Influence of Jupiter's South Equatorial Disturbance on jet-stream speed

The eastward jets on Jupiter are, with few exceptions, remarkably invariant. However, we now report considerable diversity of speeds in one of the fastest jets, at 7oS. We present groundbased observations from 1999 to 2006, which are shown to agree with measurements of the same region from Cassini spacecraft images. All the measurements refer to small cloud features seen as 'chevrons' in spacecraft images, which appear to represent the peak speed of the jet at or near cloud-top level. Previous discrepancies in measurements for this jet are resolved by showing that the speeds vary with longitude relative to a large wave-like feature, the South Equatorial Disturbance (SED). The SED has persisted from 1999 to 2006, and a similar feature was present during the Voyager flybys. When the SED is present and active, as it was during the Voyager and Cassini flybys, the observed jet speed is slow to the east of it (~116-128 m/s) but rapid to the west of it (~142-162 m/s). When the SED is absent (as in the mid-1990s) or weak (as in 2002-2005), it no longer modulates the observed speeds in the jet: a rapid jet speed of ~155 m/s is observed at all longitudes, but some individual features move more slowly over shorter intervals. These velocity patterns have implications for the physical nature of the SED and for the vertical structure of the jet at 7oS.