Whistler-mode Waves Upstream of Saturn

Whistler-mode waves are generated within and can propagate upstream of collisionless shocks. They play a role in the dissipation process and, under certain conditions, are markedly observed as wave trains preceding the shock ramp. In this letter, we take advantage of Cassini's presence at 10 AU to explore the importance of whistler-mode waves in a new parameter regime typically characterized by higher Mach number shocks compared to near Earth. We identify electromagnetic precursors preceding a small subset of crossings with properties which are consistent with whistler-mode waves. We find these monochromatic, low-frequency, circularly-polarized waves to have a typical frequency range of 0.2 - 0.4 Hz in the spacecraft frame. The waves are observed as predominantly right-handed in the spacecraft frame, the opposite sense to what is typically observed near Earth. Our results suggest that whistlers in this parameter regime are more likely to be associated with electron acceleration than their thermal heating.