Spatial structure of the cusp/cleft ion fountain: A case study using a magnetic conjugacy between Interball AP and a pair of SuperDARN radars

The spatial structure of low-energy ion outflows associated with heating processes in the dayside cusp/cleft region is investigated by using a magnetic conjugacy between Interball Auroral Probe (AP) and the Saskatoon-Kapuskasing pair of the Super Dual Auroral Radar Network (SuperDARN). The interplanetary magnetic field during this event is characterized by B-y < B-z < 0 components, which breaks the symmetry of morning and afternoon convection cells relative to the noon meridian. As a result, plasma convection over the afternoon polar cap is not antisunward, but almost azimuthally oriented. The three-dimensional thermal ion distributions measured by the Hyperboloid experiment on board Interball AP are used as input of numerical simulations in order to investigate the spatial structure of the ion heating processes. The simulations include the complete guiding center motion of ions under the effect of gravity, geomagnetic field, and convection field measured by SuperDARN radars. In contrast to the classical cleft ion fountain picture, we demonstrate that the observed ions originate from a wide latitudinal interval and that the heating region likely coincides with the polar cusp. The numerical simulations allow us to reconstruct the downstream (relative to convection) boundary of the heating region, as well as ion distributions along this boundary. Ions are found to cross this boundary and to exit the heating region over a broad range of altitudes (up to at least 15,000 km) with an average altitude increasing with ion mass, their average pitch angle increasing with altitude in agreement with altitude cumulative heating scenarios.