Constraining the hard X-ray properties of the quiet sun with new RHESSI observations

We present new RHESSI upper limits in the 3-200 keV energy range for solar hard X-ray emission in the absence of flares and active regions, i.e., the quiet Sun, using data obtained between 2005 July and 2009 April. These new limits, substantially deeper than any previous ones, constrain several physical processes that could produce hard X-ray emission. These include cosmic-ray effects and the generation of axions within the solar core. The data also limit the properties of ''nanoflares,'' a leading candidate to explain coronal heating. We find it unlikely for nanoflares involving nonthermal effects to heat the corona because such events would require a steep electron spectrum E -δ with index δ\gt5 extending to very low energies (łt1 keV), into the thermal energy range. We also use the limits to constrain the parameter space of an isothermal model and coronal thin-target emission models (power-law and kappa distributions)