Statistical properties of molecular ions in the ring current and their possible supply mechanisms from the ionosphere: Arase and EISCAT radar observations

It is observationally known that the terrestrial heavy ion contribution to the magnetospheric plasma increases with increasing geomagnetic activities, while the mechanisms of the enhanced ionospheric supply are far from understood. O+ ions are the main species of the terrestrial heavy ions. The heavier molecular ions such as NO+ and O2+ have been also observed in the various regions of the magnetosphere during geomagnetically active periods [e.g., Klecker et al, 1986; Peterson et al., 1994; Christon et al, 1994]. In order to get the molecular ion outflows from the deep ionosphere, they need to be transported upward within a short time scale ( order of minutes) to overcome the dissociative recombination lifetime at the source altitudes. The observations of the high-energy ( 100keV) molecular ions in the ring current and outer magnetosphere suggest an effective acceleration mechanism is in operation during geomagnetically active periods.

In this paper, we report on statistical properties of molecular ions in the ring current observed by the Arase satellite and their relations to the solar wind and geomagnetic conditions. The ion composition data of the Arase satellite, which detects the ions less than 180 keV/q, were analyzed in details. The investigated period from late March to December 2017 includes 13 geomagnetic storms with peak Dst less than 40 nT. The molecular ions are observed in the region of L=2.5-6.6 and clearly identified at energies above 12 keV during geomagnetically active periods. During quiet times, molecular ions are not observed. The typical O₂⁺ /O⁺energy density ratio is a few percent.

We also analyzed simultaneous observation data of EISCAT radar during the September 7, 2017 storm to investigate the outflow mechanisms of the molecular ions. The results suggest that the rapid upward transport in the deep ionosphere due to the enhanced ion heating and resultant pressure gradient is the promising candidate to cause the molecular ion outflows.

References:

Klecker et al., Geophys. Res. Lett., 13, 632-635, 1986.

Peterson et al., J. Geophys. Res., 99, 23257-23274, 1994.

Christon et al., Geophys. Re. Lett., 21, 3023-3026, 1994.