Growing “Alfvenic” modes in the upstream region of Saturn

Abstract

Recent studies of low‐frequency electromagnetic waves upstream of the Saturn bow shock have shown that these waves, in contrast to those at Earth, are observed not in one, but in at least two, distinct frequency bands. The results of wave mode identification based on the Hall‐MHD model of plasma and observed wave polarization suggested that these waves propagate in the high beta intermediate mode. However, the underlying instability was not unambiguously determined. In the present paper we use the full electromagnetic dispersion relation derived from linear Vlasov theory in order to examine which of the plasma modes, with observed properties, are unstable in an isotropic Maxwellian plasma in the presence of backstreaming proton beams consistent with Voyager 2 observations at Saturn. As a result we find that the unstable “Alfvenic” beam mode, as well as resonant and non‐resonant fast magnetosonic modes have properties consistent with the data. Moreover, we find that in contrast to the Earth' upstream waves, at Saturn no “kinetic” normal mode can account for the observed magnetic polarization.