The South Temperate Region

The South Temperate Region generally extends from latitude 27° south to 37° south. In the past, this region has displayed some of the most interesting features on the planet.

The STB can be thought of as the southern counter part to the NTB. Like the NTB, it is a relatively narrow belt and usually not nearly as dark as the SEB. Its coloration is often described as gray, but I often note a touch of reddish-brown in the belt also. During the apparitions of 2001, 2002, and 2003, the STB was faint and some segments of the belt were totally faded, or at best, broken and fragmented (Fig. 3.21). Visually, features in this belt can be difficult to observe, as often the contrast between the belt and the surrounding zones can be very subtle. But, on occasion, a feature of some prominence can be seen. This belt can be continuous, broken into segments, or faded and completely absent. Dark spots have been observed here as well as bright ovals. According to Rogers, the STB was always quite distinct, at least until the 1980s [59]. However, since 1997 I have found the STB to be very subtle and difficult to observe visually. When seen, the STB has often been a narrow, faint gray belt, broken and discontinuous.

In 1998, the STB immediately following the longitude of the GRS presented a mottled but otherwise continuous appearance. By 1999, many segments of the STB were quite faded. In 2002, the STB was so faded as to be mostly absent, punctuated by short, darker segments in a few locations around the planet. By the end of 2002, the belt was so faded as to make it difficult visually to distinguish the STB from the northern edge of the SPR, since the South-South Temperate Belt (SSTB) was also very faint or absent. This condition continued into 2003. However, in 2003 a dark segment, approximately 50° long, was seen for many months. This dark segment was so prominent as to be easily seen visually in telescopes as small as 8-inches aperture. CCD images revealed that this segment was not solid, but composed of a string of condensations close together. In February 2003, this dark segment was positioned immediately south of the GRS, giving the impression of an eyebrow over the GRS (Fig. 3.24). The appearance of this segment allowed transit timings and longitude measurements to be made, providing a welcome opportunity to track the drift rate at this latitude. By July 2004, the STB was again presenting a mostly continuous, thin, although faint belt around most of the planet. I found this to be so even visually in an 8-inch telescope. Some segments of the belt continued to be faded. In 2006 the STB was absent or at least very difficult to see around most of the planet, being most easily seen following south temperate oval (STO) BA. In some locations where the STB was faded, a very light remnant could sometimes be made out.

As mentioned, short dark segments and condensations or "spots" may occasionally be present in the STB. As recent as 2003, a very short, dark segment or condensation was observed on the southern edge of the STB (STBs). This segment was followed for several months, and was seen visually as well as being detected on CCD and webcam images. A darker segment was seen for a short distance following oval BA during 2006. Segments and spots like this are not seen during every apparition. However, one of the most prominent spots ever seen in the STB appeared in 1998.

The South Temperate Dark Spot of 1998 was a fascinating feature to observe. While there is evidence of this spot on CCD images by Miyazaki in 1997, the spot did not come to notoriety until 1998 (Fig. 3.25). As the Assistant-Coordinator for Transit Timings of the A.L.P.O. Jupiter Section, I first received a report of the spot on 1998 June 20 from Harry Pulley, who had observed the spot and recorded a transit timing on that date. The spot was subsequently observed and reported by — q q other observers over the next several weeks and I initially determined the spot icancn had a drift rate of -5° per 30 days. On July 22, 1998 I issued an alert message from Jfl q -2

A.L.P.O. via the Internet and gave this feature the provisional name of STB Dark Jj q ^

Spot #1. Visually, the spot appeared as a very small dark spot, like a pencil spot on & <D £

Fig. 3.24. Dark segment of the STB, giving the appearance of an 'eyebrow' south of the GRS on February 18, 2003. The STB is discontinuous. The NEB is narrow and the NTB is absent. Note the five bright ovals in the SSTB. Note also the coloration of the northern two-thirds of the EZ. South is up. (Credit: Ed Grafton).

Fig. 3.24. Dark segment of the STB, giving the appearance of an 'eyebrow' south of the GRS on February 18, 2003. The STB is discontinuous. The NEB is narrow and the NTB is absent. Note the five bright ovals in the SSTB. Note also the coloration of the northern two-thirds of the EZ. South is up. (Credit: Ed Grafton).

Fig. 3.25. Jupiter on August 3, 1998 and the South Temperate Dark Spot of 1998. Also seen is STB Dark Spot #2, an elongated spot following close behind. Two major ground telescopes and the Galileo spacecraft made investigations of the 'dark spot'. South is up. (Credit: Donald C. Parker).

a page, almost as dark as a moon shadow. Being so small, the spot was difficult to see visually, but once found it seemed to jump out at the observer. The visual effect was remarkable.

After July 14, 1998 the drift rate of the spot increased to -15° per 30 days, prompting me to issue another alert message. This second message caught the attention of Dr. Glenn Orton, of the Jet Propulsion Laboratory, who requested additional information. Dr. Orton was one of the Galileo Spacecraft Mission Interdisciplinary Scientists. Dr. Orton and I began corresponding on a regular basis. From the date the first alert message was issued until the end of the 1998-1999 apparition, the spot was observed continuously around the world. In fact, worldwide participation was so intense that there were few occasions that a transit of the spot was not observed at least twice a week! There was tremendous cooperation, especially among amateurs; and the interest by the professional community was a great example of professional-amateur cooperation!

On September 21, 1998 I issued another alert over the Internet announcing a second dark feature had been seen in the STB. This feature was given the provisional name of STB Dark Spot #2. Dr. Orton informed me that a decision had been made in the professional community to examine Dark Spot #1 more closely. It was Dr. Orton's intention to observe Jupiter at 5-|jm with the NASA Infrared Telescope Facility (IRTF) Telescope and Near Infrared Camera (NSFCAM) from Mauna Kea for three full nights starting on or about September 27, 1998. At the same time, Dr. Terry Martin was to observe from Palomar Observatory using a middle-infrared camera and a spectrometer. Dr. Orton anticipated they would also capture the feature with 27-^m observations made with the Galileo Spacecraft Photo-Polarimeter-Radiometer (PPR) sequence on Galileo's orbit number 17 [60]. Dr. Orton later informed me that all three observations were successful. You can imagine how excited we were in the amateur community, having contributed to the professional interest in this feature that led to the allocation of observing time with such important resources!

These observations revealed that the feature was not a spot at all, but a hole in Jupiter's cloud tops. The "spot" was bright at 5-|jm (the infrared). Since infrared detectors detect warmth, this brightness indicated the spot was a warm feature. In other words, we were detecting the warmer temperature below Jupiter's cloud tops escaping up through this hole. In visible light, this hole was dark, since we were seeing through it to lower cloud decks.

This "spot" was observed for two apparitions with great success, and caught the attention of amateurs and professionals alike. Not only was this an exciting feature to observe and record, the international cooperation that ensued was a great accomplishment for astronomers [61]. When features like these are seen, Jupiter observers are presented with an opportunity to monitor the drift rates in this belt with great accuracy.

The features most prominently seen on the STB are white ovals [62]. Many of these ovals are quite small and difficult to observe visually due to the low contrast of the ovals against the zones on either side of the belt. The most famous STO today is the larger oval BA (Fig. 3.26).

Oval BA is the loan survivor of three ovals that originated in the South Temperate Zone (STZ) between 1939 and 1940 [63]. The history of these ovals is quite fascinating, and the eventual demise of two of them presented astronomers, professional and amateur alike, with an adventure in observational astronomy we shall not soon forget.

The three STOs began their existence as bright sections of the STZ. During 1939-1940, three bright segments or spots appeared in Jupiter's south temperate region. The spots rapidly contracted during the 1940s to form three large white ovals by 1950, eventually shrinking to sizes of ~ 10,000 km by the 1990s. During their lifetimes, the ovals demonstrated wandering motions (movement in longitude), with 'mutual close approaches without mergers' [64].

APRIL 22nd. 2005 01:19 UTC

Fig. 3.26. The bright South Temperate Oval 'BA' in the STB near Jupiter's central meridian on April 22, 2005. Note how the color of oval 'BA' is very close to that of the neighboring zones on this date. A small bright oval is in the SSTB just south of 'BA'. South is up. (Credit: Damian Peach).

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