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- Jackson, Illiam, 1986-
(författare)
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Morphometric analysis of Cambrian fossils and its evolutionary significance
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Extended Evolutionary Synthesis (EES) is currently emerging as a theoretical alternative to the Modern Synthesis (MS) in which to frame evolutionary observations and interpretations. These alternative frameworks differ fundamentally in their understanding of the relative roles of the genotype, phenotype, development and environment in evolutionary processes and patterns. While the MS represents a gene-centred view of evolution, the EES instead emphasizes the interactions between organism, development and environment. This novel theoretical framework has generated a number of evolutionary predictions that are mutually incompatible with the equivalent of the MS. While research and empirical testing has begun on a number of these in a neontological context, the field of palaeontology has yet to contribute meaningfully to this endeavour. One of the reasons for this is a lack of methodological approaches capable of investigating relevant evolutionary patterns in the fossil record. In this thesis morphometric methods capable of providing relevant data are developed and employed in the analysis of Cambrian fossils. Results of these analyses provide empirical support for the process of evolution through phenotypic plasticity and genetic assimilation hypothesized by the EES. Furthermore, theoretical revision to the species concept in a palaeontological context is suggested. Finally, predictions of the EES specific to the fossil record are made explicit and promising directions of future research are outlined.
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| 2. |
- Richard, Louis
(författare)
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Energy Conversion and Particle Acceleration at Turbulent Plasma Jet Fronts
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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