Abstract
Abstract. Measurements of equatorial thermospheric winds, temperatures, and 630 nm relative intensities were obtained using an imaging Fabry–Perot interferometer (FPI), which was recently deployed at Bahir Dar University in Ethiopia (11.6° N, 37.4° E, 3.7° N magnetic). The results obtained in this study cover 6 months (53 nights of useable data) between November 2015 and April 2016. The monthly-averaged values, which include local winter and equinox seasons, show the magnitude of the maximum monthly-averaged zonal wind is typically within the range of 70 to 90 ms−1 and is eastward between 19:00 and 21:00 LT. Compared to prior studies of the equatorial thermospheric wind for this local time period, the magnitude is considerably weaker as compared to the maximum zonal wind speed observed in the Peruvian sector but comparable to Brazilian FPI results. During the early evening, the meridional wind speeds are 30 to 50 ms−1 poleward during the winter months and 10 to 25 ms−1 equatorward in the equinox months. The direction of the poleward wind during the winter months is believed to be mainly caused by the existence of the interhemispheric wind flow from the summer to winter hemispheres. An equatorial wind surge is observed later in the evening and is shifted to later local times during the winter months and to earlier local times during the equinox months. Significant night-to-night variations are also observed in the maximum speed of both zonal and meridional winds. The temperature observations show the midnight temperature maximum (MTM) to be generally present between 00:30 and 02:00 LT. The amplitude of the MTM was ∼ 110 K in January 2016 with values smaller than this in the other months. The local time difference between the appearance of the MTM and a pre-midnight equatorial wind was generally 60 to 180 min. A meridional wind reversal was also observed after the appearance of the MTM (after 02:00 LT). Climatological models, HWM14 and MSIS-00, were compared to the observations and the HWM14 model generally predicted the zonal wind observations well with the exception of higher model values by 25 ms−1 in the winter months. The HWM14 model meridional wind showed generally good agreement with the observations. Finally, the MSIS-00 model overestimated the temperature by 50 to 75 K during the early evening hours of local winter months. Otherwise, the agreement was generally good, although, in line with prior studies, the model failed to reproduce the MTM peak for any of the 6 months compared with the FPI data.
Highlights
Temporal and spatial variations in thermospheric neutral atmosphere parameters, such as density, velocity and temperature, drive a complex system of ionospheric currents and electric fields which are necessary to accurately describe equatorial thermosphere–ionosphere dynamics
This instability – which commonly manifests itself in a variety of ways, including a spread in the F-region echoes of an ionogram, intensity depletions in thermosphere airglow, plumes in VHF radar echoes, and total electron content (TEC) depletions in GPS measurements – will affect radio signals propagating through the disturbed region, causing scintillation and subsequent disruptions in communication and navigation systems (Mendillo et al, 1997; Pimenta et al, 2007; Haase et al, 2011)
Previous researches reported that the time lag between the wind surge and the appearance of midnight temperature maximum (MTM) is longer in winter and shorter in equinox season, but our results show that the longer time exists in the equinox season and the shorter time in the winter season (Meriwether et al, 2008, and references therein)
Summary
Temporal and spatial variations in thermospheric neutral atmosphere parameters, such as density, velocity and temperature, drive a complex system of ionospheric currents and electric fields which are necessary to accurately describe equatorial thermosphere–ionosphere dynamics. The equatorial zonal neutral wind dominates the F-region plasma circulation by providing momentum transfer through ion–neutral collisions This F-region coupling is manifested by significant nighttime fluctuations of the observed neutral winds and temperatures and has a strong impact on the evolution of plasma irregularities through the control of the dynamo electric field (Rishbeth, 1971; Heelis, 2004; Huba and Krall, 2013). The evolution of the early evening ionospheric dynamics causes the bottomside F-region plasma density gradient to become unstable and cause the formation of a complex equatorial plasma structure commonly referred to as equatorial spread F (Kelley, 2009) This instability – which commonly manifests itself in a variety of ways, including a spread in the F-region echoes of an ionogram, intensity depletions in thermosphere airglow, plumes in VHF radar echoes, and total electron content (TEC) depletions in GPS measurements – will affect radio signals propagating through the disturbed region, causing scintillation and subsequent disruptions in communication and navigation systems (Mendillo et al, 1997; Pimenta et al, 2007; Haase et al, 2011)
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