Transport of Thermal-energy Ionospheric Oxygen (O+) Ions between the Ionosphere and the Inner Magnetosphere at Quiet Times Preceding Magnetic Storms

The presence of energetic O+ ions in the ring current at the onset of a magnetic storm prompts the question of the possible role of "in-transit" ionospheric O+ ions between the ionosphere and the inner magnetosphere (plasma sheet and ring current) in the quiet periods immediately preceding the main phase of a magnetic storm. Thermal-energy O+ ions are often observed in the quiet-time high-altitude (>7000 km) polar ionosphere on Akebono, at temperatures of ∼0.2-0.3 eV and flow velocities of a few km/s. In this paper, we use single-particle trajectory simulation to study the transport of these ions in the periods preceding a number of large magnetic storms (Dst < -100 nT). Our simulation shows that due to centrifugal ion acceleration at higher altitudes (above ∼3 RE altitude), about 10-20% of polar wind and other low-energy O+ ions reaches the plasma sheet during such periods; the actual percentage is a factor of ∼3 larger in the dusk sector on average compared with the dawn sector and dependent on the IMF and the O+ ion temperature. This provides a low but non-negligible flux of O+ ions between the ionosphere and the inner magnetosphere, which is believed to constitute a significant "in-transit" oxygen ion population over a period of a few (∼4) hours preceding a magnetic storm. Such a population could explain the presence of energetic O+ ions at the onset of the main phase of the storm, when the heavy ions could potentially modify the evolution of the ring current.