The medium-energy particle ion mass analyzer (MEPi) onboard the Japanese Arase satellite is a high-sensitivity time-of-flight (TOF) ion mass spectrometer capable of detecting both major and minor species of energetic ions in the ~10 to 100 kilo-electron-volt (keV) energy range in the Earth’s inner magnetosphere.
The observed ion TOF spectra from MEPi presented by Seki et al.  reveals the frequent presence of energetic molecular (N2+, NO+ and/or O2+) ions in the ring current, not only during times of extreme geomagnetic activity, which is expected based on previous episodic observations, but also at times of moderate activities, which is rather surprising.
These observed molecular ions are believed to originate from the Earth’s ionosphere, where they are expected to undergo recombination to form a pair of neutral atoms within minutes of their creation. Therefore, their detection in the ring current provides a time tag for the maximum time available—and correspondingly minimum ion acceleration rate required—to extract a heavy ion from the ionosphere and energize it to ring current energies.
Coupled with our previous knowledge of thermal ion upwelling from Dynamic Explorer 1 (DE-1), Akebono, Polar and other satellites, the Arase results provide an important metric with which to evaluate large scale models of the Earth’s magnetosphere.
Citation: Seki, K., Keika, K., Kasahara, S., Yokota, S., Hori, T., Asamura, K., et al. . Statistical properties of molecular ions in the ring current observed by the Arase (ERG) satellite. Geophysical Research Letters, 46, 8643– 8651. https://doi.org/10.1029/2019GL084163
—Andrew Yau, Editor, Geophysical Research Letters