INTERPLANETARY LYMAN-ALPHA OBSERVATIONS FROM PIONEER-VENUS OVER A SOLAR-CYCLE FROM 1978 TO 1992

The Pioneer Venus Orbiter ultraviolet spectrometer (PVOUVS) routinely obtained interplanetary hydrogen Lyman alpha data while viewing eclipt ic latitudes near 30 degrees S from 1978 to 1992 (during solar cycles 21 and 22). We describe ''hot'' models for this interplanetary Lyman a lpha data that include the solar cycle variation of (1) the solar flux , as a function of latitude and longitude; (2) the radiation pressure on hydrogen atoms; (3) the salar wind flux; (4) the solar EUV flux; an d (5) the multiple scattering correction to an optically thin radiativ e transfer model. These models make use of solar radiation flux parame ters (solar wind, solar EUV, and solar Lyman alpha) from spacecraft an d ground-based solar proxy observations. Comparison of the upwind data and model indicates that the ratio of the solar Lyman alpha line cent er flux (responsible for the interplanetary signal) to the observed so lar Lyman alpha integrated flux is constant to within similar to 20%, with an effective line width near 1.1 Angstrom. Averaging the solar ra diation pressure and hydrogen atom lifetime over 1 year before the obs ervation reproduces the upwind intensity time variation but not the do wnwind. A better fit to the downwind time series is found using the 1 year average appropriate for the time that the atoms passed closest to the sun. Solar Lyman alpha measurements from two satellites are used in our models. Upper Atmosphere Research Satellite (UARS) solar Lyman alpha measurements are systematically higher than Solar Mesosphere Exp lorer (SME) values and have a larger solar maximum to solar minimum ra tio. UARS-based models work better than SME-based models in fitting th e PVOUVS downwind time series Lyman alpha data.