The Earth hydrogen exobase near a solar minimum

Abstract

Measurements of the solar Lyman alpha flux and of the exobase hydrogen density distribution were made by the University of Paris experiment aboard the Orbiting Solar Observatory (OSO‐5) during the period August 1974 to August 1975. These data were obtained near the end of the solar cycle # 20, at a time of nearly minimum solar activity. To compare most accurately solar minimum and solar maximum conditions, these data were subjected to the same analysis procedures used for OSO‐5 data from 1969‐1972 (Vidal‐Madjar et al., 1973).The results are presented individually and the average exobase hydrogen density is observed to reach 3.5 × 105 atoms cm−3 (at 500 km) for an average exospheric temperature as low as 725 K. This result shows that the non‐thermal escape mechanism increases even more than previously observed. For low solar activity conditions, the non‐thermal escape flux can be more than a factor seven larger than the Jeans escape flux.The non‐thermal mechanisms become increasingly important at low solar activity; the minimum‐to‐maximum density ratio deduced from only dayside measurements is also observed to increase at solar minimum and may reach values as high as 3.7, although the average value is of the order of 2.1.Finally, the dayside, latitudinal sub‐solar bulge monitored from 1969 to 1972 is still observed at solar minimum and presents the same amplitude as during solar maximum conditions.