The Solar System at Radio Wavelengths 71 5
higher density plasma, which produces the characteristic
type III spectral shape. The 40-minute QP bursts were
correlated with energetic (∼1 MeV) electrons observed by
Ulysses.Chandradetected similar periods in X-rays from
the auroral region, although not directly correlated with
QP bursts themselves. Such observations suggest that the
QP bursts are related to an important particle acceleration
process, but the details of the relationship and the details
of the process remain elusive.
3.5.4 GANYMEDE
Jupiter’s satellite Ganymede has its own magnetosphere
embedded within Jupiter’s magnetic field. It presents a rich
plasma wave spectrum, similar to that expected from a plan-
etary magnetosphere. It also is the source of nonthermal
narrowband radio emissions at 15–50 kHz, very similar to
the escaping continuum emissions from Jupiter. The more
intense cyclotron maser emission, seen from the auroral re-
gions of all giant planets and Earth, is absent, however. This
is almost certainly because the electron plasma frequency is
greater than the cyclotron frequency; hence, the cyclotron
maser instability does not operate.
3.6 Saturn
Saturn’s nonthermal radio spectrum consists of several com-
ponents, as displayed in Fig. 21, and discussed in the fol-
lowing section.
3.6.1 SATURN KILOMETRIC RADIO EMISSIONS
Saturn’s kilometric radiation (SKR) is characterized by a
broad band of emission, 100% circularly polarized, cover-
ing the frequency range from 20 kHz up to several hun-
dred kHz. When displayed in the frequency–time domain,
it is sometimes organized in arc-like structures, reminiscent
of Jupiter’s DAM arcs (see Fig. 21a).Cassinihas revealed
some fine structure characteristic of cyclotron maser emis-
sions (Fig. 21b). As on Earth, the SKR source appears to
be fixed at high latitudes primarily in the local morning to
noon sector, but it also appears at other local times. The SKR
intensity is strongly correlated with the solar wind ram pres-
sure, perhaps suggesting a continuous transfer of the solar
wind into Saturn’s low-altitude polar cusps. In fact, a de-
tailed comparison between high-resolutionHubble Space
Telescope(HST) images of Saturn’s aurora with SKR sug-
gests a strong correlation between the intensity of UV au-
roral spots and SKR.
Even though the emission is highly variable over time, a
clear periodicity at 10h 39 m 24 s± 7 swas derived from the
Voyagerdata, which was adopted as the planet’s rotation pe-
riod. Because the emission is tied to Saturn’s magnetic field,
which is axisymmetric, the cause of the modulation remains
a mystery, although it may be indirect evidence of higher or-
der moments in Saturn’s magnetic field. Even more mysteri-
ous, however, is that the SKR modulation period measured
byUlyssesandCassinivaries by 1% or more (several min-
utes) on timescales of a few years or less. Clearly, this change
in period cannot represent a change in the planet’s rotation
itself, but there is no commonly accepted explanation.3.6.2 VERY LOW FREQUENCY EMISSIONS
While the spacecraft was within Saturn’s magnetosphere, it
detected low-level continuum radiation (trapped radiation)
at frequencies below 2–3 kHz (VLF, very Low Frequency).
At higher frequencies, the emission can escape and appears
to be concentrated in narrow frequency bands. It is believed
that both the “trapped” and narrowband radio emissions are
generated by the same mechanism, that is, mode conversion
from electrostatic waves near the upper hybrid resonance
frequency. However, the source location has not been de-
termined. In particular, one source that has been suggested
is related to Saturn’s icy moons.
During the passage of theCassinispacecraft through
the inner region of the Saturnian system on July 1, 2004,
the Radio and Plasma Wave Science (RPWS) instrument
detected many narrowband emissions in a plasma density
minimum over the A and B rings. These have been shown
to be propagating in the z-mode, at least partially. It is not
clear how these narrowband emissions are related, if at all,
to those measured well beyond the planet.3.6.3 SATURN ELECTROSTATIC DISCHARGES
Saturn electrostatic discharges (SEDs) are strong, impul-
sive events, which last for a few tens of milliseconds from a
few hundred kHz to the upper frequency limit of theVoy-
agerplanetary radio astronomy experiment (40.2 MHz),
and are also detected by theCassinispacecraft. Struc-
ture in individual bursts can be seen down to theVoy-
agertime resolution limit of 140 μs, which suggests a
source size less than 40 km. During theVoyagerera,
episodes of SED emissions occurred approximately every
10 h 10 m, distinctly different from the periodicity in SKR.
In contrast to SKR, the SED source is fixed relative to
the planet-observer line. The emissions are likely electro-
static discharge events as a counterpart of lightning flashes
in Saturn’s atmosphere. Some SED episodes have been
linked directly to cloud systems observed in Saturn’s at-
mosphere by theCassinispacecraft.Cassini, however, has
found SEDs to be much less common, generally speak-
ing, thanVoyager. Cassini can go months without seeing
the discharges. Perhaps it may be a seasonal effect or re-
lated to the extent of ring shadowing on the atmosphere
(or ionosphere, if propagation is an issue).Cassinishould
continue to observe through similar seasonal and ring