  Context: The common flare scenario comprises an acceleration site in the
corona and particle transport to the chromosphere. Using satellites available
to date it has become possible to distinguish between the two processes of
acceleration and transport, and study the particle propagation in flare loops
in detail, as well as complete comparisons with theoretical predictions.
  Aims: We complete a quantitative comparison between flare hard X-ray spectra
observed by RHESSI and theoretical predictions. This enables acceleration to be
distinguished from transport and the nature of transport effects to be
explored.
  Methods: Data acquired by the RHESSI satellite were analyzed using full sun
spectroscopy as well as imaging spectroscopy methods. Coronal source and
footpoint spectra of well observed limb events were analyzed and quantitatively
compared to theoretical predictions. New concepts are introduced to existing
models to resolve discrepancies between observations and predictions.
  Results: The standard thin-thick target solar flare model cannot explain the
observations of all events. In the events presented here, propagation effects
in the form of non-collisional energy loss are of importance to explain the
observations. We demonstrate that those energy losses can be interpreted in
terms of an electric field in the flare loop. One event seems consistent with
particle propagation or acceleration in lower than average density in the
coronal source.
  Conclusions: We find observational evidence for an electric field in flare
loops caused by return currents.
