Populating of the cusp and boundary layers by energetic (hundreds of keV) equatorial particles

We examine the dynamics of charged particles in the frontside magnetosphere. We show that the existence of a magnetic field minimum in the outer cusp region has significant implications for the large-scale transport of energetic (a few hundreds of keV) particles that are trapped near the equator. Upon approach (within a few Earth radii) of the magnetopause in the dayside sector, these particles are subjected to a mirror force pointing away from the equator and may escape toward high latitudes. We demonstrate that via this process, energetic electrons and ions from the outer radiation belts and ring current may leak into the outer cusp and be scattered back into the nightside plasma sheet. During transport, the particle behavior critically depends upon gradient drift timescale as compared with convection timescale. We show that by diverting energetic equatorial particles toward the frontside magnetopause, dynamical reconfiguration of the magnetospheric field lines during substorms may favor such injections toward high latitudes and loading of the outer cusp. At a given energy, equatorial particles with higher charge state (i.e., larger drift timescale) reside longer in the dayside magnetosphere and are thus more susceptible to transport toward the field minimum.