THE VARIATION OF THE BRIGHTNESS OF PLUME H

The observations of the H plume from ESO La Silla, Chile, using the
TIMMI camera in the far infrared (10 micron) have now been processed
into a light-curve. This shows very clearly the rapid brightening as
the plume comes into full view round the limb of Jupiter. Most
interestingly, however, it also shows a less marked peak earlier on.
  
There are two graphs showing the variation before the rapid
brightening and the peak and its aftermath.  

This first and smallest peak (at 19:32:57 +- 3 seconds UTC) is clearly
seen on the first graph. It most likely represents the direct
observation of the plume when it was still on the hidden hemisphere of
Jupiter. At this moment, its light became visible from Earth. This is
probably because it was refracted around the limb by the Jovian
atmosphere (as the solar light is in the Earth's atmosphere after
sunset or before sunrise), or because of multiple reflections off
clouds in this atmosphere (as distant lightning on the Earth is
sometimes seen by reflection in thunderclouds), or possibly both.

The observations were made using the TIMMI system through a filter
transmitting between 9.10-10.41 micron. The time resolution is
3.5s. The time axis is seconds since UTC midnight, July 18th, 1994.

The observers were Tim Livengood (NASA), Ulli Kaeufl (ESO), Benoit Mosser
and Marc Sauvage (Observatoire de Paris-Meudon, France).

This is ESO PR Photo SL9J/94-15.  It may be reproduced, if credit
is given to the European Southern Observatory.