Solar extreme ultraviolet irradiance: Present, past, and future

Abstract

[1] New models of solar extreme ultraviolet (EUV) irradiance variability are constructed in 1 nm bins from 0 to 120 nm using multiple regression of the Mg II and F 10.7 solar activity indices with irradiance observations made during the descending phase of cycle 23. The models have been used to reconstruct EUV spectra daily since 1950, annually since 1610, to forecast daily EUV irradiance and to estimate future levels in cycle 24. A two-component model developed by scaling the observed rotational modulation of the two solar indices underestimates the solar cycle changes that the Solar EUV Experiment (SEE) reports at wavelengths shorter than 40 nm and longer than 80 nm. A three-component model implemented by including an additional term derived from the smoothed Mg II index better reproduces the measurements at all wavelengths. The three-component model is consistent with variations in the EUV energy from 0 to 45 nm that produces the far ultraviolet (FUV) terrestrial dayglow observed by the Global Ultraviolet Imager (GUVI). However, the spectral structure of this third component is complex, and its origin is uncertain. Analogous two-and three-component models are also developed with absolute scales determined by the NRLEUV2 spectrum of the quiet Sun rather than by the SEE average spectrum. Assessment of the EUV absolute spectrum and variability of the four different models indicate that during solar cycle 23, the EUV irradiance (0 to 120 nm) increased 100 ± 30%, from 2.9 ± 0.2 to 5.8 ± 0.9 mWm −2 , and may have been as low as 1.9 ± 0.5 mWm −2 during the 17th-century Maunder Minimum. Near the peak of upcoming solar cycle 24, EUV irradiance is expected to increase 40% to 80% above the 2008 minimum values.

Solar forcing for CMIP6 (v3.2)

“…The Naval Research Laboratory (NRL) family of SSI models (Lean, 2000;Lean et al, 2011) is based on the premise that changes in solar irradiance from background-quiet Sun conditions can be described by a balance between bright facular and dark sunspot features on the solar disk. These two contributions are determined by linear regression between solar proxies, and direct observations of TSI and SSI by satellite missions such as SORCE (Rottman, 2005).…”

Are the sunspots really vanishing?

“…Exceptionally low levels of the UV irradiance were measured by several instruments. Relative to the solar cycle 22-23 minimum, the SEM UV fluxes were 10% lower and the MgII core-to-wing ratio, a key proxy for the chromosphere and low corona, was 10% lower, an unprecedented level Solomon et al 2010Solomon et al , 2011Woods 2010b;Lean et al 2011a). Tapping & Valdés (2011) also show a drop of the minimum F 10.7 radio flux below the average of the four previous minima, but it is comparatively small (3%).…”

How does ionospheric TEC vary if solar EUV irradiance continuously decreases?

“…It is unclear how solar EUV and also the ionosphere varied during the extended extremely low solar activity periods such as the Maunder Minimum. Some researchers have attempted to derive the EUV irradiance during these extreme periods by solar irradiance models (e.g., Lean et al 2011); the results reveal that EUV irradiance potentially decreased to an extremely low level during the Maunder Minimum period (see Figure 1). Solar activity has recently undergone a deep minimum during 2007 to 2009.…”