Daily Variability in the Terrestrial UV Airglow

Thomas J Immel,Richard W Eastes,Colin Triplett,Harald U Frey,William E Mcclintock,Scott L England,Steven B Mende

doi:10.3390/atmos11101046

Abstract

New capability for observing conditions in the upper atmosphere comes with the implementation of global ultraviolet (UV) imaging from geosynchronous orbit. Observed by the NASA GOLD mission, the emissions of atomic oxygen (OI) and molecular nitrogen (N2) in the 133–168-nm range can be used to characterize the behavior of these major constituents of the thermosphere. Observations in the ultraviolet from the first 200 days of 2019 indicate that the oxygen emission at 135.6 nm varies much differently than the broader Lyman-Birge-Hopfield (LBH) emission of N2. This is determined from monitoring the average instrument response from two roughly 1000 km2 areas, well separated from one another, at the same time of each day. Variations in the GOLD response to UV emissions in the monitored regions are determined, both in absolute terms and relative to a running 7-day average of GOLD measurements. We find that variations in N2 emissions in the two separate regions are significantly correlated, while oxygen emissions, observed in the same fixed geographic regions at the same universal time each day, exhibit a much lower correlation, and exhibit no correlation with the N2 emissions in the same regions. This indicates that oxygen densities in the airglow-originating altitude range of 150–200 km vary independently from the variations in nitrogen, which are so well correlated across the dayside to suggest a direct connection to variation in solar extreme-UV flux. The relation of the atomic oxygen variations to solar and geomagnetic activity is also shown to be low, suggesting the existence of a regional source that modifies the production of atomic oxygen in the thermosphere.

Highlights

The research effort described here focuses on variability in the relative abundance of the two major constituents of the upper atmosphere, atomic oxygen (OI) and molecular nitrogen (N2 ).It addresses this topic using remote spectrographically-separated, photometric measurements of the photo-emissions of the upper atmosphere from the NASA Global Observations of the Limb and Disk (GOLD) mission [1,2] that provides repeatable, daily measurements, with nearly identical observing geometry
The GOLD hyperspectral imager offers simultaneous observations of thermospheric oxygen and nitrogen emissions, and we utilize the Level 1 daytime product: geolocated raw instrument counts, reported for UV wavelengths from 134 to 168 nm. These are obtained using the instrument in its daytime, high spectral resolution mode, and so provide signatures of several key features of Earth’s airglow, and allow for clear separation of the emissions of atomic oxygen and atomic and molecular nitrogen
The open question that should be addressed is whether the indications of different behavior of the OI and N2 emissions are related to modification of the photoelectron flux, or changes in the relative density of the thermospheric constituents? In either case, is it due to waves that are caught in the act of modifying the region, or is it a day-to-day change in the properties of the atmosphere well below that are responsible, introducing changes that are balanced as the thermosphere maintains diffusive equilibrium? On the other hand, is the larger variation in OI emissions due to the fact that the atomic oxygen population is excited by solar radiance variations that do not affect N2 in the same way?

Summary

Introduction

The research effort described here focuses on variability in the relative abundance of the two major constituents of the upper atmosphere, atomic oxygen (OI) and molecular nitrogen (N2 ). It addresses this topic using remote spectrographically-separated, photometric measurements of the photo-emissions of the upper atmosphere from the NASA GOLD mission [1,2] that provides repeatable, daily measurements, with nearly identical observing geometry. Earth’s upper atmosphere is the region above ∼100 km where densities drop to a level below which diffusion effectively promotes the separation of atmospheric species by mass with altitude In this stratifying atmosphere, the ratio of light to heavy constituents (for instance, atoms of He vs Ar) increases with altitude. The temperature of the thermosphere directly affects the scale height of the separate constituents, and the relative abundance of oxygen and nitrogen vs. altitude

Results

Discussion

Conclusion