Detection of Bernstein wave forbidden bands in the Jovian magnetosphere: A new way to measure the electron density

We analyze the power spectra measured by the radio receiver of the Unified Radio and Plasma Wave experiment on Ulysses during its passage through the Jovian inner magnetosphere from ∼ 9 R_J in the outskirts of the Io plasma torus to ∼ 13 R_J near the plasma sheet. Below the plasma frequency f_p, these spectra are weakly banded between gyroharmonics. These observations were interpreted by Meyer-Vernet et al. [1993] as quasi-thermal fluctuations in Bernstein waves. We show that above f_p each observed gyroharmonic band falls off very abruptly on its high-frequency side. We interpret it as the "forbidden band" predicted by the Bernstein wave dispersion equation between the so-called f_Q frequency and the consecutive gyroharmonic, that is, a region where no Bernstein wave can propagate. This allows a determination of the local cold plasma frequency and thus of the core electron density with a ∼ 16% uncertainty. As a consistency check, we show that the f_Q thus determined are very close to the frequencies of the resonances excited by the relaxation sounder on Ulysses.