Active‐region evolution and solar rotation variations in solar UV irradiance, total solar irradiance, and soft X rays

Abstract

Variations in the solar UV spectral irradiance, the total solar irradiance, and solar soft X ray emission over days and weeks are analyzed by using concurrent measurements from the NIMBUS 7 and GOES satellites. The UV variations at wavelengths (160–400 nm) of interest to ozone photochemistry and the physics of the middle atmosphere are emphasized. The nonflare variations studied are caused by two types of processes: (1) the birth, growth, peak, and decay of solar active regions and (2) solar rotation, for example, when portions of active regions are occulted near the solar limb. The observations are interpreted by using simple empirical models that relate ground‐based observations of the size and location of sunspots and plages to the full‐disk temporal variations. The major dips observed in the total solar irradiance on August 20–23 and November 6–10, 1979, and April 8, May 25, and September 1, 1980, are accompanied by only average UV and X ray enhancements. The models show that the active regions associated with the sunspots responsible for the major dips in the total solar irradiance caused strong but not outstanding emission enhancements at UV and X ray wavelengths. The solar rotation variations are studied as a function of the central meridian distance of solar active regions where the central meridian distance width at half the maximum is narrowest for decreases in the total solar irradiance (∼83°), slightly wider and also a function of wavelength for UV enhancements (∼95° at 198 nm), wider still for the 10.7 cm radio flux (∼150°), and extremely wide for soft X rays (≳205°).