Nocturnal winter increases in total electron content are observed at Bozeman, Boulder, and Dallas with the ATS 6 radio beacon. Results of this investigation suggest that the increases take place mainly in the ionosphere. Changes in plasmaspheric contents are much smaller. The downward motion of the F2 layer and the east‐to‐west movement of the nocturnal maximum (NM) suggest the presence of an electric field. A westward electric field will move plasma from higher L shells to lower L shells with smaller volumes and thus increase the plasma pressure, the result being an enhanced flow of plasma from plasmasphere to ionosphere. The total flux required to produce the NM is of the order of 1×1013 m−2 s−1.

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The Millimeter‐Wave Atmospheric Sounder (MAS) is a shuttle‐based limb‐sounding instrument designed for global spectroscopic studies of O3, and constituents important in O3 photochemistry, in the middle atmosphere. It is part of the NASA's Atmospheric Laboratory for Applications and Science (ATLAS) spacelab shuttle mission. This paper presents an overview of the instrument, operation, and data analysis. In addition, as an example of the results, we present zonal average retrievals for O3, H2O and ClO obtained in ATLAS 1. The MAS O3 and H2O measurements are shown to agree well with simultaneous observations made with the UARS MLS instrument.

Zeeman splitting of O2 molecular states in the Earth's upper atmosphere leads to polarized emission spectra. A 61 GHz radiometer operated as part of the Millimeter‐wave Atmospheric Sounder (MAS), a core payload instrument of the NASA Space Shuttle ATLAS missions, observed such emissions. This instrument's high resolution spectrometer (200 kHz) allows us to verify for the first time Zeeman effect model calculations for the upper atmosphere in some detail. The results suggest some interesting new aspects for the research of the upper atmosphere.

Latitudinal distributions of upper stratospheric ClO measured by MAS during the three ATLAS missions are presented for northern hemisphere (NH) spring equinox in 1992, southern hemisphere (SH) early fall in 1993, and NH fall in 1994. The MAS ClO results are shown along with correlative MLS observations. The results of both instruments consistently show the same latitudinal features. The ClO maximum in the NH spring occurs at mid latitudes, whereas the latitudinal ClO maximum in both the NH and SH fall occurs at high latitudes. The volume mixing ratio maxima were significantly higher in the fall (0.7–0.8 ppbv) than in spring (0.5–0.6 ppbv). Qualitatively, these results are consistent with calculations of several 2‐D models.