| Jupiter's fading SEB: A new phenomenon at the Red Spot Hollow by John H.Rogers |
The blue streak has been noted many times when the SEB is faint: "Sometimes there is a distinct bluish or grey triangle on SEB(N)s, Np. the GRS (1942/43, 1957, 1962, 1974, 1989/90, etc.).... As this colour is rarely seen at other times, it is evidence that the fading is not merely a covering of the belt with white clouds." (my book p.170). It was also visible in 1992/93, and appeared again in 2009.
The 2009 white spot developed around the start of October, and the blue streak in mid-October. Michel Jacquesson, Yuichi Iga, and Toshirou Mishina, have all made compilations and analysis of the origin of this spot. (The attached set is from Yuichi Iga; Michel and I can send a larger set of imaegs if required.) Although the white spot could have developed from a minor feature within the SEB, it seems more likely to have developed from an inconspicuous little white spot in the RSH (imaged on Sep.27 & Oct.2). Soon it became a very bright white spot interrupting the rim of the RSH (Oct.4-9), then moved fully into the narrow SEB beside the RSH (Oct.12 - still very bright white, and on this date was the first methane image, showing it methane-bright). Then the white spot emitted a less-bright white streak p. it in the SEB, with the blue streak along its N edge (Oct.19-26): both emerged together from the very bright spot.
A similar white spot has also appeared previously this year. Michel identifies an example in the excellent images taken in July, and a less conspicuous one in late April. neither was as bright as the present one.
Remarkably, the spot is also bright in methane-band images, as Antonio pointed out. They were taken on Oct.12 (K. Yunoki), Oct.14 (Don Parker), Oct.24 (A. Cidadao), Oct.25 (T. Akutsu), and Oct.28 (K. Yunoki), and showed the white spot distinctly methane-bright - brighter than the SSTB-AWOs. I don't know of any other persistent methane-bright spot recorded within a belt.
Bright interruptions in the RSH rim are common in normal times, and A. Sanchez-Lavega's team have shown that they can result from retrograding vortices being torn apart in the RSH. At present, with the SEB quiet, there are no such vortices, so the appearance of the bright spot(s) is a surprise. However we can show that identical white spots were observed along with the developing blue triangle during some earlier fadings of the SEB. Sometimes, as now, the white spot and/or the blue triangle came and went several times within a year.
The bright spot can be identified as a recurrent feature because similar white spots were also seen as the SEB faded in 1989-90 and 1992-93. According to our reports in the JBAA:
In 1992, a bright white spot appeared in the RSH in April-May (as Christophe Pellier reminds me). He points to an image from Miyazaki on 1992 April 17 which shows it:
http://www.ii-okinawa.ne.jp/people/miyazaki/planet/92-01-250.JPG
Also a colour image from the Pic du Midi on 1993 Jan.14 shows it as a bright white 'rift' with the blue-grey triangle p. it [Sanchez-Lavega et al., Icarus 121, 18: Fig.2B].
In 1989/90, the blue streak Np. the GRS was conspicuous in July, Nov., Feb., and April-May, but faint in alternate months. Hi-res photos showed that the streak in 1990 Feb. developed like a miniature SEB Revival, extending p. from a bright white spot due N of the GRS; the p. end of the blue streak extended at DL2 = -92 deg/mth for a few days. So this year's event is a repeat of this phenomenon.
In 1974, the blue triangle was seen sometimes; again it was variable (being absent during the Pioneer 10 and 11 encounters). No white spot was recorded but the RSH was particularly bright white.
Obviously it is worth continuing observations to the end of the apparition, even at low resolution, to see how the SEB progresses.
Best wishes,
John
John H. Rogers,Ph.D.
Jupiter Section Director,
[British Astronomical Association.]
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