Particle Acceleration in Cusp-like Magnetic Trapping Regions

Acceleration of charged particles is a fundamental problem of space plasma systems. Recent test particle computations in MHD magnetic and electric fields have shown significant energization of particles in magnetic trapping regions such as a cusp-like magnetic configuration. Necessary conditions for the particle acceleration are a highly efficient trapping and the presence of an electric field. In such a configuration particle trajectories are mostly a combination of gradient curvature and ExB drift. If the gradient curvature drift has a component along the electric field particles can be energized and an efficient trapping can yield large energy gain. Here we will discuss basic properties of this process, present several examples of different magnetic field geometries, basic scaling laws, current limitations of the model. The results indicate that the process is relevant to regions close to the magnetic separatrix surfaces and magnetic nullpoints where reconnection is operating such as the geomagnetic cusps. However, it is also noted that while trapping and the presence of convection electric field are necessary conditions they are not sufficient conditions for the particle acceleration.