Recirculation of plasma sheet particles into the high‐latitude boundary layer

We examine the transport of plasma sheet particles in the dayside magnetosphere using three‐dimensional single particle codes. In this region of space, whereas the equatorial plane is the locus of the field minimum at low L shells (typically, below L = 8), this field minimum is found at midlatitudes at high L shells. We show that this topological change which follows from the current system at the magnetopause, has important implications for the overall circulation of magnetospheric plasma. It is responsible for de‐trapping of plasma sheet particles originally mirroring at low latitudes. At some point during E × B transport toward the magnetopause, these particles escape from the equatorial vicinity and flow into the cusp region. Because these particles experience different energy changes in the low and high L shell field configurations, significant phasing effects are obtained between bounce and convective motions with possible violation of the second adiabatic invariant. We show that in the region adjacent to the frontside magnetopause, these phasing effects lead to characteristic energy‐latitude spectrograms with repeated streaks of enhanced flux. Most notably, we demonstrate that after their injection into the cusp, the de‐trapped particles can flow back toward the tail along the magnetopause with energies comparable to those inside the plasma sheet. Numerical calculations indicate that recirculation of these plasma sheet particles can significantly contribute to populating of the high‐latitude boundary layer, especially at high energies (a few keV up to a few tens of keV).