Temporal characteristics of the solar UV flux and He I line at 1083 nm

Abstract

Temporal variations of the Nimbus 7 measurements of solar UV flux, important for their stratospheric effects, are compared with ground‐based measurements of the solar infrared He I absorption line at 1083 nm. The close similarity of their temporal characteristics shows that the 1083‐nm line is a better estimator of the UV flux than the classical indices of solar activity, the 10.7‐cm radio flux and the sunspot number, for short time scales (days, weeks). The power spectrum of the He I line intensity matches that of the Nimbus 7 205‐nm flux at the 27‐day period peak but is weaker at the peak near 13 to 14 days period. The 27‐day peak is caused by the combination of solar rotation of active regions with one major concentration in their solar longitude distribution, and the 13‐day case involves two concentrations with solar longitude roughly 180° apart. The 13‐day periodicity is not simply a second harmonic of the 27‐day periodicity, because some episodes of activity are dominated by the 13‐day periodicity with very weak 27‐day periodicity while other episodes are dominated by 27‐day periodicity with weak 13‐day periods. These episodes of activity, which last typically 4 to 8 months, are caused mainly by groups of strong active regions that dominate the solar‐rotational variations for several months. In addition to the enhanced short‐term modulation during these episodes, the valleys in the solar‐rotational modulation also slowly rise and decay. F 10 and R tend to rise more steeply and peak earlier during these episodes than the UV flux and the He I line.