Solar EUV and XUV energy input to thermosphere on solar rotation time scales derived from photoelectron observations

Abstract

[1] Solar radiation below $100 nm produces photoelectrons, a substantial portion of the F region ionization, most of the E region ionization, and drives chemical reactions in the thermosphere. Unquantified uncertainties in thermospheric models exist because of uncertainties in solar irradiance models used to fill spectral and temporal gaps in solar irradiance observations. We investigate uncertainties in solar energy input to the thermosphere on solar rotation time scales using photoelectron observations from the FAST satellite. We compare observed and modeled photoelectron energy spectra using two photoelectron production codes driven by five different solar irradiance models. We observe about 1.7% of the ionizing solar irradiance power in the escaping photoelectron flux. Most of the code/model pairs used reproduce the average escaping photoelectron flux over a 109-day interval in late 2006. The code/model pairs we used do not completely reproduce the observed spectral and solar rotation variations in photoelectron power density. For the interval examined, 30% of the variability in photoelectron power density with equivalent wavelengths between 18 and 45 nm was not captured in the code/model pairs. For equivalent wavelengths below $16 nm, most of the variability was missed. This result implies that thermospheric model runs based on the solar irradiance models we tested systematically underestimate the energy input from ionizing radiation on solar rotation time scales.

Electron conic distributions produced by solar ionizing radiation in planetary atmospheres

“…Electron energy spectra from the Fast Auroral SnapshoT (FAST) satellite (Carlson et al, 2001) obtained under conditions of solar illumination (solar zenith angle, SZA < 90°) equatorward of the auroral field lines have previously been reported by Peterson et al (2008Peterson et al ( , 2009Peterson et al ( , 2012. Less intense fluxes of photoelectrons are also produced on the nightside of the solar terminator where there is some high altitude illumination when the SZA is larger than 90°.…”

“…Corrections for background noise and varying spacecraft potentials have been applied as described in Peterson et al (2012). After about 01:07 the satellite entered the auroral region, which is characterized by rapid changes and more energetic electron distributions.…”

Far- and Extreme-UV Solar Spectral Irradiance and Radiance from Simplified Atmospheric Physical Models

“…This variability at the very short wavelengths is comparable to the largest variability EVE observed at the lines near 28 and 34 nm, but our calculations does not include flares. When flares are included, the variations at very short wavelengths are probably larger than those shown in Figure 15 and could have significant effects on photoelectrons produced in the Earth's upper atmosphere, see Peterson et al (2012).…”

Measuring Solar Doppler Velocities in the He ii 30.38 nm Emission Using the EUV Variability Experiment (EVE)

“…This includes capturing solar flare variations that can range from seconds to hours and significantly affect the Earth's ionosphere and thermosphere (Qian et al, 2010(Qian et al, , 2011. The extension of EVE data down to the XUV wavelengths for the first time with this spectral resolution was also critical for studies of photoelectron production at Earth and Mars (Peterson et al, 2012(Peterson et al, , 2013Xu et al, 2015). The XUV and EUV solar irradiance variations have also been shown to influence the ionosphere of Mars (Withers, 2009;Lollo et al, 2012) and the charging of the lunar dust (Sternovsky et al, 2008).…”