Analysis of Jovian kilometric radiation simultaneously observed by voyagers 1 and 2

Abstract

Broadband Jovian radio emission in the kilometric wavelength range (bKOM) observed by the Voyagers 1 and 2 (V1 and V2) planetary radioastronomy, PRA, experiments have been analysed for the period January to December 1979 which includes the Jupiter encounters. The pre-encounter observations were made on the nightside, the difference in local time between the two spacecraft being about one hour. It is therefore possible to study local time, and source - observer distance effects. bKOM beaming exhibits large temporal variations. The beaming is believed to arise from the manner in which the source electrostatic upper-hybrid waves are converted to electromagnetic radiation in the plasma density gradients at the Io torus. In this case the beaming depends on the characteristic frequencies of the source plasma, and temporal variations in the beaming are expected to reflect fluctuations in the Io torus. Remote-sensed density profiles have been computed from V1 and V2 simultaneous bKOM observations and compared with in situ measurements by V1 when it passed through the torus. Good agreement is found between the density gradients obtained, and the in situ measurements. The differences between V1 and V2 profiles may possibly be explained by the difference in local time of the two spacecraft. The computed profiles from dayside and nightside observations are nearly coincident. A large temporal variability of the Io torus density, or of the latitudinal position of the source, is apparent.