Statistical Properties of Molecular Ions in the Ring Current Observed by the Arase (ERG) Satellite.

Molecular ions in the magnetosphere can be a tracer of fast ion outflows from the deep ionosphere. Statistical properties of molecular ions (O2+/NO+/N2+) in the ring current are investigated based on ion composition measurements (<180 keV/q) by medium‐energy particle experiments‐electron analyzer and low‐energy particle experiments‐ion mass analyzer instruments on board the Arase (Exploration of energization and Radiation in Geospace, ERG) satellite. The investigated period from late March to December 2017 includes 11 geomagnetic storms with the minimum Dst index 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 most magnetic storms. During quiet times, molecular ions are not observed. The average energy density ratio of the molecular ions to O+ is ~3%. The ratio tends to increase with the size of magnetic storms. Existence of molecular ions even during small magnetic storms suggests that the fast ion outflow from the deep ionosphere occurs frequently during geomagnetically active periods. Plain Language Summary: Molecular ions usually exist only in the low‐altitude (< 300 km) deep ionosphere and cannot escape to space without a fast ion outflow to overcome a rapid loss due to dissociative recombination. Thus, molecular ion escape from the terrestrial atmosphere to space can be used as a tracer of effective ion loss from the deep ionosphere. Here we report new observations by Arase satellite that enables definitive identification of molecular ions by frequent time‐of‐flight mode observations. The statistical analysis shows that molecular ions exist in near‐Earth space during most magnetic storms, while they are not detected during geomagnetically quiet periods. The existence of molecular ions even during small magnetic storms suggests that the magnetic storm is an effective driver of the ion loss from the deep terrestrial ionosphere. Key Points: Frequent TOF‐mode ion composition measurements by Arase revealed statistical properties of molecular ions in the ring currentThe molecular (O2+/NO+/N2+) ions are commonly observed during geomagnetically active periods even during small magnetic stormsThe results suggest that the rapid ion outflow from the deep ionosphere occurs frequently during active periods [ABSTRACT FROM AUTHOR]