The Solar System at Radio Wavelengths 709
FIGURE 14 A comparison of the peak flux density spectrum of
the kilometric continuum radio emissions of the four giant
planets and Earth. All emissions were scaled such that the planets
appear to be at a distance of 1 AU. Jovian emissions shown
include quasi-periodic bursts (QP), nonthermal continuum
(NTC), broadband and narrowband kilometric radiation
(bKOM, nKOM), hectometric radiation (HOM), decametric
radiation (DAM), and decimetric radiation (DIM). Saturn’s
kilometric radiation is designated SKR, and its electrostatic
discharge emissions are labeled SED. Terrestrial auroral
kilometric radiation is designated AKR. UKR and NKR refer to
kilometric radiation from Uranus and Neptune, respectively.
Uranus’ electrostatic discharges are labeled UED. (Adapted
from P. Zarka and W. S. Kurth, 2005, Radio wave emission from
the outer planets beforeCassini,Space Sci. Rev. 116 , 371–397.)
cone, in the direction of the particle’s parallel motion, and
reaches a maximum at an angle. Theoretical calculations
show that is very close to 90◦. Observed opening angles,
however, can be much smaller, down to∼ 50 ◦, which has
been attributed to refraction of the electromagnetic waves
as they depart from the source region.
The cyclotron maser instability derives energy from a
few keV electrons, which have distribution functions with
a positive slope in the direction perpendicular to the mag-
netic field. Recent observations in the source of Earth’s au-
roral kilometric radiation reveal “horseshoe”-shaped elec-
tron distributions that provide a highly efficient (of order
1%) source of free energy for the generation of the radio
waves. This distribution is thought to be the result of paral-
lel electric fields in the auroral acceleration region, the loss
of small pitch-angle electrons to the planetary atmosphere,
FIGURE 15 Radiation patterns in a magnetic field. Indicated
are the hollow cone pattern caused by cyclotron (dipole)
radiation from nonrelativistic electrons in the auroral zone. The
electrons move outward along the planet’s magnetic field lines.
The hollow cone opening half-angle is given by .Atlow
magnetic latitudes, in the Van Allen belts, the filled radiation
cone of a relativistic electron is indicated. The angle between the
particle’s direction of motion and the magnetic field, commonly
referred to as the particle’s pitch angle,α, is indicated on the
sketch. The emission is radiated into a narrow cone with a half
width of 1/γ. (I. de Pater and J. J. Lissauer, 2001, “Planetary
Sciences,” Cambridge Univ. Press.)and trapping of reflected electrons. Radio emissions gener-
ated in planetary magnetospheres by this mechanism often
display a bewildering array of structure on a frequency-
time spectrogram including narrowband tones that rise or
fall in frequency, sharp cutoffs, and more continuum-like
emissions. While it is generally accepted that emissions that
rise or fall in frequency are related to tiny sources moving
down or up the magnetic field line (hence, to regions with
higher or lower cyclotron frequencies), there is no generally
accepted theoretical explanation for the fine structure.3.1.2 OTHER TYPES OF LOW-FREQUENCY RADIO EMISSIONS
While the radio emissions generated by the cyclotron maser
instability are, by far, the most intense in any planetary
magnetosphere, other types of radio emissions do occur
that are of interest. Perhaps the most ubiquitous of these
is the so-called nonthermal continuum radiation that arises
from the conversion of wave energy in electrostatic waves
near the source plasma frequency to radio waves, usually
propagating in the ordinary mode. There are arguments
for both linear and nonlinear conversion mechanisms. The
term “continuum” was originally assigned to this class of