Beaming Geometry of the IO-Related Decametric Radiation

Abstract

The geometrical beam modeling of a component of Jovian Io-related decametric radiation (so-called Io-B) is made on the basis of the 22.2 MHz occurrence probability data from University of Texas Radio Astronomy Observatory. To see the emission directions of the 22.2 MHz radiation with respect to the orbital position of Io, the 22.2 MHz occurrence probability is plotted as a function of the sub-Io longitude and the difference between the sub-Io longitude and the sub-observer longitude (referred to as a “longitude difference”). In such a plot the longitude difference of the Io-B main region increases with increasing sub-Io longitude. A simulation of the drift of the longitude-difference is made on the basis of a hollow cone beaming model using the 04 magnetic field model. It is assumed that the disturbance caused by the Io’s motion propagates to the 22.2 MHz source location just above the cloud tops of the planet to excite the radiation. The effect of the propagation time of the disturbance to the 22.2 MHz source is taken into account by a lead angle of the Io-excited flux tube with respect to the orbital position of Io. It is also assumed that the radiation is emitted into a sector of a hollow cone, the axis of the cone being aligned with the magnetic field at the emission point. By fitting the simulated longitude-difference drift curve to the observation, the best fit lead angle and cone half-angle are estimated to be about 40º±10º and 65º∓5º, respectively. The result of the simulation is discussed in relation to the O4 field model.