The Solar System at Radio Wavelengths 711
FIGURE 16 Time variability in Jupiter’s radio emission. Panel (a) shows the radio intensity at a wavelength of 13 cm between the
years 1963 and 1998. (Courtesy M. J. Klein.) Panels (b) and (c) show Jupiter’s radio intensity at 11–13 and 21 cm, respectively, during
1994 up to the summer of 1995. The impact of comet D/Shoemaker–Levy 9 with Jupiter occurred in July of 1994 (indicated by the
vertical dashed lines). (I. de Pater and J. J. Lissauer, 2001, “Planetary Sciences,” Cambridge Univ. Press.)measurement for a plasma instrument because of spacecraft
charging effects.
3.4 Jupiter’s Synchrotron Radiation
Jupiter is the only planet from which we receive synchrotron
radiation. The variation in total intensity and polarization
characteristics during one jovian rotation (the so-called
beaming curves) indicate that Jupiter’s magnetic field is
approximately dipolar in shape, offset from the planet by
roughly one tenth of a planetary radius toward a longitude
of 140◦, and inclined by∼ 10 ◦with respect to the rotation
axis. Most electrons are confined to the magnetic equa-
torial plane. The magnetic north pole is in the northern
hemisphere, tipped toward a longitude of 200◦. The to-
tal flux density of the planet varies significantly over time
(Fig. 16). These variations seem to be correlated with solar
wind parameters, in particular the solar wind ram pressure,
suggesting that the solar wind may influence the supply
and/or loss of electrons into Jupiter’s inner magnetosphere.
In addition to variations in the total flux density, the radio
spectrum changes as well (Fig. 18).
An image of Jupiter’s synchrotron radiation obtained
with the VLA in 1994 is shown in Fig. 17a. This image
was obtained at a wavelength of 20 cm and has a spatial
resolution of∼ 6 ′′or 0.3RJ. Since Jupiter’s synchrotron
radiation is optically thin, one can use tomography to ex-
tract the 3-dimensional distribution of the radio emissivity
from data obtained over a full jovian rotation. The example
in Fig. 17b shows that most of the synchrotron radiation
is concentrated near the magnetic equator, which, due to
the higher order moments in Jupiter’s field, is warped like
a potato chip. The secondary emission regions, apparent
at high latitudes in Fig. 17a, show up as rings of emission
north and south of the main ring. These emissions are pro-
duced by electrons at their mirror points and reveal the
presence of a rather large number of electrons bouncing
up and down field lines that thread the magnetic equa-
tor at∼2.5 jovian radii. This emission may be “directed”
by the moon Amalthea. A fraction of the electrons near
Amalthea’s orbit undergoes a change in their direction of
motion, caused perhaps by interactions with low-frequency
plasma waves near Amalthea (such plasma noise was de-
tected by theGalileospacecraft when it crossed Amalthea’s
orbit), and through interactions with dust in Jupiter’s rings,
while regular synchrotron radiation losses also lead to small
changes in an electron’s direction of motion.
Figure 18 shows radio spectra of Jupiter’s synchrotron
radiation from 74 MHz up to
̃>20 GHz. These spectra show
that the electrons in Jupiter’s radiation belts do not follow