Source distributions of substorm ions observed in the near-Earth magnetotail

This study employs Geotail plasma observations and numerical modeling to de termine sources of the ions observed in the near-Earth magnetotail near mid night during a substorm. The growth phase has the low-latitude boundary lay er as its most important source of ions at Geotail, but during the expansio n phase the plasma mantle is dominant. The mantle distribution shows eviden ce of two distinct entry mechanisms: entry through a high-latitude reconnec tion region resulting in an accelerated component, and entry through open f ield lines traditionally identified with the mantle source. The two entry m echanisms are separated in time, with the high-latitude reconnection region disappearing prior to substorm onset.