71 4 Encyclopedia of the Solar System
FIGURE 20 A representative
dynamic spectrum of several of
Jupiter’s low-frequency radio
emissions. The color bar is used to
relate the color to the intensity of
the emission. The emission is
plotted as a function of frequency
(along they-axis) and time (along
thex-axis). (A. Lecacheux, 2001,
Radio Observations During the
Cassini Flyby of Jupiter, in
Planetary Radio Emissions V,
edited by H. O. Rucker, M. L.
Kaiser, and Y. Leblanc, Austrian
Academy of Sciences Press,
Vienna, pp. 1–13.)plasma torus. The source of these emissions is at high mag-
netic latitudes and appears fixed in local time. The forward
lobe near the north magnetic pole is of opposite polarization
than a “back lobe” of the same source. The nKOM emissions
last longer (up to a few hours) than bKOM, are confined to
a smaller frequency range, 50–180 kHz, and show a smooth
rise and fall in intensity. The recurrence period for nKOM
events suggests the source lags behind Jupiter’s rotation by
3–5%, which was the first indication that this emission, in
contrast to any other low-frequency emissions, is produced
by distinct sources near the outer edge of the plasma torus.
GalileoandUlyssesstudies have shown that these emissions
occur as a part of an apparently global magnetospheric dy-
namic event. There is a sudden onset of these emissions,
they are visible for a few to several planetary rotations, and
finally, they fade away.
3.5.3 VERY LOW FREQUENCY EMISSIONS
TheVoyagerspacecraft detected continuum radiation in
Jupiter’s magnetosphere at frequencies below 20 kHz, both
in its escaping and trapped form. As discussed in Section
3.1, radiation can be trapped inside the magnetic cavity if it
cannot propagate through the high plasma density magne-
tosheath. This trapped emission has been observed from a
few hundred Hz up to∼5 kHz. Occasionally, it has been de-
tected up to 25 kHz, suggesting a compression of the mag-
netosphere caused by an increased solar wind ram pressure.
Outside the magnetosphere the lower frequency cutoff of
the freely propagating radiation corresponds to the plasma
frequency in the magnetosheath and appears to be well
correlated with the solar wind ram pressure. This escaping
component is characterized by a complex narrowband spec-
trum, attributed to a linear or nonlinear conversion of elec-
trostatic waves near the plasma frequency into freely prop-
agating electromagnetic emissions. The linear mechanism
favors ordinary mode radiation, but the trapped emission
appears to be a mix of both ordinary and extraordinary radi-
ation, perhaps from the multiple reflections off high density
regions in the magnetosphere and at the magnetopause.
The quasi-periodic (QP), or jovian type III emissions
(in analogy to solar type III bursts, because of their similar
dispersive spectral shape) often occur at intervals of 15
and 40 min as observed byUlysses, but neitherGalileonor
Cassinifound particularly dominant periodicities at these
or other intervals (see Fig. 20). The emission likely origi-
nates near the poles. Simultaneous measurements by the
GalileoandCassini spacecraft, both in the solar wind but
at different locations, observed similar QP characteristics,
suggestive of a strobe light pattern rather than a search
light rotating with the planet. Within the magnetosphere,
the QP bursts can then appear as enhancements of the
continuum emission. At the magnetosheath, the lower
frequency components of the bursts are dispersed by the