Whistler-mode waves are generated within and can propagate upstream of
collisionless shocks. They are known to play a role in electron
thermodynamics/acceleration and, under certain conditions, are markedly
observed as wave trains preceding the shock ramp. In this paper, we take
advantage of Cassini's presence at ~10 AU to explore the importance of
whistler-mode waves in a parameter regime typically characterized by higher
Mach number (median of ~14) shocks, as well as a significantly different IMF
structure, compared to near Earth. We identify electromagnetic precursors
preceding a small subset of bow shock 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. This is due to the lower ion and electron
cyclotron frequencies near Saturn, between which whistler waves can develop.
The waves are also observed as predominantly right-handed in the spacecraft
frame, the opposite sense to what is typically observed near Earth. This is
attributed to the weaker Doppler shift, owing to the large angle between the
solar wind velocity and magnetic field vectors at 10 AU. Our results on the low
occurrence of whistler waves upstream of Saturn also underpins the
predominantly supercritical bow shock of Saturn.Comment: Published in Journal of Geophysical Research: Space Physics (January
2017) 21 pages, 4 figure
