A study of Jupiter's UV Great Dark Spot and tropopause to stratosphere winds in the high northern latitudes as seen by Cassini imaging

Abstract

Images of Jupiter taken during the year 2000 Cassini flyby revealed the early stages and evolution of a high northern-latitude UV Great Dark Spot over a three-month period. They also provided a window on the circulation of Jupiter's polar stratosphere. Jupiter's rarely-detected UV Great Dark Spot (UVGDS) is unusual in several respects. It is an ephemeral feature, twice appearing as an oval with size and shape similar to Jupiter's Great Red Spot (GRS). Like the GRS, it is an anticyclonic feature. It is unlike the GRS in that: (1) A gas, and/or very small particles (radius < < 1 micro meter, of unknown composition but likely hydrocarbon) is responsible for its contrast with its surroundings; (2) It appears to be a shallow feature, confined to the stratosphere; (3) It evolves (moves in System III longitude, elongates and tilts) over a period of months; and (4) It may be produced by an auroral event as inferred by its proximity to the southern extent of Jupiter's main auroral oval. However, we did not identify a specific auroral trigger event. We measure the rotation rate for the UVGDS to have been ∼20° per Earth day. To investigate the dynamical regime of this feature, we applied an automated cloud feature tracking algorithm to Cassini Imaging Science Subsystem images of Jupiter's northern hemisphere (30°N – 80°N) in the near-IR continuum, methane band, and ultraviolet filters. From these images we were able to derive zonal wind profiles from the middle troposphere cloud tops to the lower stratosphere and track the evolution of the UV Great Dark Spot. Differences in mean zonal wind speed between the continuum and methane band filters indicate a weakening of jets with altitude as far north as 75°N latitude at the altitudes sampled by these filters. We also observe notable differences between the ultraviolet and the other two filters: (1) Winds are significantly weaker and mostly westward in the ultraviolet relative to lower altitudes at all latitudes equatorward of 60°N latitude; (2) Poleward of 60°N latitude, winds observed in the ultraviolet may be faster than those observed in the continuum and methane bands, though we are less confident in this result as Jupiter's haze morphology and the lack of small, discrete, unambiguous trackable features present unique challenges to obtaining reliable mean zonal winds in the ultraviolet at high latitudes.