[2] Detailed Interim Report. May 20th, 2008.
Jupiter in 2008 May
(2) Detailed interim report
John Rogers,
British Astronomical Association
Thanks are due to all the observers who have sent images, and to Hans-Joerg
Mettig and the JUPOS team for producing an interim set of charts in April,
and to Michel Jacquesson, Marco Vedovato, and Yuichi Iga for producing their own
maps, analysis, and discussions. The following account uses
information from the JUPOS charts, but latitudes are simply estimated. (Also I
have not checked histories of single spots before 2006.)
SPR: There are 2 white ovals at ~60 deg.S. Both have been tracked at least since
2006.
SSTZ & SSTB: Yet another small anticyclonic white oval (AWO) has appeared
(probably the one labelled A7. though it could be A8), bringing the total
to nine. Most of these have lasted for many years, but with some mergers and
some new appearances, they are now numbered A0 to A8. Closed cyclonic
circulations (white bars) between A1-A2 and between A4-A5 have continued to
expand as usual, forcing these AWOs apart. A cyclonic white oval f. A6 is
also expanding. Between the other pairs of AWOs, cyclonic 'folded filamentary
regions' can be discerned.
STB: Oval BA still contains an orange annulus although it is much fainter than a
year earlier, having faded last summer. It is soon to pass the GRS
(as happens every 2 years) and the appearance of the belt segments around it
often changes at this time. As usual, f. oval BA is the only dark
segment of STB. About 120 deg. f. it is the second major long-lived circulation
complex, the cyclonic 'STB Remnant', very pale blue.
STropZ: The excitement continues, as the unusual eddying tendency that developed
in summer 2006 has taken on another novel form. Instead of the
two STrD's of 2007 there are now two dark anticyclonic ovals, slowly prograding.
'Oval 1' may be derived from STrD-1, but indirectly; it is
probably the big dark anticyclonic oval which formed at L2=261 on 2007
Sep.1 from merger of all the spots that had recirculated onto the STBn jet.
'Oval 2' may be derived either from the same source, or from
circulation flanking STrD-2. Remarkably, as T. Akutsu noted on March 1, it is
bright in methane and annular in near-IR; and hi-res visible images show it as
Little Red Spot, with a red core and bluish dark rim (first shown by A. Wesley
on March 2). The only previous example of a prograding LRS in the
STropZ was in 1986, which also arose out of a STrD; and there was probably a
similar example in 1889-1890 (see my book pp. 200 & 206).
This LRS has just been overtaken by reddish oval BA, and both are approaching
encounter with the Great Red Spot. We watch keenly to see what
will happen. The GRS, at L2 = 125 (May 1), has (surprisingly) not been visibly
affected by the SEB fading and revival. It is still a well-defined
symmetrical orange oval.
SEB: While the SEB is fully restored after last year's Revival, it is still
highly disturbed, with rift activity arising in 3 sectors (though
prograding to encompass most of the belt). They are:
(1) F. the GRS: the site of the Revival outbreak in 2007, and of
perennial activity in normal times.
(2) A mid-SEB outbreak first reported by Wesley on 2008 March 8, immediately p.
the GRS (L2 = 100). There had been a stable pale white spot
up to then, but on March 8 it erupted as a brilliant white spot, and proceeded
to expand and proliferate in classic style. This outbreak is as
unexpected as last year's. It may be significant that the SED was passing just
as it appeared, and on March 13 the expanding, twisting mid-SEB
outbreak connected briefly to the SED as a rift, such as often appears when the
SED has just passed the GRS. The approach of the SED may have actually
triggered the mid-SEB outbreak: in 1975 there was evidence that secondary SEB
outbreaks were triggered by a feature that may have been a SED, and the
2007 outbreak also appeared as the SED was approaching that longitude though it
was not so close. So I suspect this is not just a coincidence. It
is also notable that this mid-SEB outbreak appeared immediately p. the GRS: this
is very rare. However there have been precedents, most notably the
origin of the SEB Revival in 1993.
(3) Another mid-SEB outbreak started soon after, first seen by Chris Go on March
21 at L2 = 258. This probably appeared within a dark
'mini-barge' (like the 2007 SEB outbreak). Further white spots appeared at about
the same site on March 29, April 4, 12, 21, and May 1: one every 7-10
days. As usual these expanded and prograded as bright rifts, which soon became
continuous with the rifted region f. the GRS. As late as May 13,
another bright white spot appeared near the original source, at L2=266, again
within a dark mini-barge.
Meanwhile on SEBs, the only distinct retrograding ring had the modest speed of
DL2 = +30 deg/mth, and arrived at the Red Spot Hollow on
April 8. Conditions prevented adequate imaging of the GRS thereafter, but the
spot probably ended up as a white rift seen extending Np. from the
northernmost point of the Hollow on April 16-18 (exactly as reported for a
previous such event by A. Sanchez-Lavega's group).
Otherwise, SEBs is marked
by tangled strings of tiny dark spots without obvious rapid motion. One string
is of the 'mini-barges' at ~17
deg.S, DL2 ~ +10 deg/mth; another is of dark streaks in STropZ at ~22 deg.S, DL2
~ 0. However, close scrutiny of images on May 13-14 shows that a
string of tiny dark spots at ~20 deg.S is indeed retrograding at ~ +3 deg/day,
so the SEBs jetstream is still operating, although not especially
fast.
SEBn/EZ(S): The S. Equatorial Disturbance (SED) has been visible since Feb. as a
very bright white bay. It passed the GRS on March 7 and briefly interacted with
(or even induced) the mid-SEB outbreak described above. It passed the GRS again
on April 29 and then developed its classic form, again
connecting with rifts to the mid-SEB outbreak and becoming quite prominent in
early May. Its L1 was approx. 250 (Feb.26), 265 (Mar.13), 295 (Apr.7-9), 320
(May 7), indicating DL1 = +30 deg/mth.
EZ(N)/NEBs: The EZ this year has few conspicuous features: all the unusual
darkness of 2006-2007 has disappeared, leaving only faint tenuous streaks,
which are however very complex and beautiful in hi-res images. There are few
large NEBs projections, and these consist of plateaux with generally
unimpressive festoons. They are virtually static in L1. However an interesting
novelty is an orange-tinted strip along the north edge of the EZ - apparently
colour spilling over from the NEB. In recent years such colour has been seen in
individual white bays, but not
continuously.
It is worth noting that Akutsu's methane images also show a novel pattern in
that northern EZ and southern NEB form a moderate-intensity band
between the bright EZ(S) and dark NEB(N). This may be the orange haze, appearing
reflective in the methane band. It marks a large-scale change
from the previous appearance of the EZ in methane, to be discussed in a later
bulletin.
NEB & NTropZ: The NEB is still strongly reddish (as in 2007) and has also
acquired numerous small dark barges on the N edge. Both phenomena typically
occur about a year after a NEB expansion or rifting event, so we were surprised
to see them now. The last expansion event was in 2004, and in
2008, the belt has actually narrowed again.
However, a review of 2007 data shows that, while our attention was diverted
elsewhere, there was actually much activity in the NEB: A large,
long-lived rifted region grew to encompass one third of the circumference and
there were other rifts elsewhere; and with these dynamic rifts in the
NEB, plus the extreme turbulence in NTropZ during the NTBs outbreak, most of the
barges and AWOs were destroyed. In summer 2007 some new barges did develop, as
did the strong reddish colour.
As the NEBn edge has retreated southwards, the barges are on the N edge, often
associated with prominent cusps from which festoons curve Np.
into the NTropZ - a pattern familiar from previous 'retreat' years such as 1994.
There are at least 9 barges (though many are small), and 8 AWOs in
NTropZ. Although most are slow-moving in L2, there is still white spot Z (WSZ),
revived in brightness and speed since its obscurity in 2007, with
DL2 = -11 deg/mth. Some of the barges and AWOs p. it share this rapid drift, and
are colliding with others which don't. Most important was the
collision of two AWOs p. WSZ, in early April (L2 ~320). Although lo-res obs'ns
would have suggested they merged, hi-res images showed they did not!
Both AWOs shrank as they approached, especially the Sp. one, which then squeezed
S of the Nf. spot, breaking up as it did so, and then disappeared.
The tiny remaining spot then drifted to lower L2. All this is exactly what
happened when a white oval encountered white spot Z in 2006 June. It shows
that the paradigm for mergers of AWOs which we have reported in temperate
latitudes (Rogers et al., Icarus, 2006b) does not hold for the NTropZ. In
fact, no true merger of AWOs has ever been recorded in this latitude.
Theoretical study is needed to understand why these AWOs behave
differently.
Another interesting phenomenon was the 'rocking' of the little dark barge on the
Sp. edge of white spot Z, during the encounter of the
AWOs on its p. side. It might have a period of about 5 days although there were
not enough data to prove it.
All the other encounters
have been between small (cyclonic) barges, and they probably do merge, according
to a pattern previously
observed (Rogers et al., Icarus, 2006b), although this has not been clearly
observed yet this year because of small sizes and incomplete obs'ns. It is
notable that all but one of these interactions are occurring just p. white spot
Z, so I imagine that white spot Z is causing them, as with previous
mergers of barges. These encounters have been: 1) L2 ~ 110, early April (one
partner very small). 2) L2 ~ 290, late April: This pair of barges was
just p. the site of the AWOs collision, and presumably were also impelled by WSZ.
Although the crucial conjunction was not observed (April 22-28),
the appearance on April 29 showed the formerly Nf. barge protruding from the Np.
end of the other one, just as in our published examples - but in
this case a small fragment actually detached and continued prograding. However
the greater length of the surviving barge confirms that a typical
merger did take place. 3) L2 ~ 310, mid-May; ) Two further collisions p. WSZ,
one involving the 4) L2 ~ 270, late May ) product of merger (2); not
yet observed.
NTB: Last year's dramatic NTBs jetstream outbreak (DL1 ~ -160 deg/mth; 'NTC-D')
led to the revival of the NTB as a prominent orange belt in the
latitude of the NTB(S) component, with paler reddish tint flanking it. In 2008,
this orange NTB(S) is still very strong, and a fainter, greyer,
sinuous (N) component is present as well, with pale ochre shading between them.
Thus the full NTB is now reviving, with continuing reddish colour as
an aftermath of the outbreak. (However in Akutsu's methane images the NTB is
paler than before, apparently because the orange colour is a high-level
haze which masks the usual belt.)
On the NTBs edge are at
least 6 pale orange lozenges, each about 8 deg. long. Measurements on 3 of them
give a consensus speed DL1 = -78
deg/mth (133 m/s in L3). This is a very important development which gives
insight into the deep structure of the jetstream. It is one more parallel
with the apparently identical NTB outbreak of 1990: in that case, jetstream
spots (anticyclonic vortices) were first seen on NTBs 20 months after the
outbreak, with DL1 = -57 ('NTC-C'), and persisted with very similar speed for
the next 11 years. Now, with better imaging, we can see (probably, incipiently)
similar spots from only 11 months after the outbreak, with the cloud-top
jetstream speed still not fully back to normal. The rapidity of the change,
together with modelling published by Sanchez-Lavega's group, supports a model in
which the jetstream has a permanent speed of at least
DL1 ~ -160 deg/mth (170 m/s) below the cloud-tops, but the cloud-top wind speed
alternates between this 'NTC-D' speed (when outbreaks driven from
below are frequent) and 'NTC-C' speed at other times (sometimes, or perhaps
always, when jetstream vortices are present). **I will post a more detailed
discussion of this topic soon.**
NNTB & NNTZ: A jet-stream spot outbreak is in progress on the NNTBs: a
volley of small dark spots with DL2 = -90 deg/mth. (There had been many
such spots in 2006, but they disappeared during the NTB outbreak in 2007.)
The NNTB consists of irregular streaks. In the NNTZ latitude, there are several
anticyclonic ovals: two are white (L2 = 45, 250) and one is pale reddish (L2 =
90 on May 9). As usual, this LRS is prograding (DL2 = -12 deg/mth) and
methane-bright (Akutsu images). All 3 have been tracked since 2006 or earlier.
I'm not including any more images with this report, but if you want to see image
sets documenting any of these phenomena, I could send some (and
Yuichi Iga, Marco Vedovato, and Michel Jacquesson have produced some too).
___________
John
Rogers
BAA
Jupiter Section Director