Storms and Discrete Features
Ingersoll
9/27/04
The westward jet at 36 south planetocentric latitude (fav1) was the most active region of
the planet during Cassini’s long approach to Saturn in the first half of 2004. “Activity” is
here defined to mean having a large number of compact oval spots with diameters larger
than 500 km and lifetimes longer than a week. Most of the spots sat in the shear zone just
to the south of the jet. Spots in the jet moved westward at speeds up to 25 m s-1; spots a
few degrees to the south moved eastward at comparable speeds. The spots themselves
rotated counterclockwise (anticyclonic in the southern hemisphere), like ball bearings
between conveyor belts, with the same sense of rotation as the ambient shear flow
(fapi1). The westward jet at 48 south was also active and exhibited the same phenomena,
with anticyclonic vortices rolling in the anticyclonic shear zone to the south of the jet.
This picture of anticyclonic vortices sitting in anticyclonic shear zones is typical of the
giant planets, and was seen during the Voyager flyby of Saturn in 1981 (api1). The
present observations refer only to the southern hemisphere, since the rings blocked
Cassini’s view of the northern hemisphere during the approach phase. They also refer to
high-contrast visible spots that were seen by the cameras of the Cassini ISS. Other lessvisible features may be of equal dynamical significance (api2), but they are outside the
scope of this investigation. It was not possible to determine the sense of rotation for all of
the spots, especially the smallest ones, but the impression one gets from movies of timelapse images is that they were all anticyclonic.
The interaction of westward-moving spots with eastward-moving spots in adjacent
latitude bands often produced a merger or sometimes a near-merger, with bright material
suddenly forming between the two spots or around them. The compact spots were mostly
dark, but they often had bright haloes (fapi1). The new spot that resulted from a merger
sometimes continued on for weeks and sometimes split apart after a few days. Since
mergers represent a net decrease in the number of spots, there must be another process
that causes spots to appear. On Jupiter, appearances are of three types (api3): growth
from a point over a period of a few days, emergence from turbulent regions, and
gathering contrast over a period of weeks. Similar studies will be possible for Saturn as
the Cassini data accumulate.
Contrast is lower for Saturn than it is for Jupiter. Also, there are fewer spots larger than
1000 km, and no spots comparable to Jupiter’s Great Red Spot or the three white ovals
that merged in 1998 after carefully avoiding each other for 58 years (api2, api4). Despite
the low contrast, Saturn’s atmosphere is anything but featureless. Linear bands encircle
the planet, each with its own mottled texture. The edges of the bands are wave-like and
often break into turbulence. On the largest scale, rotational symmetry is the dominant
feature. A curious manifestation of this symmetry occurs at the South Pole, where a small
(2000 km diameter) feature sits undisturbed. The pole is a global-scale maximum of the
Coriolis force, so it is hard to see why a small-scale feature should be locked so precisely
to the pole, unless the perturbing forces are very small. Decreasing turbulence as one
moves away from the sun is a prominent feature of giant planet atmospheres (api4), so
perhaps the absence of such a polar feature at Jupiter should come as no surprise.
Two relatively large storms, each characterized by eruptions of white material, occurred
near the 36 jet in mid-July and mid-September, respectively (fapi2). At roughly the same
time, the Radio and Plasma Wave Science (RPWS) instrument (api5) began detecting
Saturn electrostatic discharges (SED’s), which are short burststens of milliseconds in
duration, with very large bandwidthspanning 2-40 MHz in frequency. The SED’s were
grouped into episodes that recurred with a distinct periodicity approximately equal to the
10 hr 39.4 min Saturn rotation period deduced from the Saturn kilometric radiation
observations. Voyagers 1 and 2 also detected SED’s, and they were grouped into
episodes, but the recurrence period was ~10 hr 10 min (api6). Two hypotheses were
proposed at that time, that the bursts originated in the rings and that they originated in the
atmosphere. As shown below, the Cassini data favor the latter hypothesis.
The Voyager and Cassini periods span the range at which atmospheric features at
different latitudes cross the center of the disk as viewed by a stationary observer. The
shorter period is characteristic of the high-speed equatorial jet, which blew eastward at
speeds close to 500 m s-1 at the time of Voyager (arv4). The longer period is
characteristic of the spots just south of the westward jet at 36S, where the storms
occurred in July and September of 2004. Since the SED bursts continued over 25-30
rotations of the planet, which is more than 10 days, it was possible to match the phase of
the bursts with the times that the storms crossed the center of the disk. A constant phase
would suggest that the radio bursts are originating in the atmosphere and are probably
caused by lightning discharges associated with the storms.
The results of this analysis are shown in fapi3. The abscissa is time, measured in units of
Saturn’s rotation period. The tick mark at the number 4 coincides with the time that the
storm crossed the central meridian (CM) as viewed from the Cassini spacecraft. Since the
CM crossings were highly periodic, a determination of longitude from any ISS image
could be extrapolated to a CM crossing for that rotation of the planet. These measured
CM crossing times are indicated on the figure by the plus signs. The SED burst frequency
is indicated by the bell-shaped histograms. The SED “on” and “off” periods each last for
~5 hours. As seen in the figure, the bursts began ~5 hours before the storms crossed the
CM and ended near the CM crossing time. The fact that the phase of the SED episodes
relative to the CM crossings remains constant and is the same for both storms is a good
indication that they are related.
Nevertheless, the details of the relation are somewhat mysterious. If the storm is
producing lightning at the same longitude as the visible cloud, why should the SED’s
begin when the storm is still below the horizon and end when the storm is on the CM?
Perhaps the lightning source is 90 to the east of the visible cloud. The smaller features at
the storm’s latitude are moving westward, although the storm center is nearly stationary.
If the storm were moving with the winds at a deeper level than the white clouds, with the
winds becoming more westward with altitude, the clouds from the storm would trail off
to the west relative to the storm and its lightning discharges. Then the SED’s would lead
the visible storm, as observed.
The Voyagers detected low-frequency SED burstsbelow 5 MHz only when the
spacecraft was above the night side of the planet (api6, api7). The experimenters pointed
out that if the ionospheric plasma density near the noon meridian was ~3  105 cm-3,
escape of atmospheric radio bursts below 5 MHz would be prevented (api7). If the
nightside ionosphere had low-density regions or holes, the observed low-frequency
emission could escape. This explanation could partly explain the phase shift observed by
Cassini, since the storm was in darkness before it crossed the CM and was in sunlight
after CM crossing. The ionospheric cutoff would explain why SED’s appeared to turn off
after the storm crossed the CM, but it does not explain why the SED began while the
storm was still below the horizon. Undoubtedly there will be other periods of SED
activity and other storms, perhaps with direct optical detection of lightning, which will
further test these hypotheses.
References and Notes
api1. A. P. Ingersoll, R.F. Beebe, B.J. Conrath, and G.E. Hunt, in Saturn, T. Gehrels and
M.S. Matthews, Eds, (University of Arizona Press, Tucson, 1984), pp. 195-238,
api2. A. Youssef and P. S. Marcus, Icarus 162, 74 (2003).
api3. L. Li, A. P. Ingersoll, A. R. Vasavada, C. C. Porco, A. D. Del Genio, S. P. Ewald,
Icarus, in press (2004).
api4. A. P. Ingersoll et al., in Jupiter: The Planet, Satellites, and Magnetosphere, F.
Bagenal, W. McKinnon, and T. E. Dowling, Eds. (Cambridge University Press,
Cambridge, 2004), pp. 105-128.
api5. D. A. Gurnett et al., Science, this issue (2004).
api6. M. L. Kaiser, in Saturn, T. Gehrels and M.S. Matthews, Eds, (University of
Arizona Press, Tucson, 1984), pp.
api7. M. L. Kaiser, (1983).
Figure Captions
fapi1. Southern hemisphere of Saturn in near-infrared light showing compact oval dark
spotsstormsat the latitude of the westward jet at 36 planetocentric latitude. The
image was taken on May 5, 2004. [The figure is “Two Pairs of Storms,” the press release
on June 4, 2004]
fapi2. White plume emanating from a large storm near the westward jet at 36
planetocentric latitude. The storm first appeared on September 6, and the image was
taken on September 12, 2004. [The figure shows the “Dragon Storm.”]
fapi3. Timing of the storms’ central meridian crossings compared to the timing of the
Saturn electrostatic discharges (SED’s). The abscissa is time in units of Saturn’s rotation.
The plus signs are the times that the visible storm crosses the central meridian as seen
from the Cassini spacecraft. The bell-shaped histograms are the number of SED bursts in
a given time interval. Note that the burst episodes lead the central meridian crossings by
~one-quarter of a Saturn rotation. [There is a figure for the July storm and for the
September storm].