Nov 11, 1996

Correction and Apology --- Glenn Orton served two terms as chair of the IJW for a total of 6 years. Sorry, Glenn!!


November 8, 1996

Exciting and unusual events are in store for Galileo and its team today. An unusual opportunity will occur this morning to measure the gases that are in the upper part of Jupiter's atmosphere. We will take advantage of Galileo passing behind Jupiter as seen from Earth, and look at how the spacecraft's radio signals are changed as they go through different places in Jupiter's atmosphere. As Galileo goes just behind Jupiter, and when it comes out again, the radio signal will be affected by the gases it passes through, and the amount of change is determined by the kind of gas it goes through. To add some drama to this event, Galileo will be out of communications with Earth for about 5 hours, and we will eagerly await for the first signals again around 9 or 10 AM PST.

In the evening, the spacecraft will enter Jupiter's shadow, and turn back to look at Jupiter's thin rings, which are normally hard to see but visible from the angle we'll be looking from. At the same time, we'll be watching for lightning and aurora on Jupiter. Also, Galileo will image Europa looking for evidence of icy volcanoes.

For information on the Galileo spacecraft and its mission to Jupiter, see the Galileo home page.

There will be special sessions on Galileo Results during the next AGU meeting. Atmospheres session including and earth-based support will be on Sunday, Dec. 15 in the afternoon in San Francisco's Moscone Convention Center.


Third interim report.

John H. Rogers,
British Astronomical Association,
1996 October 23.


There have been no major changes since our last interim report dated July 27. The array of white ovals in the STB continues on a stable course. The NEBs dark projections have been very prominent since the NEB expansion event, and show diverse drifts; one set are moving unusually slowly but others, including the 'PES Hotspot', have become stationary in System I and are changeable. There are no stable NEBs 'plumes', nor NEB 'barges', nor jetstream spot outbreaks, at present.

This is a further report on Isao Miyazaki's CCD images, up to 1996 Sep. 23. A set of Don Parker's images, April to July, has also just been received but has not been included in this report. Several visual observers have also sent observations, and their transits of the NEBs dark projections agree well with the data from images. We may get some additional data over the next month, but after that, there is unlikely to be much if anything more until next May.

Longitudes are given in Systems I and II (\1, \2), with conversion from \2 to \3 at the end. \\2 [\\1] is drift in System II [I] in degrees per 30 days. Latitudes are zenographic.

The two Little Red Spots at 42.5 deg.N (bright in methane) have varied irregularly in longitude but, over the whole apparition, have had sustained but different average drifts. Thes drifts reinforce the view that they were the same spots imaged in 1995.

The North Temperate Belt has a well-separated north component spanning ~150 degrees, so the whole N. Temperate domain is shaded in these longitudes. In other longitudes, NTZ is clear white with a narrow NNTB separating it from a yellowish NNTZ.

There are no more NTBs jetstream spots. In keeping with the historical tendency for features to appear in the 'N. Temp. Current B' shortly after a NTBs jetstream outbreak ends, there has been a short faint sector of NTB moving at a rate of -106 to -150 deg/month (Sys II) from June to September.

The NEB remains broad. Cyclonic dark 'barges' may well develop at 16 deg.N within the next year, following the NEB expansion event, but we do not see any yet. If the Galileo team wish to locate one for targetting, IRTF 5-micron images the spring, 1997, may offer the best chance of doing so.

On the NEBs edge, the dark projections (5-micron hotspots) have been very large and dark and bluish since the NEB expansion event. They show diverse drifts. Four (listed in the table below) were large 'plateaux' with unusually slow rotation periods (i.e. positive drifts in System I ranging from +13 to +20 deg/month). Their shapes have varied a lot, and in September they were more compact dark projections. In the remaining ~180 degrees of longitude, the dark projections are smaller and have lesser but variable drifts. On Sep.8 they were at 48, 68, 88, 122, 158, 184 deg. in System I longitude. The first two had positive drifts early in the year but were stationary in September. (Thus the 'PES Hotspot' was fixed at longitude (Sys I) = 68 deg from late July to mid Sep., but it then merged with a projection 20 deg. west of it, so it is no longer distinct as of Sep.22.) The others have been roughly stationary since first observed in June, but variable in drift and appearance; the last one developed from a string of tiny dark spots left in the wake of retrograding projection (a) in the table below. I have no confidence in any of these dark projections persisting unchanged until December.

Vigorous white spot activity continues in the SEBZ immediately f. the GRS. Its f. edge was at ~ 93deg (Sys II) on Aug.31 but a distinct outbreak of new white spots occurred up to 98 deg by Sep.2 [according to visual observers as well as images]. So far it has not extended further.

On the SEBs edge, most of the almost-stationary dark spots in the STropZ have been transient, though a tiny orange streak in the SEB, at 16.5 deg.S, persisted at least till Sep.3.

The prominent white oval, which has been tracked since 1987, is steadily approaching the GRS. Its present drift would bring it to the edge of the Red Spot Hollow in 1997 April. If it persists, this will be a very interesting interaction to observe, because no such long-lived S. Tropical oval has ever been seen to interact with the GRS: will it merge with the GRS like SEBs jetstream spots do, or keep some distance away like the S. Temperate ovals do from each other, or just dissipate before the encounter?

The GRS has gradually lost the very dark grey collar around the S. edge.

The aspect is little changed, including the cluster of four anticyclonic white ovals, BC-DE-XY-FA, now accompanied by three cyclonic ovals in the spaces between them. The white cyclonic ovals between BC-DE and between XY-FA have swung to and fro as drift rates have varied slightly, but they remain distinct; also a grey cyclonic oval has developed between DE-XY. The average longitudinal spacing of the anticyclonic ovals has slowly shrunk from 20 deg. (May) to 18 deg. (Sep.); also, two more tiny anticyclonic ovals have appeared 18 deg. on either side, so the array now has 6 anticyclonic white ovals. Oval DE has maintained an absolutely constant drift rate of -12 deg/month in System II. Therefore we can expect this oval (and perhaps its interesting cyclonic neighbours) to be correctly in place for the Galileo targetting in 1997 February.

7/49/812/19Drift Rate(deg/mo)
NEBs dark plateaux

The Galileo viewing range (290-330 deg in Sys III as of Dec.17.00 UT) corresponds to 174-214 deg in Sys I. This is the sector presently between the stationary features (see text) and these retrograding ones.

(Conversion to \3 is given at the bottom)
7/49/812/19Drift Rate(deg/mo)
N.N. Temperate ovals
Dark NTB(N)
p. end254~290~344~+16
f. end42~ 89~160~+21
S. Tropical ovals
Tiny orange streak in SEB (16.5 deg.S)221230245+4.5
White oval on SEBs51532+4.9
S. Temperate ovals
To convert SYS II to III-98-80-53+8.0

Longitudes are given at:
(a) July 4 (opposition) (value from best fit line);
(b) Sep.8 (day after G2 perijove, 2.20 mth after opp.) (actual value);
(c) Dec.19 (day of E4 perijove, 5.60 mth after opp.) (extrapolated value).
Drift rates are defined as degrees/30 interval in System I and II.

No account is taken of phase effect, which may be negligible on these high-quality images; further measurements are required to determine it. If there is a phase effect, the longitudes for Dec.19 may be overestimated by up to 3 deg.

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