Title: Improved Modeling of the Solar FUV Irradiance Using the Mg II C/W Index
Author: C. Pankratz
Affil: LASP / Univ. of Colorado
Email: chris.pankratz@lasp.colorado.edu
Authors: T. Woods, G. de Toma, O. R. White, and G. Rottman
Affils: LASP/CU and HAO/NCAR
Abstract: The solar far ultraviolet (FUV: 120-200 nm) radiation is a principle energy input to the upper atmosphere above 50 km. Much of this solar FUV energy is deposited via the photodissociation of molecular oxygen in the upper stratosphere, mesosphere, and thermosphere. In addition, the solar FUV radiation plays a key role in the chemistry of the minor species in the upper atmosphere, such as ozone, water, and nitric oxide. Models of the solar FUV irradiance and its variability are thus needed for detailed studies of the upper atmosphere, especially for time periods when the solar FUV irradiance is not measured. The solar FUV irradiance varies over the 11-year solar cycle by about 20% near 200 nm and by a factor of about 2 at 120 nm. The FUV spectral range includes emissions from the upper photosphere, chromosphere, and transition region between the chromosphere and corona. Understanding the evolution of the different features on the solar disk and formation of their FUV emission provide insight into the causes of the solar FUV irradiance variations with different characteristic times. Relating the irradiance at one wavelength to a solar measurement at another wavelength through a proxy is a simple method for estimating irradiances. Such proxy models are easy to use; but to be accurate, the proxy needs to be carefully chosen and based on the physical characteristics relating the proxy and the irradiance to be estimated. The plages associated with sunspots, active network, and quiet network are the primary sources of variations in the solar FUV irradiance. The plage emission, modulated by the 27-day solar rotation, is the primary source for the short-term (< 40 days) variations. The dispersal of plages into active network followed by the slow decay of the active network into quiet network is the primary source for the long-term (> 40 days) variations. By determining both long-term and short-term components, we derive a new irradiance model with the specific contribution of the active network to the irradiance at different wavelengths. Further splitting of the proxy's short-term component into 27-day and 13-day components permits the irradiance model to account for differences in the center-to-limb variation (CLV) at different wavelengths. Our improved model of the solar FUV irradiance based on the SOLSTICE Mg II core-to-wing index has four components: quiet Sun as a constant, 27-day short-term variation, 13-day short-term variation, and long-term variations with periods > 40 days.