Directional trends in thermospheric neutral winds observed at Arecibo during the past three solar cycles

Abstract

Since 1980, we have observed the thermospheric neutral wind at the Arecibo Observatory using a Fabry‐Perot interferometer to measure the O(1D) 630 nm emission. Burnside and Tepley (1989) examined the first 8 years of this extended data set and found that there were no significant or systematic solar cycle influences on the magnitude or direction of the neutral wind field, nor on its horizontal gradients. Such affects have been observed previously at other locations around the globe, and their absence at Arecibo may have been due to the limited data set. Thus, we have extended the period of acquisition and analysis of our neutral wind measurements to include nearly three complete solar cycles (or approximately 30 years) and will present our results within the framework of the earlier work. While the earlier conclusion that no major systematic solar cycle influence on the neutral winds at Arecibo generally remains intact, we did find a slight increase in wind magnitude and a gradual, yet consistent rotation of the thermospheric neutral wind vector from a general southeast to a more eastward flow during 30 years of observation. We explain the magnitude and directional variations in terms of long‐term changes in the density and temperature of the upper atmosphere and their possible dissimilar influences on each wind component that appear as a rotation of the neutral wind vector.

Long‐term changes in the thermospheric neutral winds over Arecibo: Climatology based on over three decades of Fabry‐Perot observations

“…As a function of time, the estimated long‐term trend presented inFigure 11 is in agreement with Tepley et al [2011], i.e., the zonal/meridional component's 30 year trend progressively increase/decrease throughout the night. However, related to the nighttime magnitude of the TNW components, our results show smaller magnitudes than Tepley et al [2011]. Integrated throughout the whole night (18:00 LT to 06:00 LT) they found 10 m s −1 and 15 m s −1 and we found 3.3 m s −1 and 11.1 m s −1 toward the east and north, respectively.…”

“…In general, the TNW zonal component is eastward during almost the entire night with a peak around 22:00 LT (∼75 m s −1 ) with a change in its direction near sunrise. The meridional component tends to have a maximum equatorward value of ∼35 m s −1 just before local midnight and then tends toward zero, or reverses slightly poleward in the early morning [ Tepley et al , 2011]. In addition, Figure 4exhibits large day‐to‐day variation of the thermospheric wind over Arecibo, which means that there are many TNW patterns that must be identified in order to produce a consistent climatology.…”

“…The calibration process was identical over the entire period consisting of regular stabilized HeNe laser samples to keep track of potential instrumental drifts while observing, and intensity calibrations referenced to a “tracking” photometer that also measured O ( 1 D ) referenced to a 14 C source. The same analysis programs and equipment were used as well, and often the same people were involved in the process, thus establishing a consistent picture of the TNW behavior over Arecibo [ Tepley et al , 2011].…”

“…Despite the good description of the TNW in general, we found a large range of values (i.e., “dispersion”) in both components in all 36 bins but, especially during low solar activity. This is likely due to the weaker airglow emissions measured during those periods, which translates into more statistical scatter [ Tepley et al , 2011], i.e., the intensity of the 630.0 nm emission is about 8 times lower at solar minimum [ Burnside and Tepley , 1989].…”

High time and height resolution neutral wind profile measurements across the mesosphere/lower thermosphere region using the Arecibo incoherent scatter radar

“…These include studies that focus on the mesosphere, where the procedure is especially straightforward [Mathews et al, 1981;Rottger et al, 1981;Fukao et al, 1982Fukao et al, , 1985Rottger et al, 1983;Tsuda et al, 1985;Maekawa et al, 1986;Zhou and Morton, 2006]. The techniques and related databases have become sufficiently reliable to support studies of long-term trends over Arecibo [Maekawa et al, 1987;Tepley et al, 2011;Santos et al, 2011;Brum et al, 2012].…”