Summary of Rings Session, SL-9 workshop, Maryland, Jan 1994.
Session Chair: Phil Nicholson
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Two main areas of Rings observations during and after the impacts
were identified:

1) monitoring of the rings at near-IR wavelengths for brightening
induced by the impacting dust tail; and

2) observations of the impact flashes reflected off the eclipsed
portion of the rings.

I. DUST-INDUCED BRIGHTENING.

Due to huge uncertainties in the mass of dust associated with the
tail of SL9, it is not possible to make reliable predictions of the
effects of the dust impact on the rings. However, the rings are
believed to represent an equilibrium state between the production of
dust by interplanetary micrometeoroid impact on the embedded
satellites, Metis and Adrastea, and perhaps on larger ring particles.
According to calculations by Doug Hamilton at MPI-Heidelberg, the flux
of 100-micron projectiles from the comet, at 60 km/sec, may
significantly exceed the average interplanetary flux, and thus result
in substantial brightening of the ring as more dust is ejected from
the parent bodies. At the low optical depth of the Jovian ring
(5\times1-^{-6}), this enhanced dust will have a decay time of order
100 yrs.

Initially, the brightening may be restricted to the neighborhood(s)
of the unseen parent bodies of the rings and of the two satellites. Over a few
weeks, the newly-produced dust should spread azimuthally around the
ring, and over a few years it should also spread radially.  Monitoring
the ring at regular intervals of the days, weeks, and even years after
the impact should shed considerable light on the locations and number of
the source bodies, and also constrain models of dust re-accretion.

II. FLASH REFLECTIONS.

Although much fainter then the bright satellites, the jovian ring is
well-placed to reflect the flashes predicted to arise from the impact
fireballs. It is closer to the planet, is always present, and a 6
arcsec segment of the ring adjacent to Jupiter's eastern limb will be
in eclipse at the time. Calculations for a peak fireball luminosity of
1\times10^{25} erg/sec (Ahrens, et al.) predict that the
flash-illuminated ring may be 40% of the normal brightness of the ring
in full sunlight.  In addition, the brightness of the ring may vary
substantially in azimuth, due to the strongly forward scattering phase
function of the ring particles. Thus measurements of the ring
brightness with longitude may help determine the poorly-known
intermediate angle phase function, and thus constrain the size
distribution of the ring particles. The azimuthal extent of the
reflected light will also be sensitive to the effective height of the
fireball above the visible limb.

These observations will require near-IR imaging (best at 2.2 microns,
with a K filter) with a fairly large telescope, and integration times
of 30 sec or less. A sequence of images must be taken during the
predicted impact period, as each flash may last only 90-120 sec,
according to impact models.

III. OBSERVING PLANS.

Several groups will be observing the impacts using near-IR cameras,
and they are encouraged also to take longer exposure frames of the
ring, both before and after the impacts.  Because of the asymmetry of
the collision, and of the flash illumination, it is important to take
images of both east and west ansae for comparison purposes. This may
also serve as a calibration for attempts to monitor the flash
reflections. All observations are probably best made in the 2.1-2.4
micron region, where Jupiter is very dark. Because of the long
predicted decay time, post-impact observations are not extremely
time-critical, but should be made as observing schedules permit.  A
convenient recent reference on IR imaging of the jovian ring is:

Nicholson & Matthews (1991) Icarus 93, 331.

P. Nicholson,
10 May 1994.