  We present analyses of the spatial and spectral evolution of hard X-ray
emission observed by {\it RHESSI} during the impulsive phase of an M1.7 flare
on 2003 November 13. In general, as expected, the loop top (LT) source
dominates at low energies while the footpoint (FP) sources dominate the high
energy emission. At intermediate energies, both the LT and FPs may be seen, but
during certain intervals emission from the legs of the loop dominates, in
contrast to the commonly observed LT and FP emission. The hard X-ray emission
tends to rise above the FPs and eventually merge into a single LT source. This
evolution starts first at low energies and proceeds to higher energies. The
spectrum of the resultant LT source becomes more and more dominated by a
thermal component with an increasing emission measure as the flare proceeds.
The soft and hard X-rays show a Neupert-type behavior. With a non-thermal
bremsstrahlung model the brightness profile along the loop is used to determine
the density profile and its evolution, which reveals a gradual increase of the
gas density in the loop. These results are evidence for chromospheric
evaporation and are consistent with the qualitative features of hydrodynamic
simulations of this phenomenon. However, some observed source morphology and
its evolution cannot be accounted for by previous simulations. Therefore
simulations with more realistic physical conditions are required to explain the
results and the particle acceleration and plasma heating processes.
