New results on equatorial thermospheric winds and the midnight temperature maximum

J Meriwether,O Veliz,M Faivre,P Sherwood,C Fesen

doi:10.5194/angeo-26-447-2008

J Meriwether, O Veliz + Show 3 more

Open Access

https://doi.org/10.5194/angeo-26-447-2008

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Abstract

Abstract. Optical observations of thermospheric winds and temperatures determined with high resolution measurements of Doppler shifts and Doppler widths of the OI 630-nm equatorial nightglow emission have been made with improved accuracy at Arequipa, Peru (16.4° S, 71.4° W) with an imaging Fabry-Perot interferometer. An observing procedure previously used at Arecibo Observatory was applied to achieve increased spatial and temporal sampling of the thermospheric wind and temperature with the selection of eight azimuthal directions, equally spaced from 0 to 360°, at a zenith angle of 60°. By assuming the equivalence of longitude and local time, the data obtained using this technique is analyzed to determine the mean neutral wind speeds and mean horizontal gradients of the wind field in the zonal and meridional directions. The new temperature measurements obtained with the improved instrumental accuracy clearly show the midnight temperature maximum (MTM) peak with amplitudes of 25 to 200 K in all directions observed for most nights. The horizontal wind field maps calculated from the mean winds and gradients show the MTM peak is always preceded by an equatorward wind surge lasting 1–2 h. The results also show for winter events a meridional wind abatement seen after the MTM peak. On one occasion, near the September equinox, a reversal was observed during the poleward transit of the MTM over Arequipa. Analysis inferring vertical winds from the observed convergence yielded inconsistent results, calling into question the validity of this calculation for the MTM structure at equatorial latitudes during solar minimum. Comparison of the observations with the predictions of the NCAR general circulation model indicates that the model fails to reproduce the observed amplitude by a factor of 5 or more. This is attributed in part to the lack of adequate spatial resolution in the model as the MTM phenomenon takes place within a scale of 300–500 km and ~45 min in local time. The model shortcoming is also attributed in part to the need for the model to include a hydrodynamical mechanism to describe the merging of the zonal wind with the meridional tidal winds that converge onto the geographical equator. Finally, a conclusion of this work is that the MTM compressional heating takes place along the perimeter of the pressure bulge rather than within the bulge, an issue previously not appreciated.

Highlights

The midnight temperature maximum (MTM) is a low latitude thermospheric phenomenon that has a long history in aeronomy from the first detection in the early 1970s
We adopted a new observing strategy for the Arequipa Fabry-Perot interferometer (FPI) similar to that used by Burnside et al (1981), who studied the MTM dynamics at Arecibo Observatory with observations in 1979 and 1980
Results from 11 nights illustrated the development of the MTM peak is preceded by a meridional flow of air toward the equator, that appears near midnight

Summary

Introduction

The midnight temperature maximum (MTM) is a low latitude thermospheric phenomenon that has a long history in aeronomy from the first detection in the early 1970s. None of the current physics-based numerical models is able to reproduce the MTM phenomenon with the amplitudes of 150–200 K that are commonly observed with the Arequipa Fabry-Perot interferometer (Faivre et al, 2006). The implication of the inability of grid point models such as that of the NCAR TIME-GCM (Roble et al, 1988, 1994, 1996) to reproduce the MTM appearance indicates that the ion-neutral coupling between the low-latitude F-region plasma and the thermosphere is poorly understood, and the crucial interplay of the neutral dynamics with the background ionosphere is not being represented correctly within the model. The MTM represents an excellent test of current physics-based models and how well they simulate the low latitude thermosphere and ionosphere.

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