THE GALILEO AND PIONEER-VENUS ULTRAVIOLET SPECTROMETER EXPERIMENTS - SOLAR LYMAN-ALPHA LATITUDE VARIATION AT SOLAR MAXIMUM FROM INTERPLANETARY LYMAN-ALPHA OBSERVATIONS

The Galileo ultraviolet spectrometer and Pioneer Venus Orbiter ultraviolet spectrometer (PVOUVS) experiments performed nearly simultaneous observations of the interplanetary Lyman-alpha-emissions (IP Lyman-alpha) at solar maximum in 1990. The Galileo ultraviolet spectrometer experiment consists of an ultraviolet spectrometer (UVS) and an extreme-ultraviolet spectrometer (EUVS) both of which began acquiring IP Lyman-alpha data late in 1989. The PVOUVS has been in operation since 1978. A comparison is made of the latitude variation of the IP Lyman-alpha signal in 1986 at solar minimum from Pioneer Venus (PV) and in 1990 at solar maximum from Galileo. The Galileo EUVS shows a large enhancement of the IP Lyman-alpha emission has occurred over the intervening 4 years near the solar equator. An IP Lyman-alpha model has been developed which considers the latitude variation of the solar Lyman-alpha flux. The model is based upon the fraction of the solar disk occupied by plage regions. Low-latitude plage regions are the source of the enhanced Lyman-alpha flux near the solar equator. The model fit to the data shows a 25% decrease of the full disk solar Lyman-alpha flux from solar equator to solar pole in 1990. At the extremes of the solar cycle the dominant causes of IP Lyman-alpha asymmetries have now been determined from UVS observations spanning the region ecliptic pole to equator. The latitudinal variations in the solar Lyman-alpha radiation field are the explanation at solar maximum and the solar wind field is the basis at solar minimum. A detailed study of the Galileo IP Lyman-alpha observations on day-of-year (DOY) 190, 193, 197, and 200 in 1990 reveals large daily-variations occur in response to the 27 day solar variation. Analysis of these data shows a maximum variation of 20% can be expected in the IP Lyman-alpha upwind intensity over this 27 day period.