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We discuss high-resolution, time-resolved spectra of the caustic exit of thebinary microlensing event OGLE 2002-BUL-69 obtained with UVES on the VLT. Thesource star is a G5III giant in the Galactic Bulge. During such events, thesource star is highly magnified, and a strong differential magnification aroundthe caustic resolves its surface. Using an appropriate model stellar atmospheregenerated by the NextGEN code we obtained a model light curve for the causticexit and compared it with a dense set of photometric observations obtained bythe PLANET microlensing follow up network. We further compared predictedvariations in the H alpha equivalent width with those measured from ourspectra. While the model and observations agree in the gross features, thereare discrepancies suggesting shortcomings in the model, particularly for the Halpha line core, where we have detected amplified emission from the stellarchromosphere as the source star's trailing limb exited the caustic. Thisachievement became possible by the provision of the OGLE-III Early WarningSystem, a network of small telescopes capable of nearly-continuousround-the-clock photometric monitoring, on-line data reduction, dailynear-real-time modelling in order to predict caustic crossing parameters, and afast and efficient response of a 8m-class telescope to a``Target-Of-Opportunity'' observation request.Comment: 4 pages Latex, 3 figures, accepted for publication to astronomy and  astrophysics letter

Probing the atmosphere of the bulge G5III star OGLE-2002-BUL-069 by analysis of microlensed Hα line