Saturn Kilometric Radiation Research Articles

The Source of Saturn's Kilometric Radiation and the Magnetic Field Line's Spirality

AbstractWe put forward a model and deduce the distribution of Saturn's Kilometric Radiation (SKR) Source. The model is as follows, the Kelvin‐Helmholtz Instability (KHI) will generate kinds of waves, in which there is the Alfven wave. It will propagate to the Saturn's polar area by following the bending magnetic field line. It will also cause the Cyclotron Maser Instability (CMI) leading to the generation of extraordinary wave (R‐X mode) in the process. In our views, the region of the CMI, near the ionosphere of Saturn (1Rs above the polar area, Rs is Saturn radius), is regarded as the same area as the Saturn's Kilometric Radiation Source.In this paper, we deduce that the KHI will appear before 11 LT (Saturn's Local Time), the Alfven wave will take 38.3 min to reach Saturn, the sunward magnetic field line will bend almost 80°. Finally, we get the distribution of SKR source. It locates from 7~9 LT at a relatively lower latitude to 18 LT at the eveningside.

A Saturnian longitude system based on a variable kilometric radiation period

This paper describes a longitude system for Saturn which is locked to the period of Saturn kilometric radiation. Because the apparent radio emission period varies with time, the period used in the system is allowed to vary. The resulting system results in the ‘diurnal’ peak of the radio emission occurring when the subsolar longitude is 100°, as was the case during the Voyager epoch. The variable period used in this system is shown to be statistically the same as periodicities recently reported for residuals in Saturn's magnetic field. It is expected that this longitude system will be more useful for organizing magnetospheric phenomena and even spoke creation in the rings than the existing longitude system based on the fixed period determined from Voyager observations which is fully 1% shorter than the currently‐measured period.

Linear prediction studies for the solar wind and Saturn kilometric radiation

Abstract. The external control of Saturn kilometric radiation (SKR) by the solar wind has been investigated in the frame of the Linear Prediction Theory (LPT). The LPT establishes a linear filter function on the basis of correlations between input signals, i.e. time profiles for solar wind parameters, and output signals, i.e. time profiles for SKR intensity. Three different experiments onboard the Cassini spacecraft (RPWS, MAG and CAPS) yield appropriate data sets for compiling the various input and output signals. The time period investigated ranges from DOY 202 to 326, 2004 and is only limited due to limited availability of CAPS plasma data for the solar wind. During this time Cassini was positioned mainly on the morning side on its orbit around Saturn at low southern latitudes. Four basic solar wind quantities have been found to exert a clear influence on the SKR intensity profile. These quantities are: the solar wind bulk velocity, the solar wind ram pressure, the magnetic field strength of the interplanetary magnetic field (IMF) and the y-component of the IMF. All four inputs exhibit nearly the same level of efficiency for the linear prediction indicating that all four inputs are possible drivers for triggering SKR. Furthermore, they act at completely different lag times ranging from ~13 h for the ram pressure to ~52 h for the bulk velocity. The lag time for the magnetic field strength is usually beyond ~40 h and the lag time for the y-component of the magnetic field is located around 30 h. Considering that all four solar wind quantities are interrelated in a corotating interaction region, only the influence of the ram pressure seems to be of reasonable relevance. An increase in ram pressure causes a substantial compression of Saturn's magnetosphere leading to tail collapse, injection of hot plasma from the tail into the outer magnetosphere and finally to an intensification of auroral dynamics and SKR emission. So, after the onset of magnetospheric compression at least ~1.2 rotations of the planet elapse until intensified SKR emission is visible in a Cassini-RPWS dynamic spectrum.

Proposed model for Saturn's auroral response to the solar wind: Centrifugal instability model

We present a model of Saturn's global auroral response to the solar wind as observed by simultaneous Hubble Space Telescope (HST) auroral images and Cassini upstream measurements of the solar wind taken during the month of January 2004. These observations show a direct correlation between solar wind dynamic pressure and (1) auroral brightening toward dawn local time, (2) an increase of rotational movement of auroral features to as much as 75% of the corotation speed, (3) the movement of the auroral oval to higher latitudes, and (4) an increase in the intensity of Saturn Kilometric Radiation (SKR). Our model, referred to as the centrifugal instability model, provides an alternative to the reconnection model of Cowley et al. (2004a, 2004b, 2005); we suggest the above observations result from Saturn's magnetosphere being a fast rotator. Since the torques on Saturn's outer magnetosphere are relatively low, its outer magnetosphere will tend to conserve angular momentum. When compressed on the dayside, the outer magnetosphere spins up to higher angular velocities, and when it expands, the outer magnetosphere spins down to lower angular velocities. This response occurs since Saturn's ionosphere is unable to enforce corotation. The outer boundary of the plasma sheet at L ∼ 15 is identified as the primary source location for the auroral precipitating particles. Enhanced wave activity, which can precipitate the auroral producing particles, may be present at this boundary. If radial transport is dominated by centrifugally driven flux tube interchange motions, when the magnetosphere spins up, outward transport will increase, and the precipitating particles will move radially outward (since the radial gradient in electron energy flux is negative). This mechanism will cause the auroral oval to move to higher latitudes as observed. The Kelvin‐Helmholtz instability may contribute to the enhanced emission along the dawn meridian, as observed by HST, via enhanced wave activity and corresponding charged particle precipitation.

Model of a variable radio period for Saturn

We propose an explanation for the variations at the 1% level of Saturn's radio rotation period measured at kilometer wavelengths. Because Saturn's kilometric radiation (SKR) is strongly controlled by the solar wind, we suggest that nonrandom variations of solar wind characteristics, especially its velocity, at Saturn may result in systematic displacement of the auroral sources in local time and finally in modifications of the apparent radio period. Alternatively, it may result in the superposition of two apparent periods, as also observed in Voyager data. We develop two simple models of local time variations of the SKR sources and analyze the conditions under which the measured radio period may be shifted by up to a few percent from the planet's sidereal period. Our results provide a possible explanation for the 1% variation observed and suggest that the dominant peak in the harmonic analysis of SKR variations seen by Voyager may be different from Saturn's sidereal rotation period. We relate the limitation in the accuracy of planetary rotation period determination to long‐term variations of “control” parameters (like the solar wind velocity). One and a half to three years of continuous SKR observations with Cassini will be required to reliably and accurately derive Saturn's true sidereal period.

Interplanetary conditions and magnetospheric dynamics during the Cassini orbit insertion fly‐through of Saturn's magnetosphere

The primary purpose of this paper is to discuss the interplanetary conditions that prevailed during the Saturn orbit insertion (SOI) fly‐through of Saturn's magnetosphere by the Cassini spacecraft in June–July 2004 and the consequent magnetospheric dynamics. We begin by examining concurrent interplanetary magnetic field (IMF) and Saturn kilometric radiation (SKR) data from Cassini together with images from the Hubble Space Telescope (HST) from an interval in January 2004, which show the effect of the arrival at Saturn of a corotating interaction region (CIR)‐related compression region. We then examine the IMF data obtained over five solar rotations bracketing the SOI fly‐through and show that a similar CIR compression and embedded crossing of the heliospheric current sheet (HCS) is expected to have impinged on Saturn's magnetosphere at some time during the fly‐through. Examination of the IMF direction on either side of the fly‐through confirms the HCS crossing. Observations of SKR show relatively weak emissions modulated at the planetary rotation period on the SOI inbound pass. Strong bursts extending to low frequencies, which are not in phase with the previous emissions, were observed on the outbound pass, similar to the CIR‐related SKR bursts seen in the January data. We thus suggest that the inbound pass occurred under uncompressed conditions of SKR and auroral quiet, while much of the outbound pass occurred under compressed conditions of SKR and auroral disturbance, probably of the same general character as observed in association with CIR compressions during the HST‐Cassini campaign in January 2004. We also examine the in situ magnetic field data observed outbound by Cassini in the predawn sector and find that the largest emission bursts are associated with concurrent variations in the predawn magnetic field, which are indicative of the injection of hot plasma at the spacecraft. Specifically, after an initial field strength increase, the field becomes depressed in strength and highly variable in time. These observations are consistent with an injection of hot plasma into the nightside magnetosphere from the tail, which we suggest is connected with auroral processes of the same nature as observed by the HST during the January 2004 campaign. The injection may be associated with compression‐induced reconnection in the tail, as has been proposed to explain the auroral signatures observed in the HST‐Cassini campaign data.

A nightside source of Saturn's kilometric radiation: Evidence for an inner magnetosphere energy driver

During Cassini's orbit insertion about Saturn, the spacecraft passed within 1.4 Rs of the planet passing from dayside into the nightside region. During this nightside passage, the onboard Radio and Plasma Wave (RPWS) instrument surprisingly detected Saturn kilometric radiation (SKR). Prior to this encounter, it was believed that SKR originated from a high‐latitude dayside source, and radio beams from such a source would not be viewable in this nearplanet night‐side location. Subsequent analysis presented here reveals that this SKR did indeed originate from the near‐midnight region on field lines near L ∼ 10–15. Such a radio source suggests the presence of an active region in the night‐side inner magnetosphere; this source possibly being near the outer edge of the icy‐moon created plasma torus surrounding the planet. The implication is that some of the SKR is driven by an internal energy source that may also account for recent UV aurora observations.

High spectral and temporal resolution observations of Saturn kilometric radiation

This paper presents the first high‐resolution dynamic spectra of Saturn kilometric radiation acquired upon Cassini's approach and first orbits of Saturn. The emissions display upward and downward drifting features with bandwidths down to ∼200 Hz and drift rates of a few kHz per second. At other times, the emissions are much more diffuse or continuous, showing little spectral structure on scales of 10 or 20 kHz. The fine structure is strikingly similar to Earth's auroral kilometric radiation (AKR) and Jovian auroral radio emissions in many respects. The dynamic spectral features provide insight into the highly nonlinear nature of the cyclotron maser instability believed to generate the emissions. We use ideas developed to explain the fine structures at Earth to suggest features and processes in the auroral acceleration region which may result in Saturn's fine structures.

In situ observations of a solar wind compression‐induced hot plasma injection in Saturn's tail

During the Saturn orbit insertion (SOI) fly‐through of the Cassini spacecraft, Saturn's magnetosphere underwent a significant corotating interaction region (CIR) related compression. Such compressions have recently been suggested to produce rapid bursts of tail reconnection, enhanced Saturn kilometric radiation (SKR), and consequent auroral dynamics. On the outbound pass the spacecraft became engulfed by hot plasma, associated with a reduction in field strength, and a change in orientation indicative of a dipolarisation. Concurrently, a substantial enhancement in SKR emissions took place, together with a disruption of the typical planetary modulation. We suggest this is the first in situ evidence of compression‐related tail collapse via magnetic reconnection and hot plasma acceleration in Saturn's magnetotail.

Energetic ion acceleration in Saturn's magnetotail: Substorms at Saturn?

The Magnetospheric Imaging Instrument (MIMI) Ion and Neutral Camera (INCA) on the Cassini spacecraft has recorded abrupt increases in energetic neutral atom flux coming from the general direction of Saturn's magnetotail. These bursts of ion activity in the tail are well correlated with enhancements in the Saturn kilometric radiation. Given the similarities between these events and substorm activity on Earth, including their dependence on interplanetary conditions, we conclude that Earth‐like substorms occur within Saturn's magnetosphere.

Variations of Saturn's radio rotation period measured at kilometer wavelengths

The Unified Radio and Plasma Wave (URAP) experiment on the interplanetary spacecraft Ulysses is able to detect the Saturnian kilometric radiation (SKR) thanks to the high sensitivity of the receiver and in spite of the remoteness of the planet (8–13 AU). Our knowledge about Saturn comes essentially from the observations by the two Voyager spacecraft. Ulysses allows us to reassess the main properties of the SKR as they had been observed by Voyager 1 and 2: average flux density, spectrum, periodicity, and polarization. A striking difference between the results obtained from Voyager and those obtained from Ulysses is the periodicity of the radio emission linked to the planetary rotation (10 hours 39 min 24 s): The period deduced from Ulysses' observations is not constant and may differ by 1% from that of Voyager. The northern and southern sources of SKR are distributed along magnetic field lines which are fixed in local time. We interpret the source location by a Kelvin‐Helmholtz instability developing on the flanks of Saturn's magnetopause, in the morningside. This instability could be at the origin of the acceleration of the particles responsible for the radio emission. Because of the solar wind fluctuations, the position of the magnetopause and the magnetohydrodynamic flow around it are modified so that the zone of Kelvin‐Helmholtz instability slowly moves, implying a drift of the radio sources in local time. We examine and discuss the possibility for this drift to be the cause of the variations observed for the SKR period.

Source location of Saturn's kilometric radiation: The Kelvin‐Helmholtz instability hypothesis

A new determination of the location of Saturn's kilometric radiation (SKR) sources is deduced from a limited set of hypotheses: assuming that the radio sources are fixed in latitude and local time, and that SKR is emitted on the X mode near its local cutoff frequency, we obtain source location footprints for northern and southern radio sources. The sources in both hemispheres appear magnetically conjugate and are consistent over the whole SKR frequency range. They are centered about 1300 local time (LT), at magnetic latitudes ≥80°, extending toward lower latitudes on the morning side (down to 60° at 0800–0900 LT) and on the evening side (down to 75° at 1900 LT). We interpret these source locations in terms of electron precipitations possibly caused by Kelvin‐Helmholtz instability arising on the flanks of the Saturnian magnetopause (mainly the morning one). Constraints on the average SKR beaming are derived.

Comment on “Reply to Comment on ‘Evidence of Saturn's magnetic field anomaly from Saturnian kilometric radiation high‐frequency limit’” by Patrick Galopeau and Philippe Zarka

Comment on “Reply to Comment on ‘Evidence of Saturn's magnetic field anomaly from Saturnian kilometric radiation high‐frequency limit’” by Patrick Galopeau and Philippe Zarka

Reply [to “Comment on ‘Evidence of Saturn's magnetic field anomaly from saturnian kilometric radiation high‐frequency limit’”

Reply [to “Comment on ‘Evidence of Saturn's magnetic field anomaly from saturnian kilometric radiation high‐frequency limit’”

Comment on “Evidence of Saturn's magnetic field anomaly from Saturnian kilometric radiation high‐frequency limit” by P. Galopeau et al.

Comment on “Evidence of Saturn's magnetic field anomaly from Saturnian kilometric radiation high‐frequency limit” by P. Galopeau et al.

Evidence of Saturn's magnetic field anomaly from Saturnian kilometric radiation high‐frequency limit

The detailed analysis of Voyager observations of Saturn kilometric radiation (SKR), taking into account the polarization response of the planetary radio astronomy experiment, has allowed us to study separately the high‐frequency limit of SKR emitted from the northern and southern auroral regions of the planet. Interpretation of the variations of these high‐frequency limits, in the frame of a model inspired by the cyclotron maser mechanism, has allowed us to demonstrate unambiguously the existence of a magnetic “anomaly”, postulated for years, in the near‐surface Saturnian field. Its location and amplitude are estimated, and it is shown that, although it accounts for the observed SKR modulation, it could not be detected by the spacecraft‐borne magnetometers which passed too far from the planetary surface. This demonstrates that the current degree of comprehension of planetary radio emissions makes them a unique and very pertinent tool for the fine study of planetary magnetic fields.

Influence of the solar wind/interplanetary medium on saturnian kilometric radiation.

Previous studies on the periodicities of the Saturnian kilometric radiation (SKR) suggested a considerable solar wind influence on the occurrence of SKR, so it was obvious to investigate the relationship between parameters of the solar wind/interplanetary medium and this Saturnian radio component. Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the external control of SKR. Out of the examined quantities known to be important in controlling magnetospheric processes this investigation yielded a dominance of the solar wind momentum, ram pressure and kinetic energy flux, in stimulating SKR and controlling its activity and emitted energy, and confirmed the results of the Voyager 1 analysis.

Theoretical model of Saturn's kilometric radiation spectrum

A theoretical model is developed here in order to determine an envelope for the average spectrum of the Saturnian kilometric radiation (SKR). The microscopic generation mechanism is supposed to be the so‐called synchrotron (or cyclotron) maser instability. As in recent works on the terrestrial kilometric radiation, the effect of the magnetic field inhomogeneity on the generation process must be taken into account. Then, assuming that the emission is nonlinearly saturated by trapping, our calculation allows us to put an upper limit on the SKR spectral intensity very simply: the maximum level of the wave electric field within the source region is indeed linked to a few macroscopic plasma parameters, which can be derived from the observations (the structure of the magnetospheric magnetic field, the cold plasma density, and the characteristic energy of the hot emitting electrons). We have used a dipolar magnetic field model, while the plasma distribution results from the superposition of two components: an ionospheric population and a plasma disc, whose scale heights have been roughly determined from Voyager measurements. The energetic electrons responsible for the emission are supposed to precipitate along the high‐latitude magnetic field lines where SKR emission is known to take place. The width of the source region can be self‐consistently estimated from the model. A very good agreement is obtained between the theoretical spectrum and the observational radio data. The calculated spectral intensities exceed the most intense observed intensities by up to 1 order of magnitude, suggesting that the SKR emission is only marginally saturated by nonlinear processes.

Power spectrum of electrical discharges seen on Earth and at Saturn

We present a method for deriving the radio spectrum of electrical discharges from the properties of the time series of charges crossing the discharge gap. For the simple case considered, the resulting spectrum is the product of the power spectrum of emission from a single charge crossing the gap and of the power spectrum of the time series of charges crossing a given point in the gap. This result is applied to the observed spectra of both terrestrial lightning and Saturn electrical discharge(s) (SED). These observations are discussed, particularly in the case of SED. SED occurrence and power density are shown to have subtle, yet important, differences from these observables as they have been described in the last 5 years. It is demonstrated that throughout the episode of Voyager 1's (V1) closest approach to Saturn, SED probably occurred continuously in frequency upward at least from the upper limit of Saturn kilometric radiation at about 800 kHz. This is so despite the fact that in the dynamic spectra a strip in time and frequency in which SED do not occur extends in frequency from 1.3 MHz up to the oft‐discussed “lower limit” of SED in the leading edge of the episode of closest approach. This cutoff band can be well interpreted through the combined effects of lower receiver sensitivity resulting from spacecraft interference and the increasing efficiency of power transfer from a given small antenna as frequency increases up to about the quarter‐wave value (above 7 MHz for the planetary radio astronomy antennas). The greater power in SED that occurred after V1 closest approach is emphasized; it is shown to be consistent with the lower frequency of the maximum in their power spectra. This is a result of the greater length of the discharge gap as it generates SED after closest approach. The variable gap length factor is also invoked to explain the variable frequency cutoff in the range 5–15 MHz of the episodes before closest approach. The SED source moved along a single arc defining both preencounter and postencounter events. The discharge gap lengths were a continuous function of position along this arc, with the shortest gaps lying about 5° west (as seen from the spacecraft) of the noon meridian of Saturn and the longest gaps lying on the nightside of the planet.

The centrifugal flute instability and the generation of Saturnian kilometric radiation

We show that the source region for Saturnian kilometric radiation (SKR) which originates in the high latitude near‐noon dayside ionospheric can be mapped via the Saturn magnetic field to the outer edge of the dayside equatorial plasmasheet. The plasmasheet is known to be unstable at this boundary due to the outward centrifugal forces generated by the heavily mass‐loaded equatorial Saturnian magnetosphere. Also previous work has shown that Voyager 1 observations of MHD waves near the plasmasheet are consistent with their being driven by the centrifugal flute instability. The resulting field‐aligned MHD waves at higher latitudes can trap and accelerate electrons. For the plasma environment measured by Voyager, field‐aligned accelerated electrons up to energies of several keV are expected to be deposited into the SKR ionospheric source region. A necessary condition for the acceleration of the electrons to several keV is a population of hot electrons inside of the plasmasheet, near the unstable boundary. The hot electrons are produced as a byproduct of the pick‐up of exospheric atoms from Saturn's moons. The solar wind dependence of the SKR is thus a consequence of the elongation of the Saturn nightside magnetosphere with greater solar wind pressure leading to greater centrifugal pressures on the nightside plasmasheet. Although the theory presented correctly gives the local time dependence of the emissions, we suggest that the strong emission peak at a fixed Saturnian longitude is a consequence of locally reduced electron plasma frequency to electron gyrofrequency ratio. The lack of a nightside source of SKR is consistent with the disruption of the plasmasheet due to loss of plasma down the Saturn magnetotail.