Staircase ion signature in the polar cusp: A case study

On 15 October 1981 Dynamics Explorer 2 crossed the polar cusp at 1015 MLT and observed three distinct ion populations as it was moving poleward. These three populations had peak‐flux energy around 2.7 keV, 850 eV and 360 eV. At the time of observation, the IMF was southward. The first step coincided with a rotation of the flow from westward to poleward and then eastward. The second and third steps showed a flow directed principally poleward. Furthermore, the magnetic and electric perturbations in the first step are well fitted by an elongated flux tube footprint model. These results suggest that three consecutive Flux Transfer Events (FTEs) have injected solar wind plasma into the ionosphere forming the polar cusp. The individual FTE signatures can only be identified by the jumps in the precipitation pattern. The newest reconnected FTE footprint was crossed near the edge of the event while the two oldest ones were crossed around the event center. The small latitudinal size of these FTE footprints (∼ 40 km) and their short recurrence rate (3, 6 min) is consistent with an intermittent reconnection taking place at the subsolar point on a short time scale.