New results on equatorial thermospheric winds and the midnight temperature maximum

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.Correspondence to: J. Meriwether (john.meriwether@ces.clemson.edu)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.

Equinoctial asymmetry of a low-latitude ionosphere-thermosphere system and equatorial irregularities: evidence for meridional wind control

“…In the late winter to spring season of the Southern Hemisphere (e.g., the bottom panel of Fig. 7 of Meriwether et al, 2008), the wind is directed poleward during the early evening hours, followed by the equatorward surge immediately before midnight, and then it is reversed poleward again.. The periodic change in the wind direction is consistent with that represented in Fig.…”

“…2 systematically varies through the night. Meriwether et al (2008) observed nightly variations of thermospheric winds and temperatures with high accuracy at a low-latitude station Arequipa, Peru (16.4 • S, 71.4 • W), which allowed to draw local time-latitude maps of horizontal wind. In the late winter to spring season of the Southern Hemisphere (e.g., the bottom panel of Fig.…”

Simultaneous 6300 Å airglow and radar observations of ionospheric irregularities and dynamics at the geomagnetic equator

“…The FPI uses the Doppler shift and Doppler width of the 6300 Å emission to determine these parameters (Meriwether et al, 2008).…”

“…Another equatorial phenomenon we discuss in this paper, the MTM, has been studied directly using a variety of instruments such as satellites, incoherent scatter radars (ISR), and FPIs, (Herrero and Spencer, 1982;Hickey et al, 2014;Meriwether et al, 2008). At Jicamarca, the ISR has been used to study the seasonal dependence of the MTM (Bamgboye and McClure, 1982) and the MTM has been observed with FPIs at locations near the magnetic equator (Meriwether et al, 2008;Fisher et al, 2015).…”

“…At Jicamarca, the ISR has been used to study the seasonal dependence of the MTM (Bamgboye and McClure, 1982) and the MTM has been observed with FPIs at locations near the magnetic equator (Meriwether et al, 2008;Fisher et al, 2015). A signature of the MTM, known as a brightness wave, can be observed using an ASI.…”