Properties of Saturn Kilometric Radiation measured within its source region

TL;DR

This study uses in situ measurements from Cassini to characterize the source region of Saturn kilometric radiation, revealing hot plasma, upward currents, and the role of cyclotron maser instability in radio wave amplification.

Contribution

First in situ characterization of Saturn's SKR source region, linking plasma conditions, magnetic field topology, and radio emission mechanisms.

Findings

SKR sources are surrounded by hot, tenuous plasma and upward field-aligned currents.

Magnetic field lines form a high-latitude, spiral-shaped auroral oval.

Observed cutoff frequencies support cyclotron maser instability driven by resonant electrons.

Abstract

On 17 October 2008, the Cassini spacecraft crossed the southern sources of Saturn kilometric radiation (SKR), while flying along high-latitude nightside magnetic field lines. In situ measurements allowed us to characterize for the first time the source region of an extra-terrestrial auroral radio emission. Using radio, magnetic field and particle observations, we show that SKR sources are surrounded by a hot tenuous plasma, in a region of upward field-aligned currents. Magnetic field lines supporting radio sources map a continuous, high-latitude and spiral-shaped auroral oval observed on the dawnside, consistent with enhanced auroral activity. Investigating the Cyclotron Maser Instability (CMI) as a mechanism responsible for SKR generation, we find that observed cutoff frequencies are consistent with radio waves amplified perpendicular to the magnetic field by hot (6 to 9 keV) resonant electrons, measured locally.