Source location of the Jovian hectometric radiation via ray‐tracing technique

Abstract

Ray tracing using realistic magnetic field and plasma models has been performed to locate the Jovian hectometric radiation (HOM) sources. To consider only a few possible source locations, we have assumed that the radiation mechanism is the cyclotron maser instability (CMI), and taken into account the characteristics of the HOM including the recent results by Ladreiter and Leblanc (1989) (hereafter paper 1). The emission is assumed to escape from the planet in the extraordinary wave mode within a thin beam of hollow cones whose apexes are distributed uniformly in longitude at the northern and southern hemispheres at altitudes where the gyrofrequency fc is less but nearly equal to the observed wave frequency. The rays are traced from possible sources located at f/fc = 1.05 and labeled by the dipole shell parameter L. For each source, we have determined the emergence wave normal angle θcrit above which the rays no longer travel within the HOM emission beam nearby the magnetic equator (paper 1). We show that the HOM sources cannot be located on field lines within the Io flux tube. However, wave refraction due to the Io torus plays a crucial role for the propagation and beaming of the waves toward the magnetic equator. It is found that HOM sources located at field lines 15 ≤ L ≤ 30 agree best with the observations when assuming that the cone half angle of emission is about 70°–90° with respect to the local magnetic field. Thus, the source is associated with the tail field aurora where the magnetic field lines connect the polar region to the Jovian magnetic tail and is extended in altitude from 2 to 7 Jovian radii (RJ). Our results explain in detail the radio phenomenology of the hectometric radiation, in particular the strong latitudinal beaming, the lowest observed frequencies (≈ 40 kHz), the solar wind control, the absence of HOM emission when Voyager 1 was within the Io torus, and the absence of Io control on the HOM. The results are discussed in the context of the other Jovian radio sources, namely the sources of the non‐Io decametric and kilometric emissions.