Variability of the solar flux in the far ultraviolet 1175–2100 Å

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Abstract
We examine the expected range of variability over the solar cycle of the far UV flux (irradiance) in selected lines and continua within the wavelength range 1175–2100 Å which results from a simple two‐component model of the full‐disk flux. We determine contrast factors I(plage)/I(quiet region) for various lines and continua and estimate the fraction of the disk that is covered by plages at various times in the solar 11‐year cycle. We assume (1) that enhanced far UV emission arises from the same plage areas visible in Ca II emission, (2) that our contrast values are typical, and (3) that both the plage and quiet region intensities per unit surface area remain constant over the solar cycle, so that flux variability is caused only by a greater or lesser fraction of surface area covered by plages. We discuss available observational evidence for these assumptions. Our results are in good agreement with the Lα fluxes published by Vidal‐Madjar [1975] and the recent broadband observations by Hinteregger (private communication, 1979), but they are in disagreement with the flux variability reported by Heath and Thekaekara [1977] over the 1700–3000‐Å range.