The legacy of Cassini RPWS: Radio and plasma wav es at Saturn
Abstract
Introduction:
The 13
-
year exploration of Saturn with Cassini provided enormous scientific return from the Radio and P
lasma
Wave Science (RPWS) investigation. While it is not possible to achieve absolute concensus on the most important results from
this
investigation, here we attempt to show the breadth of contributions to the study of the Saturnian system with this inst
rument.
Saturn Kilometric Radiation:
Cassini’s RPWS determined that not only does the SKR rotational modulation vary in time, there
are typically two different periods in the northern and southern hemispheres. Cassini crossed the SKR source region, conf
irming that
the cyclotron maser instability (CMI) can drive the auroral radio emissions. These are the first in situ observations of a n
on
-
terrestrial CMI radio source. In addition, SKR was shown to be generated on field lines threading the UV auroras an
d diagnositic of
magnetospheric dynamics imposed by solar wind compressions and tail reconnection.
Enceladus and the E ring:
RPWS observations contributed to the mapping of dust from the plumes of Enceladus and the
resulting E ring. The discovery of aur
oral hiss generated by electron beams accelerated from the moon informed our understanding of
the electromagnetic interaction of the moon with the magnetosphere. Auroal hiss also provided evidence of this interaction ve
ry close
to the planet on field lines
threading the moon. Plasma resulting from the ionoization of material coming from Enceladus was
modeled through the determination of the electron density in Saturn’s inner magnetosphere. The depletion of the electron den
sity in
the plume led to the real
ization that charged micron
-
sized dust grains were a major component of a dusty plasma in the vicinity of the
moon.
Lightning:
High frequency radio emissions initiated in lightning strokes enabled RPWS to characterize the occurrence of
lightning, hence, c
onvective storms in Saturn’s atmosphere and tracked the development of a Great White Spot storm beginning in late
2010. Coupled with amateur and ISS images, an extensive compilation of thunderstorm activity in Saturn’s atmosphere was poss
ible.
Titan’s Ion
osphere:
The RPWS Langmuir Probe was the first instrument to confirm the existence of a substantial ionosphere at
Titan. The Solar EUV dominates the ionization of the upper atmosphere of Titan, although energetic particles in Saturn’s
magnetosphere do con
tribute substantially
at all Solar Zenith Angles
.
Observations over the orbital mission showed the effect of the
vaiation of EUV over the solar cycle in the ionospheric density.
One of the more intriguing discoveries was the formation of complex
negative
organic ions and aerosol pre
-
cursors in the deep ionosphere of Titan, which may have implications for how pre
-
biotic
chemistry occurred on the early Earth.
RPWS observations also helped characterize the interaction of Titan with the solar wind and
the deve
lopment of a compound bowshock encompassing both Titan and Saturn’s magnetosphere.
Saturn’s Ionosphere:
An obvious result of Cassini’s Grand Finale was the first in situ obervations of electron densities and
temperatures in Saturn’s topside ionosphere and
the revelation of strong interactions between the rings and the ionosphere.