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Exoplanets orbiting close to their host star are expected to support a largeionosphere, which extends to larger pressures than witnessed in our SolarSystem. These ionospheres can be investigated with ground-based transitobservations of the optical signatures of alkali metals, which are the sourceof the ions. Here, we present new observations and analyses of optical transitspectra that cover the Na doublet in the atmosphere of the exoplanet XO-2 b. Toassess the consistency of our results, observations were obtained from twoseparate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematicerrors, we chose XO-2, because it has a binary companion of the same brightnessand stellar type, which provides an ideal reference star to model Earth'satmospheric effects. We find that interpretation of the data is highlysensitive to time-varying translations along the detector, which changeaccording to wavelength and differ between the target and reference star. Itwas necessary to employ a time-dependent cross-correlation to align ourwavelength bins and correct for atmospheric differential refraction. Thisapproach allows us to resolve the wings of the Na line across 5 wavelength binsat a resolution of $\sim$1.6nm and limit the abundance of Na. We obtainconsistent results from each telescope with a Na amplitude of 521$\pm$161 ppmand 403$\pm$186 ppm for GMOS and KOSMOS respectively. The results are analyzedwith a radiative transfer model that includes the effects of ionization. Thedata are consistent with a clear atmosphere between $\sim$1--100 mbar whichestablish a lower limit on Na at 0.4$^{+2}_{-0.3}$ ppm([Na/H]=-0.64$^{+0.78}_{-0.6}$), consistent with solar. However, we can notrule out the presence of clouds at $\sim$10 mbar which allow for higher Naabundances which would be consistent with stellar metallicity measured for thehost star ([Na/H]=0.485$\pm$0.043).

Ground-based Spectroscopy of the Exoplanet XO-2b using a Systematic Wavelength Calibration