Solar Ultraviolet Variability Over Time Periods of Aeronomic Interest

The solar ultraviolet (UV) radiation is a primary energy source for planetary atmospheres and is also a tool for remote sensing of the planets. The solar UV radiation from mostly below 300 nm dissociates molecules, ionizes the neutral atmosphere, and affects many chemical cycles in the atmospheres. In addition, solar photons scatter off the molecules and atoms in the atmospheres and provide a method of remote sensing the composition and density of the atmospheres. For such aeronomic studies, accurate values of the solar UV irradiance, primarily shortward of 200 nm, are needed over time periods of days to decades. A planet's orbital motion around the Sun and the intrinsic solar variability are the primary causes of the variation of the solar intensity at a planet. The insolation changes caused by the orbital motion are easily calculated as the inverse square of the Sun-planet distance; however, the intrinsic solar irradiance variability is more complicated and is strongly dependent on wavelength and time. The primary short-term irradiance variability over several days is caused by the solar rotation, which has a mean period of 27 days. The primary long-term variability is related to the solar dynamo that reverses the solar magnetic field and is known as the 11-year solar cycle. The variations observed during solar cycle 22 (1986-1996) are the basis for creating some reference spectra of the solar UV irradiance for use in comparative aeronomic studies. From these reference spectra, the solar cycle variability as a function of wavelength can be characterized as 15% and less at wavelengths longward of 160 nm, as 15% to 70% between 160 and 65 nm, and as a factor of 1.5 to 7 between 65 and 1 nm. The magnitude of the 27-day rotational variability is usually no more than one third of the solar cycle variability. There is not a smooth transition of variability between wavelengths, but instead the amount of intrinsic solar variability depends on the source of the radiation in the solar atmosphere. In general, the coronal emissions vary the most, then the transition region emissions vary somewhat less, chromospheric emissions vary even less, and finally the photospheric emissions vary the least, perhaps only 0.1% at the longest UV wavelengths.