Hello,

Here too are some thoughts on the reddened oval BA and its forthcoming passage past the GRS (conjunction forecast for mid-July).  They will, of
course, not collide nor be disrupted.  Oval BA, like its predecessors on the STB, passes the GRS about every two years.  However structures in the STB
are sometimes destabilised as they pass the GRS: for example, these 'white' ovals have sometimes appeared slightly reddened or veiled,  and segments of
STB have either darkened or faded. So we might see changes in the appearance of oval BA and of the dark STB f. it, during the passage.  Whether its
redness will change is anyone's guess!

The colour in the GRS is always strongest when the SEB is faint (whitened) ? as last seen in early 1993.  Although large sectors of the STB are often
faint (as now p. BA), this has long been a common event and does not seem to resemble the fading of the SEB. Nevertheless, it will be interesting if
there are correlated changes in the colour of oval BA and of the surrounding STB after it passes the GRS. 

It is interesting that the reddish colour has appeared as a ring within the white oval BA (possibly around its radius of highest wind speeds??we await
the analysis of Hubble images).  The GRS also may have developed reddish colour first as a ring, in the 19th century.  From its discovery in 1831 up
to 1856, only the light Hollow was seen.  In 1857 a dark arch appeared around its S side. Then when the SEB was faint in 1858/59, and again in
1870, it was drawn for the first time as a dark ring. In 1870, this was described as a 'ruddy elliptical line' by one observer (Mayer). As he used
only a 15-cm refractor, I would not trust the colour, but this was the first mention of reddish colour in it.  The reddish colour was first clearly seen,
filling the Spot, by Rosse and Copeland in 1873.  I suspect that these observations traced the true origin of red colour in the GRS, and that oval
BA may be recapitulating this development now.  (Or it may just be going through a funny phase.)

Reddish colours on Jupiter may be associated with high vorticity, ranging from the GRS, to tiny  eddies in cyclonic 'rift' regions in Voyager images.
We still don't know why only some anticyclonic ovals become reddish and methane-bright ? especially in the NNTZ (e.g. the present LRS,
methane-bright in the Lazzarotti-Olivetti image of April 10).  Reddish colours over belts or zones are also associated with recent energetic
outbreaks (e.g. a segment of NNTB is reddish following last year's numerous NNTBs jetstream spots, and the NEB was recently reddish after the broadening
event).  It is still not clear whether these disparate phenomena really share any physical properties, but they all appear to involve high vorticity
or high energy in some form, and to lead to the appearance of reddish haze over the clouds. 

Best wishes,
John Rogers, 2006 May 19


John H. Rogers : Jupiter Section Director,
[British Astronomical Association.]