Impact of Solar EUV, XUV, and X‐Ray Variations on Earths's Atmosphere

Solar extreme ultraviolet (EUV and XUV) radiation (1-103 nm) is the main source of energy and ionization of the upper atmosphere. Changes in EUV flux over the solar cycle and solar rotation time-scales give rise to large changes in neutral temperature and density, and ion density. Thermal expansion of the upper atmosphere over the solar cycle changes the drag on low-Earth orbiting spacecraft by a factor of ten. The increase in plasma density impacts propagation of high-frequency (HF) radio communication signals, and introduces positioning errors by increasing the phase delay in single frequency Global Positioning System navigation signals. Reasonable agreement between observed and globally averaged models of the energy budget and ionospheric production indicates a high level of maturity in our understanding of aeronomic processes initiated by solar photon impact of Earth's atmosphere. Much of our current understanding is based on satellite data from over twenty years ago, but higher resolution observations at EUV and XUV wavelengths may shed light on some of the remaining discrepancies in the energy budget. On time-scales less than a day, the flux at X-ray wavelengths (0.1-0.8 nm) are highly variable, and can increase by three orders of magnitude during a major solar flare. During these events, D-region ion density increases and follows the time history of the flare, causing absorption of HF radio signals and disruption of low frequency navigation systems. EUV flares have also been observed on these same time-scales and can change EUV flux by ∼50% for a short period (less than an hour) and have a modest impact on neutral and plasma density in the upper atmosphere. Ionization by soft X-rays (XUV; 1-30 nm) are an important source of plasma in the E-region ionosphere, contribute significantly to the higher energies in the photoelectron spectrum, and are dominant in the time-varying production of low-latitude nitric oxide in the lower thermosphere. Recent measurements of the flux of soft X-rays by SNOE have largely resolved discrepancies in the observed and modeled photoelectron spectrum and E-region electron density profile. It remains to be seen if modeled solar produced NO densities can now be brought into agreement with observations at low latitudes, including the diurnal and latitude structure and the relative contribution of molecular ions.