Venus' O 5577 Å Oxygen Green Line: A Global Diffuse Proton-Induced Aurora

Abstract

The Venusian O(¹S–¹D) 5577 Å “oxygen green line” has been an enigmatic feature of the Venusian atmosphere since its first attempted observation by the Venera spacecraft. Its first detection in 1999 and subsequent detections point to a unique auroral phenomena. However, the lack of (¹D–³P) 6300 Å “oxygen red line” emission suggests that the green line originates from deep in the ionosphere, much lower than current models predict. Here, we present 16 years of ground-based observations of the Venusian green line, comparing its behavior to the solar wind and spacecraft observations of the Venusian ionosphere. We find that all instances of green line emission occur during solar energetic particle (SEP) events, with a Matthews correlation coefficient of 0.93 between emission and the presence of SEPs. Coordinated observations between Venus Express and ground-based observatories show enhanced nightside ionospheric peak densities during the time of green line emission, with the lowest peak occurring at 115 km near local midnight. Such high density yet low altitude peaks suggest the presence of highly energetic particle precipitation. Initial modeling indicates $\ge$50 keV protons are needed to penetrate to such low altitudes. Comparisons of solar wind data confirm that such protons are present during all green line detections and nightside ionosphere enhancements. The association of SEP storms with green line emission and low nightside ionospheric peaks indicates that the green line is a unique global diffuse aurora, likely originating deep in the ionosphere and driven by proton precipitation, something that could be common for all non-magnetic planetary atmospheres.