Lyman-alpha measurements of neutral hydrogen in the outer geocorona and in interplanetary space

Hydrogen Lyman-α (1216 A) measurements were made on a continuous basis by a two-channel photometer on Ogo 5 from March 1968 to June 1971. The highly elliptical orbit provided measurements of both the outer geocorona and of the 1216-A sky background emission, since geocoronal scattering is minimal at the apogee distance of 150,000 km. We present selected data (through 1970) and an interpretation of the three principal discoveries to date, namely, (1) a pronounced antisolar enhancement of the geocoronal scattering beyond 70,000 km, which we believe is evidence for a hydrogen 'geotail' produced by solar Lyman-alpha radiation pressure; (2) a clear correlation of periodic variations in the sky background emission with solar activity associated with solar rotation; and (3) an annual variation of the 1216-A sky background emission, caused by the earth's orbital motion within the cavity created by the solar wind in the nearby interstellar hydrogen.