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BIMF & Radiations studies

Effects of the southward turning of the Interplanetary Magnetic Field (IMF) on the injection of Energetic Interplanetary Particles into the magnetosphere, as well as on the mirror point altitudes and precititation rate of trapped Van Allen belt particles into the Earth atmosphere are modelled.

The entry of Cosmic Rays (CR) and Energetic Interplanetary Particles (EIP) into the closed allowed zone of Störmer is controlled by the orientation and by the modulus of the Interplanetary Magnetic Field. When the IMF turns south the equatorial width of Störmer's forbidden zone is reduced for EIP of a given kinetic energy. The inner and outer allowed zones may become interconnected when Bz is sufficiently negative. The energy threshold for which EIP can penetrate in the inner magnetosphere is then diminished, and the equatorial distance of the geomagnetic cut-off surface shifts away from Earth. When the southward component of the IMF exceeds some critical value (Bz < - 15 nT), EIP of all energies can freely invade the cusps regions (Lemaire, 2003).

During the southward turning of the IMF, which generally corresponds to the initial phase of a geomagnetic storm, the geomagnetic cut-off surface shifts toward higher altitudes, as well as the mirror points altitudes of all trapped RB particles. This lessens their collisional loss-rate in the atmosphere, and therefore favors the refilling of the slot region and reduces the precipitation rate of trapped Van Allen particles.

Van Allen belt particles of all energies can be energized by the betatron acceleration mechanism induced by the decrease of the IMF Bz component. This betatron effect is also associated with the enhancement of the Ring Current magnetic field component wich affects also the energisation processes taking place in the outer and inner regions of the magnetosphere, during southward turnings of the IMF.

A dynamical model for the injection of EIP into the magnetsophere as well as plasmasheet particles into the Ring Current, their acceleration by the betatron mechanism, scattering by VLF waves and precipitation in the magnetosphere will be develloped and the role of the southward component of the IMF and of Ring Current Bz magnetic field will be examined and modelled in this theoretical study.

New classes of periodic trajectories of charged particles in a dipole magnetic field have been identified. After 1, 2, 3 or more bounce periods the meridian projection of their trajectory returns to the same points (i.e. the same mirror point altitudes, latitudes, and the same points of crossing of the equatorial plane in the meridian plane drifting around the Earth with the longitudinal velocity of the particle)

A mathematical method has been found to compute the pitch angle (a) of a particle (in the drifting Guiding Center frame of reference) once its velocity (modulus and orientation) and pseudo-pitch-angle (b) are determined (given or measured) in fixed inertial frame of reference. An algorithm has been developed at the CSR to calculate a once b is known. Another inverse algorithm has been developed to calculate b once a is determined.

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