First direct measurement of the beaming of Jupiter's decametric radiation

Abstract

ONE of the most important features of the jovian decametric radioemission comes from the geometry of the observed radiation. The emission occurrence probability appears as a well defined function of the respective positions of Jupiter, the observer and the innermost galilean satellite Io, exhibiting several major sources which correspond to narrow ranges in zenocentric observer's longitude L (the so-called central meridian longitude) and in the synodic orbital phase of Io, Φ. These statistically established directivity properties were defined by Warwick¹ and Alexander², who showed that the repetition of the distinguishable dynamic spectra associated with each source occurring in a longitude range of 20°, is attributable to a ±10° beaming cone into which the radiation escapes in the ecliptic plane. Dulk³ stated more precisely that the emission is received on the Earth when Io makes certain angles with the line of sight, and that the dynamic spectrum varies in shape and frequency range with the magnetic longitude of Io. The beaming in latitude is also important⁴: the ±3.3° variation of the zenocentric declination of the Earth is sufficient to lead to a 15° variation of the statistical apparent sub-Io longitude of the emission. But these conclusions, based on statistics and on the repeatability of similar dynamic spectra in equivalent geometrical configurations of Jupiter, Io and the observer, are not decisive, as they were obtained from a single direction at a given time. We present here the first direct experimental proof of a narrow beaming of Jupiter radiation, using simultaneous observations in two different directions, during the French–Soviet space radio astronomy experiment Stereo-5.