Interpretation of Ogo 5 Lyman alpha measurements in the upper geocorona

Abstract

Lyman α intensity measurements (1216 A) were obtained by a photometer on board the Ogo 5 spacecraft outside the geocorona. The Lyman α emission originating from hydrogen atoms in the upper part of the geocorona was derived from the total measured intensity after subtraction of the extraterrestrial emission. The hydrogen density distribution between 5 and 16 RE was deduced. This distribution is consistent with the evaporative atmospheric model of J. W. Chamberlain on the night side. A depletion of atoms found above a distance of 6 RE on the day side could be accounted for by the action on satellite particles of charge exchange with solar wind protons and by the solar Lyman α radiation pressure, according to numerical computations of both effects. It was also calculated that orbits with a perigee of 2 RE are rapidly distorted by radiation pressure. The mean time spent in the exosphere by satellite particles on such orbits is only 5 × 105 sec, one‐half the ionization lifetime.

Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff

“…Therefore, the interstellar flow vector has been the objective of investigations for years (Axford 1972). First, remote measurements of neutral hydrogen (Bertaux & Blamont 1971) and helium (Weller & Meier 1974) were performed using backscattered solar UV observations. Later on, in situ measurements using pickup ions (PUIs) (Möbius et al 1995) and direct neutral particle observations (Witte 2004) complemented the previous studies.…”

Synopsis of the interstellar He parameters from combined neutral gas, pickup ion and UV scattering observations and related consequences

“…Through the interplay between this wind, the ionization of the neutrals upon their approach to the Sun, and the Sun's gravitational field (distinctly modified by radiation pressure for H), a characteristic flow pattern and density structure is formed with a cavity close to the Sun and gravitational focusing on the downwind side (for all species except H). Starting with the analysis of backscattered solar Lyman α intensity sky maps (Bertaux & Blamont 1971;Thomas & Krassa 1971) the parameters of H became accessible to measurements. Based on early modeling using a cold interstellar gas flow (e.g., Blum & Fahr 1970;Holzer & Axford 1971;Axford 1972;Fahr 1974) the general flow direction and an order of magnitude estimate for the density could be deduced.…”

The inner magnetosphere imager mission

“…The geocorona can be seen in scattered ultraviolet light from strong solar emission in the hydrogen Lyman-a line at 1216 A. Geocoronal Lyman-a observations have been made from rocket [Kupperian et al, 1959;Purcell and Tousey, 1960] and satellite [Mange and Meier, 1970;Meier and Mange, 1973] experiments. Although the geocorona is generally spherical in distribu- lion with a power law dependence on radial distance [Wallace et al, 1970], a tail ward extension [Thomas and Bohlin, 1972], dayside high-altitude depletion [Bertaux and Blamont, 1973], dawn-dusk asymmetry [Meier and Mange, 1973], and high-latitude depletion [Thomas and Vidal-Madjar, 1978] have all been observed. A source of error in observing the geocorona is from background extraterrestrial Lyman-a sources.…”