SATURNS CENTRAL FLASH FROM THE 3-JULY-1989 OCCULTATION OF 28-SGR

We present observations of Saturn's central flash obtained from Paloma r and McDonald Observatories during the 3 July 1989 occultation of 28 Sgr. As the star passed close to the geometric center of Saturn's shad ow, the focusing of the incident starlight by the planet's atmosphere formed multiple stellar images along the limb which were detected in i nfrared images obtained at wavelengths of 3.9 (Palomar) and 2.1 mu m ( McDonald). These are the first reported observations of a central flas h due to Saturn, and the first far any planet in which the signal from each stellar image could be determined separately, permitting a compa rison of both intensity and position for each image with model predict ions. The starlight forming the central flash was strongly modulated b y passage through Saturn's rings. Four separate flashes were observed from each observatory, corresponding to the points on the limb where t he starlight passed through the Cassini Division and the relatively tr ansparent C Ring, with maximum brightnesses reaching 1-2% of the unocc ulted stellar intensity. The more usual brightening seen during previo us central flashes as the observer crossed the caustic formed by the p lanet's oblate limb was not detectable in this event due to strong att enuation by the B Ring. The timing of the flashes is quite sensitive t o the shape of Saturn's limb, which depends in turn on the planet's zo nal gravity harmonics and on the zonal wind profile in the lower strat osphere, near the 2.5-mbar pressure level. The locations of the images along the limb, as well as the timing, shapes, and amplitudes of the individual flash light curves, are well matched by a smoothed model ba sed on a tropospheric zonal wind profile obtained from tracking cloud features in Voyager images and the Saturn ring optical depth profile o btained from the Voyager Photopolarimeter experiment. The smoothing re quired to give the best match to the data exceeds that attributable to the finite angular extent of the occulted star and may be due to refr active scattering by turbulence or wave structure in Saturn's atmosphe re. The detailed agreement of the models and observations is improved by assuming a uniform zonal wind speed of 40 msec(-1) at 25-70 degrees north latitude in the stratosphere, suggesting that midlatitude zonal winds on Saturn decay with height above the troposphere to the nonzer o mean of local tropospheric winds. There is no evidence of significan t atmospheric absorption at the observed wavelengths, which correspond to spectral regions of weak methane absorption. (C) 1995 academic Pre ss, Inc.