active substorm. We show that a proton with a gyroradius smaller than the scale of the Earthward convected structures gain energy from the bulk flow. On the other hand, we show that, depending on the time scale of the electromagnetic fluctuations with respect to the proton scale, protons with larger gyroradius get accelerated via resonant interaction with the jet front or via direct acceleration by the dawn-dusk electric field in a spatially limited electric field pulse.