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Temporal variability of the Earth's magnetic field and its influence on the near-Earth space environment

Abstract : The Earth's magnetic field undergoes strong temporal variabilities with characteristic periods as short as ten seconds (magnetospheric substorms triggering the polar aurora) and as long as a million years (geomagnetic reversals). Its temporal variations, although of very different origin and characteristics, affect the dynamics of the near-Earth space environment.The first part of this thesis is dedicated to the development of a new kinetic theory of instabilities in the magnetospheric tail which could explain the origin of substorms. Starting from a known theory of drift instabilities linked to the presence of a pressure gradient in the magnetotail, the proposed model includes trapped bouncing electrons which can enter into resonance with drift Alfvén instability modes if the density gradient in the tail becomes large. Taking this the bouncing motion into account significantly increases the growth rate of this universal instability. To try to validate this new model, an example of an auroral observation by the THEMIS mission (February 3, 2008) was analyzed. This event was chosen because it corresponds to an isolated auroral arc observed both by the All-sky cameras located on the ground and by the THEMIS satellites orbiting at 10 RE. This auroral activation seems to have been triggered by a sudden compression of the magnetospheric tail towards 10 RE significantly increasing the pressure gradient and causing significant fluctuations in the magnetic field. The orders of magnitude of the period and the growth rate of these oscillations are compatible with the dispersion curves deduced from the theoretical model.Second part of the thesis is devoted to changes in the radiation situation on Earth, the radiation belts and the terrestrial atmosphere during Earth's magnetic field reversal. We calculated the variations in galactic cosmic proton flux during a geomagnetic reversal to infer the radiation doses to which human population and astronauts could be exposed. The radiation background should increase by a factor of about three during the solar minimum period, and the elevated radiation regions should be redistributed and their areas will apparently increase due to the dipole field decrease, such radiation doses are not dangerous for humans and other living creatures. At the same time, for astronauts aboard the ISS orbiting at 400 km above the ground, during a reversal period a 14-fold radiation increase can be dangerous. Undoubtedly, in this case, a correction of the orbits of space vehicles would be required. Classical Störmer theory was generalized to the case of an axisymmetric superposition of dipole and quadrupole fields. [...]
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Olga Tsareva. Temporal variability of the Earth's magnetic field and its influence on the near-Earth space environment. Astrophysics [astro-ph]. Université Paul Sabatier - Toulouse III; Académie des sciences de Russie, 2020. English. ⟨NNT : 2020TOU30122⟩. ⟨tel-03184813⟩

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