A. Lecacheux

An update to a Saturnian longitude system based on kilometric radio emissions

[1]xa0The period of Saturn kilometric radiation modulation as determined by Voyager forms the basis for a longitude system (SLS) recognized by the International Astronomical Union. However, Ulysses and Cassini observations have shown that this modulation period varies by the order of one percent on timescales of a few years and, hence, does not represent the internal rotation period of the planet. A new longitude system was proposed based on ∼2 years of Cassini observations of the kilometric radio emissions and accounts for the variable radio period (SLS2) valid over the time interval from day 001, 2004 through day 240, 2006. Early uses of this longitude system have revealed a number of magnetospheric phenomena which appear to be locked to the radio period, such as variations in the external magnetic field, the plasma density in the inner magnetosphere, and enhanced intensities of energetic ions. Analysis of the radio emissions since the new system was proposed revealed that the radio period continued to evolve, even showing a second, shorter period at times. The subsolar longitude of the peak of Saturn kilometric radio emissions begins to deviate from that given by the SLS2 system almost immediately after the previous analysis interval. Here, we provide a definition for SLS3, an extension to the longitude system valid over the interval from day 001, 2004 through day 222, 2007 based on variable period radio emissions.

A Saturnian longitude system based on a variable kilometric radiation period

[1]xa0This 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 Saturns 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.

The reversal of the rotational modulation rates of the north and south components of Saturn kilometric radiation near equinox

[1]xa0It has been known for many years that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies slowly on time scales of years. Recently it has been shown that the radio emission consists of two components that have different rotational modulation rates, one emitted from the northern auroral region and the other emitted from the southern auroral region. In this paper we show using radio measurements from the Cassini spacecraft that the rotational modulation rates of the northern and southern components reversed near Saturns recent equinox, which occurred on 11 August 2009. We show that a similar reversal was also observed by the Ulysses spacecraft near the previous equinox, which occurred on 19 November 1995. The solar control implied by these reversals has important implications on how Saturns rotation is coupled into the magnetosphere.

Direction finding study of Jovian hectometric and broadband kilometric radio emissions: evidence for their auroral origin

Abstract Taking advantage of the direction finding capabilities of the Ulysses unified radio and plasma wave (URAP) experiment we derive the source locations and emission characteristics of the Jovian hectometric (HOM) and broadband kilometric (bKOM) emissions. Unlike previous studies we additionally determine the systematic error of the source direction due to the uncertainty of the antenna/ receiver calibration parameters. To obtain maximum accuracy when doing direction finding, we use HOM and bKOM events observed close to the Ulysses encounter with Jupiter. It is found that both emissions have their sources at the auroral zones of Jupiter, at dipole L shells between 7 and 11 for HOM in the northern hemisphere, and between 9 and 15 for the bKOM radiation observed at high southern latitudes after the Ulysses Jupiter encounter. For the analyzed HOM events the source location is usually between 40° and 130° central meridian longitude (CML) and 2200 and 0600 local time (LT), but probably there exist emissions from other longitudes and local times too. In contrast, the bKOM is emitted from a wide range of longitudes and local times as well. There is some evidence that both the kilometric and hectometric emissions are composed of a dominant component in the fast extraordinary (R-X) mode and a weaker ordinary (L-O) mode component. Due to the uncertainty in source direction determination the emissions cone half-angle (i.e. half-angle of the cone in which the emission is beamed) cannot be accurately determined. It is some 30°–90° for the HOM and about 40°–80° for the bKOM. The source location of bKOM is likely to be associated with open magnetospheric field lines whereas the HOM is located at field lines that connect the HOM radio sources with the inner Jovian plasma sheet and/or outer plasma torus. There is some evidence that the bKOM radio emissions are correlated with ultraviolet auroral activity.

Observation of similar radio signatures at Saturn and Jupiter: Implications for the magnetospheric dynamics

We report on radio signatures observed at Saturn by the Cassini RPWS experiment which are strikingly similar to the Jovian “energetic events” observed by Galileo. They consist of sudden intensifications of the auroral radio emission (SKR) followed by the detection of a periodic narrowband radiation which most likely originates from Saturns plasma disk. About ten “events” have been observed in 2006, showing on average temporal scales ∼3 times longer than their Jovian counterparts. We analyze the conditions of generation and the visibility of the narrowband radiation and conclude that the Kronian “events” are most likely associated with plasma evacuation from the disk. These observations provide new insights on the role of internal energy releases in Saturns magnetosphere, known from other observations to be mainly driven by the solar wind.

Source locations of narrowband radio emissions detected at Saturn

[1]xa0Since Cassinis arrival at Saturn in 2004, the Radio and Plasma Wave Science instrument has detected numerous narrowband (NB) radio emission events. These emissions, mostly detected around 5 and 20 kHz, usually occur periodically for several days after intensifications of Saturn kilometric radiation. We present calculations based on an electron density profile of Saturns plasma torus and a dipole magnetic field model showing that the NB emissions originate from the northern and southern edges of Saturns plasma torus at L shells ∼ 8 to 10 for 5-kHz NB and L ∼ 4 to 7 for 20-kHz NB. In many cases, Cassini passes through the source region of the 20-kHz NB, as indicated by intense electrostatic upper hybrid (ESUH) waves in close proximity to electromagnetic emissions on spectrograms. The positions of the spacecraft when intense ESUH waves are observed agree with the model predictions of the NB source locations. Source locations determined by goniopolarimetric (also known as direction-finding) analysis of the NB emissions also support the above results, although sometimes the directions of arrival point toward the region interior to Saturns plasma torus. A polarization reversal technique is applied to localize the NB emissions observed during spacecraft rotation, on the basis of the fact that the source is within the antenna plane when the apparent circular polarization degree switches sign. The NB emissions are found to be L-O mode polarized, which is consistent with the prediction of linear/nonlinear mode conversion theory. It is also found that sometimes right-hand polarized NB emissions are generated at second harmonic frequencies of the 20-kHz NB; in which case, wave-wave interactions between oppositely propagating ESUH waves may play an important role in the mode conversion process.

Ulysses observations of escaping VLF emissions from Jupiter

The Ulysses URAP experiment has detected Jovian radio emissions in the VLF range at distances from Jupiter in excess of 1.5 A.U. The URAP observations represent the first synoptic observations of Jupiter in the VLF band, 3 to 30 kHz. In this band lie the low-frequency extent of the bKOM emission, the escaping continuum emission, and the Jovian type IIIs. Initial results indicate that the continuum varies in frequency with the solar wind ram pressure at Jupiter, whereas, the Jovian type IIIs appear to be controlled to some extent by the planetary rotation, often appearing when system III longitude 100° faces the spacecraft.

In ecliptic observations of Jovian radio emissions by Ulysses comparison with Voyager results

During the Ulysses inbound cruise to Jupiter the Unified Radio and Plasma Wave (URAP) experiment observed a variety of the planets radio components in the frequency range below 1 MHz. Most of these emissions were already detected by the Voyager Radio Astronomy (PRA) and Plasma Wave (PWS) experiments, however with much less sensitivity and different spectral coverage. We identify these different radio components within the URAP dynamic spectra and compare their appearance with the previous Voyager observations.

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

[1]xa0During Cassinis 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

[1]xa0This paper presents the first high-resolution dynamic spectra of Saturn kilometric radiation acquired upon Cassinis 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 Earths 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 Saturns fine structures.