  Hard x-ray spectral breaks are explained in terms of a 1D model with a
co-spatial return current. We study 19 flares observed by RHESSI (Ramaty High
Energy Solar Spectroscopic Imager) with strong spectral breaks at energies
around a few deka-keV, that cannot be explained by isotropic albedo or
non-uniform ionization alone. We identify these breaks at the HXR peak time,
but we obtain 8 s-cadence spectra of the entire impulsive phase. Electrons with
an initially power-law distribution and a sharp low-energy cutoff lose energy
through return-current losses until they reach the thick target, where they
lose their remaining energy through collisions. Our main results are: (1) The
return-current collisional thick-target model (RCCTTM) provides acceptable fits
for spectra with strong breaks. (2) Limits on the plasma resistivity are
derived from the fitted potential drop and deduced electron-beam flux density,
assuming the return-current is a drift current in the ambient plasma. These
resistivities are typically 2-3 orders of magnitude higher than the Spitzer
resistivity at the fitted temperature, and provide a test for the adequacy of
classical resistivity and the stability of the return current. (3) Using the
upper limit of the low-energy cutoff, the return current is always stable to
the generation of ion acoustic and electrostatic ion cyclotron instabilities
when the electron temperature is lower than 9 times the ion temperature. (4) In
most cases the return current is most likely primarily carried by runaway
electrons from the tail of the thermal distribution rather than the bulk
drifting thermal electrons. For these cases, anomalous resistivity is not
required.
