Thermospheric neutral temperatures derived from charge-exchange produced N₂⁺ Meinel (1,0) rotational distributions

Abstract. Thermalized rotational distributions of neutral and ionized N₂ and O₂ have long been used to determine neutral temperatures (T_n) during auroral conditions. In both bright E-region (≲150 km) auroras, and in higher-altitude auroras, spectral distributions of molecular emissions employed to determine T_n in the E-region cannot likewise be used to obtain T_n in the F-region. Nevertheless, charge-exchange reactions between high-altitude (≳130 km) species provide an exception to this situation. In particular, the charge-exchange reaction O⁺(²D) + N₂(X) → N⁺₂(A²Π_u, ν' = 1 + O(³P) yields thermalized N₂⁺ Meinel (1,0) emissions, which, albeit weak, can be used to derive neutral temperatures at altitudes of ~130 km and higher.

In this work, we present N₂⁺ Meinel (1,0) rotational temperatures and brightnesses obtained at Svalbard, Norway, during various auroral conditions. We calculate T_n at thermospheric altitudes of 130–180 km from thermalized rotational populations of N₂⁺ Meinel (1,0); these emissions are excited by soft electron (≲1 keV) impact and charge-exchange reactions. We model the contributions of the respective excitation mechanisms, and compare derived brightnesses to observations. The agreement between the two is good. Emission heights obtained from optical data, modeling, and ISR data are consistent. Obtaining thermospheric T_n from charge-exchange excited N₂⁺ Meinel (1,0) emissions provides an additional means of remotely sensing the neutral atmosphere, although certain limiting conditions are necessary. These include precipitation of low-energy electrons, and a non-sunlit emitting layer.