Particle Acceleration in the Magnetotail and Aurora

This paper deals with acceleration processes in the magnetotail and the processes that enhance particle precipitation from the tail into the ionosphere through electric fields in the auroral acceleration region, generating or intensifying discrete auroral arcs. Particle acceleration in the magnetotail is closely related to substorms and the occurrence, and consequences, of magnetic reconnection. We discuss major advances in the understanding of relevant acceleration processes on the basis of simple analytical models, magnetohydrodynamic and test particle simulations, as well as full electromagnetic particle-in-cell simulations. The auroral acceleration mechanisms are not fully understood, although several, sometimes competing, theories and models received experimental support during the last decades. We review recent advances that emphasize the role of parallel electric fields produced by quasi-stationary or Alfvénic processes.     

Laboratory and Numerical Simulations of the Impulsive Penetration Mechanism

Plasma interaction at the interface between the magnetosheath and magnetosphere has been extensively studied during recent years. As a consequence various theoretical models have emerged. The impulsive penetration mechanism initially proposed by Lemaire and Roth as an alternative approach to the steady state reconnection, is a non-stationary model describing the processes which take place when a 3-D solar wind plasma irregularity interacts with the outer regions of the Earth's magnetosphere. In this paper we are reviewing the main features of the impulsive penetration mechanism and the role of the electric field in driving impulsive events. An alternative point of view and the controversy it has raised are discussed. We also review the numerical codes developed to simulate the impulsive transport of plasma across the magnetopause. They have illustrated the relationship between the magnetic field distribution and the convection of solar-wind plasma inside the magnetosphere and brought into perspective non-stationary phenomena (like instabilities and waves) which were not explicitly integrated in the early models of impulsive penetration. Numerical simulations devoted to these processes cover a broad range of approximations, from ideal MHD to hybrid and kinetic codes. The results show the limitation of these theories in describing the full range of phenomena observed at the magnetopause and magnetospheric boundary layers.     

Test-Particle Trajectories in a “Sheared” Stationary Field: Newton–Lorentz and First Order Drift Numerical Simulations1

We study a magnetic field distribution that is nonuniform and sheared like in tangential discontinuities. This distribution is an input parameter for the numerical integration of the equations of motion of the test-particle and of its guiding center. Two different electric field distributions are alternatively tested. In the first case, the electric field is uniform and constant like the electric field prescribed in the large-scale, steady-state reconnection models. The numerical solution shows that in this case the test-particle is trapped within the discontinuity into a region where (i) B goes to zero or (ii) the magnetic vector becomes exactly parallel to the electric field. In the second case, we consider an electric field, which is nonuniform. Its components are computed such that the zero order (or electric) drift is everywhere perpendicular to the discontinuity surface and its value is conserved throughout the simulation. In this case the numerically integrated trajectory of the test-particle penetrates the discontinuity for any angle of shear of B. Direct comparison between exact (Newton–Lorentz) and approximated (first order drift) numerical solutions shows that the mathematical singularities of the latter do not correspond to any physical singularity of the exact equation of motion of the particle.