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Figure 5.18. The merger of the white ovals from 1997 to 2000 observed by HST. Courtesy of NASA.

Figure 5.18. The merger of the white ovals from 1997 to 2000 observed by HST. Courtesy of NASA.

1979. Two of these ovals were observed to merge together in 1998, and in March 2000 the resultant two remaining ovals coalesced to form a single white oval (Figure 5.18), known as Oval BA. More recently, Oval BA was observed by amateur astronomers in 2005 to turn the same color of red as the GRS and the storm is now known to many as the "Little Red Spot" (LRS) or "Red Spot Jr.". Oval BA has been extensively observed in its new red state by HST and also by the New Horizons spacecraft during its flyby in February 2007. HST observations (Simon-Miller et al., 2006) have shown that Oval BA appears to be getting stronger, with wind speeds reaching 400 mph (645 km/h), similar to those seen in the GRS (Figure 5.19, see color section). Its current size is about the diameter of Earth. Approximately every 2 years, Oval BA passes close by the GRS; previous encounters in 1998 and 2000 were accompanied by dramatic changes in the white ovals. The most recent encounter occurred in June 2008 and was widely monitored with ground-based telescopes. During the encounter between the GRS and Red Spot Jr. a third, smaller red oval, called the "Baby Red Spot", became involved, which was caught between the two larger ovals and was torn apart and absorbed by the GRS.

The region to the northwest of the GRS is an area of cyclonic vorticity and usually appears to be particularly chaotic and rapidly changing. Small bright clouds regularly appear, which have been widely interpreted as thunderstorm clouds and the base of these clouds appear to be at pressures greater than 4 bar (Banfield et al., 1998 and Figure 4.11) suggesting a moist convective cumulus cloud rising from the base of the expected water cloud. In addition, the spectral signature of ammonia ice has been detected in these bright white clouds indicating rapid updraft and formation of pure white ammonia crystals (Baines et al., 2002 and Figure 4.10). The absence of clear ammonia ice features elsewhere, except in the NEB plumes, indicates that these crystals are rapidly modified or perhaps coated in some way as to hide their pure spectral signature (Atreya et al., 2005; Irwin and Dyudina, 2002; Irwin et al., 2005; Kalogerakis et al., 2008), as was described in Chapter 4.

A number of thunderstorms have now been observed in Jupiter's atmosphere. The first thunderstorms were observed by Galileo at latitudes of cyclonic shear (Dyudina and Ingersoll, 2002; Gierasch et al., 2000; Little et al., 1999) by observing their flashes on the night side of Jupiter (Figure 4.12). These storms have a lifetime of —4 days and the size of lightning spots in images suggest they result from point sources within or below the expected water cloud, which appear to be much more energetic than the average terrestrial lightning bolt. Lightning flashes on Jupiter's night side were also observed by the Cassini ISS instrument using an HQ filter (Dyudina et al., 2004). Dyudina et al. found that the flashes were 10x less bright than expected compared with the earlier clear-filter Galileo SSI observations, indicating that the flashes are generated at levels deeper than 5 bar, consistent with supersolar abundances of water vapor. Most recently, during the flyby of the New Horizons spacecraft in February 2007, the LORRI instrument reported many lightning flashes in Jupiter's polar regions (latitude >60°) (Baines et al., 2007a; Weaver et al., 2007). Until this observation, polar lightning had only ever been observed in Earth's atmosphere. The most poleward lightning flashes were seen at 80°N and 74°S. The energies of these lightning strikes are estimated to be between 0.2 GJ and 13 GJ, which are comparable with values seen by Galileo and Cassini at midlatitudes, but much larger than values estimated in the equatorial region, which is consistent with the hypothesis that the release of internal heat is mainly driving Jupiter's convection. The spatial extent of the flashes is again consistent with them originating in the 5 bar to 8 bar water-rich region of Jupiter's atmosphere. Although cyclonic shear latitudes are generally cloud-free regions, higher occurrence of lightning flashes is not thought to be purely an observational effect in that the flashes are simply more visible where there are fewer overlying clouds. Instead, modeling of the scattering properties of the clouds indicates that deep flashes occurring in zones would also be clearly visible. Hence, the correlation suggests that regions of cyclonic shear are simply more susceptible to moist convection (as was mentioned in Section 5.3.3). The moist convection scenario is also supported by an indication of increased water humidity in these areas (Roos-Serote et al., 2000). Although the Galileo entry probe descended in just such a cyclonic shear zone at 6.5°N, no lightning was detected within 10,000 km and indeed the nearest lightning strike detected in images was observed at 8.6°N.

At higher altitudes, in the stratosphere, other transient spot-like features have been noted in the polar regions of Jupiter by HST, Galileo, and Cassini at UV wavelengths, sounding approximately the 1 mbar level. These wavelengths are sensitive to the abundances of stratospheric hazes and the ovals appear with sizes comparable with the GRS. An example of such a UV spot can be seen in the middle UV image of Figure 5.16 (on the top, right-hand limb). What these spots are is not yet fully known. However, during the Cassini flyby, the Cassini ISS instrument recorded the birth, development, and subsequent decay of a large dark UV spot at 60°N with a total lifetime of approximately two months (Porco et al., 2003). This spot was found to lie within the main auroral oval, strongly suggesting a link with auroral processes. How auroral processes might affect stratospheric hazes is unclear, although the Cassini CIRS instrument found the upper stratosphere within the auroral oval to have anomalously high temperatures at pressures less than 4mbar (Flasar et al., 2004b; Kunde et al., 2004; Nixon et al., 2007). This region is also coincident with an area of enhanced X-ray emission observed in December 2000 by the Chandra X-Ray Telescope, launched into Earth orbit in July 1999.

In 2007 Jupiter went through a period of massive global upheaval, last seen in 1990 (Sanchez-Lavega et al., 1991). The changes (Go et al., 2007)started in mid-2006 with a darkening of the central and southern EZ, and the detachment of the GRS from the SEB, leading to cessation of the usually strong convective activity seen to the northwest of the GRS (Baines et al., 2007a). At the start of 2007, long-running activity in the SEB had ceased and two South Tropical Zone (STrZ) disturbances had formed, which appeared as dark hooks emanating from the southern edge of the SEB. Small dark spots running along the northern edge of the STrZ were observed to be deflected by these "hooks" and then run back in the opposite direction along the southern edge of the STrZ. In March 2007, two brilliant white spots emerged in Jupiter's North Tropical Zone (NTrZ), reaching an altitude 30 km above the surrounding clouds (Sanchez-Lavega et al., 2008) and initially separated by 55° longitude, which then took about 6 weeks to travel around the planet at a high speed relative to the System III longitude system, before they merged and subsided, leaving behind a trail of dark material, which slowly darkened, reviving the North Temperate Belt. Modeling of the three-dimensional form of the disturbance suggests that the plumes originated from great depth, and their observed high speed suggests that Jupiter's zonal wind system extends well below the depths that can be penetrated by sunlight, adding weight to the theory that Jupiter's dynamics are driven mostly by the release of internal heat. During the NTB disturbance, the SEB slowly faded, but was then revived by an SEB outbreak that appeared in mid-May. Unlike in 1990, this upheaval was observed by astronomers all over the world and also by the HST and partially imaged during the flyby of the New Horizons spacecraft in February 2007.

Although warm cyclonic polar vortices have been found on Saturn and Neptune, it is not yet clear if the same happens on Jupiter as we rarely see the poles. However, the only spacecraft to have flown over one of Jupiter's poles, Pioneer 11, reported higher temperatures at the cloud tops at the pole than at the equator (Ingersoll, 1990), suggesting such polar vortices may also exist on Jupiter.

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