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.

[1] Examples of storm enhanced density (SED) formation over Russia and Northern Europe are presented. These events, which persisted 15-20 hours, were fixed in local time near noon over Europe and then later observed over the American sector. The amount of total electron content (TEC) at the base of the SED erosion plume is found to be greatest in the American sector. A persistent, repeatable pattern is apparent in the time evolution of the latitude location of the SED plume base, although the latitudinal rate of change differs between the two sectors. In the European sector the invariant latitude (L) of the SED plume base is observed to be between 61°-63°L and at a time close to local noon. In the American sector, the position of the base of the plume shifts from local noon towards dusk, and moves to a lower latitude at a nearly fixed longitude.

[1] We have examined quantitatively the influence a low-latitude, premidnight sporadic E layer might have on the daily and hourly development of equatorial spread F (ESF). In particular, we calculated changes in the flux tube -integrated Pedersen conductivity as it affects the growth rate of the Rayleigh-Taylor instability, which governs the initial development of ESF. We find that the growth rate is lowered by an order of magnitude with a density of 1 Â 10 6 cm À3 in a slab from 115 to 120 km. Since sporadic E layers observed after dusk do not regularly reach these values, they are not a likely source of the daily variability in ESF. However, even a mild enhancement in the postsunset E region could lead to a significant suppression of ESF if it also inhibits the upward plasma drift of the prereversal enhancement, a key variable in the growth rate of the equatorial spread F instability. Thus, consistent with the nature of an instability, the second-order effect (suppressed upward drift) is more important than the first-order cause (reduced F region to E region conductivity) of inhibited ESF onset.

The thermospheric midnight temperature maximum (MTM) is an upper atmospheric e ect found at low latitudes. It is accompanied by an increase in pressure and a signature poleward abatement or reversal in the meridional neutral winds. The MTM exhibits a poleward propagation away from the geographic equator with two secondary maxima developing at approximately ±15• latitude. In this paper, we review early works and recent e orts regarding the MTM. Outstanding questions dealing with seasonal and longitudinal dependencies of the MTM's basic characteristics are discussed. All-sky imaging systems at Arequipa and El Leoncito observed the propagation of 6300 A airglow enhancements related to the MTM past 35• S latitude. This provides useful information on the upper latitude limit of the MTM. TIEGCM modeling e orts simulate the MTM through upward propagating semi-diurnal tides but have di culty reproducing accurately its amplitude and occurrence time. It is suggested that the role of the terdiurnal tidal mode may be more important than previously thought. Recent comparative observation and modeling studies of MTM related 6300 A emission proved unsuccessful. We report that the amplitude of the modeled MTM was not strong enough to instigate the 'midnight collapse' of the F-region needed to produce the airglow signature.

Abstract. The thermospheric midnight temperature maximum (MTM) is a highly variable, but persistent, large scale neutral temperature enhancement which occurs at low latitudes. Its occurrence can impact many fundamental upper atmospheric parameters such as pressure, density, neutral winds, neutral density, and F-region plasma. Although the MTM has been the focus of several investigations employing various instrumentation including photometers, satellites, and Fabry-Perot interferometers, limited knowledge exists regarding the latitude extent of its influence on the upper atmosphere. This is largely due to observational limitations which confined the collective geographic range to latitudes within ±23 • . This paper investigates the MTM's latitudinal extent through all-sky imaging observations of its 6300Å airglow signature referred to by Colerico et al. (1996) as the midnight brightness wave (MBW). The combined field of view of three Southern Hemisphere imaging systems located at Arequipa, Peru, and Tucuman and El Leoncito, Argentina, for the first time extends the contiguous latitudinal range of imager observations to 8 • S-39 • S in the American sector. Our results highlight the propagation of MBW events through the combined fields of view past 39 • S latitude, providing the first evidence that the MTM's effect on the upper atmosphere extends into mid-latitudes. The observations presented here are compared with modeled 6300Å emissions calculated using the NCAR thermosphere-ionosphereelectrodynamic general circulation model (TIEGCM) in conjunction with an airglow code. We report that at this time TIEGCM is unable to simulate an MBW event due to the model's inability to reproduce an MTM of the same magnitude and occurrence time as those observed via FPI measurements made from Arequipa. This work also investigates the origins of an additional low latitude airglow feature referred to by Colerico et al. (1996) as the pre-midnightCorrespondence to: M. J. Colerico (colerico@haystack.mit.edu) brightness wave (PMBW) and described as an enhancement in 6300Å emission which occurs typically between 20:00-22:00 LT and exhibits equatorward propagation. We present the first successful simulation of a PMBW event using the TIEGCM and the airglow code. We find that the PMBW's origin is electro-dynamical in nature, resulting from the expected evening decay of the inter-tropical arcs.