The magnetic moment of Jupiter, as befitting the largest planet in the solar system, is also the largest of the planetary system over 10,000 times that of the earth. Its equatorial surface field is over 10 times that of the Earth. The strength of its magnetic field combined with the weakness of the solar wind at Jupiter produces a magnetosphere that is enormous. The sun could easily fit inside the magnetosphere. Its tail is thought to extend past Saturn, over 5 AU away. If Jupiter's magnetosphere could be seen from Earth it would appear to be larger than the Earth's moon. (Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometers in the direction toward the Sun.)

Deep inside the jovian magnetosphere orbit the Galilean satellites. One of these, Io, has a volcanic- ally produced atmos- phere that is constantly being bombarded by the intense radiation belts of jupiter. This bombard- ment knocks atoms out of the atmosphere of Io into the magnetosphere of Jupiter where they become ionized. This process produces a torus, or doughnut, of hot ions circling Jupiter near Io's orbit. This torus together with the enormous electrical and magnetic forces in the Jovian magnetosphere leads to intense radiation belts and radio emissions. These emissions can be detected from Earth and were the first indication of Jupiter's enormous magnetic field well before the first interplanetary spacecraft were launched.

The Jovian Decametric Emission

The Jovian decametric emission was discovered in 1955 by B.F. Burke and K.L. Franklin at the frequency of 22.2 MHz. The emission has an upper cutoff frequency of 39.5 MHz. It can be detected from ground based stations from the upper cutoff frequency of the emission down to the cutoff frequency of the terrestrial ionosphere which is usually around 5 to 10 MHz. The peak of the intensity of the emission occurs at around 8 MHz.

The emission occur in episodes called "storms". A storm can last from a few minutes to several hours. 2 types of bursts can be received during a storm.

The L bursts (L for Long) are bursts that vary slowly in intensity with time. They last from a few seconds to several tens of seconds and have instantaneous bandwidth of a few MHz.

The S bursts (S for Short) are very short in duration, have instantaneous bandwidth of a few kHz to a few tens of kHz, and drift downward in frequency at a rate of typically -20 MHz/sec. They arrive at a rate of a few to several hundred bursts per second. In a 5 kHz bandwidth receiver they last only a few milliseconds.

Sometimes both types of bursts can be heard simultaneously. The emission is believed to be beamed into a thin hollow cone with axis parallel to the direction of the magnetic field lines in the region where the emission originates (near the magnetic poles).

The probabilities of detecting the emission depend strongly on the values of the Jovian central meridian longitude (CML), the Io Phase, and the Jovicentric declination of the Earth (DE).

Lambda III is the System III longitude (CML = Central Meridian Longitude) of Jupiter facing the Earth (0 degrees mean CML is facing Earth).
The Io Phase is the angle of Io, with respect to the line of Jupiter and Earth (0 degree means Jupiter, Io and Earth are in one line).
The regions in the CML-Io phase plane that have increased probabilities of emission are called sources. The sources are named Io-A, Io-B, and Io-C for the Io- controlled emission and A, B, and C for the Non-Io controlled emission.

The Jovian decametric emission is detectable with a simple half wavelength dipole, a long wire or a loop antenna. Such low gain antennas may allow the detection of only very strong bursts.
Antennas with gains of 6-10 dB with respect to a half wavelength dipole are more suitable for detecting the emission. Yagi (5-elements) and log periodic antennas usually have gains in this range.
These higher gain antennas connected to HF amateur radio receivers can easily detect most of the strong part of the Jovian decametric radio emission. It will be necessary to disable the AGC of the receiver otherwise the signal will be badly compressed and enable a narrowband filter to ensure that no station is disturbing.

Emissions are best detectable between 18 - 22 MHz. At frequencies below 18 MHz strong interference from stations is expected and frequencies higher than 22 MHz implicates a sharp drop in the intensity of the emission.