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 frequenc
y bands. The results of wave mode identification based on the Hall-MHD
model of plasma and observed wave polarization suggested that these w
aves propagate in the high beta intermediate mode. However, the underl
ying instability was not unambiguously determined. In the present pape
r we use the full electromagnetic dispersion relation derived from lin
ear 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 ''Alfven
ic'' beam mode, as well as resonant and non-resonant fast magnetosonic
modes have properties consistent with the data. Moreover, we find tha
t in contrast to the Earth's upstream waves, at Saturn no ''kinetic''
normal mode can account for the observed magnetic polarization.