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Session 74 - The Quiet & Active Sun.
Display session, Friday, January 09
The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite has been measuring the sun's UV spectral irradiance for wavelengths from 115 to 410 nm since October 1991. We describe the behavior of the solar UV spectral irradiance during the decline of solar cycle 22 with special emphasis on the recent solar minimum period. Although the time series of the spectral irradiances have behavior similar to that of the Mg II core-to-wing ratio, there are differences among them. We describe the behavior near solar minimum of both the irradiance in selected UV wavelength intervals, the the Mg II core-to-wing ratio, and the integrated Ly-\alpha line irradiance. Overall, each of these time series follows a similar pattern: a broad and flat minimum, with some 27-day solar rotation modulation still evident at nearly all times. The date found for solar minimum is often used as an input parameter used in predicting the timing of the following solar cycle maximum. The minimum strongly depends on both the algorithm and the averaging time period used. For sunspot number, the generally accepted minimum is defined as the lowest value of a 12-month running average of monthly averages of daily values. We analyze various methods for finding the solar minimum in the UV including that used for sunspot number. For wavelengths longer than the Al edge at 208 nm, systematic errors due to instrument responsivity degradation (\sim \pm\hbox1%) become a significant fraction of the long-term variation during the minimum, making determination of the date of the minimum increasingly uncertain. We find that the minimum for Mg II occurs during April 1996 while that for Ly-\alpha occurs later, during October 1996. These results are compared with solar minimum behavior of other solar indices to better understand the relationship among the regions of the solar atmosphere where each emission originates.
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