Neptune's Stratospheric Winds from Three Central Flash Occultations

Details Neptune's Stratospheric Winds from Three Central Flash Occultations Neptune's Stratospheric Winds from Three Central Flash Occultations

Jun 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0509f&link_type=abstract

American Astronomical Society, DPS meeting #28, #05.09; Bulletin of the American Astronomical Society, Vol. 28, p.1078

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Neptune has extremely strong differential rotation at the cloud-top level, with retrograde equatorial winds =~ 400 m sec (-1) [Sromovsky et al. (1993), Icarus 105, 131]. Voyager IRIS observations indicate that the winds decay with height in the 30-120 mbar region [Conrath et al. (1989) Science 246, 1454], but the strength of the zonal winds in the stratosphere has remained unknown. In a ``central flash'' stellar occultation, the apparent path of the occulted star passes almost directly behind the center of the planet. During this phase of the occultation, the planet's limb acts like an enormous lens, due to refractive bending of the starlight in the stratosphere, and the detailed structure of the central flash light curve can be used to determine the shape of the limb and the strength of the zonal winds. We applied this technique to three central flash events: the 20 August 1985 occultation of n39 from ESO, the 12 September 1988 occultation of N51 from the 2-m telescope at OPMT, and the 8 July 1989 occultation of N55 from ESO. We assumed that the latitude-dependence of the stratospheric winds followed the zonal wind profile of Sromovsky \etal, and we scaled the overall wind profile by a multiplicative factor that produced the best-matching light curves. Neptune central flash occultations are sensitive to winds near the 0.38 mbar level, and from our joint solution to all three events, we found the winds in this region to be =~ 0.6 times their strength at 100 mbar. The corresponding average vertical shear in the zonal wind between the 100 mbar and 0.38 mbar levels is very close to the Voyager IRIS results of Conrath \etal, at the latitudes to which the central flash is most sensitive. We also determined the shape of the limb at the 0.7 mu bar level from the ``half-light'' points of the atmospheric immersion and emersion light curves of five multiple-station occultations, using the same zonal wind model as for the central flash analysis. We found that the winds in this pressure regime had decayed to about =~ 0.2 times their strength at 100 mbar level. This implies that 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. This work supported by NASA Grant NAGW-1368.

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