Interplanetary Lyman α observations from Pioneer Venus over a solar cycle from 1978 to 1992

Abstract

The Pioneer Venus Orbiter ultraviolet spectrometer (PVOUVS) routinely obtained interplanetary hydrogen Lyman α data while viewing ecliptic latitudes near 30°S from 1978 to 1992 (during solar cycles 21 and 22). We describe “hot” models for this interplanetary Lyman α 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 solar wind flux; (4) the solar EUV flux; and (5) the multiple scattering correction to an optically thin radiative transfer model. These models make use of solar radiation flux parameters (solar wind, solar EUV, and solar Lyman α) from spacecraft and ground‐based solar proxy observations. Comparison of the upwind data and model indicates that the ratio of the solar Lyman α line center flux (responsible for the interplanetary signal) to the observed solar Lyman α integrated flux is constant to within ∼20%, with an effective line width near 1.1 Å. Averaging the solar radiation pressure and hydrogen atom lifetime over 1 year before the observation reproduces the upwind intensity time variation but not the downwind. 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 α measurements from two satellites are used in our models. Upper Atmosphere Research Satellite (UARS) solar Lyman α measurements are systematically higher than Solar Mesosphere Explorer (SME) values and have a larger solar maximum to solar minimum ratio. UARS‐based models work better than SME‐based models in fitting the PVOUVS downwind time series Lyman α data.