| Interim report on the NTropZ outbreak by John H.Rogers |
by John Rogers (BAA) & Michel Jacquesson (JUPOS team) 2009 July 14
In early June we drew attention to a remarkable dark spot that had suddenly appeared in the NTropZ, due N of the GRS. The outbreak at this location has now become more elaborate, and we still think it is probably the beginning of a new NEB expansion event - the northern equivalent of an SEB Revival. Here we report on the outbreak to date, with a series of images (Fig. 1) and a longitude chart of the dark spots (Fig. 2). (Thanks to everyone who has sent images; only a small selection are shown here.)
Three similar dark spots have appeared in succession at the same location: L2 ~ 136 (approx. L3 ~ 223). Previously, in May, there was a projection from NEBn here, which marked the position of a dark barge that had been prominent in March, though in May only the projection was well-defined. In late May, one of the mid-NEB outbreaks (bright rifts) was passing this point, and within it a new bright cloud erupted (May 21-23) and projected north through the dark projection (May 28), thus crossing the NEBn retrograding jetstream. By June 1, it formed the new dark spot in NTropZ, which quickly became the darkest spot on the planet, at latitude 20 deg.N, prograding at DL2 = -62 deg/month. On June 16, a second dark spot appeared at the original longitude, and prograded like the first (19.5 deg.N, -42 deg/month). But meanwhile, the first spot moved south again and retrograded (18.5 -> 16.5 deg.N, +20 deg/month). The two spots were in contact on June 30, but passed each other without merging, although spot 1 then became difficult to track as it ran along the disturbed NEBn towards the original source. Meanwhile a third, similar but smaller spot emerged from the original source on June 30, and also prograded.
There has often been reddish colour in the projection, but the NTropZ spots have been dark grey. In methane-band images they are dark.
The long-lived white spot Z is rapidly approaching the source of the outbreak, and may cause further agitation as it approaches. This year, white spot Z consists of a bright bay in NEBn plus a small brilliant spot in NTropZ at 20 deg.N, moving at -12 deg/month. From June 5-20, a NEBn dark projection squeezed past it with DL2 = +39 deg/month, one of the fastest retrograding speeds ever observed for a coherent feature here on the NEBn jet. This feature could have been created from the big NEB rift just before the main outbreak. White spot Z was at L2 = 175 on July 1 (40 deg. f. the outbreak), and the NEBn shows small-scale disturbance in this interval.
The outbreak is an example of an uncommon but well-documented phenomenon, in which a turbulent NEB rift, passing a cyclonic barge, generates an eruption of bright and/or dark material which is vigorously ejected northwards, crossing the retrograding NEBn jet, and forms an anticyclonic dark spot in the NTropZ. The dark spot may be brown or grey, and is methane-dark. (Thus they resemble jetstream spots, which are anticyclonic vortices on jetstreams, whereas slower-moving anticyclonic ovals are usually methane-bright.) These dark spots may be prograding or oscillating, like the present ones.
Very similar outbreaks have initiated NEB expansion events in the past, notably those of 1993 and 1996, and similar outbreaks occurred in the early stages of the more broadly distributed NEB expansion events of 1999-2000 and 2004. These events have occurred at intervals of 3-5 years since 1988, so we are due for another one in 2009. The present outbreak is very probably the start of this event, which will lead to broadening of the NEB into the NTropZ at all longitudes within the next year.
APPENDIX: List of recent NEB broadening events:
1988 (BAA reports): -- focal origin, poorly observed just before solar conjunction.
1993 (interim report in JBAA 103 p.157, & unpublished BAA report) - focal origin, similar to the present events.
1996 (BAA report) - similar focal origin.
1999- 2000 (BAA reports) - Irregular projections from NEBn in 1999, esp. a suddenly-formed Little Brown Spot (LBS), led to a slow and broadly distributed NEB broadening event that developed in 2000.
2004 (our e-mail bulletins) - Several LBSs appeared in the wake of NEB rifts, and the fourth of these was one focus for the more extensive NEB broadening event that then occurred.
(The LBSs were similar to the present dark grey spots. In both 1999 and 2004, the LBSs oscillated in the early stages of the NEB expansion events, as spot 1 is doing now.)
(Earlier such events are described in 'The Giant Planet Jupiter' Chapter 8.)
John H. Rogers,Ph.D. Jupiter Section Director,
[British Astronomical Association.]
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