No abstract

As part of the NSF/CEDAR program (Coupling Energetics and Dynamics of Atmospheric Regions) in Multi‐Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA), an all‐sky CCD airglow imaging system has been in operation in Arequipa, Peru, since October 1993. Here we report on the first such use of a wide‐field imager to document the optical signature and variability of a brightness feature associated with the so‐called midnight temperature maximum (MTM). While theo observational driver of this study is a “brightness wave” (BW) seen in 6300 Å and 5577 Å airglow images, detailed case studies are conducted during two campaign periods when Fabry‐Perot interferometer (FPI) and digital ionosonde data were also available. During the passage of a BW, the FPI observed enhancements in thermospheric temperatures, reversals (from equatorward to poleward) of the meridional neutral winds, and local minima in the zonal neutral winds. The ionosonde recorded decreases in the height of the F‐layer during BW events. This lends support to the concept that the poleward winds generated by the MTM pressure bulge cause the lowering of the F‐layer to regions of enhanced loss (h < 300 km) and corresponding airglow production. The two‐dimensional field‐of‐view of the imager allows identification of the geographical orientation of the BW pattern. We use the orientation angle of the BW as an indicator of the geographical orientation of the MTM. Significant day‐to‐day variability in these patterns suggests a complex mix of tidal mode interactions that lead to the overall MTM phenomena.

Pioneer Venus (PV) orbiter ultraviolet spectrometer (OUVS) images of the nightside airglow in the (0, 1)/5 band of nitric oxide showed a maximum whose average location was at 0200 local solar time just south of the equator. The average airglow brightness calculated over a portion of the nightside for 35 early orbits during the Pioneer Venus mission was a factor of 4 lower than this maximum. Recent recalibration of the PV OUVS instrument and reanalysis of the data yield new values for this statistical maximum (1.9 -+ 0.6 kR) and the nightside average (400-460 -+ 120 R) nightglow. This emission is produced by radiative recombination of N and O atoms transported from their source on the dayside to the nightside by the Venus thermospheric circulation. The Venus Thermospheric General Circulation Model (VTGCM) has been extended to incorporate odd nitrogen chemistry in order to examine the dynamical and chemical processes required to give rise to this emission. Its predictions of dayside N atom densities are also compared with empirical models based on Pioneer Venus measurements. Calculations are presented corresponding to OUVS data taken during solar maximum (F10 = 180-200). The average production of nitrogen atoms on the dayside is about 9.0 x 109 atoms cm :• s-1. Approximately 30% of this dayside source is required for transport to the nightside to yield the observed dark-disk nightglow features. The statistical location and intensity of the bright spot are well reproduced, as well as the altitude of the airglow layer. The importance of the large-scale transport and eddy diffusion on the global N(4S) distribution is also evaluated. Uncertainties in measured rate coefficients and branching ratios and their effects on the calculated airglow are discussed.It is found that model parameters can be adjusted to reproduce either the OUVS nightglow or the observed dayside N densities; a best match to both implies a VTGCM global wind system that is a factor of 1.5-2.0 slower than at present. Predictions are also made for comparison with OUVS solar minimum data.

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) is utilized to evaluate the role of upward propagating diurnal and semidiurnal tides in determining the zonal mean states of the thermosphere and ionosphere above 100 km. Differences between various zonal mean fields from TIGCM simulations are examined with and without tidal forcing at the lower boundary. The following effects are observed: (1) A westward jet of the order of 10–30 m s−1 in the equatorial lower thermosphere and weaker eastward flanking mid‐latitude jets are induced by eddy and molecular dissipation of the tidal motions. (2) Salient characteristics of the equatorial jet are independent of tidal phase. (3) Increases in the zonal mean N2 and O2 number densities of the order of 10–30% are produced above 110 km and between ± 30° latitude, mostly by virtue of the net heating due to tidal dissipation; these are accompanied by electron density depletions of the order of 10–15% above about 160 km due to the enhanced chemical loss. (4) A low‐latitude (< 30°) depletion (∼ 30–50%) of atomic oxygen occurs due to the increase in effective recombination rate within a tidally displaced air parcel. The latter may explain the low‐latitude depression in the OI (5577 Å) airglow intensity from satellite‐based observations. In addition, variations in the zonal mean densities of NO, N(4S), and N(2D) of the order of 30–60% are induced by reactions with the tidally modified distributions of O, O2, and temperature. The occurrence frequency of these effects is influenced by the amplitude of diurnal propagating tide assumed at the lower boundary of the TIGCM, which according to radar observations is exceeded about 20–40% of the time, depending on season.

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.