Properties of Saturn Kilometric Radiation measured within its source region

摘要

On 17 October 2008, the Cassini spacecraft crossed the southern sources ofSaturn kilometric radiation (SKR), while flying along high-latitude nightsidemagnetic field lines. In situ measurements allowed us to characterize for thefirst time the source region of an extra-terrestrial auroral radio emission.Using radio, magnetic field and particle observations, we show that SKR sourcesare surrounded by a hot tenuous plasma, in a region of upward field-alignedcurrents. 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 MaserInstability (CMI) as a mechanism responsible for SKR generation, we find thatobserved cutoff frequencies are consistent with radio waves amplifiedperpendicular to the magnetic field by hot (6 to 9 keV) resonant electrons,measured locally.