Observation of the 40 keV field‐aligned ion beams

Recent observations by CEPPAD/IPS from the new perspective of the POLAR orbit reveal a close association of field‐aligned ionospheric beams with convecting plasmasheet “nose” ions. April 15, 1996 was marked by a large southward B Z accompanying a fast solar wind shock. This event triggered a strong cross‐tail convection electric field that pushed plasmasheet ions deep into the magnetosphere. When the POLAR spacecraft passed through the inner magnetosphere, it observed both an unusually energetic plasmasheet nose ion injection at ∼90 keV, as well as a peculiar field‐aligned beam at ∼40 keV. These beams appear to be enriched in oxygen ions (as inferred from other instruments), which would place their origin in the ionosphere. Both populations existed from L =7–3, for a duration of at least 2 hours. We speculate that the nose ions create a parallel electric field that is responsible for the extended ionospheric beam signature.