The Solar System at Radio Wavelengths 707
FIGURE 12 Time evolution of the observed production rates of
comet C/Hale–Bopp as a function of heliocentric distance, with
superposed fitted power laws (dashed lines). (N. Biver et al.,
1999, Post-perihelion observations of the distant gaseous activity
of comet C/1995 O1 (Hale–Bopp) with the Swedish–ESO
Submillimeter Telescope (SEST),Asteroids, Comets and
Meteors.)
ARO 12 m telescope and the Berkeley–Illinois–Maryland
Association (BIMA) array. Transitions at several frequencies
are shown, as well as a contour map from BIMA at 72.8 GHz
(in bold) superposed on the ARO 225.7 GHz image. These
observations show that formaldehyde indeed originates
both from the nucleus and in the coma, where the coma-
source appears dominated by a single fragment in this case.
E S A’sRosettaspacecraft, currently on its way to comet
67P/Churyumov–Gerasimenko, carries a microwave instru-
ment, MIRO, with receivers centered at 190 and 562 GHz.
Upon rendezvous at a heliocentric distance of 3.5 AU,
Rosetta will move with the comet down to perihelion near
1.3 AU. MIRO is one of the instruments that will observe
the comet during this time. It has broadband channels on
both receivers to measure near-surface temperatures and
temperature gradients in the comet’s nucleus. Particularly
exciting is the high spectral resolution spectrometer con-
nected to the 562 GHz receiver, which will measure several
major volatile species (H 2 O, CO, CH 3 OH, and NH 3 )at
extreme high spatial (down to5matthecomet’s surface)
and spectral resolution. These measurements will provide
unprecedented information on the outgassing of the comet
as a function of heliocentric distance.3. Nonthermal Radiation
Nonthermal planetary radio emissions are usually produced
by electrons spiraling around magnetic field lines. Until the
era of spacecraft missions, we had only receivednonther-
mal radio emissionsfrom the planet Jupiter, and these
were usually limited to frequencies
̃>10 MHz, since radia-
tion at lower frequencies is blocked by Earth’s ionosphere.
Strong radio bursts at frequencies below 40 MHz were at-
tributed to emission via the cyclotron maser instability in
which auroral electrons with energies of a few to several keV
power the emission, while radiation at frequencies
̃> 100
MHz was interpreted as synchrotron radiation, emitted by
high energy (MeV range) electrons trapped in Jupiter’s radi-
ation belts, a region in Jupiter’s magnetic field analogous to
the Earth’sVan Allen belts. Like Earth, the magnetic fields
of the four giant planets resemble to first approximation that
of a dipole magnetic field. Despite several searches, no pos-
itive detections of nonthermal radio emissions from any of
the other three giant planets were made until theVoyager
spacecraft approached these objects. Now we know that all
four giant planets as well as Earth are strong radio sources
at low frequencies (kilometric wavelengths). Jupiter’s moon
Ganymede is also a source of nonthermal radio emissions.
The strongest planetary radio emissions usually originate
near the auroral regions and are intimately related to auro-
ral processes.
A graph of the average normalized spectra of the auro-
ral radio emissions from the four giant planets and Earth is
displayed in Fig. 14. All data are adjusted to a distance of 1
AU. Jupiter is the strongest low-frequency radio source, fol-
lowed by Saturn, Earth, Uranus, and Neptune. In Sections
3.3–3.7, we discuss the emissions from each planet.3.1 Low-Frequency Emissions
3.1.1 CYCLOTRON MASER EMISSIONS
Radio emission at frequencies of a few kHz to 40 MHz (for
Jupiter) is usually attributed to electron cyclotron maser
radiation, emitted by keV (nonrelativistic) electrons in the
auroral regions of a planet’s magnetic field. The radiation
is emitted at the frequency that electrons spiral around
the local magnetic field lines (the cyclotron or Larmor
frequency):νL=qB
2 πmec(6)