Abstract
Ionospheric molecular ions, such as NO+, N2+ and O2+, 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.
Highlights
Charged heavy ions observed in the magnetosphere, such as atomic N+ and O+, and molecular N+2, NO+, and O+2 ions, are sourced from the Earth’s ionosphere, and transported outward through the process of ionospheric escape
The abundance of molecular ions was correlated with the density ratio of N+/O+ (Yau et al, 1993), that is, the maximum flux of molecular ions was accompanied by the unity ratio of N+/O+, and the molecular ions flux could be at most 15% of the total ionospheric outflow fluxes during storm time
Based on the Enhanced Polar Outflow Probe (e-POP) data collected during 2013–2018 time period, most observations of molecular ions occurred in the pre-midnight sector above 50° latitude ionosphere, while the lowest count rate events were located in the 8–10 MLT range, which coincided with the peak region of energetic precipitating electrons (Foss and Yau, 2019)
Summary
Charged heavy ions observed in the magnetosphere, such as atomic N+ and O+, and molecular N+2 , NO+, and O+2 ions, are sourced from the Earth’s ionosphere, and transported outward through the process of ionospheric escape. One of the main pathways of ionospheric loss is the polar wind, an ambipolar flow of plasma from the high-latitude ionosphere to the low pressure magnetosphere This outflow (Axford and Hines, 1961; Shelley et al, 1972; Yau et al, 1991; Maggiolo et al, 2006; Schunk and Nagy, 2009; Kronberg et al, 2014; Ilie and Liemohn, 2016; Glocer et al, 2018; Lin et al, 2020) provides a pathway for atmospheric migration and escape at a rate that generally depends on solar extreme ultraviolet (EUV) photon flux striking the upper atmosphere, as well the electromagnetic driving from the solar wind. These observations provide context to understand the energization of molecular ions, as well as to help interpret plasma observations from current and past space missions
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
