  The solar corona is orders of magnitude hotter than the underlying
photosphere, but how the corona attains such high temperatures is still not
understood. Soft X-ray (SXR) emission provides important diagnostics for
thermal processes in the high-temperature corona, and is also an important
driver of ionospheric dynamics at Earth. There is a crucial observational gap
between ~0.2 and ~4 keV, outside the ranges of existing spectrometers. We
present observations from a new SXR spectrometer, the Amptek X123-SDD, which
measured the spatially-integrated solar spectral irradiance from ~0.5 to ~5
keV, with ~0.15 keV FWHM resolution, during sounding rocket flights on 2012
June 23 and 2013 October 21. These measurements show that the highly variable
SXR emission is orders of magnitude greater than that during the deep minimum
of 2009, even with only weak activity. The observed spectra show significant
high-temperature (5-10 MK) emission and are well fit by simple power-law
temperature distributions with indices of ~6, close to the predictions of
nanoflare models of coronal heating. Observations during the more active 2013
flight indicate an enrichment of low first-ionization potential (FIP) elements
of only ~1.6, below the usually-observed value of ~4, suggesting that abundance
variations may be related to coronal heating processes. The XUV Photometer
System Level 4 data product, a spectral irradiance model derived from
integrated broadband measurements, significantly overestimates the spectra from
both flights, suggesting a need for revision of its non-flare reference
spectra, with important implications for studies of Earth ionospheric dynamics
driven by solar SXRs.
