Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground‐based incoherent scatter radar and Fabry‐Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground‐based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry‐Perot observations.
An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day‐to‐day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi‐Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced‐antenna satellite scintillation, digisonde, all‐sky imager, and Fabry‐Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all‐sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.
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.
Abstract. The determination of the physical processes that cause the day-to-day variability of equatorial spread F (ESF) has long been one of the outstanding problems in terrestrial space physics. Within the context of the Rayleigh-Taylor instability model for ESF, mechanisms that either enhance or inhibit the growth of a seed perturbation offer potential sources of variability that can be tested. In this study the hypothesis that enhanced thermospheric meridional winds play a critical role in suppressing ESF is examined during the Multi-Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA) campaign of September 1998. New, high-time-resolution Fabry-Perot interferometer (FPI) observations at 6300-• nightglow made at Arequipa (Peru) provided the neutral wind measurements during the critical postsunset hours that had been sampled only sparsely in earlier morphology studies. Evidence of local ESF activity was obtained using GPS-based observations of phase fluctuations (Fp) and 6300-• all-sky optical images from the same site. Additional GPS measurements of Fp and total electron content (TEC) from Bogota (Colombia) and Santiago (Chile) were used to determine the full flux tube development of ESF plumes and to characterize the F region morphology of the interhemispheric Appleton anomaly. Correlative studies between the nightly magnitudes of the meridional winds (Urn), ESF activity (Fp), and indices describing the strength (Is) and asymmetry (I•) of the Appleton anomaly offered no convincing evidence for the wind suppression mechanism. The best available precursor for premidnight ESF appeared to be the strength of the electrodynamically driven Appleton anomaly pattern at sunset. If one assumes that the required seed perturbation for ESF onset is essentially always available, then for all practical purposes, the magnitude of the eastward electric field that causes upward drift is both the necessary and sufficient parameter to forecast ESF with reasonable success. These results reconfirm 60 years of study pointing to the dominance of electrodynamical processes in the onset and growth of plasma instabilities at low latitudes. IntroductionPerhaps the most often described enigma in ionospheric physics is the seemingly capricious occurrence patterns of equatorial spread F (ESF). The many spatial and temporal scales of F region plasma irregularities that fall under the ESF designation were initially a source of confusion and controversy. In time, morphological classification schemes provided some real benefits in guiding physical explanations, and thus today the field is better off than, say, the magnetospheric community's attempt to agree upon what constitutes a substorm. The F region's classic "plasma bubble" encountered by a satellite sensor, the radar "backscatter plume," and the "airglow depletion" captured in an all-sky imager all refer to widely accepted views of an ESF event.
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