On the Cause of the Post‐Sunset Nocturnal OI 630 nm Airglow Enhancement Over Low‐Latitude Thermosphere.

Airglow emissions serve as good tracers of the altitudinal regions at which they originate. OI 630 nm (red line) nocturnal emissions originate from around 250 km altitude. A total of 142 nights of data corresponding to the months of January, February, and March in the years 2013, 2014, and 2016, obtained from Mt. Abu (24.6°N, 72.7°E, 16°N Mag), Gurushikhar, India, a low‐latitude location, are investigated. These are compared with the column integrated emission rates calculated using, as inputs, the measured electron density profiles obtained from a digisonde from Ahmedabad (AMD, 23.0°N, 72.6°E, 15°N Mag), India. Following the expected monotonic decrement in the emissions after sunset, an enhancement is observed on several nights that peaks at around 20–21 local time (LT). The cause for this enhancement has been investigated in detail and it is found that the neutral winds, as obtained using digisondes at two locations, show almost a very good correlation between a poleward directed wind or cessation of equatorward wind over AMD and the observed airglow emission enhancement in the post‐sunset time. Further, the percentage enhancement in emissions also shows a decrease in magnitudes from January to March which has a broad similarity to the decrease in the model climatological meridional wind magnitudes in the same duration. Based on the data spanning over different years, it is inferred that, during geomagnetic quiet periods, almost all of the nocturnal variability in OI 630 nm emissions is due to the variations in the neutral wind. Plain Language Summary: Some atoms in the Earth's upper atmosphere emit light due to internal processes known as airglow. The processes prevalent at those altitudes can be understood by remotely observing the modulations/variations of this airglow brightness. One such emission at 630 nm, known as redline airglow showed large enhancements in post‐sunset time over a low‐latitude location. While such redline emission enhancements can, on occasions, possibly occur due to a process at magnetic equator, known as pre‐reversal enhancement in electric field, systematic measurements carried out in this study over a three year duration in the months of January to March revealed that the meridional wind at thermospheric altitudes is the dominant cause of such enhancements. This conclusion has been made by measuring the relative variations in winds in the post‐sunset times that vary from night‐to‐night and the climatological model output that shows a broad month‐to‐month variation. We use the fact that poleward winds bring in electrons, one of the ingredients that produces redline emission, to low‐latitudes thereby causing these emission enhancements. Key Points: Enhancement in OI 630 nm red line emissions observed during the post‐sunset time from a low‐latitude locationColumn integrated volume emission rate and relative meridional wind variations have been calculated using digisondes at two locationsPoleward meridional winds play an important role in causing the observed enhancements in the post‐sunset OI 630 nm emissions [ABSTRACT FROM AUTHOR]