The Solar System at Radio Wavelengths 71 3
FIGURE 19 Real and synthetic false color images at a
wavelength of 20 cm (1.5 GHz) of Jupiter following the impacts
of comet D/Shoemaker–Levy 9 with the planet. (a and b)
Observations of the synchroptron radiation before (June 1994)
and after several impacts (19 July 1994), respectively. (c)
Theoretical emission based on a model of the ambient relativistic
electron distribution within a multipole magnetic field
configuration. (d) Theoretical synchrotron radiation after an
enhancement in the radial diffusion coefficient by a factor of a
few million. (e) Enhancement in the theoretical synchrotron
radiation, as produced from just shock acceleration. (f)
Theoretical synchrotron radiation using the shock model and
radial diffusion combined. (S. H. Brecht et al., 2001,
Modification of the jovian radiation belts by Shoemaker–Levy 9:
An Explanation of the data,Icarus 151 , 25–38.)
complex interaction of the radiating particles with shocks
and electromagnetic waves induced in the magnetosphere
by the series of cometary impacts. Results from models sim-
ulating the effects are shown in Figs. 19c–f.
3.5 Jupiter at Low Frequencies
Jupiter has the most complex low-frequency radio spectrum
of all the planets. Examples of most of these are shown in
Fig. 20 and are discussed in this section.
3.5.1 DECAMETRIC AND HECTOMETRIC RADIO EMISSIONS
From the ground, Jupiter’s decametric (DAM) emission,
confined to frequencies below 40 MHz, has routinely been
observed since its discovery in the early 1950s, occasionally
down to frequencies of 4 MHz. The upper-frequency cutoff
is determined by the local magnetic field strength in the
auroral regions: 40 MHz for RH emissions translates into
∼14 Gauss in the north polar region, and 20 MHz for LH
into∼7 Gauss in the south.
The dynamic spectra in the frequency–time domain are
extremely complex, but well ordered. On time scales of
minutes, the emission displays a series of arcs, like open
or closed parentheses (Fig. 20). Within one storm, the arcs
are all oriented the same way. The emissions have been in-
terpreted as coherent cyclotron emissions. The satellite Io
appears to modulate some of the emissions: Both the inten-
sity and the probability of the occurrence of bursts increase
when Io is at certain locations in its orbit with respect to
Jupiter and the observer. The non-Io emission originates
near Jupiter’s aurora, and is produced by electrons that
travel along magnetic field lines from the middle-to-outer
magnetosphere toward Jupiter’s ionosphere. Particles that
enter the atmosphere are “lost.” These may locally excite
atoms and molecules through collisions, which upon de-
excitation are visible as aurora at UV and IR wavelengths.
Other electrons are reflected back along the field lines, and
produce DAM, where their motion along the field line is
reflected in the form of arcs in the radio emission (i.e., a
drift with frequency). The Io-dependent emissions are pro-
duced at or near the footprints of the magnetic flux tube
passing through Io (similar, but much weaker, emissions
originate along the flux tubes passing through Ganymede,
and perhaps Callisto).
Hectometric (HOM) emissions are, in many ways, indis-
tinguishable from DAM except that they are found at lower
frequencies, from a few hundred kHz to a few MHz, with
a local maximum near 1 MHz. The source region of HOM
must be further from Jupiter than the DAM source. Oth-
erwise, like DAM, HOM is predominantly emitted in the
extraordinary mode and is likely generated by the cyclotron
maser instability.
Because the dipole moment of Jupiter is tilted by some
10 ◦from the rotational axis, most jovian radio emissions ex-
hibit a strong rotational modulation. Given that Jupiter is
a gas giant, this modulation is thought to be the best in-
dicator of the rotation of the deep interior of the planet.
The rotation period of the interior is important, for ex-
ample, because this provides a rotating coordinate system
against which the atmospheric winds can be measured. Be-
cause these radio observations have been recorded over
many decades of time, analysis of these data lead to an ex-
tremely accurate determination of Jupiter’s rotation period,
9 h 55 m 29 s.6854.3.5.2 KILOMETRIC RADIO EMISSIONS
Between a few kHz up to 1 MHz various spacecraft detected
both broadband (bKOM) and narrowband (nKOM) kilo-
metric radiation from Jupiter (Fig. 20). The lower frequency
cutoff for bKOM,∼20 kHz (sometimes down to∼5 kHz)
is likely set by propagation of the radiation through the Io