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- Richard, Louis
(author)
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Energy Conversion and Particle Acceleration at Turbulent Plasma Jet Fronts
- 2023
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Doctoral thesis (other academic/artistic)abstract
- High-speed plasma flows (jets) are ubiquitous phenomena in the visible Universe. When the fast plasma flow encounters the ambient plasma at rest, it forms a front where its kinetic energy is dissipated. At the jet front, charged particles gain energy from the electromagnetic fields through heating and acceleration. Plasma jets carry energy away from the most powerful sources in the visible Universe (e.g., active galactic nuclei) and transfer it to the surrounding medium. High-speed plasma flows are also common in planetary magnetospheres, including the Earth’s magnetotail. In the Earth’s magnetotail, plasma jets, called bursty bulk flows, are crucial in transporting energy to the inner magnetosphere in the (sub-)storm cycle. However, the physical mechanisms through which the jet deposits its energy into the plasma are yet to be understood. This thesis focuses on plasma jets produced by magnetic reconnection in the Earth’s magnetotail. The magnetotail is a natural laboratory to probe the plasma at the kinetic scales (10-100 km). This allows us to address some of the open questions related to plasma jet fronts and the associated energy conversion and particle acceleration. We use the four Magne-tospheric Multiscale spacecraft launched in 2015. In paper I, we focus on the global effects of the plasma jets on the Earth’s magnetotail. In the wake of a plasma jet, we show that the Earth’s magnetotail current sheet undergoes a kink-like flapping motion transporting energy across the magnetotail. In paper II, we study the ion acceleration mechanisms associated with the jet. We identify three active mechanisms depending on the relative ion energy compared with the jet size. In paper III, we challenge the picture of the jet front as a sharp two-dimensional boundary. We show that the jet front is often strongly perturbed, contrary to the commonly accepted pic-ture. In paper IV, we investigate the ion dynamics in the magnetic reconnection jets. We show that the thermal ions are rapidly scattered by the strongly curved magnetic field in the magne-totail current sheet. Finally, in paper V, we focus on the turbulence in the plasma jets. We show that the turbulence substantially contributes to the magnetic reconnection energy transfer.
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2. |
- Lalti, Ahmad
(author)
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Electrostatic turbulence and electron heating in collisionless shocks
- 2024
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Doctoral thesis (other academic/artistic)abstract
- When the supersonic solar wind interacts with Earth’s magnetosphere it forms a shock wave. However, due to the low densities in space, inter-particle collisions play an insignificant role in its dynamics. Earth's bow shock is an example of a collisionless shock, ubiquitous throughout the universe. Their dynamics are complex and their physics remains an active field of research. In this thesis, we use high-resolution measurements from NASA's Magnetospheric Multiscale (MMS) spacecraft to study the plasma wave activity across Earth’s bow shock and its effects on electron heating. In Paper I we train a convolutional neural network (CNN) to identify the different plasma regions that MMS crosses. In Paper II we use the results of this CNN to compile a database of time intervals in which MMS crosses Earth’s bow shock, which we use to find suitable events to tackle the science questions of interest. In Paper III we use multispacecraft methods to properly characterize obliquely propagating whistler waves running upstream of the shock. By analyzing the ion and electron distribution functions we find that their likely source is the instability between the incoming electrons and reflected ions. Shifting our focus to Debye scale electrostatic waves, in Paper IV we develop a method to measure their 3D wave vector based on single-spacecraft interferometry. We are in the process of using this method to study the evolution of Debye scale electrostatic waves across quasi-perpendicular shocks (see Chapter 7). Finally, in Paper V we investigate the electron heating mechanism across quasi-perpendicular shocks. We find the heating mechanism to depend on the Alfvénic Mach number in the deHoffman-Teller frame . We also find that at high the heating mechanism is consistent with the stochastic shock drift acceleration mechanism.
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3. |
- Richard, Louis
(author)
-
Energy Conversion and Particle Acceleration at Turbulent Plasma Jet Fronts
- 2022
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Licentiate thesis (other academic/artistic)abstract
- 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.
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