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We use the IRS on Spitzer to observe the southern part of the reflectionnebula NGC 2023, including the Southern Ridge, which is a photodissociationregion par excellence excited by HD 37903. Five pure-rotational H2 emissionlines are detected and mapped over and around the Southern Ridge in order tocompare with predicted level column densities from theoretical PDR models. Wefind very good agreement between PDR model predictions and emission lineintensities and ratios measured with Spitzer, leading us to conclude that grainphotoelectric heating sufficiently warms the gas to produce the observed H2line emission via collisional excitation. On the Southern Ridge, we infer ahydrogen nucleus density n_H approx 2x10^5 cm^-3 and radiation field strength\chi\ approx 10^4 relative to the local Galactic interstellar radiation field.This high value for \chi\ independently predicts a distance toward HD 37903 of300 pc, and is consistent with the most recent HIPPARCOS results. Over the mapwe find that both n_H and \chi\ vary by a factor of sim 3. Such 2-D variationsprovide clues about the underlying 3-D structure of the Southern Ridge field,which appears to be the tip of a molecular cloud. We also map variations inexcitation temperature and the ortho-to-para ratio, the latter attaining valuesof sim 1.5---2.0 on the Southern Ridge, and find that PDR modeling can readilyreproduce observed ortho-to-para ratios that are < 3 for rotational excitationdominated by collisional processes. Last, the stars Sellgren C and G arediscovered to be resolved on archival HST images into two point sources each,with separations of lesssim 0.5".

PDR Model Mapping of Physical Conditions via Spitzer/IRS Spectroscopy of H2: Theoretical Success toward NGC 2023-South