Neptune's Stratospheric Winds from Three Central Flash Occultations

Abstract

We present observations of three Neptune central flash events: the 20 August 1985 occultation of n39 from the European Southern Observatory (ESO), the 12 September 1988 occultation of N51 from Pic du Midi, and the 8 July 1989 occultation of N55 from ESO. From simultaneous fits to the three central flash lightcurves, we determine the shape of Neptune's limb, and show that winds near the 0.38-mbar level have decayed to about ≃0.6 ± 0.2 times their strength at 100 mbar, assuming that the latitude-dependence of the stratospheric winds follows the zonal wind profile of L. A. Sromovskyet al.(1993,Icarus105, 110–141). From the measured decay of winds with height, we find that the average vertical shear in the zonal wind between 100 and 0.38 mbar is very close to the Voyager IRIS results for the 30- to 120-mbar region (B. J. Conrathet al., 1989,Science246, 1454–1459), at the latitudes to which the central flashes are most sensitive. We also determine the shape of the limb at the 0.7-μbar level from the “half-light” points of the atmospheric immersion and emersion light curves of five stellar occultations by Neptune. The winds in this pressure regime have decayed to ≃0.17 times their strength at the 100-mbar level, and the vertical shear in the zonal wind as determined from the thermal wind equation and Voyager IRIS measurements extends essentially unchanged all the way up to the microbar level of the stratosphere. Alternatively, a simple oblate model fit to the planetary limb gives an oblateness of ϵ = 0.0180 ± 0.0010 and an equatorial radius ofre= 25262.7 ± 3.5 km. The corresponding rotation period is 16.59 ± 0.92 h, quite similar to that of Neptune's deep interior (16.11 h). This near corotation suggests that there is some coupling between Neptune's interior and the upper stratosphere, even though the intervening atmosphere between the cloud deck and the stratosphere has strong retrograde flow.