Encyclopedia of the Solar System 2nd ed

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CHAPTER 38

The Solar System at

Radio Wavelengths

Imke de Pater

University of California, Berkeley

Berkeley, California

William S. Kurth

University of Iowa

Iowa City, Iowa

1. Introduction 4. Future of Radio Astronomy


2. Thermal Emission from Planetary Bodies for Solar System Research


3. Nonthermal Radiation Bibliography

1. Introduction

Ground-based radio astronomical observations of planetary
objects provide information that is complementary to that
obtained at other (visual, infrared, ultraviolet) wavelengths.
We distinguish between thermal and nonthermal emissions.
Thermal radio emission originates from a body’s surface
(or more appropriately subsurface) and/or atmosphere, and
nonthermal radio emissionsare produced by charged
particles in a planet’s magnetosphere. The thermal emis-
sion can be used to deduce the structure and composition
of a planet’s atmosphere and surface layers; the nonthermal
radiation provides information about its magnetic field and
charged particle distributions therein. Ground-based radio
astronomy is essentially limited to frequencies above about
10 MHz because of the shielding effects of Earth’s iono-
sphere at lower frequencies. Space-based measurements
extend the frequency range of solar system radio astron-
omy as low as a few kHz. In this chapter, we discuss radio
emissions from a few kHz up to
̃

500 GHz. Since we cover
over 9 orders of magnitude in frequency, we can include
only brief summaries of a select number of topics.

Instrumentation

A radio telescope consists of an antenna and a receiver. The
antenna can be a simple monopole, dipole, or parabolic
dish (Fig. 1). The sensitivity of the antenna depends upon

many factors, but the most important are the effective

aperture and system temperature. The effective aperture

depends upon the size of the dish and the aperture effi-

ciency. The sensitivity of the telescope increases when the

effective aperture increases and/or the system temperature

decreases.

The response of an antenna as a function of direction is

given by its antenna pattern, which consists of a “main” lobe

and a number of smaller “side” lobes, as depicted in Fig. 2a.

The resolution of the telescope depends upon the angular

size of the main lobe. It is common to express the main

lobe width as the angle between the directions for which

the power is half that at lobe maximum; this is referred to

as the half power beam width. This angle depends upon the

size of the dish and the observing wavelength: For a uniform

illumination, the beam width is approximatelyλ/Dradians,

withDthe dish diameter in the same units as the wavelength

λ. Space-borne radio telescopes at low frequencies usually

are composed of one or more long cylindrical elements since

dish antennas are prohibitive in terms of mass.

The resolution of a radio telescope can be improved by

connecting the outputs of two antennas which are sepa-

rated by a distanceS, at the input of a radio receiver. The

VLA (Very Large Array) in Socorro, New Mexico, consists

of a Y-shaped track, with 9 antennas along each of the arms

(Fig. 1b). This telescope thus provides 351 individual inter-

ferometer pairs, each of which has its own instantaneous

resolution along its projected (on the sky) baselineS′. Such

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