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 Wave 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 of 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 magnetospheric sounder observations performed by GEOS and ISEE 1 spacecraft are used here to verify this explanation. The gyroharmonic resonances triggered by magnetospheric sounders appear quite different with respect to those observed by ionospheric topside sounder; particularly, they have a longer duration than those derived by these recent studies. Their characteristics give further arguments in favor of their interpretation as oblique echoes. The gyroharmonic resonances are also observed in the large-velocity plasma encountered in the Earth's magnetosheath. The earthbound observations refute, then, qualitatively and quantitatively the explanation of the absence of harmonics because of their too short duration and are a strong support in the interpretation of URAP observations as large frequency shifts induced on the gyroharmonics by the Ulysses velocity.