710 Encyclopedia of the Solar System
emissions because they can be generated at very low fre-
quencies and can be trapped in low-density cavities in the
outer portions of the magnetosphere when the surround-
ing solar wind density is higher. The mixture of multiple
sources at different frequencies and multiple reflections
off the moving walls of the magnetosphere tend to homog-
enize the spectrum. However, at higher frequencies, these
emissions are often created as narrowband emissions from
narrowband electrostatic bands at the upper hybrid reso-
nance frequency on density gradients in the inner magne-
tosphere and can propagate directly away from the source,
yielding a complex narrowband spectrum. These emissions
were first discovered at Earth and have been found at all
of the magnetized planets. Furthermore, emissions of this
nature are also produced by Ganymede’s magnetosphere.
Another type of planetary radio emission is closely re-
lated to a common solar emission mechanism, the conver-
sion of Langmuir waves to radio emissions at either the
plasma frequency or its harmonic. The Langmuir waves are
common features of the solar wind upstream of a planetary
bow shock, which arises from the interaction of the super-
sonic flow of solar wind plasma past the planets. This mech-
anism is a nonlinear mechanism involving three waves: the
Langmuir wave, the radio wave, and either a low-frequency
wave in the case of emission near the plasma frequency or
a second Langmuir wave in the case of harmonic emission.
The resulting emissions are weak, narrowband emissions.
3.1.3 ATMOSPHERIC LIGHTNING
Radio emissions from planets are sometimes associated with
atmospheric lightning. The lightning discharge, in addition
to producing the visible flash, also produces broad, impul-
sive radio emissions. If the spectrum of this impulse extends
above the ionospheric plasma frequency and if absorption
in the atmosphere is not too great, a remote observer can
detect the high-frequency end of the spectrum. The “in-
terference” detected with an AM radio on Earth during a
thunderstorm is the same phenomenon.
3.2 Synchrotron Radiation
Synchrotron radiation is emitted by relativistic electrons gy-
rating around magnetic field lines. In essence, this emission
consists of photons emitted by the acceleration of electrons
as they execute their helical trajectories about magnetic
field lines. The emission is strongly beamed in the forward
direction (see Fig. 15) within a cone 1/γ:
1
γ=√
1 −v^2
c^2(8)withvthe particle’s velocity andcthe speed of light. The
relativistic beaming factorγ= 2 E, withEthe energy in
MeV. The radiation is emitted over a wide range of fre-
quencies, but shows a maximum at 0.29νc, with the critical
frequency,νc, in MHz:νc= 16. 08 E^2 B (9)where the energyEis in MeV and the field strengthBis
in Gauss. The emission is polarized, where the direction
of the electric vector depends on the direction of the lo-
cal magnetic field. Jupiter is the only planet for which this
type of emission has been observed. It has been mapped by
ground-based radio telescopes and byCassinito provide
some of the most comprehensive, though indirect, infor-
mation about Jupiter’s intense radiation belts.3.3 Earth
The terrestrial version of the cyclotron maser emission,
commonly referred to as auroral kilometric radiation
(AKR), has been studied both at close range and larger
distances by many Earth-orbiting satellites. The radiation
is very intense; the total power is 10^7 W, sometimes up to
109 W. The intensity is highly correlated with geomagnetic
substorms, thus it is indirectly modulated by the solar wind.
It originates in the night side auroral regions and in the day
side polar cusps at low altitudes and high frequencies and
spreads to higher altitudes and lower frequencies. Typical
frequencies are between 100 and 600 kHz. Since AKR is
generated by auroral electrons, it can be used as a proxy
for auroral activity. And, since numerous in situ studies of
the terrestrial auroral electron populations and the result-
ing radio emissions have been carried out, we can apply our
understanding of this emission process to similar emissions
at other planets where in situ studies have not yet been
carried out.
Earth is also the source of nonthermal continuum radia-
tion. Below the solar wind plasma frequency this radiation
is trapped within the magnetosphere. The spectrum is rela-
tively smooth down to the local plasma frequency, typically
in the range of a few kHz, where the emission cuts off at the
ordinary mode cutoff. A few observations of this emission
also show an extraordinary mode cutoff. Hence, the emis-
sion is either generated in both polarizations, or some of the
initially dominant ordinary mode is converted into the ex-
traordinary mode via reflections or other interactions with
the magnetospheric medium. Above the solar wind plasma
frequency, typically at a few tens of kHz, the “continuum”
radiation spectrum exhibits a plethora of narrowband emis-
sions; some of these extend well into the range of a few
hundred kHz.
While not as important as the auroral radio emissions
from an energetics point of view, the low-frequency limit of
the continuum radiation at the plasma frequency provides
an accurate measure of the plasma density, an often difficult