Observations of very-low-energy (<10 eV) ion outflows dominated by O⁺ ions in the region of enhanced electron density in the polar cap magnetosphere during geomagnetic storms

Velocity distributions of upflowing ions in the polar ionosphere are crucial to understand their destinations. Natural plasma wave observations by the plasma wave and sounder experiments and thermal ion observations by the suprathermal ion mass spectrometer onboard the Akebono satellite at ∼9000 km altitude in the polar magnetosphere during the geomagnetic storms showed that ions in the region of enhanced electron density in the polar cap were dominated by very-low-energy O⁺ ions (∼85%) with upward velocities of 4-10 km s^-1, corresponding to streaming energies of 1.3-8.4 eV. The fluxes of very-low-energy upflowing O⁺ ions exceeded 1 × 10 ⁹ cm^-2 s^-1 (mapped to 1000 km altitude) across wide regions. These signatures are consistent with high-density plasma supplied by the cleft ion fountain mechanism. Trajectory calculations of O⁺ ions based on the Akebono observations as the initial condition showed the transport paths and accelerations of the O⁺ ions and indicated that the velocities of the very-low-energy upflowing O⁺ ions through the dayside polar cap are enough to reach the magnetosphere under strong convection. The calculations suggest the importance of the very-low-energy upflowing O ⁺ ions with large fluxes in the total O⁺ ion supply toward the magnetosphere, especially the near-Earth tail region and inner magnetosphere. The initially very-low-energy O⁺ ions can contribute significantly to the ring current formation during geomagnetic storms since some of the O⁺ ions were transported into the ring current region with typical energies of ring current ions (several tens of keV) in the trajectory calculations.