Behavior of the atomic oxygen 5577 Ångström emission intensity at mid-latitudes: a climatological view

Abstract

A global mid-latitude study of the atomic oxygen green line emission intensity at 5577 A has been undertaken with the goal of developing a climatological understanding of the emission behavior and its usefulness as a tracer for the atmosphere near 97 km. Long-term observations have been analysed at nine stations covering periods of ∼8--12 years, for a total of over 90 years of measurements. The results of this investigation show that the emission typically exhibits a maximum near the summer solstice and again near the fall equinox, before falling to a low winter-time level that persists into the middle of spring. Importantly, the oft-reported maximum at the spring equinox is not a statistically significant feature on the climatological time scale. This finding has implications on our understanding of the dominant processes operating in the region. Specifically, the role of seasonally varying vertical diffusion caused by breaking gravity waves must be readdressed in light of the absence of a strong maximum at the spring equinox.This work also addresses the relationship between the green line emission intensity and geomagnetic and solar activity. Results show that failing to exclude observations taken under high geomagnetic activity conditions leads to increased springtime emission levels and may be one explanation for this feature as has been reported by others. The influence of solar activity on the green line emission over the long term is shown to exhibit a hysteresis effect within a given solar cycle, confirming that there is not a simple linear relationship between the two processes.Finally, a critical examination is made of how long of a data series is necessary to fully achieve a climatological understanding of this emission and how this understanding may reasonably be used to advance our understanding of the upper middle atmosphere region. After ∼10 years, features with periods less than one year become stable (or achieve climatology), but the data examined here show unresolved power at periods approaching the series length which need longer data coverage to fully characterize.