  Exoplanets orbiting close to their host star are expected to support a large
ionosphere, which extends to larger pressures than witnessed in our Solar
System. These ionospheres can be investigated with ground-based transit
observations of the optical signatures of alkali metals, which are the source
of the ions. Here, we present new observations and analyses of optical transit
spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2 b. To
assess the consistency of our results, observations were obtained from two
separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic
errors, we chose XO-2, because it has a binary companion of the same brightness
and stellar type, which provides an ideal reference star to model Earth's
atmospheric effects. We find that interpretation of the data is highly
sensitive to time-varying translations along the detector, which change
according to wavelength and differ between the target and reference star. It
was necessary to employ a time-dependent cross-correlation to align our
wavelength bins and correct for atmospheric differential refraction. This
approach allows us to resolve the wings of the Na line across 5 wavelength bins
at a resolution of $\sim$1.6nm and limit the abundance of Na. We obtain
consistent results from each telescope with a Na amplitude of 521$\pm$161 ppm
and 403$\pm$186 ppm for GMOS and KOSMOS respectively. The results are analyzed
with a radiative transfer model that includes the effects of ionization. The
data are consistent with a clear atmosphere between $\sim$1--100 mbar which
establish a lower limit on Na at 0.4$^{+2}_{-0.3}$ ppm
([Na/H]=-0.64$^{+0.78}_{-0.6}$), consistent with solar. However, we can not
rule out the presence of clouds at $\sim$10 mbar which allow for higher Na
abundances which would be consistent with stellar metallicity measured for the
host star ([Na/H]=0.485$\pm$0.043).
