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M. D. Hofstadter (JPL), B. J. Butler (NRAO), H. B. Hammel (SSI), M. J. Klein (JPL)
Uranus' extreme obliquity causes unusual seasonal variations in solar insolation, with the poles receiving more sunlight on an annual average than the equator. Until recently, ground-based instruments could only map the southern, summer hemisphere. As we approach the 2007 equinox, we are able to map northern winter/spring latitudes for the first time, and visible, infrared, and radio observing campaigns are underway.
In July of 2003, Very Large Array observations were made at wavelengths of 2 and 6 cm, seeing latitudes beyond 60 degrees North. They clearly show a bright region northward of +45 degrees. The south polar region (poleward of -45 degrees) has long been known to be radio bright (Briggs and Andrew 1980, Icarus 41), and this similarity between the fall and spring hemispheres is significant. It suggests that the annual-averaged insolation helps organize atmospheric structure at pressures of a few to tens of bars. There also appears to be a seasonally varying component in this pressure range (Hofstadter and Butler 2003, Icarus 165). At lower pressures (higher altitudes), visible and IR images from HST and Keck show the planet to be hemispherically asymmetric (Rages et al. 2004, submitted to Icarus; Hammel et al. 2004, submitted to Icarus). These observations are consistent with a troposphere in which cloud formation and circulation vary seasonally at pressures from hundreds of mbar to several or tens of bars. At deeper levels, there are still strong spatial variations in conditions, but they are relatively unchanging temporally. Through continued analysis and observations at additional wavelengths, we are seeking to better constrain models of the vertical structure of the atmosphere, and in particular the altitude of the transition from seasonally varying to invariant conditions.
This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.