Contributions of the solar ultraviolet irradiance to the total solar irradiance during large flares

Abstract

The solar X‐ray radiation varies more than other wavelengths during flares; thus solar X‐ray irradiance measurements are relied upon for detecting flare events as well as used to study flare parameters. There is new information about the spectral and temporal variations of flares using solar irradiance measurements from NASA's Solar Radiation and Climate Experiment (SORCE) and the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) missions. For one, the improved measurement precision for the total solar irradiance (TSI) measurements by the SORCE Total Irradiance Monitor (TIM) has enabled the first detection of flares in the TSI. These flare observations indicate a total flare energy that is about 105 times more than the X‐ray measurements in the 0.1–0.8 nm range. In addition, solar spectral irradiance instruments aboard TIMED and SORCE have observed hundreds of flare events in the 0.1 nm to 190 nm range. These solar ultraviolet measurements show that the ultraviolet irradiance changes during flares account for 50% or more of the flare variations seen in the TSI, with most of the ultraviolet contribution coming from the ultraviolet shortward of 14 nm. The remaining part of the flare energy is assumed to come from the wavelengths longward of 190 nm, typically only needing to be about 100 ppm increase for the largest flares. Another result is that the flare variations in the TSI have a strong limb darkening effect, whereby the flares near the limb indicate variations in the TSI being almost entirely from the ultraviolet shortward of 14 nm.