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 Uni fied Radio and Plasma Wave experiment on Ulysses during its passage th rough the Jovian inner magnetosphere from similar to 9 R(J) in the out skirts of the Io plasma torus to similar to 13 R(J) near the plasma sh eet. Below the plasma frequency f(p), these spectra are weakly banded between gyroharmonics. These observations were interpreted by Meyer-Ve rnet et al. [1993] as quasi-thermal fluctuations in Bernstein waves. W e show that above f(p) each observed gyroharmonic band falls off very abruptly on its high-frequency side. We interpret it as the ''forbidde n band'' predicted by the Bernstein wave dispersion equation between t he so-called f(Q) frequency and the consecutive gyroharmonic, that is, a region where no Bernstein wave can propagate. This allows a determi nation of the local cold plasma frequency and thus of the core electro n density with a similar to 16% uncertainty. As a consistency check, w e show that the f(Q) thus determined are very close to the frequencies of the resonances excited by the relaxation sounder on Ulysses.