Dynamics of cloud features on Uranus

Abstract

Near-infrared adaptive optics imaging of Uranus by the Keck 2 telescope during 2003 and 2004 has revealed numerous discrete cloud features, 70 of which were used to extend the zonal wind profile of Uranus up to 60\deg N. We confirmed the presence of a north-south asymmetry in the circulation (Karkoschka, Science 111, 570-572, 1998), and improved its characterization. We found no clear indication of long term change in wind speed between 1986 and 2004, although results of Hammel et al. (2001, Icarus 153, 229-235) based on 2001 HST and Keck observations average ~10 m/s less westward than earlier and later results, and 2003 observations by Hammel et al. (2005, Icarus 175, 534-545) show increased wind speeds near 45\deg N, which we don't see in our 2003-2004 observations. We observed a wide range of lifetimes for discrete cloud features: some features evolve within ~1 hour, many have persisted at least one month, and one feature near 34\deg S (termed S34) seems to have persisted for nearly two decades, a conclusion derived with the help of Voyager 2 and HST observations. S34 oscillates in latitude between 32\deg S and 36.5\deg S, with a period of $\sim$1000 days, which may be a result of a non-barotropic Rossby wave. It also varied its longitudinal drift rate between -20\deg /day and -31\deg /day in approximate accord with the latitudinal gradient in the zonal wind profile, exhibiting behavior similar to that of the DS2 feature observed on Neptune (Sromovsky et al., Icarus 105, 110-141, 1993). S34 also exhibits a superimposed rapid oscillation with an amplitude of 0.57\deg in latitude and period of 0.7 days, which is approximately consistent with an inertial oscillation.