  Astrophysical jets are present in a range of environments, including young
stellar objects, X-ray binaries, and active galactic nuclei, but their
formation is still not fully understood. As one of the nearest symbiotic binary
stars, R Aquarii ($D \sim 220$ pc) offers a unique opportunity to study the
inner region within $\sim$ 600 AU of the jet source, which is particularly
crucial to our understanding of non-relativistic jet formation and origin. We
present high-angular resolution ultraviolet and optical imaging from the
\emph{Hubble} Space Telescope in six emission-line regions of the inner jet.
Using these observations to obtain a range of representative line ratios for
our system and kinematic data derived from a comparison with previous studies,
we model the shocked gas in order to determine the relative roles of shock
heating and photoionization in the R Aquarii system. We find that our shock
models suggest a nonzero magnetic field is needed to describe the measured line
ratios. We also find that the Mg~II$\lambda\lambda$2795,2802 intensities are
overpredicted by our models for most of the jet regions, perhaps because of
depletion onto grains or to opacity in these resonance lines.
