Low-frequency limit of Jovian radio emissions and implications on source locations and Io plasma wake

Abstract

After deriving from Ulysses-URAP measurements the low-frequency limit of the Jovian hectometer emission spectrum ( 250±50 kHz at the −20 dB level below the emission peak), and confirming the absence of Io's control on Jovian radio emission below ∼1 MHz , we propose a single common explanation for these low-frequency limits: both are well explained by the quenching of the generation mechanism (the cyclotron-maser instability) where the ratio f pe/ f ce reaches a critical value (about 0.14) along the source field lines. This occurs in the external part of Io's plasma torus for the hectometer component, and in Io's dense plasma wake discovered by Galileo for the Io-dependent (decameter) component. As a consequence, we infer new constraints on Jovian radio source locations, which are found to extend along the L≈6 field lines intersecting Io's wake (where the electron density is up to 10–20 times higher than in the average torus) for the Io-dependent decameter emission, and along L≈7–9 field lines (with apex at ≈7– 11 R J ) for the hectometer emission. We also infer that the broadband kilometer (auroral) emission originates from L>10 field lines, with apex well beyond 12 R J , closing in the distant Jovian magnetosphere or opened to the solar wind. These results confirm and precise the source locations obtained from direction-finding studies with Ulysses. Finally, we discuss implications in terms of source extent and of the origin of the accelerated electrons responsible for the emissions, and we derive lower limits on the proton concentration in Io's plasma wake.