71 2 Encyclopedia of the Solar System
(a) (b)FIGURE 17 (a) Radio photograph of Jupiter’s decimetric emission at a wavelength of 20 cm, and a central meridian longitude of
λcml∼ 312 ◦. Magnetic field lines at equatorial distances of 1.5 and 2.5 Jupiter radii are superposed. Field lines are shown every 15◦,
betweenλcml− 90 ◦andλcml+ 90 ◦. The image was taken with the VLA in June 1994. The resolution is 0.3 Jupiter radii, roughly the
size of the high latitude emission regions. [I. de Pater et al., 1997, Synchrotron evidence for Amalthea’s influence on Jupiter’s
electron radiation belt,J. Geoph. Res., 102 (A10), 22,043–22,064; Copyright 1997 American Geophysical Union.
Reproduced/modified by permission of American Geophysical Union.] (b) Three-dimensional reconstruction of Jupiter’s nonthermal
radio emissivity, from VLA data taken in June 1994, as seen from Earth atλcml= 140 ◦(DE=− 3 ◦). The planet is added as a black
sphere in this visualization. (I. de Pater and R. J. Sault, 1998, An intercomparison of 3-D reconstruction techniques using data and
models of Jupiter’s synchrotron radiation.J. Geophys. Res. Planets 103 (E9), 19,973–19,984; Copyright 1998 American Geophysical
Union. Reproduced/modified by permission of American Geophysical Union.)FIGURE 18 Jupiter’s radio spectrum as measured in September
1998 and June 1994. Superposed are various model calculations.
(Adapted from I. de Pater et al., 2003, Jupiter’s radio spectrum
from 74 MHz up to 8 GHz.Icarus 163 , 434–448, and I. de Pater
and D. E. Dunn, 2003, VLA Observations of Jupiter’s
synchrotron radiation at 15 and 22 GHz,Icarus 163 , 449–455.
a simpleN(E)∞E−apower law. Well outside the syn-
chrotron radiation region, beyond Io’s orbit at 6 jovian radii,
the electron energy spectrum appears to follow a double
power law,N(E)∞E−^0.^5 (1+E/100)−^3 , consistent with
in situ measurements by thePioneerspacecraft. Processes
as radial diffusion, pitch angle scattering, synchrotron ra-
diation losses, and absorption by moons and rings change
the electron spectrum. The radio spectra superposed on the
data were derived from such models.
Early in the 20th century (∼1930), Jupiter captured a
comet, now known as comet D/Shoemaker–Levy 9. During
a close encounter with the planet, this comet was ripped
apart by Jupiter’s strong tidal force into over 20 pieces.
These comet fragments, all in orbit about Jupiter, were dis-
covered by the Shoemaker–Levy comet hunting team in
May 1993. About a year later, from July 16 to 22 (1994),
all comet fragments hit Jupiter. These events were widely
observed, at wavelengths across the entire electromagnetic
spectrum. At infrared wavelength these impacts were in-
credibly bright, while at optical wavelengths the impact sites
were visible as dark spots with even the smallest telescopes.
This collision also triggered large temporary changes in
Jupiter’s synchrotron radiation. The total flux density in-
creased by∼20% (Fig. 16), the radio spectrum hardened,
and the spatial brightness distribution changed consider-
ably (Fig. 19a, b). These changes were brought about by a