The basic nomenclature of Jupiter begins with the largest atmospheric features, the belts and zones. Dark streaks circling the globe at given latitudes are called belts. The lighter areas in between are called zones. These belts and zones are referenced by a standardized system according to the diagram below.
Figure 1: Belt and zone nomenclature of Jupiter. Abbreviations: N = North, S = South, Temp = Temperate, Trop = Tropical, Eq = Equatorial.
It is worth noting that the Equatorial Zone lies on both sides of the Equatorial Band. (The Equatorial Band itself is the sole exception to the dark belt nomenclature.)
For convenience, the belts and zones are usually abbreviated. S denotes south while N denotes north. Equatorial is handled by E, temperate is handled by T, and tropical is abbreviated Tr. Zone is Z and belt and band are B. Polar is P and region is R. Under this system, then, the SEB refers to the South Equatorial Belt, and NTrZ refers to the North Tropical Zone.
The SEB usually consists of two components divided by a thin, sometimes discontinuous zone, as depicted above. This zone, in addition to being incomplete, may sometimes appear with ragged edges, or it may appear as a streak of spots. The two components of the SEB are called the north SEB (nSEB) and the south SEB (sSEB), respectively. The light zone dividing the two components of the SEB is called the South Equatorial Belt Zone or SEBZ for short.
(Confused yet? Don't worry. A command of the nomenclature is not critical - there's still a lot to see on Jupiter whether or not you know the lingo. The details here are for those who need them.)
The SEB is subject to periodic disturbances, usually heralded by the fading of the sSEB. There is usually a big increase in activity within the nSEB around the same time. The fading of the sSEB is usually followed by the appearance of a white oval or two in the SEBZ, which rapidly multiply until the zone takes on a foamy appearance. Frequently the nSEB will appear to become darker at the same time. The 1999 and 2000 apparitions of Jupiter were ones in which the SEB disturbance was active, and therefore a large number of white ovals were present. The foamiest looking region was often adjacent to the Great Red Spot during these apparitions.
The tidy temperate belt-zone system depicted above is often much more clearly presented in the northern hemisphere of Jupiter than in the southern.
The Equatorial Band, in particular among Jupiters dark cloud features, is not always present.
The polar areas have long been neglected by ground-based observers. In recent years, however, there has been an increased awareness that the polar regions generate observable activity. The polar regions will take on a mottled or stippled appearance combined with a suggestion of fine banding under good seeing. In the last few years there have been repeated reports of large, indistinct, low-contrast blue triangular shaped features in the polar regions, as well.
The Great Red Spot (GRS), a high-pressure system of epic dimensions, is found in the SEB.
The Great Red Spot is contained within the Great Red Spot Hollow (GRSH). The hollow is the dark area of SEB which surrounds the spot; in fact, GRSH refers specifically to the part of the SEB which intrudes into the South Tropical Zone in order to accommodate the spot (the hollow hollows out a bit of the zone, you see). The GRSH is not the light colored area in which the Red Spot itself floats.
The GRS itself varies dramatically in color. A few years ago, the GRS was distinctly salmon-colored. A few years before this, it was a notable dark red (I am old enough to remember this). In recent years, however, the GRS has turned the same color as the zones - a pasty nothing. Give it some time - it will most likely turn red again. A blue filter, such as a Wratten 80A, 82, or 38A, can help darken the GRS and make it more easily visible against its surroundings.
In addition to these features, there are a class of transient and usually highly variable features that are seen in the Jovian atmosphere. The most stable of these are the white ovals.
White ovals are particularly common in the South Temperate Belt and South Equatorial Belt, but can occur in any belt or zone. Two white ovals in the STB, called spots BE and FA, have been observed since at least 1939. Others are more transient. White ovals, which are cold features in the Jovian atmosphere, are the site of upwelling winds; downdrafts skirt their edges. Interaction between ovals is relatively frequent; two ovals will approach one another and, as they pass, they will repel each other strongly, increasing speed quite rapidly. In early 1998, two white ovals actually collided and merged with one another.
Festoons are seen as though they were thin, dark projections of a belt into a zone. They are in fact more complicated than this. A few degrees north of the Jovian equator are a series of constantly forming, dissipating, and reforming bright clouds known as the Equatorial Plumes, which are probably locations of rapid upwelling of the Jovian atmosphere (these plumes can sometimes be observed from earth, especially when a red filter is used). A plume has a triangular shape when seen from above, with the base of the triangle at about 10° north latitude. This position is almost along the south edge of the NEB. The sides of the triangle are not perfectly straight, and the 'point' skews toward the southwest. The spaces between these plumes, called the "interplume region," contain less material than the clouds, and consequently a deeper layer of the atmosphere can be seen through them. These interplume regions are seen as festoons.
Festoons tend to have a blue tint in visible light. They usually appear to project from a festoon base, a knob which appears to be attached to the nearby belt. The base is often more strongly blue than the festoon. Bases can exist without festoons, and festoons can exist without bases. Several morphologies of festoon exist; some are straight, short lines; longer festoons are curved or can appear like the letter J; some can appear to cross an entire zone; and others will loop back to the belt they originated from. Festoons are most frequently and easily seen in the Equatorial Zone appearing to project from the North Equatorial Belt. But any region of rapid upwelling and bright cloud formation has the potential to create a festoon along its downdrafting edge, so festoons can theoretically be seen in any cloud-forming zone on the planet. Because festoons tend to be blue, red filters can help bring out the lower-contrast examples.
Dark spots are seen mainly in belts as strongly dark round or oval features, generally smaller than the white ovals.
Barges are distinguished from dark spots by their highly elongate shape. Barges are probably similar in nature to dark spots, but neither is well understood. The most plausible hypothesis advanced so far is that these are areas of high velocity downward winds, which separate clouds and reveal very deep layers of the Jovian atmosphere. Both dark spots and barges are very bright at thermal infrared wavelengths.
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