Improved solar Lyman alpha irradiance modeling from 1947 through 1999 based on UARS observations

The solar Lyman alpha radiation is the brightest solar vacuum ultraviolet ( VUV: lambda < 200 nm) emission, and this radiation is deposited in Earth's atmosphere above 70 km. The Lyman <alpha> irradiance and its variability ar e therefore important for many studies of the Earth's upper atmosphere. A l ong-term data set of the solar Lyman alpha irradiance from 1947 through 199 9 is constructed using the measurements from the Atmospheric Explorer E (AE -E), the Solar Mesospheric Explorer (SME), and the Upper Atmosphere Researc h Satellite (UARS) along with predictions from proxy models to fill in data gaps and to extrapolate back to 1947. The UARS measurement is used as the reference, and the AE-E and SME measurements and the proxy models are adjus ted to agree with the UARS values. The estimated I-a uncertainty for this l ong-term Lyman alpha time series is 10%. The average solar rotation (27-day ) variability in Lyman alpha is 9% from this composite times series, and th e solar rotation variability averaged over 2 years at solar minimum and max imum conditions is 5 and 11%, respectively. The average solar cycle (11-yea r) variability is a factor of 1.5 when the data are smoothed over 2 years, and the extreme Lyman alpha variability is a factor of 2.1. The Lyman alpha irradiances averaged over 2 years during solar minimum and maximum conditi ons are 3.7 and 5.6 x 10(11) photons s(-1) cm(-2), respectively. The proxy models include three components to better fit the UARS measurements; noneth eless, there remain differences between the proxy models and the observed L yman alpha irradiance, which are related to the source of the Lyman alpha r adiation being different than that for the proxies. The available proxies a re primarily chromospheric and coronal emissions, whereas the Lyman alpha v ariability is manifested more in the transition region. It is shown that em issions throughout the chromosphere, transition region, and corona vary dif ferently mainly because their contrasts for active network and plage compon ents are different. A transition region proxy is needed to improve the empi rical proxy models of solar irradiance, and this composite Lyman alpha time series could serve as a proxy for other transition region emissions.