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(AE-E) spacecraft about 15 years ago. These measurements revealed a considerable increase in the solar EUV flux during the ascending phase of solar cycle 21. Some emissions at wavelengths shorter than 30 nm increased by factors of 10 to 100 between solar minimum conditions in 1976 and maximum activity in 1980. These emissions emanate from the highest, hottest layers of the Sun's atmosphere (the solar corona). Radiation at wavelengths between 30 and 120 nm, formed lower in the solar atmosphere (the chromosphere), varied somewhat less, by factors of two to three from the minimum to the maximum of activity in solar cycle 21. At still longer UV wavelengths, solar cycle variability decreases from a factor of two near 100 nm to about 10 percent near 200 nm. In addition to the overall change in solar radiation between solar minimum and maximum, the AE-E data showed shorter term fluctuations on a monthly, daily, and even an hourly basis, with the coronal emissions being much more variable than the chromospheric emissions.

Essentially all interpretive studies of upper atmosphere phenomena now use scenarios of solar variability derived from the AE-E data base. However, AE-E did not monitor the highly variable soft X-rays, nor do the AE-E data agree with earlier rocket measurements about either the magnitude or the variability of the EUV irradiance (Lean, 1988). Concerns about the validity and limitations of the AE-E data base continue to be raised. AE-E's absolute irradiance calibration was derived from two Air Force Geophysics Laboratory rocket measurements, one during 1974 (which preceded the AE-E data) and another in 1979 (Heroux and Hinteregger, 1978; L. Heroux, private communication, 1981). Possible changes in the sensitivity of the AE-E instruments throughout the mission are unknown, since no provision was made for in-flight calibration. A comparison of the 1979 rocket measurement used for the AE-E calibration with a recent rocket measurement (Woods and Rottman, 1990) indicates significant inconsistencies in that only the strongest emission lines were enhanced in the 1979 spectrum, for which solar activity levels were higher. This contradicts current understanding of the origin of the EUV irradiance variations, which predicts that solar activity causes an increase in the EUV radiation at all wavelengths. The discrepancy is most likely the result of instrumental effects (see Lean, 1990 for details).

AE-E ceased operation at the end of 1980. In the ensuing decade only a few isolated measurements of the solar EUV spectral irradiance were



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