Solar Soft X-ray Variations from the 2008 - 2019 Solar Cycle inferred from CORONAS/SphinX, GOES/XRS, Hinode/XRT, MinXSS, NuSTAR, and RHESSI Instruments

The Solar spectral irradiance (SSI) is vital for understanding the physics of all layers of the solar atmosphere from the photosphere to the corona. While most of the contribution to the Total Solar Irradiance (TSI) reside in visible and infrared light, the UV and X-rays have the largest change in magnitude. Quantifying the UV and X-ray variations over the solar cycle is critical for constraining the physics of solar flares, active regions, the quiet Sun, as well as the atmospheres of planets and moons in the heliosphere. The GOES/XRS spectrally integrated 0.1 - 0.8 nm energy flux has been a longstanding diagnostic of soft x-ray variations, but is limited by non-linearities in signal response for low solar flux levels and an observed minimum detection limit. The Hinode/XRT filter images provide a unique alternative proxy for solar soft X-ray flux inferences with larger dynamic range and a lower flux sensitivity. We compare the spectral irradiance estimate from a Hinode/XRT filter-ratio technique results to the lowest spectra measured-to-date between 1.25 - 3 keV by CORONAS/SPhinX in 2009, and MinXSS CubeSat spectra in 2016 - 2019. We also highlight the large variability in the soft X-ray spectra as directly measured by CORONAS/SphinX, MinXSS, NuSTAR, and RHESSI intermittently between 2009 - 2019.