Cusp energetic oxygen ions of ionospheric origin Original Research Article

Energetic oxygen ions (∼ 70–200 keV/e) of ionospheric origin were measured by the CAMMICE experiment on board the POLAR spacecraft in the high-altitude dayside cusp. The observed cusp energetic ions were associated with a dramatic decrease and large fluctuations in the local magnetic field strength. Just like in the equatorial ring current, the measured magnetic moment spectrum of the energetic O≤ + 2 ions in the cusp follows a power law. Its spectral amplitude in the cusp was much larger than that in the equatorial ring current, indicating a higher low charge state oxygen ion flux (or phase space density) in the cusp than in the equatorial ring current at a given magnetic moment. If ring current leakage was the source of the cusp energetic O≤ + 2 ions, one would expect a spectral amplitude in the equatorial ring current larger than or equal to that in the cusp. The measured energetic oxygen ions in the cusp cannot be explained by a ring current source. To interpret these observations, one needs a new energetic ion source.