Observation of atomic oxygen O(¹S) green‐line emission in the summer polar upper mesosphere associated with high‐energy (≥30 keV) electron precipitation during high‐speed solar wind streams

Abstract

The auroral green‐line emission at 557.7 nm wavelength as arising from the atomic oxygen O(1S → 1D) transition typically peaks at an altitude of ~100 km specifically in the nightside oval, induced by auroral electrons within an energy range of ~100 eV–30 keV. Intense aurora is known as being suppressed by sunlight in summer daytime but usually occurs in low electrical background conductivity. However, in the present study in summer (July) sunlit condition, enhancements of O(1S) emission rates observed by using the Wind Imaging Interferometer/UARS were frequently observed at low altitudes below 90 km, where ice particles are created initially as subvisible and detected as polar mesosphere summer echoes, emerging to be an optical phenomenon of polar mesospheric clouds. The intense O(1S) emission occurring in summer exceeds those occurring in the daytime in other seasons both in occurrence and in intensity, frequently accompanied by occurrences of supersonic neutral velocity (300–1500 m s−1). In the mesosphere, ion motion is controlled by electric field and the momentum is transferred to neutrals. The intense O(1S) emission is well associated with high‐energy electron precipitation as observed during an event of high‐speed solar wind streams. Meanwhile, since the minimum occurrences of O(1S) emission and supersonic velocity are maintained even in the low precipitation flux, the mechanism responsible is not only related to high‐energy electron precipitation but also presumably to the local conditions, including the composition of meteoric‐charged ice particles and charge separation expected in extremely low temperatures (<150 K).