# Interaction des nuages magnétiques éjectés par le Soleil avec l'environnement terrestre

Abstract : Magnetic clouds are huge structures released from the Sun through violent eruptions, which then propagate into the solar system at supersonic speeds. They are characterised in the solar wind by an enhanced and smoothly-rotating magnetic field. They cause large disturbances in the Earth's environment which sometimes have an impact on human activity in space (telecommunications, GPS, …) and on the ground (electrical networks,...). When magnetic clouds arrive in the vicinity of Earth, they first encounter the bow shock. In this thesis, we focus on the alteration of the magnetic structure of the clouds at the bow shock's crossing and during their propagation in the downstream region. Any significant modification would indeed have important implications on the prediction of geomagnetic disturbances. Three different approaches complementing one another are employed to address this issue: we first analyse data from different spacecraft orbiting Earth, in particular from ESA's Cluster mission, then we develop a model describing as a whole the bow shock's crossing and the propagation of the magnetic cloud in the downstream region, and finally we use numerical simulations to study in more detail some aspects of the physics of this interaction. The results obtained with these three methods consistently show that the variation of the cloud's structure across the bow shock is strongly related to the magnetic configuration of the magnetic cloud relative to the shock. This can be quantified by the value of $\Theta_{\mathrm{Bn}}$, the angle between the normal to the shock's surface and the magnetic cloud's magnetic field in the solar wind. We show that a quasi-perpendicular configuration ($\Theta_{\mathrm{Bn}} \sim 90^{\circ}$) keeps the cloud's magnetic structure roughly unchanged. When the configuration becomes more oblique, a rotation of the magnetic cloud's structure is observed in some parts of the downstream region. In a quasi-parallel geometry, the magnetic cloud's structure is strongly altered. Its magnetic field direction can then reverse and a high level of turbulence is observed downstream of the shock. Using the model we developed, we estimate the location of the regions favourable to reconnection processes, which give rise to disturbances in the Earth's environment. The numerical simulations allow us to investigate the turbulent regions downstream and also upstream of the bow shock. Finally, we find that, owing to the modification of their magnetic structure across the bow shock, the impact of certain magnetic clouds on the Earth's environment can differ from that estimated from their characteristics in the solar wind.
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https://tel.archives-ouvertes.fr/tel-01112997
Contributor : Lucile Turc <>
Submitted on : Wednesday, February 4, 2015 - 10:12:46 AM
Last modification on : Monday, December 14, 2020 - 9:47:49 AM
Long-term archiving on: : Sunday, April 16, 2017 - 8:18:45 AM

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• HAL Id : tel-01112997, version 1

### Citation

Lucile Turc. Interaction des nuages magnétiques éjectés par le Soleil avec l'environnement terrestre. Planète et Univers [physics]. Ecole Polytechnique, 2014. Français. ⟨tel-01112997⟩

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