Well-resolved far-ultraviolet spectroscopic images of O I, S I, and previously undetected H ILyman-alpha emission from Io were obtained with the Hubble space telescope imaging spectrograph (STIS). Detected O I and S I lines (1250 to 1500 angstroms) have bright equatorial spots (up to 2.5 kilorayleighs) that shift position with jovian magnetic field orientation; limb glow that is brighter on the hemisphere facing the jovian magnetic equator; and faint diffuse emission extending to approximately 20 Io radii. All O I and S I features brightened by approximately 50 percent in the last two images, concurrently with a ground-based observation of increased iogenic [O I] 6300-angstrom emission. The H ILyman-alpha emission, consisting of a small, approximately 2-kilorayleigh patch near each pole, has a different morphology and time variation.

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Observations of the geocoronal Balmer α nightglow have been made from Wisconsin for more than a solar cycle with an internally consistent intensity reference to standard astronomical nebulae. These measurements were made with a double‐etalon, pressure‐scanned, 15‐cm aperture Fabry‐Perot interferometer. The resulting long time line data provides an opportunity to examine solar cycle influence on the mid‐latitude exosphere and to address accompanying questions concerning the degree to which the exosphere is locally static or changing. Our exospheric Balmer α absolute intensity measurements show no statistically significant variations throughout the solar cycle when the variation with viewing geometry is removed by normalizing the data to reference exospheric model predictions by Anderson et al. However, the relative intensity dependence on solar depression angle does show a solar cycle variation. This variation suggests a possible related variation in the exospheric hydrogen density profile, although other interpretations are also possible. The results suggest that additional well‐calibrated data taken over a longer time span could probe low‐amplitude variations over the solar cycle and test predictions of a slow monotonic increase in exospheric hydrogen arising from greenhouse gases.

Plasma measurements were made with a detector aboard the Explorer 10 satellite, launched on a highly elongated elliptical trajectory with the line of apsides about 33° to the antisolar direction. Magnetic field measurements were also carried out on Explorer 10 by the Goddard Space Flight Center of NASA. A plasma moving with a velocity of about 300 km sec−1 was first observed when the satellite reached a distance of about 22 earth radii. During the rest of the observations (which terminated about 40 hours later, at a distance of 42 earth radius, periods in which substantial plasma fluxes were recorded alternated with shorter periods in which the plasma flux was below or just above the detection limit. There was a striking correlation between the plasma flux and the magnetic field: in the absence of plasma the magnetic field direction was nearly radial from the earth, whereas in the presence of plasma, the field was irregular and generally formed large angles with the earth‐satellite direction. The plasma probe did not provide accurate information on the direction of the plasma flow, but placed the direction within a ‘window’ of about 20° by 80°. This window includes the direction pointing radially away from the sun. The flux densities of the positive ions (presumably protons) corresponding to the observed currents were of the order of a few times 108 cm−2 sec−1. They fluctuated over a range of about a factor of 2 during the periods when plasma was observed.

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Ground‐based observations of geocoronal Balmer α (Hα) emission were performed with a large‐aperture, dual‐etalon Fabry‐Perot spectrometer at a resolving power of about 25,000, which clearly isolates the geocoronal emission from galactic and zodiacal light backgrounds. During each observing night, observations were conducted alternately between two directions, one at 60° zenith distance in the sun's azimuth and the other in the antisolar direction, to compare the emission primarily from single scattering of solar Lyman β with that primarily from multiple scattering, and to evaluate the intensity variations over various time scales. Accurate intensity calibration was obtained by observing astronomical photometric standard sources on each night of observation. The morning enhancement of the observed Hα intensities in both directions is 10%. Assuming single scattering dominates, the observed intensity variation seems difficult to reconcile with the standard diurnal variation model which implies an increase at mid‐latitudes of about 30% in the exobase hydrogen density between the evening period and the post‐midnight period. In contrast to the standard diurnal variation model, some satellite measurements indicate a morning hydrogen trough in northern mid‐latitudes, a result which appears to be consistent with the present measurements. Detailed radiative transfer calculations and model refinements are required to clarify the situation. Comparisons of Hα intensities observed on different nights with the same scattering geometry reveal a clear inverse relationship between the geocoronal Hα emission rate and the exospheric temperature based on the Jacchia 1977 model. Preliminary calculation shows this Hα intensity variation agrees with the results of several satellite experiments which found that the variation of the exobase hydrogen density with temperature is less rapid than that predicted by consideration of Jeans escape as the only hydrogen escape mechanism.