Solar ultraviolet irradiance variations: A review

Abstract

Despite the geophysical importance of solar ultraviolet radiation, specific aspects of its temporal variations have not yet been adequately determined experimentally, nor are the mechanisms for the variability completely understood. Satellite observations have verified the reality of solar ultraviolet irradiance variations over time scales of days and months, and model calculations have confirmed the association of these short‐term variations with the evolution and rotation of regions of enhanced magnetic activity on the solar disc. However, neither rocket nor satellite measurements have yet been made with sufficient accuracy and regularity to establish unequivocally the nature of the variability over the longer time of the 11‐year solar cycle. The comparative importance for the long‐term variations of local regions of enhanced magnetic activity and global scale activity perturbations is still being investigated. Solar ultraviolet irradiance variations over both short and long time scales are reviewed, with emphasis on their connection to solar magnetic activity. Correlations with ground‐based measures of solar variability are examined because of the importance of the ground‐based observations as historical proxies of ultraviolet irradiance variations. Current problems in understanding solar ultraviolet irradiance variations are discussed, and the measurements planned for solar cycle 22, which may resolve these problems, are briefly described.

Long-term variations in tweek reflection height in the D and lowerEregions of the ionosphere

“…[28] The Lyman-a intensity increases by 1.7-2.7 times at solar maximum compared with that at solar minimum [Lean, 1987;Danilov, 1998]. If the ionization production rate of NO + (Lyman-a) increased threefold, the electron density would increase by about 4.0 × 10 2 cm −3 , which corresponds to an almost doubling in the electron density at an altitude of 95 km.…”

A neural network‐based local model for prediction of geomagnetic disturbances

“…These two parts cannot be separated un- [Greener and Schlapp, 1979], and at high latitudes much of the variability depends on the solar wind conditions and on magnetospheric processes. The role of F•0.7 is mainly to account for variations of the solar ionizing radiation on a solar-rotational or solar-cycle time scale [Lean, 1987], and not to account for the day-to-day variability.…”

Extension of the F10.7 Index to 1905 using Mt. Wilson Ca K Spectroheliograms

“…Donnelly et al, 1983), which arise predominantly from the plage and network structures, and vary mainly due to changes in their projected area on the solar disk (e.g. Lean, 1987). It is superior in this use to the CaK plage index (Swartz and Overbeek, 1971) because it covers the full disk, i.e., it captures the contribution to UV emissions arising from changes in the chromospheric network, as well as from changes in the active region plages.…”