Scintillation Caustics in Planetary Occultation Light Curves

We revisit the GSC5249-01240 light curve obtained during its occultation bySaturn's North polar region. In addition to refractive scintillations, thepower spectrum of intensity fluctuations shows an enhancement of power betweenrefractive and diffractive regimes. We identify this excess power as due tohigh amplitude spikes in the light curve and suggest that these spikes are dueto caustics associated with ray crossing situations. The flux variation inindividual spikes follows the expected caustic behavior, including diffractionfringes which we have observed for the first time in a planetary occultationlight curve. The presence of caustics in scintillation light curves require aninner scale cut off to the power spectrum of underlying density fluctuationsassociated with turbulence. Another possibility is the presence of gravitywaves in the atmosphere. While occultation light curves previously showed theexistence of refractive scintillations, a combination of small projectedstellar size and a low relative velocity during the event have allowed us toidentify caustics in this occultation. This has led us to re-examine previousdata sets, in which we have also found likely examples of caustics.Comment: 4 pages, 3 figures; ApJL submitte