Cl

According to Rogers, the three 'proto ovals' appeared when the STZ became subdivided by the gradual appearance of three dark features, which initially appeared either as dusky sections of the zone or as segments of a south component of the STB. These dark features expanded longitudinally until they confined the three intervening bright sectors of the STZ into gradually contracting ovals. Elmer Reese had originally named the dark segments Ab, CD, and EF. Subsequently, the bright ovals formed between them became known as BC, DE, and FA [65]. The STOs are anticyclonic like the GRS, rotating counterclockwise.

Voyager images in 1979 revealed the STOs to be anticyclonic vortices similar dynamically to the GRS. In 1998, two of the ovals BC and DE merged into a larger one, later designated BE, while Jupiter was too close to the Sun to be observed [66]. At the beginning of the 1998-1999 apparition, it was discovered that one of the three ovals was missing! Apparently, something unexpected happened while Jupiter was at conjunction, hidden from view behind the sun. The answer to this did not come immediately. Dr. Reta Beebe of NMSU and the International Jupiter Watch (IJW) contacted me seeking the longitudinal positions of the two surviving ovals. I had just assumed the duties of the Assistant Coordinator for Transit Timings of the A.L.P.O. Jupiter Section in 1997 and had the responsibility of keeping track of the positions of features in Jupiter's visible atmosphere. Dr. Beebe's intention was to use the Hubble Space Telescope (HST) to study the remaining ovals. In her words, no one was sure whether an oval had simply faded or two had merged. Eventually it was decided that the most likely scenario was that ovals BC and DE had indeed merged. The merged oval was renamed BE.

Previously the three ovals had drifted close to each other on many occasions, only to drift apart again. Often, smaller intervening cyclonic ovals were present, separating the larger anticyclonic ovals. It was always assumed that the dynamics of these systems would simply cause the three large ovals to bounce off of each other should they come together. News that two of the ovals had actually merged stunned Jupiter scientists.

As the 1999-2000 apparition began, the possibility of another STO merger, this time between ovals BE and FA, again presented itself. On April 30, 1999 the separation between the ovals' centers was only 18°. From April 30, 1999 to November 20, 1999 the distance between BE and FA varied in a dance that would see them drift closer together only to drift further apart again. The ovals drew closer then drifted apart several times. By November 20, 1999 the ovals were at or approaching conjunction with the GRS. Astronomers wondered if the passing of the GRS would change the ovals' drift rates. The ovals continued their dance of drifting closer together and then farther apart, but closing the overall distance over time. By January 2000, CCD images by the Pic du Midi Observatory revealed that a cyclonic cell between the two ovals had disappeared, which could make it easier for the two ovals to collide and merge (Sanchez-Lavega personal communication). On February 8, 2000 the ovals centers were measured as 12° apart. More significantly, the following edge of BE and the preceding edge of FA were only 5° apart! At this time as a staff member of the A.L.P.O., I issued an alert over the Internet, receiving immediate response from astronomers at JPL and Cornell University. By March 17, 2000 the ovals were in contact with each other but undisturbed. From this time forward, events developed rapidly. On March 19 and 20, 2000 oval BE shifted northward with FA overtaking it. Indeed, infrared images from the Infrared Telescope Facility (IRTF) on Mauna Kea, showed the bright methane cloud-caps over the ovals rotating around each other. On

March 21 and 23, 2000 CCD images showed BE was disrupted. By April 7, 2000 CCD images showed BE/FA as a single object, very diffuse [67].

And so, during a three week period beginning in March 2000, ovals FA and BE completed their merger in the south temperate region. The merger took place when the ovals were southeast of the GRS, and after the disappearance of a smaller clockwise rotating oval that had been between them. The high altitude oval clouds of ovals BE and FA demonstrated counter clockwise rotation around each other, then merged and began shrinking. The interaction of deeper clouds did not show mutual rotation [68].

Prior to the merger of the two remaining ovals BE and FA, HST images revealed that oval BE had a diameter of ~9,000 km and oval FA had a diameter ~7,700 km. Situated between them was a smaller oval, designated O1. Oval O1 had a diameter of ~5,000km. In latitude, oval BE was located at -32.7°, oval FA at -33.6°, and smaller oval O1 was at -31°, or south of both STOs. Oval O1 was first observed in May 1998 and was similar to one observed between ovals BC and DE before they merged in 1998 [69].

Close interactions between these three vortices (BE, FA, and O1) began in November 1999 when the ovals passed the GRS. At that time oval O1 began a southward migration, moving from latitude -31° to -36°, "moving along an arc of 35° that encircled oval BE in the anticyclonic (counter clockwise) sense", according to Sanchez-LaVega et al. Oval O1 moved from the cyclonic region to the adjacent anticyclonic (counter clockwise) one [70].

It would appear that the removal of oval O1 allowed ovals BE and FA to interact directly. According to Sanchez-LaVega et al. "as the ovals approached each other, BE moved with a velocity of u = 0.6 ms-1, whereas FA decreased its velocity from u = 1.6 ms-1 in January 2000 to u = 0.9 ms-1 in early March 2000." On March 17, 2000 FA pushed BE northward by ~3.2° (4,000 km) as observed at high altitudes.

Then on March 21, 2000 the pair began an anticyclonic (counter clockwise) orbit — q q about each other. By April 3, 2000 the ovals had merged, and by April 14, 2000 had y j¿ C

decreased in size to a compact state [71]. Jfl g —

At middle and lower levels (altitudes) the interaction was a little different, and hyar P

the first interaction occurred March 12-15, 2000. Between April 7 and 14, 2000 new £L ©

oval BA was still forming with a double nucleus of 5° separation still seen. Later, e pp t on September 2, 2000 the area of BA was determined to be ~70% of the sum of the ¡£ ^

According to Sanchez-LaVega et al. the orbiting action observed in oval O1 and in ovals FA and BE can be explained by the presence of a velocity field induced by the vortices on each other. The observed interactions consisted of well-known phenomena that had been observed and predicted by models in computer simulations. It appears that most interactions of smaller scale spots observed in Jupiter lead to mergers. However, prior to 1998 the three STOs had always just bounced off of each other. In spite of their long history as separate objects, the two remaining STOs had collided and merged together. So, after 60 years the three ovals have finally coalesced into one vortex [73]. As of this writing, the surviving oval BA continues to thrive. With the last of the two STOs having merged, the remaining oval became known as oval BA [74, 75]. Like the GRS, oval BA is a huge counter-clockwise rotating vortice. Oval BA is still visible today, being especially well seen in CCD imaging. Visually, this oval can be difficult to make out due to its low contrast, especially if the dark collar of material that often surrounds it is absent.

Amateur and professional astronomers alike kept faithful watch over STO BA during the 2000-2001 apparition. In October 2000, oval BA presented a large, bright object. However, with a faded and broken STB, oval BA was often difficult to make out visually. A CCD image taken by Donald Parker on October 5, 2000 revealed a large oval BA surrounded by a collar of dark material, and trailed for a short distance by a dark segment of the STB. Without this dark material, BA would have been difficult to see even on CCD images, due to low contrast with the rest of the south temperate region. I used this dark segment of the STB to help pinpoint the following edge of oval BA when observing visually. However, by December 2001 the dark collar of material surrounding oval BA had almost vanished, leaving the contrast between the oval and its surroundings so subtle as to make it near impossible to visually make out the oval. Only low intensity dark material following the oval in the STB made it possible to see it at all. Even on CCD images by Ed Grafton in December 2001 and Maurizio Di Sciulla in January 2002, the low contrast of oval BA was startling. Eventually, some of the dark collar material returned. A CCD image by Eric Ng taken on February 22, 2003 revealed an oval surrounded by dark material, although oval BA itself was rather dusky and subdued, contributing to low contrast. By March 2004, oval BA was once again easier to see due to dark material surrounding it on its south, preceding, and following edge. CCD images by Rolando Chavez, Cristian Fattinnanzi, and Parker reveal this quite well.

Since they first appeared in 1939-1940, the STOs contracted and continued to contract during their lifespan. The three ovals were of different lengths longitudinally, with FA being the most diminutive of the three before the demise began. In the 1980s, ovals BC and DE were 8-9° in length while oval FA was 5° in length [76]. On March 11, 2004 I measured the length of surviving oval BA as 7°. The morphology, intensity, and drift rate of STO BA continues to be of great interest to Jupiter observers. Could the GRS have gone through the same process somewhere in its history? Keeping an eye on the behavior and condition of oval BA will be a valuable contribution that amateur astronomers can make, possibly for years to come.

Fig. 3.27. Jupiter on July 3, 2006. South Temperate Oval 'BA' has turned red. Note how the color of 'BA' is very similar to that of the GRS. South is up. (Credit: Christopher Go).

Jupiter with the GRS and Oval BA

July 3 2006 12 26UT I 270 II 125 III 200 S 7/10 T 3/5 <D Christopher Go {Cebu. Philippines)

With great surprise, oval BA changed color in late 2005 and became noticeably red in early 2006 (Fig. 3.27). Many amateurs began to refer to the oval as "Red Spot, Jr." I prefer to continue referring to this feature as oval BA, since this is what it truly is and because we need to preserve the continuity of our observational record. This change in the oval's color was first noted and reported by amateur astronomers, and by April 2006 professional astronomers had made a detailed study and analysis of this event. This historic change will be more fully discussed in Chap. 4.

It seems to me that, compared to the NTB, the STB is a much more active belt with a greater variety of features. What will the future hold? Only future observations will reveal how many ovals, spots, and belt fadings will appear. And what about oval BA? How long will it survive? Forever? That now seems unlikely. However, with no other large ovals to collide with, is there anything there to disrupt it? The south temperate region will continue to be one of the most important regions of the planet to patrol.

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