Observation of summer daytime aurora in the noctilucent cloud layer and its link to high-energy particle precipitation during high-speed solar wind streams

Abstract:

Aurora produced by precipitating low-energy electrons can be suppressed in summer daytime. However, the high-energy electrons (>30 keV) that are unsuppressed by sunlight are capable of penetrating deep into the mesosphere, where they can produce the odd hydrogen (HO_x) and eventually lead to catalytic ozone (O₃) loss. By elevating the D-region ionization level, they also play the important role of facilitating the production of polar mesospheric summer echoes (PMSE) as a precursor of polar mesospheric clouds (PMC). In the present study, it was discovered that high-energy electrons induce supersonic luminous phenomena, including the enhancement of O(¹S) 557.7-nm emission with an intensity of up to 300 kR (horizontally integrated) and a supersonic velocity (300–1500 m s^-1) as seen within a field of view that is 150-km wide, also called a supersonic burst (SB). SB-accompanied O(¹S) emission enhancement is differentiated from aurora because the former occurs only in summer daytime, at a low altitude of ~80 km, and in the form of an intense localised burst. The source of the SB energy might be linked to the precipitation of high-energy electrons (>30 keV), especially as observed during high-speed solar wind streams (HSSs). In producing O(¹S) emission, the secondary electron number flux of the precipitated primary electrons increases in magnitude by as much as an order of four, and a local process is required to provide the supplement. The supplementary local process may involve a supersonic velocity possibly caused by ion acceleration in a strong electric field, resulting in the inducement of electron acceleration in the field.