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SHOCK ACCELERATION OF PARTICLES IN THE NONSTATIONARY EVOLUTION OF COROTATING INTERACTION REGIONS

One-dimensional hybrid simulations are used to investigate the particle energization process during the nonstationary evolution of corotating interaction regions (CIRs) in the heliosphere. The simulation model, where fast and slow solar wind streams interact with each other, allows the formation of a pair (forward/reverse) of shocks at the CIR boundaries and the stream interface interior, which prevents the interchange of both streams. While both shocks are quasi-perpendicular and are not capable of accelerating thermal particles (hundreds of eV) up to a suprathermal energy (tens to hundreds of keV) in the early phase of their development, the reverse shock in the fast wind experiences a transition to a quasi-parallel regime in the later phase. The quasi-parallel reverse shock can efficiently accelerate particles to the suprathermal range. The different timescale of the adiabatic expansion between the fast and slow wind leads to a transition of the shock geometry that can take place more easily in the reverse shock than in the forward shock, where the magnetic field in the fast wind remains more radial to the propagation direction than in the slow wind. The difference in the acceleration efficiency between these shocks follows a well-known observed asymmetry in the profile of the energetic particle fluxes, where the larger intensity increases more in the reverse shock than in the forward shock. The present results suggest that the solar wind thermal plasma, as well as interstellar pickup ions, can contribute to the composition of energetic particles associated with the CIRs.