SOLAR LYMAN-ALPHA IRRADIANCE MEASUREMENTS DURING 2 SOLAR-CYCLES

The solar Lyman alpha radiation is the dominant source of energy for t he Earth's mesosphere; thus accurate knowledge of its variability is i mportant for many studies of the middle and upper atmosphere. The meas urements of the solar Lyman a irradiance taken during solar cycles 21 and 22 by the Atmospheric Explorer-E (AE-E), the Solar Mesospheric Exp lorer (SME), and the Upper Atmosphere Research Satellite (UARS) are co mpared and then combined to construct a long-term data set. The UARS m easurements, from both the Solar Ultraviolet Spectral Irradiance Monit or (SUSIM) and Solar Stellar Irradiance Comparison Experiment (SOLSTIC E), have the best photometric accuracy yet achieved with 2-sigma uncer tainties of 10%; thus the SME and AE-E measurements, which have 2-sigm a uncertainties of 40% and 60% respectively, are adjusted to agree wit h the UARS values. Because none of these satellite measurements overla pped in time, solar indices representative of the upper chromosphere a re used in determining these adjustments. The solar indices considered for this analysis are the National Oceanic and Atmospheric Administra tion (NOAA) Mg II core-to-wing ratio, the National Solar Observatory ( NSO) He 1083 nm equivalent width, the Penticton (Ottawa) 10.7 cm flux, and the Pioneer Venus Langmuir probe solar signal. For this long-term data set, the SME Lyman alpha irradiances are adjusted up by 25%, and the AE-E Lyman alpha irradiances are scaled down by a time dependent factor ranging from 10% to 40%. The relationships to the solar indices are also presented so that the estimates for the solar Lyman alpha ir radiance can be extended back as far as 1947 using the solar 10.7 cm f lux with an estimated uncertainty of 20%.