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The South Tropical Region extends from latitude 9° south to 27° south. The South Tropical Region is considered by some to be the most interesting region of the planet. This is no doubt due to the fact that the GRS resides in this region, situated on and actually protruding into the southern edge of the SEB.

The South Tropical Zone lies just south of the SEB, between the SEB and the South Temperate Belt (STB). This zone, like other zones on the planet, normally displays a bright cream or alabaster appearance. Sometimes shadings can be seen in the zone, and disturbances and dislocations can also be seen from time to time. Markings in this zone are often difficult to see visually although easily detected by CCD or web cam imaging.

A further discussion of the SEB is appropriate here, as this is where many of the cyclical events on Jupiter occur. In addition to being the home of the GRS, the South Tropical Region is home to the famous SEB fadings, SEB Revivals, partial fadings, mid-SEB outbreaks, South Tropical Disturbances (STrDists), and South Tropical Dislocations.

The SEB is often wide and dark like the North Equatorial Belt. Sometimes the SEB is slightly wider. Normally, the SEB displays a reddish-brown color, just as the NEB does. At times, the SEB can split into two parts, a northern and southern component. When this occurs, the bright zone between these two components is referred to as the SEB zone (SEBZ). In November 1998 the SEB was a solid belt at longitude 292° System 2, with a very narrow belt of turbulent white clouds running along the northern edge of the belt. However, at 236° System 2 in September 1998 the northern half of the SEB was very bright, with turbulent white clouds. Later, during September 2000, Jupiter's SEB was split into two components, with the northern component, SEBZ, and southern component occupying the width of a normal, solid SEB (Fig. 3.21). The southern edge of the SEB (SEBs) was uneven, with bright bays protruding into the southern edge. Through February 2002 and beyond, the SEB continued to be divided into this double component. Although faint, in May 2002 the northern and southern components displayed a slight reddish-brown color, with the northern component the darker of the two. By February 2003, the SEB was again solid along most of its length, except for the usual bright, turbulent region following the GRS. During 2006, the SEB was again broad with a general reddish-brown color. As usual, there was much bright turbulence following the GRS. In April 2006, there was a really large white oval in the wake of turbulence following the GRS.

Although the SEB does not form large condensations or barges like the NEB does, sometimes small, reddish condensations or spots can form on or near the southern edge of the SEB. These very tiny versions of the GRS, while sometimes difficult to make out visually in a small telescope, can provide us with an opportunity to track the drift rate and currents at this latitude of the planet.

5 Sept. 2000 09:23 UT

Fig. 3.21. Jupiter on September 6, 2000 with SEB split into a northern and southern component. Note STB fragment. South is up. (Credit: Donald C. Parker).

CM1=334.4CM2=219.4CM3=167.0

I think one of the most dramatic and interesting phenomena to observe on Jupiter is an SEB Revival. An SEB Revival is actually a series of events, the cause of which is not fully understood. During an SEB Revival, the SEB will fade, taking several months to completely disappear. The fading will normally begin in the southern component. The northern component does not always disappear; but it will often fade completely away. Once the SEB has faded, the GRS will often darken _ ^

and become more intense. Then, after remaining faint for one to three years, the § v c

Revival will begin. The Revival starts at a single point in the belt, and from this *!« q .2

point dark and bright spots begin to appear. From this point source spots are car- hyr P

ried along with the currents of the SEB. Other spots well up from the point source £L <D 2

and are carried along in the SEB currents, spreading out across the width of the <d belt. Eventually the material from these spots reaches the GRS. When that happens |2E *o the GRS fades. The SEB continues to be filled with the dark and bright material and T

is finally restored. At present, a complete SEB fading followed by a Revival is well overdue. When it happens, chances are an amateur astronomer will be the first to notice the fading and report it.

Another SEB phenomenon is the Mid-SEB Outbreak. A Mid-SEB Outbreak in appearance mimics the beginnings of an SEB fading. When a Mid-SEB Outbreak occurs, there is an apparent fading of a portion of the SEB for several degrees of longitude. However, the remainder of the belt remains visible. The typical Mid-SEB Outbreak first appears as a small white streak or spot in the southern portion of the SEB, removed from the longitude of the GRS. This spot stretches out toward the following direction of the belt with other spots continuing to develop at the place of origin of the first spot. Soon, the SEB is covered with a very turbulent region of bright and dark streaks and spots resembling the continuous turbulent area following the GRS. This condition of the SEB can exist for several months. A Mid-SEB Outbreak occurred in 1998 and was easily observed, both visually and with CCD imaging. In 2006 there was another SEB outbreak that encircled much of the planet. As it progressed around the planet, it gave the SEB the appearance of being split into two components, a northern and southern component (Fig. 3.22).

The STrDist is another interesting phenomenon. Rogers describes a STrDist as "a coherent sector of shading and disturbance that spans the South Tropical Zone (STrZ) [19]". This zone resides between the SEB and the STB. The disturbance is characterized by a dark coloration or shading in the STrZ. This dark coloration is initially short in length, concave on the following and preceding ends, and usually bounded on each end by a small white oval. Often, the disturbance begins near the preceding edge of the GRS. The Disturbance normally fades when the SEB fades prior to a SEB Revival. Disturbances normally last several months [20].

South Tropical Dislocations are also seen in the STrZ. A South Tropical Dislocation occurs when a faded section of the STB passes the GRS. When this happens the STrZ preceding the GRS darkens, so that the normal pattern of belts and zones is reversed [21]. According to Rogers, in the 1970s a sector of the STB turned white, and since then this 'STB Fade' has repeatedly induced structures like STrDists preceding the GRS, giving rise to a dislocation of the normal pattern of alternating belts and zones. Moving with the South Temperate Current this STB Fade passes the GRS every two years. On some of these passages, it has induced a darkening of the STrZ on the preceding side of the GRS, usually in the form of a South Tropical Band although additional shading may be present. The term 'South Tropical Dislocation' has been applied to this combination of the white STB and dark STrZ, and also to the remarkable suite of structures that develop from it [22]. Although the STB itself in recent years has often been difficult to make out visually, including any STrDist, the dislocation is fascinating and valuable to observe. CCD and webcam images normally show these events quite well.

D.Parker 2 March 2006 16 in [email protected]

Coral Gables. FL 10 18 UT Lumenera 075M

Astromik RGB Filters See poor 4 Ail: 4B

CM1=199 3 CM2=273 5 CM3=35 6

Fig. 3.22. Jupiter with a prominent Mid-SEB Outbreak underway on March 2, 2006. South is up. (Credit: Donald C. Parker).

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