Jupiter's Decametric Radio Emission Modes

There three major factors not related to observing conditions on Earth which have been identified to affect the probability of hearing Jupiter's decametric emissions at any given time.

Three major regions of longitude called simply, A, B, and C regions seem to have the most effect. A fourth region, D, is sometimes recognized, though not universally.  Jupiter's magnetic field is tilted about 10 degrees in relation to the planet's rotational axis and thus appears to wobble from our perspective.  The active regions are tied to this magnetic field. Jupiter's gaseous makeup provides us with no solid surface features to monitor it's rotation. The clouds at different latitudes rotate at different rates. The rotation of the magnetic field has proven to be the best indicator and so a System III system of longitude was established for Jupiter. Systems I and II were based on cloud rotation at certain latitudes.  The A, B, and C emission mode regions are based on System III central meridian longitude (CML III).

In the 1960's it was recognized that the position of Io also strongly affects the probability that a given region will produce emissions. This is probably related to Io's volcanic nature and it's constant spewing of ionized particles along its orbit. The trail of ionized particles that lie along it's orbital plane is referred to as the Io torus. A longitudinal band of electromagnetic activity extends from the magnetic poles of Jupiter to Io. This band, known as a flux tube, plays a role in the radio noise storms as electrons spiral back and forth along it's path.  

We measure Io's position in its orbit around Jupiter in relation to the way we see it from Earth. When Io is on the back side of Jupiter, 180 degrees from its closest Earthward position we say it is in Superior Geocentric Conjunction or SGC.  The angular distance which Io has from SGC is called the Io Phase. Taken together Io Phase and CML III account for the major emission modes. The last major factor identified, the Jovicentric declination of the Earth is a long term variance in probability related to the the orbits of Jupiter and the Earth and is treated elsewhere in this document.

The following chart is adapted from "Physics of the Jovian Magnetosphere" published by Cambridge University Press. Other ranges have been published as one would expect with the statistical nature of these parameters. The default ranges for Radio-Jupiter Pro software are based on published ranges of the radio observatory at the University of Florida and do not not include the D mode. The University of Florida parameters are based on 40 years of ground-based observations and seem to be most applicable to our needs. As a practical matter, however, there is a general agreement as to within a few degrees to the parameters shown below which also take into account Voyager space probe data. Amateur observations should begin sometime before and end sometime after the predicted occurrences of these positions (See * note below.) 

Table Headings:

MODE - Also referred to as source designation. Emissions identifiable by a unique set of parameters.

 CML RANGE *- The System III Longitude of Jupiter associated with the emission. (degrees)

 Io RANGE * - The position of Io relative to Superior Geocentric Conjunction, that is, the point at which Io is directly opposite from Earth with respect to Jupiter, associated with the emission. (degrees)

 MAX F - Maximum observed frequency in megahertz.

 POLAR - Dominant polarization of the radio wave. RH = right hand. LH = Left Hand.

 ARC - Spectral arc curvature. "Late" and "Early" refer to arrival time of the vertex of the spectral feature of the emission.

* When ranges are in parentheses they are based on widths of the half maximums for the major sources as viewed from Earth at 20 MHz. Other ranges will be primarily based on Voyager observations.

Io-D 0-200 95-130 18 LH Early Also called "fourth source"
Io-B (105 - 185) (80-110) 39.5 RH Early Also called "early source"
non Io-B 80-200 0-360 38 RH Early Voyager info
Io-A (200-270) (205-260) 38 RH Late Also called "main source"
non-Io-A (230-280) 0-360 38 RH Late
Io-C (300-20) (225-260) 36 RH&LH Late Also called "third source"
non-Io-C 300-360 0-360 32 RH&LH Late Voyager info