Comparative Solar Seeing and Scintillation Studies at the Fuxian Lake Solar Station

Abstract

Starting November 1999 we are carrying out simultaneous seeing observations with the Solar Differential Image Motion Monitor (S-DIMM) at the Fuxian Lake station of the Yunnan Observatory and a solar scintillometer of the type used in the recent site survey by one of us (Beckers et al., 1997). The purpose was to compare the two methods of assessing the daytime atmospheric seeing for a lake site. We report here the first results of this comparison. We find that the relation between the seeing as measured by the S-DIMM (the Fried parameter r ₀) and the scintillation in the solar irradiance (σ_I) differs greatly from the relation found by Seykora (1993) for NSO/Sac Peak. We conclude that the σ_I measurements give a good indication for the amount of near-Earth seeing but that they are a poor proxy for the total atmospheric seeing. We interpret the simultaneous (r ₀, σ_I) observations in terms of an atmospheric seeing model and find good quantitative agreement with a model in which a fraction (α) of the seeing originated near the Earth (ground or water) and the rest (1−α) originates at higher layers. For lake sites α is small all day and the seeing is determined primarily by the refractive index variations at higher atmospheric layers. For land sites α is small in the early morning but rapidly increases as the day progresses, near-Earth seeing dominating there most of the time.