P. Canu

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.

Solar wind electron parameters from quasi‐thermal noise spectroscopy and comparison with other measurements on Ulysses

Plasma thermal noise spectroscopy was used for the first time on a large scale on the Ulysses radio receiver data to measure the solar wind electron density and temperature in the ecliptic plane. The validity and limitations of the results obtained with this method are discussed. Nearly simultaneous measurements of the electron density and temperature from the radio receiver, the sounder, and the electron analyzer on Ulysses are intercompared. The thermal noise measurements are found to compare quite well with the other measurements, apart from some discrepancies, which are discussed. The uncertainties on the core temperature, derived from a least squares model fitting of the radio data, are shown to be statistically consistent and significant.

Detection of Bernstein wave forbidden bands in the Jovian magnetosphere : A new way to measure the electron density

We analyze the power spectra measured by the radio receiver of the Unified Radio and Plasma Wave experiment on Ulysses during its passage through the Jovian inner magnetosphere from ∼ 9 RJ in the outskirts of the Io plasma torus to ∼ 13 RJ near the plasma sheet. Below the plasma frequency ƒp, these spectra are weakly banded between gyroharmonics. These observations were interpreted by Meyer-Vernet et al. [1993] as quasi-thermal fluctuations in Bernstein waves. We show that above ƒp each observed gyroharmonic band falls off very abruptly on its high-frequency side. We interpret it as the “forbidden band” predicted by the Bernstein wave dispersion equation between the so-called ƒQ frequency and the consecutive gyroharmonic, that is, a region where no Bernstein wave can propagate. This allows a determination of the local cold plasma frequency and thus of the core electron density with a ∼ 16% uncertainty. As a consistency check, we show that the ƒQ thus determined are very close to the frequencies of the resonances excited by the relaxation sounder on Ulysses.

ULF waves in the Io torus: Ulysses observations

Throughout the Io torus, Ulysses has observed intense ultralow frequency (ULF) wave activity in both electric and magnetic components. Such ULF waves have been previously suggested as the source of ion precipitation leading to Jovian aurorae. The peaks of the wave spectra are closely related to the ion cyclotron frequencies, which is evidence of the waves being ion cyclotron waves (ICWs). Analysis of the dispersion relation using a multicomponent density model shows that at high latitudes (approximately 30 deg), peak frequencies of the waves fall into L mode branches of guided or unguided ICWs. Near the equator, in addition to the ICWs below f cO(2+) , there are strong signals at approximately 10 Hz which require an unexpectedly large energetic ion temperature anistropy to be explained by the excitation of either convective or nonconvective ion cyclotron instabilities. Their generation mechanism remains open for the future study. Evaluation of the Poynting vector and the dispersion relation analysis suggest that the waves near the equator had a small wave angle relative to the magnetic field, while those observed at high latitudes were more oblique. The polarization of the waves below f cH(+) is more random than that of the whistler mode waves, but left-hand-polarized components of the waves can still be seen. The intensity of the ICWs both near the equator and at high latitudes are strong enough to meet the requirement for producing strong pitch angle scattering of energetic ions.

Correlations between magnetic field and electron density observations during the inbound Ulysses Jupiter flyby

The spacecraft Ulysses flew through the Jovian magnetosphere during February 1992. This paper compares the magnetic field observations recorded during the inbound pass of the flyby with the electron density as derived from the URAP instrument. In general, it is expected that the density variations will anti-correlate with the magnetic field strength in order to maintain pressure balance, although there may be instances when a temperature or energy rise alone could balance the static stress. Furthermore, there is the possibility that a dynamic process could occur which would cause both the density and field magnitude to rise in unison. In the middle magnetosphere, anti-correlation is found to exist between the two data sets; however, in the outer magnetosphere (which was characterized by very disturbed fields) and in the transition region between the outer and middle magnetospheres, there is no simple relationship between the density and field. Examples of anti-correlation, temperature or energy increases and dynamic processes are found.

Ulysses observations of auroral hiss at high Jovian latitudes

During the Ulysses flyby of Jupiter, a whistler-mode emission was periodically detected by the unified Radio and Plasma wave (URAP) experiment during intervals when the spacecraft extended to high magnetic latitudes. The signal was detected between the local electron plasma frequency and lower hybrid resonance and appears as a funnel-shaped structure on frequency-versus-time spectrograms; these characteristics are very reminiscent of whistler-mode auroral hiss observed at high latitudes at Earth. Ray tracing of the emission occurrences suggests the emission source is on magnetic field lines extending out to at least 65 R(sub J). This location associates the emission with the boundary between open and closed field lines -- not the Io torus. The emission radiates about 10(exp 7) W of power. Consequently, the auroral input power derived from the solar wind to drive the emission is believed to be 10(exp 10-12) W (or about 1% of the energy associated with Io torus electrical processes).

POSSIBLE ROLE OF ELECTROMAGNETIC LOW FREQUENCY WAVES IN THE IO TORUS IN THE PRODUCTION OF JOVIAN AURORAE

Abstract The possible role of ULF waves observed in the Io torus in the generation of Jovian autorae is investigated. Aaroral emissions have been observed, not only at high latitude, but also at the footprint of magnetic field lines connected to the Io torus in a limited loagitudinal range around 150°. As Ulysses passed by Jupiter, it explored the Io torus in latitude and longituds, allowing to test x possible localization of the waves on field lines mapping down into the surors. A maximum intensity of the waves is observed in the high latitude part of the torus around the longitude where the surorae were observed. Another maximum is also observed at the magnetic equator. The propagation mode of the waves is magnetic equator. The propagation mode of the values necessary for scattering ions, showing that the ULF waves could possibly be at the origin of the aurorae.

Correction to “ULF wave identification in the magnetosheath: The k‐filtering technique applied to Cluster II data”

[1] In the results presented in ‘‘ULF wave identification in the magnetosheath: The k-filtering technique applied to Cluster II data’’ by Sahraoui et al. (Journal of Geophysical Research, 108(A9), 1335, doi:10.1029/2002JA009587, 2003), there is an error in the matrix transformation from the GSE frame to the MFA (Magnetic Field-Aligned) one: the z axis of the adopted frame was not really aligned with the local magnetic field. The MFA frame has in fact to be corrected with respect to the old one by the following angles between the axes: (ex, ex 0 ) = 3 , (ey, ey 0 ) = 179 and (ez, ez 0 ) = 167 . Because of this angle shift, some of the characteristics of the identified waves are slightly modified. This concerns particularly the propagation angles with respect to B0. In the figure below, which is to be compared with Figure 7a in the original paper, we illustrate such a modification concerning the dominant mirror mode previously identified: the modulus of the wave vector remains unchanged k 0.012 rd/km; however, its direction is now 80 with respect to B0 instead of 62 given previously. Nevertheless, all the main physical conclusions brought by the paper (mixture of the LF linear modes with the predominance of the mirror mode, importance of the Doppler shift, . . .) remain fully valid. The error in the rotation matrix was discovered thanks to a comparative study done on the same data by S. Walker and M. A. Balikhin, to whom we are grateful.