Long-Term Variation of the Interplanetary H Lyα Glow: Voyager UVS Measurements and Implications for the Solar H Lyα Irradiance

Giuliana de Toma (1) , Eric Quémerais (2) , Bill R. Sandel (3)

1 HAO - High Altitude Observatory
2 SA - Service d'aéronomie
3 LPL - Lunar and Planetary Laboratory [Tucson]

In this paper we study interplanetary (IP) Lya data taken with the Voyager 1 and Voyager 2 spacecraft from 1980 to 1995. The coverage in time is equal to about 156 and 220 points yr~1 for Voyager 1 and Voyager 2, respectively, with almost no gaps. The IP Lya data are normalized for spatial changes in the emissivity, which arise from variations in observing geometry, by using a radiative transfer model. The normalized data show the variation of the solar H Lya line-center Ñux during the solar cycle. We compare this variation with the solar H Lya irradiance measurements of integrated Ñux from the Solar Mesosphere Explorer and the Upper Atmosphere Research Satellite/Solar-Stellar Irradiance Comparison Experiment (SOLSTICE), and, when direct solar measurements are not available, we use estimated irra-diances from magnesium and helium indices. The comparison between Voyager IP data and solar data shows that the best agreement is found with the SOLSTICE set of measurements, when no di †erences in the variation of the line-center Ñux and the integrated Ñux are taken into account.

Domaines: Astrophysique stellaire et solaire [astro-ph.SR]

Giuliana de Toma, Eric Quémerais, Bill R. Sandel. Long-Term Variation of the Interplanetary H Lyα Glow: Voyager UVS Measurements and Implications for the Solar H Lyα Irradiance. The Astrophysical Journal, 1997, 491 (2), pp.980 - 992. ⟨10.1086/304966⟩. ⟨insu-02902249⟩