A Review of Observations of Molecular Ions in the Earth's Magnetosphere-Ionosphere System

Ionospheric molecular ions, such as NO+, N-2(+) and O-2(+), are gravitationally bound, and are expected to undergo recombination to form a pair of neutral atoms, due to short dissociative recombination lifetime. Therefore, they are expected to be relatively cold in the Earth's atmosphere, compared with light ions such as H+ and He+, or even heavier ions such as N+ or O+. However, several spacecraft missions observed their presence in the high-altitude ionosphere and the magnetosphere, predominantly during the geomagnetically active times. This hints to the possibility that molecular ions have the ability to acquire sufficient energy in a very short time, and can be used as tracers of mass differentiated vertical transport to understand the mechanisms responsible for "fast ionospheric outflow. " In this letter, we review the observational data sets that reported on the abundances of molecular ions in the Earth's magnetosphere-ionosphere system, starting from their first observations by the Sputnik III mission, to the current Arase (ERG) satellite and Enhanced Polar Outflow Probe (e-POP) missions. The available data suggests that molecular ions are quite abundant in the lower atmosphere at all times, but are only seen in the high-altitude ionosphere and magnetosphere during the times of increased geomagnetic activity.