On the arc structure of the DAM Jupiter emission

Abstract

An analysis of the dynamic spectra of the Jovian DAM emission (1.3–40 MHz) has been made from Voyager data; it appears that the different Jovian ‘sources’ can be defined by spectral characteristics, rather than by occurrence probability. The non‐Io emission consists of two families: vertex early arcs (VEA) and vertex late arcs (VLA). These two families are superimposed at all longitudes, but one is always more intense than the other. The characteristics of the two families are specified; in particular, it is shown that the VEA family is more stable in time than the VLA family. The Io‐controlled emission consists of the four sources already known from the ground‐based observations in addition to a new source, (Io‐A')sp, identified by its dynamic spectrum alone. All of the sources are partially superimposed on non‐Io emission. The (Io‐B)sp and (Io‐A')sp sources are made up of low‐curvature arcs having low‐frequency limits above 5 MHz. The high‐frequency limit of the (Io‐B)sp source is strongly modulated by Io‐phase. The (Io‐A)sp source has a spectrum similar to the non‐Io VLA emission. The other two sources, (Io‐C)sp and (Io‐D)sp, are not structured into well‐defined arcs. A comparison is made between the occurrence of these sources in the Io‐CML plane with the sources defined from ground observations by probability of occurrence. Local time effects are observed only in the non‐Io emission when compared before and after encounter. Before encounter, the VEA family is very weak and the VLA family very intense. After encounter, the opposite effect is observed. The Io‐controlled sources are not affected by these local time effects.