Saturn's Central Flash from the 3 July 1989 Occultation of 28 Sgr

Abstract

We present observations of Saturn's central flash obtained from Palomar and McDonald Observatories during the 3 July 1989 occultation of 28 Sgr. As the star passed close to the geometric center of Saturn's shadow, the focusing of the incident starlight by the planet's atmosphere formed multiple stellar images along the limb which were detected in infrared images obtained at wavelengths of 3.9 (Palomar) and 2.1 μm (McDonald). These are the first reported observations of a central flash due to Saturn, and the first for any planet in which the signal from each stellar image could be determined separately, permitting a comparison of both intensity and position for each image with model predictions. The starlight forming the central flash was strongly modulated by passage through Saturn's rings. Four separate flashes were observed from each observatory, corresponding to the points on the limb where the starlight passed through the Cassini Division and the relatively transparent C Ring, with maximum brightnesses reaching 1-2% of the unocculted stellar intensity. The more usual brightening seen during previous central flashes as the observer crossed the caustic formed by the planet's oblate limb was not detectable in this event due to strong attenuation by the B Ring. The timing of the flashes is quite sensitive to the shape of Saturn's limb, which depends in turn on the planet's zonal gravity harmonics and on the zonal wind profile in the lower stratosphere, 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 based on a tropospheric zonal wind profile obtained from tracking cloud features in Voyager images and the Saturn ring optical depth profile obtained from the Voyager Photopolarimeter experiment. The smoothing required 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 refractive scattering by turbulence or wave structure in Saturn's atmosphere. The detailed agreement of the models and observations is improved by assuming a uniform zonal wind speed of 40 msec -1 at 25-70° north latitude in the stratosphere, suggesting that midlatitude zonal winds on Saturn decay with height above the troposphere to the nonzero mean of local tropospheric winds. There is no evidence of significant atmospheric absorption at the observed wavelengths, which correspond to spectral regions of weak methane absorption.