Showing papers by "Baptiste Cecconi published in 2005"

Radio and plasma wave observations at Saturn from Cassini's approach and first orbit.

Abstract: We report data from the Cassini radio and plasma wave instrument during the approach and first orbit at Saturn. During the approach, radio emissions from Saturn showed that the radio rotation period is now 10 hours 45 minutes 45 ± 36 seconds, about 6 minutes longer than measured by Voyager in 1980 to 1981. In addition, many intense impulsive radio signals were detected from Saturn lightning during the approach and first orbit. Some of these have been linked to storm systems observed by the Cassini imaging instrument. Within the magnetosphere, whistler-mode auroral hiss emissions were observed near the rings, suggesting that a strong electrodynamic interaction is occurring in or near the rings.

An Earth-like correspondence between Saturn's auroral features and radio emission

Abstract: Saturn is a source of intense kilometre-wavelength radio emissions that are believed to be associated with its polar aurorae, and which provide an important remote diagnostic of its magnetospheric activity. Previous observations implied that the radio emission originated in the polar regions, and indicated a strong correlation with solar wind dynamic pressure. The radio source also appeared to be fixed near local noon and at the latitude of the ultraviolet aurora. There have, however, been no observations relating the radio emissions to detailed auroral structures. Here we report measurements of the radio emissions, which, along with high-resolution images of Saturn's ultraviolet auroral emissions, suggest that although there are differences in the global morphology of the aurorae, Saturn's radio emissions exhibit an Earth-like correspondence between bright auroral features and the radio emissions. This demonstrates the universality of the mechanism that results in emissions near the electron cyclotron frequency narrowly beamed at large angles to the magnetic field.

Direction finding and antenna calibration through analytical inversion of radio measurements performed using a system of two or three electric dipole antennas on a three-axis stabilized spacecraft

Abstract: [1] We present an analytical inversion method to achieve direction-finding (DF) (i.e., retrieve the direction of arrival of an incoming electromagnetic wave, its flux, and its full polarization state) using radio measurements performed using a system of two or three electric dipole antennas on a three-axis stabilized spacecraft. The Radio and Plasma Wave Science (RPWS) radio receiver on board Cassini includes such instantaneous DF capabilities, and so does the Solar Terrestrial Relations Observatory (STEREO) Waves radio receiver. We also present an analytical solution of the inverse problem which consists of calibrating the electric dipole orientations and effective lengths using a known radio source. Error sources (imperfect knowledge of antenna parameters, digitization errors, signal to noise ratio, etc.) and their propagation through the analytical inversion have been studied. The typical expected accuracy of our DF inversion is 1 dB [V2/Hz] for flux measurements, about 1–2° for source position and a few percent for degrees of polarization. For the antenna calibration procedure the expected accuracy is of the order of 2° on antenna direction and of 1% on antenna length. We define the data selection criteria to be used during both DF analysis and antenna calibration. We also discuss the limitations of the methods and the ways to improve their accuracy.

Quasi thermal noise spectroscopy in the inner magnetosphere of Saturn with Cassini/RPWS: Electron temperatures and density

Abstract: [1] On 1 July 2004, the Cassini spacecraft performed its Saturn orbit insertion, twice crossing the equatorial plane between the G and F rings. The radio HF receiver observed a peak at the upper-hybrid frequency and weakly banded emissions having well-defined minima at gyroharmonics. We show that through most of the encounter, these emissions do not result from instabilities, but instead are the quasi-thermal noise that can be calculated from the classical theory of plasma fluctuations. The spectroscopy of this noise yields the electron density, the core and the halo temperatures in the range 2.3 < L/RS < 7, −1.2 < z/RS < +0.1. For the first time, we measure the core temperature of the Kronian plasma torus to be about 0.5 eV in the ring plane at ∼2.5RS, and increasing to ∼6 eV at 7RS. From the noise minima at the gyroharmonics, we also deduce the magnetic field strength, which agrees with the Cassini's magnetometer data to better than 2%.

Model of a variable radio period for Saturn

Abstract: [1] 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

Abstract: [1] 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.

High spectral and temporal resolution observations of Saturn kilometric radiation

Abstract: [1] 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.

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

Abstract: [1] 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.