It had been thought that these features formed at random longitudes around the planet, but a closer inspection of the data revealed that this was not true once an appropriate drift rate was chosen. Ortiz et al., concluded that both the pattern and observed speeds were consistent with a Rossby wave .
Ortiz, define a hotspot as a region in Jupiter's atmosphere whose equivalent brightness temperature at 4.8 ||m is greater than 240 K at nadir viewing. The hotspots extend from the southern edge of the NEBs and into the equatorial zone (EZ). They are not referred to as NEB or EZ hotspots, but by their central latitude, that is 6.5° N planetocentric .
High-resolution red and near-infrared images reveal that regions of exactly the same morphology as the hotspots at 4.8 |im are very dark visually. However, not all the dark features seen in the red and near infrared are bright at 4.8 |im. Thus, while every hotspot is associated with a bluish-gray feature, not every bluish-gray feature (festoon) is associated with a hotspot . According to Orton, all of the bluish-gray features are warm, even if slightly so, in the infrared; however, as Ortiz writes, not every bluish-gray feature would be classified as a hotspot (Orton, personal communication, August 2005).
Long-term observations of the NEBs bluish-gray features (the visible festoons) reveal that their drift rates and locations show similarities to the 5-| m hotspots; that is, their quasi-periodic, but often symmetric, spacing in longitude as well as their time variable numbers around the full circumference of the planet. According to Rogers (1995), and the data of other organizations, these 'dark NEBs projections' often have lifetimes of months with a faded feature reappearing in the same location .
We have previously discussed the physical appearance of these features (festoons) as seen visually in amateur instruments and how during one period the A.L.P.O. made a special effort to follow the festoons from one apparition into the next. I think it would be a wonderful project for amateurs to repeat that endeavor on a more continued basis with the goal of tracking the morphology just described.
These bluish-gray features appear "bright" at 5 |im, that is to say, in the infrared. Since infrared senses differences in temperature, we know they are hot spots. Since Earth's atmosphere shields us from infrared wavelengths, work in the infrared must be done at high altitude. Orton has gathered his data with the NASA infrared telescope on Mauna Kea, Hawaii. While infrared work is still much the domain of the professional astronomer, how far off in the future can amateur instrumentation and capabilities be?
While we often see extended features in Jupiter's belts and zones that are somewhat dark, true condensed, independent spots that are truly dark are rare in Jupiter's atmosphere. We have already had some discussion of the South Temperate Dark Spot of 1998. It was one of the darkest spots ever seen (Fig. 4.8). The Galileo spacecraft was used to examine this spot. Data indicated that the spot was warmer than the environment in which it resided. The warm cloud-free conditions indicated it was a region where dry upper atmosphere gas flow had converged, made a hole in the cloud as it was forced to descend, and warmed as its density increased .
The 1998 Dark Spot (South Temperate Dark Spot of 1998) was unlike the cloud-free hotspot into which the probe had descended. According to Dr. Glenn Orton, JPL, both appear warm at 5 |im, but the dark spot was truly warmer than its surroundings, whereas the '5-| m hotspots' actually appear to be the same temperature as their surroundings .
There is also a distinct color contrast. The 5-| m hotspots, festoons as we see them, are normally colored a dark blue-gray. The 1998 Dark Spot was spectacularly
Fig. 4.8. The South Temperate Dark Spot of 1998 as Imaged by the Galileo spacecraft. The single arrow in the upper image, which is a map of Jovian temperatures, identifies a warm area that is further identified with the "black spot", shown in the visible light image of the middle image. The spot may be the result of a downward spiraling wind that blows away high clouds and reveals deeper, very dark cloud layers. The bottom image is a thermal radiation image sensitive to cloud-top temperatures. The warm temperatures and cloud-free conditions im ply that th is feature is a region where dry upper-atmospheric gas is being forced to converge, is warmed up and then forced to descend,
Fig. 4.8. The South Temperate Dark Spot of 1998 as Imaged by the Galileo spacecraft. The single arrow in the upper image, which is a map of Jovian temperatures, identifies a warm area that is further identified with the "black spot", shown in the visible light image of the middle image. The spot may be the result of a downward spiraling wind that blows away high clouds and reveals deeper, very dark cloud layers. The bottom image is a thermal radiation image sensitive to cloud-top temperatures. The warm temperatures and cloud-free conditions im ply that th is feature is a region where dry upper-atmospheric gas is being forced to converge, is warmed up and then forced to descend, a clearing out clouds. (Credit: NASA/JPL-Caltech). W
black. Visually when I observed the spot, it was incredibly small, yet almost as s O
In late 2005, the remaining white south temperate oval BA changed color and "Ö O
became noticeably red in early 2006. I think this was quite unexpected by the astro- V V 0
nomical community, and proved to be of great interest to professional astronomers. Oval BA is the remnant of three long-lived white ovals that formed in the 1930s south of the GRS. The previous two surviving ovals, BE and FA collided and merged in April 2000 (Fig. 4.9). Oval BA had always displayed an off-white or dusky-white appearance, but images obtained by amateur astronomers in late 2005 caught the oval displaying a tawny appearance. Images taken in December 2005 revealed that oval BA was red for the first time in its history. This was confirmed by further imaging in early 2006  (Fig. 4.10).
According to Simon-Miller et al., Hubble Space Telescope (HST) images from 1998 to 1996, and additional Galileo spacecraft data from 1997, indicated that oval BA was similar to the rest of Jupiter's white zone regions with high, optically thick, white clouds and haze. In contrast, the darker belt regions appeared to be comprised of deeper white clouds covered with a thick, blue-wavelength, absorbing (or scattering) haze. This haze is actually widespread, but appears darkest in locations with the deepest clouds, allowing for the longest path length through the haze. As previously mentioned, the GRS with its high, thick, colored clouds differs from Jupiter's other anticyclonic regions and ovals. A separate coloring agent is required to explain its color. This coloring agent may be material that is dredged up from deep within the atmosphere in the upwelling central region of the GRS' strong, high-pressure system. Neither the coloring agents in the belts nor the
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