UV and visible planetary auroral emissions: Jupiter and Saturn

In the giant planets upper atmosphere, collisions of
auroral electrons with atmospheric H atoms and H2
molecules, following acceleration along magnetic
field lines, give rise to excitation of these primary
neutrals. Excited H and H2 almost immediately loose
part of (~15%) their excess energy through radiative
decay processes implying emission of FUV, EUV,
NUV and visible light. An observer located near
Earth orbit will only see the sunlit portion of the
giant planets for which the reflected sunlight
outshines a large portion of the hydrogen auroral
emissions. Fortunately, the solar spectrum drops by
several orders of magnitude in the FUV-EUV
bandpass and is further attenuated by low altitude
hydrocarbon haze produced in the polar regions. This
makes it possible to observe Jupiter and Saturn EUV
and FUV auroras from Earth orbit with, for example,
the UV cameras onboard the Hubble Space
Telescope. These cameras provided numerous
fantastic views of Jupiter and Saturn's polar auroral
emissions. By contrast, the dimmer NUV and visible
auroral emissions cannot compete with the solar light
and can only be observed on the night side
hemisphere of Jupiter and Saturn; out of visibility
from Earth orbit. This region is accessible to in situ
spacecraft, like Galileo, Cassini or NewHorizons,
which have to share their precious observing time
among several different scientific topics. As a result,
images of the NUV and visible auroral emissions are
rare, in comparison with the huge HST database.
Nevertheless, the fact that they are only captured in
the night side implies that the origin of the energetic
particles that gave rise to them is principally found in
the immense magnetospheric tail; a vast region where
energetic electromagnetic processes and plasma
motions are still poorly documented. This makes
these emissions invaluable in terms of scientific
return.