Observation of long-duration gyroharmonic resonances: A refutation of the short-duration explanation for interpreting the anomalous URAP sounder spectra observed in the Io torus

The resonance spectra collected by the Ulysses Unified Radio and Plasma Wav e relaxation sounder in the Jovian magnetosphere at the outskirts of the Io torus exhibit significant differences from those gathered from earthbound spacecraft. The main deviation is the lack of resonances at the harmonics o f the electron gyrofrequency. Extrapolated from statistics of gyroharmonics observed by ionospheric topside sounders, recent works have suggested that the duration of the gyroharmonics can be too short to be detected by URAP, because of the large velocity of Ulysses in the torus. Actual magnetospher ic sounder observations performed by GEOS and ISEE 1 spacecraft are used he re to verify this explanation. The gyroharmonic resonances triggered by mag netospheric sounders appear quite different with respect to those observed by ionospheric topside sounder; particularly, they have a longer duration t han those derived by these recent studies. Their characteristics give furth er arguments in favor of their interpretation as oblique echoes. The gyroha rmonic resonances are also observed in the large-velocity plasma encountere d in the Earth's magnetosheath. The earthbound observations refute, then, q ualitatively and quantitatively the explanation of the absence of harmonics because of their too short duration and are a strong support in the interp retation of URAP observations as large frequency shifts induced on the gyro harmonics by the Ulysses velocity.