[Report drafted 2014 July, finalised for posting 2016 Nov.]

Summary

In early 2014, our measurements on amateur images of the Great Red Spot (GRS) showed that its circulation period had decreased to an exceptionally short 3.6 days, which was largely due to shrinkage of the GRS, but possibly also due to acceleration of the wind speed [Ref.1].  Imaging by the Hubble Space Telescope (HST) was obtained on 2014 April 21 to investigate this further, and the wind speed was indeed found to have increased to a maximum of ~150 m/s, although it was not quantitated in detail [Ref.11].  Here, I report manual measurements of the GRS circulation from HST images taken in 2009 Sep. and 2012 Sep. as well as 2014 April, and offer a comparison with other HST and ground-based results. 

            In order to obtain a simple parametrisation of the GRS circulation, and compatible with simple image projection techniques, the GRS circulation is approximated as an elliptical projection of circular motion, and angular displacements are measured.  Therefore the results are presented primarily in terms of the implied rotation periods, with wind speeds derived therefrom.

            The results confirm that amateur images can track the circulating features accurately, though they do not always capture the fastest wind speeds.  HST and ground-based measurements gave exactly the same rotation period for a large streak in 2012, but other features in the HST images rotated faster, indicating that the rapid rotation seen in 2014 had actually begun earlier.

            In all 3 sets of HST data, the deduced rotation period did not vary significantly with radius from r~6300-7400 km, at least. Streaks at larger radius and (in 2014) at smaller radius had longer rotation periods. Between 2009 Sep. and 2014 April, the implied rotation period decreased significantly (from 91 to 79 hours). This was mainly due to the physical shrinkage of the GRS.  The peak wind speeds showed no significant increase: 147 m/s in 2009, 150 m/s in 2012, 150 m/s in 2012. However, these speeds were significantly faster than those previously reported in 2006. 

            The consistency of the 2009 parameters with the long-term trend, despite an SEB Fade at the time, contradicts the prevailing view that influx of vortices on the retrograde SEBs jet sustains the circulation of the GRS.  Likewise the unchanged speed in 2014, when spots on the STBn jet were impinging on the GRS, shows that these spots do not affect the circulation either.

The report can be downloaded as a PDF here:

Report-on-GRS-2009-2014_TO-POST(2016).pdf

Also, here is a list of our reports on ground-based tracking of the GRS circulation from 2005 to 2016:

Our-reports-on-GRS-2005-2016.pdf