Solar Soft X-ray Spectral Measurements and the Temperature Structure of Active Regions and Flares

How solar and stellar atmospheres are heated to millions of degrees is a fundamental problem in astrophysics. The Parker nanoflare model, in which the topological complexity created by turbulent photospheric motions is dissipated by magnetic reconnection, is perhaps the most widely studied theory of coronal heating. Although this model is conceptually similar to our understanding of how large flares work, recent results suggest that they may be fundamentally different. Large flares, for example, have a peak in the emission measure distribution near 10 MK, while active regions appear to have relatively little plasma at that temperature. For large flares, several studies have indicated a composition close to that of the photosphere, while active region structures show a clear enhancement in elements with low first ionization potential. These results rely on observations at extreme ultraviolet wavelengths, which do not provide the rich array of temperature and abundance diagnostics that are available at soft X-ray wavelengths. In this talk we will review these recent results and explore the potential for observations from new soft X-ray instrumentation such as MinXSS to advance our understanding of coronal heating mechanisms.