It is now well established that high-latitude forcing (heating and convection) can lead to significant thermospheric disturbances on a global scale. The basic scenario is upwelling of air from the lower thermosphere within the auroral oval due to Joule/particle heating followed by horizontal advection of this molecular rich air through a combination of diurnal tidal motion and ion convection effects. During storms, a strong increase in ion convection can produce a wind surge (due to ion/neutral coupling) out of the polar cap directed toward the midnight/postmidnight midlatitude region. The increased convection currents are caused by enhanced ionization within the oval and increased ion drift associated with intensification of the polar cap electric field. The resulting wind pattern mimicks the two-cell ion convection pattern, provided storm conditions last for several hours. An extensive bibliography exists on the response of the global thermosphere to geomagnetic disturbances. Examples of recent papers on the subject are those by Burns et al. [1991, 1995], and Fuller-Rowe# et al. [1994, 1996], who base their findings on general circulation model results. Numerous references to past work may be found in these papers.
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A selected set of far ultraviolet images of the Earth have been analyzed quantitatively to establish their validity for studying thermospheric "weather." The set of images chosen for study was restricted to mostly geomagnetically quiet conditions in order to obtain a baseline understanding of the relationship between the observations and thermospheric phenomenology. The images included low to moderate solar activity levels. A new model was developed to generate global dayglow images using first principles methods. The mass spectrometer/incoherent scatter (MSIS-86) model was used to predict the thermospheric concentrations. The analyses of thermospheric images observed in the 123 to 160-nm nominal passband show that the spectral composition for observations on the projected Earth disk is dominated by O I 130.4-nm radiation (85-90 %), with contributions from O I 135.6-nm and N2 Lyman-Birge-Hopfield (LBH) bands of about 5-8% each. The synthetic images reproduce the global features of the observed images rather well. Differences between the model and the data are attributed to real atmospheric effects, such as atomic oxygen depletions which are not well reproduced by the MSIS model when geomagnetic activity is elevated. The absolute values recorded were 38-54 % higher than predicted. We attribute this discrepancy to low values of the solar extreme ultraviolet irradiances used in the model. Images obtained in the 136 to 165-nm nominal passband are a factor of 2.7 greater than the model. The excess signal observed is most likely due to a long wavelength tail in the instrument sensitivity which allowed Rayleigh scattered sunlight between 180 and 250 nm to be detected. The understanding of the DE 1 images gained by this study provides the basis for future work on the global response of the thermosphere to geomagnetic forcing. Introduction The Dynamics Explorer (DE 1) mission was launched on August 3, 1981 into a 90' inclination orbit with perigee and apogee altitudes of 570 km and 3.65 R•, respectively. Conrained within the DE 1 payload were three spin-scan photometric imagers provided by the University of Iowa. A description of the Iowa instruments was published by Frank et al. [1981]. The Iowa instruments acquired over 600,000 far ultraviolet (FUV) and visible images before the effective end of the mission in November 1990. Highlights of imaging results from the mission through 1987 were reviewed by Frank and Craven [1988]. distorted, compared with quiet time images [Craven and Frank, 1984; Craven et al., 1993] (see also a recent paper by Craven et al. [1994]). These reports are consistent with earlier satellite observations of depletions in the O I 130.4-and 135.6 nmdayglow at high latitudes [Meier, 1970; Strickland and Thomas, 1976; Conway et al., 1988; Picone et al., 1993; Parish et al., 1994]. The implication of such observations is that thermospheric "weather" can be observed on a global scale by imaging the O (and N2) FUV dayglow, which is directly (but not linearly) related to the atmospheric composition. In par...
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