Low‐frequency waves in the foreshock of Saturn: First results from Cassini

Abstract

We present the first characterization of low‐frequency upstream waves associated with Saturn's foreshock from observations by the Cassini spacecraft. A classification based on their frequency in the spacecraft frame (s/c) yields two groups: (1) a large majority of waves with frequencies below the local proton cyclotron frequency ΩH+ and (2) waves with frequencies above ΩH+. The waves within the first group are usually phase steepened and have a left‐hand polarization in the spacecraft frame. In addition, they present left‐hand‐polarized (s/c) dispersive wave packets attached to the steepening front. An analysis of these waves suggests that these are sunward propagating ion/ion resonant right‐hand mode waves that steepen and emit a whistler precursor to stop the steepening. These waves seem to populate the deep ion foreshock. Within the second group we find quasi‐monochromatic and steepened waves with a right‐hand polarization (s/c). Among the first we find noncompressive and slightly compressive waves, whereas the steepened ones are very compressive, show oblique propagation, and also display dispersive wave packets. Assuming that these packets are also whistlers, we suggest that these waves could be generated by the ion/ion resonant left‐hand mode. Then, during the nonlinear regime they would become compressive and eventually steepen, emitting a whistler precursor. However, we are unable to provide evidence for hot ion beams that could be related to this instability. This is the first time that such waves are observed at Saturn, and observations confirm that these waves contribute to Saturn's quasi‐parallel shock reformation.