P. Zarka

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

[1]xa0The 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 Saturns 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 Saturns magnetosphere.

On the possibility of coherent cyclotron emission from extrasolar planets

A model of the coherent cyclotron emission from extrasolar planets is presented. Scaling laws known to operate in our solar system (including scaling laws of planetary magnetic fields and the radiometric Bodes law of radio power generation) are applied to the extrasolar systems. We consider the possibility that each of the extrasolar planets possesses a substantial planetary magnetic field which is in quasi-continuous interaction with the local stellar wind. Cyclotron emission from extrasolar planets is then driven by the stellar wind/magnetospheric interaction, much like the coherent cyclotron radio emission processes associated with planets in our solar system. Based on the model results, the best candidate for solar-wind-driven cyclotron emission is Tau Bootes, with an expected median amplitude of about 2 janskys (1 Jy = 10−26 W m−2 Hz−1) at 28 MHz, an intensity level of about a factor of 100 below the current limit of detectability. However, variations in the local stellar medium could conceivably increase power levels by a factor of 100 for short periods of time. Like the solar planets, the extrasolar planets should radiate episodically, with emission reoccurring at the planetary rotation period. Thus spectral integration techniques could also be applied to improve the likelihood of detectability.

Jupiter's low‐frequency radio spectrum from Cassini/Radio and Plasma Wave Science (RPWS) absolute flux density measurements

[1] We apply the calibration method developed by Dulk et al. [2001] to the data from the CassinilRadio and Plasma Wave Science (RPWS) High-Frequency Receiver in order to derive flux density measurements of six components of the Jovian low-frequency radio spectrum over the full frequency range of the instrument (3.5 kHz to 16.1 MHz). The estimated accuracy is better than 50%, i.e., much less than the intrinsic variations of the flux densities of these radiosources. It is mainly limited by the accuracy of the model used for the radio galactic background. Instrumental parameters such as the antennas effective lengths and base capacitance are constrained in the calibration process. From 6 months of calibrated data centered on the Cassini-Jupiter flyby, we derive the average and peak Jovian radio spectrum between 3.5 and 16.1 MHz and its range of fluctuations, from which we deduce constraints on the beaming of the various radio components and estimate the power emitted by each component. Our calibration procedure also allows us to compare Cassini measurements of the Jovian radio spectrum with ground-based measurements performed, e.g., in Nancay above the ionospheric cutoff (10-15 MHz). It will be used to derive absolute flux measurements during the Saturn tour.

Model of the Jovian magnetic field topology constrained by the Io auroral emissions

[1]xa0The determination of the internal magnetic field of Jupiter has been the object of many studies and publications. These models have been computed from the Pioneer, Voyager, and Ulysses measurements. Some models also use the position of the Io footprints as a constraint: the magnetic field lines mapping to the footprints must have their origins along Ios orbit. The use of this latter constraint to determine the internal magnetic field models greatly improved the modeling of the auroral emissions, in particular the radio ones, which strongly depends on the magnetic field geometry. This constraint is, however, not sufficient for allowing a completely accurate modeling. The fact that the footprint field line should map to a longitude close to Ios was not used, so that the azimuthal component of the magnetic field could not be precisely constrained. Moreover, a recent study showed the presence of a magnetic anomaly in the northern hemisphere, which has never been included in any spherical harmonic decomposition of the internal magnetic field. We compute a decomposition of the Jovian internal magnetic field into spherical harmonics, which allows for a more accurate mapping of the magnetic field lines crossing Io, Europa, and Ganymede orbits to the satellite footprints observed in UV. This model, named VIPAL, is mostly constrained by the Io footprint positions, including the longitudinal constraint, and normalized by the Voyager and Pioneer magnetic field measurements. We show that the surface magnetic fields predicted by our model are more consistent with the observed frequencies of the Jovian radio emissions than those predicted by previous models.

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.

Non-detection at Venus of High-Frequency Radio Signals Characteristic of Terrestrial Lightning

The detection of impulsive low-frequency (10 to 80u2009kHz) radio signals, and separate very-low-frequency (∼100u2009Hz) radio ‘whistler’ signals provided the first evidence for lightning in the atmosphere of Venus. Later, a small number of impulsive high-frequency (100u2009kHz to 5.6u2009MHz) radio signals, possibly due to lightning, were also detected. The existence of lightning at Venus has, however, remained controversial. Here we report the results of a search for high-frequency (0.125 to 16u2009MHz) radio signals during two close fly-bys of Venus by the Cassini spacecraft. Such signals are characteristic of terrestrial lightning, and are commonly heard on AM (amplitude-modulated) radios during thunderstorms. Although the instrument easily detected signals from terrestrial lightning during a later fly-by of Earth (at a global flash rate estimated to be 70u2009s -1, which is consistent with the rate expected for terrestrial lightning), no similar signals were detected from Venus. If lightning exists in the venusian atmosphere, it is either extremely rare, or very different from terrestrial lightning.

Jovian S burst generation by Alfvén waves

[1]xa0Jupiters radio emissions are dominated in intensity by decametric radio emissions due to the Io-Jupiter interaction. Previous analyses suggest that these emissions are cyclotron-maser emissions in the flux tubes connecting Io or Ios wake to Jupiter. Electrons responsible for the emission are thought to be accelerated from Io to Jupiter. We present simulations of this hot electron population under the assumption of acceleration by Alfven waves in the Io flux tube. Outside of limited acceleration regions where a parallel electric field associated with Alfven waves exists, the electrons are supposed to have an adiabatic motion along the magnetic field lines. Near Jupiter a loss cone appears in the magnetically mirrored electron population, which is able to amplify extraordinary (X) mode radio waves. The X-mode growth rate is computed, which allows us to build theoretical dynamic spectra of the resulting Jovian radio emissions, whose characteristics match those observed for Jovian S bursts.

Variation of Saturn's UV aurora with SKR phase

It is well known that a wide range of kronian magnetospheric phenomena, including the Saturn kilometric radiation (SKR), exhibit oscillations near the planetary rotation period. However, although the SKR is believed to be generated by unstable auroral electrons, no connection has been established to date between diurnal SKR modulations and UV auroral power. We use an empirical SKR phase determined from Cassini observations to order the ‘quiet time’ total emitted UV auroral power as observed by the Hubble Space Telescope in programs during the interval 2005–2009. Our results indicate that both the northern and southern UV powers are dependent on SKR phase, varying diurnally by factors of ∼3. We also show that the UV variation originates principally from the morning half of the oval, consistent with previous observations of the SKR sources.

Source characteristics of Jovian narrow-band kilometric radio emissions

New observations of Jovian narrow-band kilometric (nKOM) radio emissions were made by the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses-Jupiter encounter in early February 1992. These observations have demonstrated the unique capability of the URAP instrument for determining both the direction and polarization of nKOM radio sources. An important result is the discovery that nKOM radio emission originates from a number of distinct sources located at different Jovian longitudes and at the inner and outermost regions of the Io plasma torus. These sources have been tracked for several Jovian rotations, yielding their corotational lags, their spatial and temporal evolution, and their radiation characteristics at both low latitudes far from Jupiter and at high latitudes near the planet. Both right-hand and left-hand circularly polarized nKOM sources were observed. The polarizations observed for sources in the outermost regions of the torus seem to favor extraordinary mode emission.

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.