HORIZONTAL DISTRIBUTION OF HYDROGEN IN THE SUMMER THERMOSPHERE FROM AE-C SATELLITE MEASUREMENTS

We describe satellite observations and analyses leading to the abundan ce of neutral atomic hydrogen near 300 km and its horizontal distribut ion with latitude and local time through the summer thermosphere. Thes e H concentrations were derived from the charge-exchange equilibrium r elationship between hydrogen and oxygen atoms and ions, with basic in situ Measurements accomplished in the F region with the AE-C satellite around the June 1974 solstice near a solar minimum. Critical observat ional information concerning the low ion densities of H+ was provided by the magnetic ion mass spectrometer (MIMS) on board this spacecraft. A partitioning of the available database serves to illustrate the diu rnal variability of thermospheric hydrogen in several prechosen zones of latitude to the 68 degrees orbital inclination of AE-C; an alternat ive approach accentuates the dependence on latitude in select local li me intervals. Observed seasonal changes reflect a general decrease in hydrogen density from the equatorial region toward the summer pole, wi th the largest latitudinal gradients near predawn and the smallest dur ing afternoon. We demonstrate reasonable consistency of the broad hori zontal behavior with other available observations and the mass spectro meter/incoherent scatter 1986 atmospheric model, both for the extensiv ely studied diurnal variation at low latitude and the lesser-known lat itudinal dependence. Of special significance are apparent effects on t he thermospheric H abundance and distribution associated with several geomagnetic disturbances during the extended time period of data acqui sition. These features are most discernible as perturbations in the re corded diurnal variation but also alter the latitudinal gradients. We include a brief discussion regarding the observed relationship between local H concentration and the temperature as they vary through the su mmer thermosphere, noting distinctly different power law behaviors in the hydrogen distribution for temperatures lesser and greater than sim ilar to 800 degrees K.