Apparition 2011 Note n°3

Since the return of Jupiter in the night sky after the last solar conjunction, observers have witnessed the revival of the South equatorial belt (SEB), whose fading had been the major subject during the last two apparitions of 2009 and 2010. The revival phase of the SEB has begun in november 2010 but has been developing further during the year 2011. Some of the usual aspects of the belt when it's in a "normal" phase of activity did reappear only this fall - and some are still missing.

In every atmosphere of the universe, cyclones and anticyclones are vortices created by imbalances of pressure or temperature. They allow airmasses to mix between the poles and the equator - or between the equator and the tropics... they work with each other - a cyclonic structure must be accompanied by its high-pressure counterpart. We know that on Jupiter the GRS is strong, ancient anticyclone. But where is the low-pressure zone that must exist on its side ?

When the SEB is in a normal phase of activity, we always see white structures on its north-following flank inside the SEB. Those spots are convective eddies, local cumulonimbus that has a short life-time (a few days). But, if they are short-lived the convective region itself is permanent and this is where the low-pressure counterpart of the GRS is, even if it's not organized as a steady vortex like the NEB barges are for example. This region is often nicknamed the "GRS wake" by amateurs. During 2011 fall, this region is reviving strongly, and it's an event in itself because it had been missing for two years and a half : this convective turbulence is the first one to stop abruptly when the SEB is about to fade. This what had been observed in 2009 may or 2007 january. Its revival is as important as its halt ; this is a sign that may allow us to conclude that the fade and revival cycle of the SEB that started in 2006 has come to its end.

Schema of the GRS region, with it associated low-pressure region inside the SEB

The Great red spot is the high-pressure (H) of the South tropical zone STrZ. At north-west (north-following), is a SEB low-pressure region designed "L". Green arrows draw the jetstreams that mark the STrZ and the SEB. Black arrows outline the global rotating wind movements.

As it must be in a southern hemisphere, H is rotating counter-clockwise and L, in the clockwise sense.

At bottom is an animation cropped from a Cassini movie that show the movements. Despite its messy aspect in a still image, the clockwise movement of the GRS wake is easy to see.

Images (c) STScL/HST and (c)NASA/JPL/Caltech

The revival of the GRS wake in images

During the first months of the 2011 apparition (may-august), the aspect of the SEB north-following of the GRS is still in the post-revival phase. Despite some further outbreaks near the source of the original 2010 outbreak farther in longitude (250-260°), this aspect is characterized by an absence of big convection, a reddish tint and the existence of very small red spots (mini barges). The aspect of the zone in Akutsu's image of 27 june 2011 is really comparable with the typical aspect depicted in the 1991 march HST shot, also taken months after a SEB revival. From late september 2011, the situation has been changing, with the reappearance of white convection just near the GRS (Yann Le Gall's image). From september to december 2011, the white spots has been intensifying quite a lot and many amateurs have noticed them. On Marc Delcroix's image of october 8th, the region is already much more active than two weeks ago.

These white spots show the revival of the low-pressure SEB region associated to the GRS. Images taken in the methan band are equally interesting...

Images taken in the 890 nm CH4 absorption band, showing the phenomenon

CH4 images reveal that the white spots, being brillant, are of a relative high altitude. This show their outbreaking nature - they are local big cumulonimbus. Images taken during 2011 summer (Peach, Prost) never showed any sign of brightening there. Yet in october (Pellier, Akutsu), oubreaks are now showing up. We can also note that the strange CH4 aspect the belt had last summer looks to have darkened. This change, if confirmed, might also show a slow return to normal of the SEB.

The persistenced of the massive grey south tropical belt (in the STrZ) on the southern flank of the SEB is remarkable, and this is not a normal aspect of the region. Some others usual phenomenons of the active SEB are still missing, like the SEBs jetstreams spots that have not been seen for at least six years now ; and in CH4 the SEB it still too narrow. The return too "normal" is not complete and the region deserved to be followed during the coming months.