Saturation of nonthermal hard X-ray emission in solar flares

We consider a number of flares spanning a range of magnitudes over two decades in GOES soft X-ray luminosity and demonstrate an observational confirmation of the saturation of the nonthermal hard X-ray photon flux in large solar flares. Using the unprecedented spectral and spatial resolution of the RHESSI telescope, we find that the integrated photon flux above 20 keV asymptotically approaches a limiting value with increasing flare intensity, suggesting a saturation of the nonthermal photon production for high electron-beam fluxes in flares. This result is strengthened by considering each of the large flares as an aggregation of multiple fiducial energy release events identified using the high spatial resolution of RHESSI. We argue that the saturation of the photon production sets in as a result of the transition from Coulomb collisions to return current ohmic dissipation dominating the energy losses as the flare magnitudes increase.