[9]  Interim reports on STB (Oval BA passing GRS), STropB, GRS (internal rotation measured), EZ( S. Eq. Disturbance; dramatic darkening; NEB interactions), & NNTB:  (Report,  2006 July 30)

These image sets show the GRS with oval BA passing it.  They also show the features in the equatorial region which have become so striking this year, including the SED.
Belts are labelled on the image of July 12 – the narrow bands p. the GRS may be quite confusing! (My designation SEB(SS) is unofficial but seems necessary.)

You can no doubt see many interesting motions in these images which I won't discuss, for example in the bright rifts in the SEB f. the GRS.

Also see the superb adaptive-optics near-infrared images from Hawaii (Gemini on July 13/14 and Keck on July 20/21). The 1.3-1.6 micron channel is similar to red light; 1.65 microns is a methane band. Web links were provided by Chris Go, viz:




STB: Oval BA's red ring is unscathed as it drifts past the GRS:

Oval BA marks the p. end of the only large dark sector of STB: this is the major complex of activity in this domain.

Note that p. oval BA, the STB is largely faint (white) except for a narrow STB(N), which is probably composed of small dark spots prograding from the active complex in the STBn jet stream.  (See the Cassini movie for a dramatic view of the dynamics in this region, showing how the active complex emits dark spots on al adjacent jetstreams.)  This STB(N) has become longer and darker since oval BA started passing the GRS.

As it started to pass the GRS, the whitened STB immediately p. oval BA dramatically turned very dark (see images of June 21—July 7).  But since then, the dark patch has become smaller again (July 8-27).

Albedo changes like this are quite common as these great S. Temperate ovals pass the GRS, as the compression of jetstreams here clearly destabilises the cloud layers.

However no change was expected to the internal circulation of oval BA, and there has been no change in its colour: the orange ring is still a striking feature in hi-res images (as of July 27).


STropZ: South Tropical Band progrades p. the GRS:

The S. Trop. Band detached from the p. end of the GRS around July 1, and continues to prograde; it is now strikingly oblique in latitude.

Oval Q is now a very small white spot (arrowed on the July 12 image), and is stationary relative to the GRS, so we will not see them merge in this apparition.


GRS:  Direct measurement of the internal rotation period confirms acceleration:

This is an amazing testament to the quality of your images: due to the number of international observers, the high resolution of images, and the fortunate  presence of some distinct dark streaks inside the GRS, we can produce the first ground-based measurements of its internal circulation period since it was first measured by professional photography in the 1960s. And we find that the circulation period has speeded up a lot!  This is being written up as a paper for the Journal of the BAA.

Since the 1960s, only spacecraft images have been able to track the GRS circulation, and they have shown the period to be shortening – largely due to the gradual shrinkage of the GRS, but also recently due to a real acceleration of the winds in 2000 (Galileo Orbiter data; Simon-Miller et al., 2002).  The shrinkage of the GRS in longitude is part of a long-term trend since the 19th century, and I suspect this GRS is actually a new one that arose in the 18th century. Cassini's spot in 1665-1713 was an old one, and showed the condition towards which the present GRS is evolving (see my book pp.188-196 and pp.262-264). 

In 2006, your images have shown dark streaks rotating inside the GRS in April (already reported), June 21 (see first set of images), and July (see second set of images). I have plotted the position angle of the dark streak against time, and derived the following rotation periods:


Dates               Short intervals                        Longest interval No. of rotations

April 12-24            5.0 – 5.5 d                4.4 d                            2.8

June 21            (~4.2 d            )                                                           0.2

July 8-20            4.8 – 5.2 d                4.5 d                            2.9


The circulation rate seems to vary somewhat around the GRS, but I think the mean period measured over the longest intervals is most reliable, 4.4 – 4.5 days. Of course with these images we cannot be certain that we are measuring the physical rotation rate, as the streak could be a wave propagating at a different speed, but it agrees well with spacecraft data. It confirms that the period has shortened since the Voyager images which gave a period of 6-8 days, both from tracking distinct features like these, and from mapping the wind speeds.

This shortening of period is proportional to the shortening of the length of the GRS, so it does not confirm that the wind speeds within it have increased; however, that could still be the case as the streaks we track may not represent the most rapidly-circulating radius in the GRS.


EZ(S), SEBn:  Colour change, and revival of the S. Equatorial Disturbance (SED).

As previously noted, the narrow EZ(S) has gradually changed from brilliant white to a yellowish colour during 2006 (thus it is also now dark in UV images).  In late June, 3 dusky grey-brown veils were noted equally spaced around it, which were unusual features for this latitude. They were at L1 = 30, 150, and 282 (July 1). 

The first one was associated with the long-lived SED, as tracked by Hans-Joerg Mettig. (We are now writing another paper about the recent dynamics of the SED). 

When most active, the SED is distinguished by a bright rift in the SEBn, and this was recorded passing the GRS on June 2 and July 18. In late June this rift was sometimes detectable just p. the grey-brown veil at L1=30.  In mid-July the rift broke through more clearly ~20 deg. further p., at L1 ~ 0-10. Unusually, this rift broke through as it approached the GRS alongside the turbulent SEBZ – but it did become even stronger as it passed the GRS on July 18.  Its linkage to the bright SEBZ rifts confirms observations made by Voyager and may be a source of energy to sustain the SED. Its persistence thereafter is shown in:

Very recently, a whole chain of bright rifts and dark streaks has appeared on SEBn p. another of the brown veils (July 26, CM1=249, Don Parker; see montage below:


Click for full version.

EB, EZ(N), NEBs:  Dramatic darkening, and interactions with NEB thunderstorms.

It is notable this year that dark features are accumulating all across the EZ.  Most obviously, the bluish-grey festoons in EZ(N) and the EB have become much bigger and darker. 

The EB is very broad and quite dark.  Some images suggest a greenish colour!—but this is not confirmed by colour sampling, which suggests that it is really grey (though depending on the overall colour balance of the image).  Any greenish impression is either due to visual contrast with the adjacent yellowish EZ(S) and bluish EZ(N) festoons, or due to actual mixing of these colours.

In EZ(N) the dark festoons are attached to the even darker blue-grey NEBs projections, which are now numerous and intense. They are also fixed in L1, i.e. they have returned to a normal drift rate after the exceptionally slow drift rates (positive DL1) of the last few years.  All this represents a reversion to the state last seen in 1999. 

The montage from July 8-20 shows an interesting interaction involving small white spots associated with rifts in the NEB, first pointed out by Fabio Carvalho.  These white spots are known to be thunderstorms. Note how one of a pair of bright white spots in the southern NEB (red arrows) breaks through into the EZ(N) at the p. edge of a dark projection, establishing a new rift. Such events have been seen on other dates, including one in the Cassini movie in 2000. The interaction strongly resembles the way the SEB rifts coupled to the SED in the same sequence of images. 


NNTB:  Trace of  'red smoke' persists:

The diffuse reddish patch left by the disappearance of the dark NNTB 'barge' was still faintly visible at L2 ~135 in some images in June and July, esp. by Chris Go.


John Rogers, 2006 July 30