OGO 5 Measurements of the Lyman Alpha Sky Background

We describe a series of sateffite measurements of the Lyman alpha sky background radiation at radial distances from the earth of more than 80000 km. A two-channel ultraviolet photometer on OGO 5, responding to the spectral regions 1050 A-1800 A and 1225 A-1800 A, measured the spatial variations of the 1216 A background during three spin-up manoeuvres in September 1969, December 1969 and April 1970, Observations of a large number of hot stars were useful in determining the viewing direction as a function of time. It is shown that a weighted difference of the signals from the two channels represents the Lyman alpha 1216 A background, free from the contribution of unresolved stars. We present the results in the form of iso-intensity contour maps in ecliptic, celestial, and galactic coordinate systems. The most significant result is the discovery of a prominent maximum in intensity (570 Rayleighs) and a less pronounced minimum (240 Rayleighs) both located near the ecliptic and separated by about 180 degrees. Comparison of the results from the September and April observations indicates the existence of time variations in the spatial structure of up to 70 Rayleighs. The present results are in excellent agreement with those of Bertaux and Blamont (this issue) and with less extensive measurements by previous investigators. We place an upper limit of about 250 Rayleighs for the contribution of galactic sources, which would be expected to be very nearly isotropic. The most likely origin of the observed emission is scattering of solar Lyman alpha from interstellar hydrogen that is swept into the vicinity of the earth from the direction of the apparent apex of motion of the solar system. The observed displacements of the maximum and nnnimum along the ecliptic over a 6-month period are explained by a parallax effect due to the Earth's motion in its orbit. This explanation requires that the effective scattering region be located well within the solar system. Key words: OGO 5 - Lyman alpha - interstellar medium - interplanetary medium.