Magnetic Local Time dependency of the modeling of the Earth radiation belts

Since the beginning of the Space Era with the launch of Spoutnik 1 spacecraft in 1957, the radiation belts have been studied by scientists due to their hazardousness on both spacecrafts and humans. Indeed, particles are injected during strong solar activity and can induce fluxes thousand times higher than quiet time ones. Thus, it's very important to understand the nature of physics-based processes and the consecutive dynamic during a geomagnetic storm. For this purpose, the Department Physics Instrumentation Environment and space (DPhIEE) at ONERA has been developing the Salammbo models family for more than 20 years which robustly reproduce in three dimensions the dynamics of trapped particles in the radiation belts. Nevertheless, although they are reliable at high energy, the physics-based processes and the hypothesis taken into account are inadequate at lower energies, below a few hundreds of keV. Indeed, the radiation belts can no longer be considered as homogeneous around the Earth. The aim of this PhD research work was to take into account a fourth coordinate, the Magnetic Local Time (MLT), in order to better reproduce the thinnest structures occurring during a geomagnetic storm. The first part of this work focused on the optimization of the numerical scheme. By the emergence of an advective term, the inclusion of a fourth coordinate induces a strong numerical diffusion that has to be controlled and limited without deteriorating the computing efficiency. So, the implemented statistical equation was discretized using a Beam-Warming scheme coupled with the Superbee limiter, which guarantee us an adequate propagation of the initial distribution. Once numerical issues have been resolved and controlled, all the different mechanisms driving the dynamics of the trapped particles have been implemented into the code, taking into account their MLT dependency, especially for the wave-particle interaction. Taking into account the magnetospheric electric fields was also necessary. Indeed, they correspond to one of the main drivers of the low energy particles motion. Then, the Salammbo 4D model has been validated by a comparison with the 3D one on a simulation of the March 2015 geomagnetic storm. The results showed a good restitution of the dynamics of the radiation belts, refining the storm main phase resolution. Thereafter, this event has been simulated at lower energy to analyze the asymmetry of the dynamics of trapped electrons highlighting the prime role of the convection electric field. The comparison with data from THEMIS spacecraft showed a good modeling of the different physics-based processes, in particular regarding dropouts as controlled by magnetopause shadowing effect. Finally, the implementation of a solar wind driven outer boundary condition opens up new prospects. (author)