Whistler mode waves upstream of Saturn

TL;DR

This study investigates whistler-mode waves upstream of Saturn's bow shock using Cassini data, revealing their properties, occurrence rate, and differences from Earth's shock environment, highlighting the influence of Saturn's unique plasma conditions.

Contribution

It provides the first detailed analysis of whistler-mode waves at Saturn's bow shock, emphasizing their distinct characteristics and occurrence in a high Mach number, different IMF environment.

Findings

Whistler waves observed are monochromatic and circularly polarized.

Frequency range of waves is 0.2 - 0.4 Hz in spacecraft frame.

Low occurrence rate of whistler waves supports Saturn's supercritical bow shock.

Abstract

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.