Vela 4 Lyman-α observations: evidence for an aspherical hydrogen geocorona at 18 R/sub E/

Observations of scattered solar hydrogen Lyman-α radiation made with detectors carried by satellites Vela 4A and 4B show a repetitive and marked asymmetry in the Lyman-α flux. The detectors look radially outward from the earth at an altitude of 110,000 km. The axis of the 34° angle of view lies in the plane of the circular satellite orbit, which is inclined 32° to the equator. The distinctive characteristic of the data for the period May, 13, 1967, to July 31, 1967, is a broad maximum in the flux. In a preliminary interpretation, we assumed the detectors were above the hydrogen geocorona and that the scattering hydrogen, which gives rise to the broad maximum, must be in interplanetary space. Detailed analysis of data for many orbits indicates that the most precisely located feature of the data, the leading edge of the broad maximum, moves in right ascension at about the same rate and in the same direction as the motion of the earth projected on the celestial sphere. Thus, a significant part of the scattering hydrogen moves with the earth, and the observations are interpreted as evidence for an aspherical geocorona with an increased density of hydrogen beyond 18 RE on the night side. The view direction is above the ecliptic plane when the maximum is observed. After an estimated interplanetary component is subtracted from the observed maximum flux, the scattered Lyman-α intensity from telluric hydrogen beyond 18 RE is about 150 rayleighs. The column density of hydrogen above 18 RE when the maximum is observed is 6 to 7 × 1010 atom cm−2 in the plane of the satellite orbit.