The Io decametric emission cone


  • James Lauer Green


The Planetary Radio Astronomy (PRA) Experiment on board Voyager 1 and 2 observed intense Jovian decametric (or DAM) radiation from Jupiter for several months before and after encounter with the giant planet. As measured by the PRA experiment the DAM emissions from 2 to 40 MHz have a characteristic “arclike” shape in frequency versus time spectrograms. Much work has already been done relating observations of DAM emission with the position of the satellite Io. The PRA data are reorganized in this study in order to account for the relative positions of Io and Jupiter and to aid in determining the causes of the DAM arclike structure. Nearly 2 months of the PRA data were transformed into a coordinate system for which Io and Jupiter remain fixed and the spacecraft data are sorted into bins of spacecraft system III (65°) longitude. A 1/RJ2 normalization factor is applied to all the sorted data to take into account the radial distance dependence of this emission. The results of this analysis technique indicate that the Io-dependent DAM emissions, as seen by both Voyager 1 and 2, are generated in hollow cones at all frequencies over the fixed Io longitudes studied (from 200° to 260°). Comparing the characteristics of the DAM emission at the edges of the emission cones (in two ranges of spacecraft longitude), distinct differences can be found. In addition, the edges of the emission cones are, in many cases, broad in longitude (30° or more). Three-dimensional raytracing calculations within a model Jovian magnetosphere are used in an effort to explain the shapes of the DAM emission cones. Using a general emission mechanism which ultimately generates radiation in the R-X mode above the R=0 cutoff, qualitative agreement with the observations can be made which indicates that the observed arc structure is largely a propagation effect. The greatest bending of the DAM model rays occurs on one edge of the emission cones and is almost entirely due, at frequencies below 25 MHz, to the large region of enhanced (above dipole) magnetic field believed to exist around 155°. Enhanced plasma densities found in the Jovian ionosphere may produce an additional propagation effect at frequencies greater than 25 MHz.