Research Article

The low-frequency source of Saturn’s kilometric radiation

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Science  05 Oct 2018:
Vol. 362, Issue 6410, eaat2027
DOI: 10.1126/science.aat2027

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Cassini's final phase of exploration

The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere.

Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382

Structured Abstract

INTRODUCTION

Planetary auroral radio emissions are powerful nonthermal cyclotron radiation produced by magnetized planets. Their remote observation provides information on planetary auroral processes and magnetospheric dynamics. Understanding how they are generated requires in situ measurements from within their source region. During its early 2008 high-inclination orbits, the Cassini spacecraft unexpectedly sampled two local sources of Saturn’s kilometric radiation (SKR) at 10-kHz frequencies, corresponding to the low-frequency (LF) portion of its 1- to 1000-kHz typical spectrum (hence only observable from space). These provided insights into the underlying physical excitation mechanism. The combined analysis of radio, plasma, and magnetic field in situ measurements demonstrated that the cyclotron maser instability (CMI), which generates auroral kilometric radiation (AKR) at Earth, is a universal generation mechanism able to operate in widely different planetary plasma environments. The CMI requires accelerated (out-of-equilibrium) electrons and low-density magnetized plasma, such as auroral regions where the ratio of the electron plasma frequency fpe to the electron cyclotron frequency fce is much lower than unity.

RATIONALE

Intensifications of the SKR spectrum in general, and of its LF part in particular, have been widely used as a diagnostic of Saturn’s large-scale magnetospheric dynamics, such as the SKR rotational modulation or major auroral storms driven by the solar wind. However, the limited set of events encountered in 2008 did not provide a comprehensive picture of the source of SKR LF emissions and of the conditions in which the CMI can trigger them.

During the 20 ring-grazing high-inclination orbits and the preceding 7 orbits, which together spanned late 2016 to early 2017, the Cassini spacecraft repeatedly sampled the top of the SKR emission region, at distances of a few planetary radii (RS), corresponding to the lowest-frequency part of the SKR spectrum. SKR emission frequency is close to fce, itself proportional to the magnetic field amplitude, and so decreases with increasing distance from the planet. We conducted a survey of these orbits to extract the average properties of the radio sources encountered and assess the ambient magnetospheric plasma parameters that control them.

RESULTS

Throughout this set of trajectories, we were able to identify only three SKR sources. They covered the 10- to 20-kHz range (3.5 to 4.5 RS distances from the planet’s center) and were solely found on the northern dawn-side sector. The source regions hosted narrow-banded emission, propagating in the extraordinary wave mode, and radiated quasi-perpendicularly to the magnetic field lines. Their emission frequency, measured in situ, was fully consistent with the CMI mechanism driven by 6- to 12-keV electron beams with shell-type velocity distribution functions, as for AKR at Earth.

The SKR sources were embedded within larger regions of upward currents, themselves coincident with the ultraviolet (UV) auroral oval, which was observed simultaneously with the Hubble Space Telescope. However, unlike the terrestrial case, the spacecraft exited the radio source region before exiting the main oval, itself strictly coincident with the upward current layer, when the ratio fpe/fce exceeded the typical CMI threshold of 0.1. This occurred at times of sudden local increases of the magnetospheric electron density.

CONCLUSION

The generation conditions of SKR LF emission appear to be strongly time-variable. The SKR spectrum additionally displays a significant local time dependence, with the lowest frequencies (the highest altitudes) reached for dawn-side radio sources. The characteristics of CMI-unstable electrons at 3.5 to 4.5 RS imply that downward electron acceleration took place at farther distances along the auroral magnetic field lines and brings new constraints to particle acceleration models. Finally, the LF SKR component is mainly controlled by local plasma conditions, namely time-variable magnetospheric electron densities, which can quench the CMI mechanism even if accelerated electrons are present. This explains why the magnetic field lines hosting SKR LF sources can map to a restricted portion of the UV auroral oval and associated upward current region.

Schematic view of Saturn’s auroral emissions.

Auroral radio emissions observed above the atmosphere were remotely mapped from Cassini (the most intense emissions in red) when the spacecraft encountered a low-frequency radio source. The magnetic field lines hosting it, sampled in situ by the probe, map to the (atmospheric) UV auroral oval, as observed simultaneously by the Hubble Space Telescope.

Abstract

Understanding how auroral radio emissions are produced by magnetized bodies requires in situ measurements within their source region. Saturn’s kilometric radiation (SKR) has been widely used as a remote proxy of Saturn’s magnetosphere. We present wave and plasma measurements from the Cassini spacecraft during its ring-grazing high-inclination orbits, which passed three times through the high-altitude SKR emission region. Northern dawn-side, narrow-banded radio sources were encountered at frequencies of 10 to 20 kilohertz, within regions of upward currents mapping to the ultraviolet auroral oval. The kilometric waves were produced on the extraordinary mode by the cyclotron maser instability from 6– to 12–kilo–electron volt electron beams and radiated quasi-perpendicularly to the auroral magnetic field lines. The SKR low-frequency sources appear to be strongly controlled by time-variable magnetospheric electron densities.

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