A. M. Persoon

Multi-instrument analysis of electron populations in Saturn's magnetosphere

We analyze the radial distribution of electron populations inside 20 R-s in Saturns magnetosphere, and we calculate moments for these populations by a forward modeling method using composite spectra produced by the CAPS/ELS (0.6 eV to 26 keV) and the MIMI/LEMMS (15 keV to 10 MeV) instruments on board Cassini. We first calculate and harmonize both data sets in physical units and apply corrections taking into account biases introduced by spacecraft interaction with the magnetospheric environment. We then test different bimodal isotropic electron distribution models, deciding on a model with two kappa distributions. We adjust our isotropic model to the flux composite spectra with a least square method to produce three sets of fluid parameters (density, temperature, spectral index) per electron population. The radial profiles are then analyzed, revealing a relevant boundary at 9 R-s in both thermal and suprathermal electron populations. Observed discontinuities in the moment profiles (sudden drop-off in cold density profile outside 9 R-s, hot electrons drop-off inside 9 R-s) coincide with the known outer edge of Saturns neutral OH cloud. Farther out, thermal electrons disappear completely beyond 15 R-s while suprathermal electrons are still observed in the middle and outer magnetosphere.

Radar Soundings of the Ionosphere of Mars

We report the first radar soundings of the ionosphere of Mars with the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on board the orbiting Mars Express spacecraft. Several types of ionospheric echoes are observed, ranging from vertical echoes caused by specular reflection from the horizontally stratified ionosphere to a wide variety of oblique and diffuse echoes. The oblique echoes are believed to arise mainly from ionospheric structures associated with the complex crustal magnetic fields of Mars. Echoes at the electron plasma frequency and the cyclotron period also provide measurements of the local electron density and magnetic field strength.

Dusty plasma in the vicinity of Enceladus

We present in situ Cassini Radio Plasma Wave Science observations in the vicinity of Enceladus and in the E ring of Saturn that indicate the presence of dusty plasma. The four flybys of Enceladus i ...

Magnetic signatures of plasma-depleted flux tubes in the Saturnian inner magnetosphere

Initial Cassini observations have revealed evidence for interchanging magnetic flux tubes in the inner Saturnian magnetosphere. Some of the reported flux tubes differ remarkably by their magnetic signatures, having a depressed or enhanced magnetic pressure relative to their surroundings. The ones with stronger fields have been interpreted previously as either outward moving mass-loaded or inward moving plasma-depleted flux tubes based on magnetometer observations only. We use detailed multi-instrumental observations of small and large density depletions in the inner Saturnian magnetosphere from Cassini Rev. A orbit that enable us to discriminate amongst the two previous and opposite interpretations. Our analysis undoubtedly confirms the similar nature of both types of reported interchanging magnetic flux tubes, which are plasma-depleted, whatever their magnetic signatures are. Their different magnetic signature is clearly an effect associated with latitude. These Saturnian plasma-depleted flux tubes ultimately may play a similar role as the Jovian ones.

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.

The Plasma Wave Environment of Europa

Abstract The Galileo spacecraft has executed nine close flybys of Jupiters moon Europa for which plasma wave observations were obtained. This paper presents an analysis of the observations from these flybys taking into consideration the variable geometry of the trajectories in an attempt to characterize the general plasma-wave environment associated with the interaction of the Jovian magnetosphere with the moon. A wide variety of plasma-wave phenomena are found to be associated with this interaction. While there are apparently temporal variations which complicate the analysis, a crude model of the distribution of these phenomena around Europa is derived. Primarily on the upstream side of Europa, and working inward to the moon, electron–cyclotron harmonics are first observed, followed by a region within about two Europa radii of the moon with whistler-mode hiss or chorus, and culminating in a region closest to the moon where a band at the upper hybrid resonance frequency is sometimes enhanced over its ambient intensity. The wake region is approximately two Europa radii across and comprises a broadband, highly variable, and bursty electrostatic phenomenon. Upon closer inspection, these bursty emissions appear as solitary structures similar to those in Earth s auroral zone and plasma sheet boundary layer. In addition to the survey of wave phenomena in the vicinity of Europa, we provide density profiles derived primarily from the upper hybrid resonance frequency which is readily apparent throughout most of each of the flybys. Finally, we suggest that the whistler mode, electron cyclotron harmonic, and upper hybrid resonance emissions are driven by some combination of factors including variations in the magnetic field near Europa and the loss and production of plasma at Europa as a result of the interaction of the Jovian magnetosphere with the moon. By analogy with studies of the ion and electron holes and broadband electrostatic noise at Earth and Jupiter, we argue that the electrostatic solitary structures in the wake are associated with currents and beams coupling Europa to Jupiters ionosphere.

Saturn's ring current: Local time dependence and temporal variability

Radial profiles of the azimuthal current density between similar to 3 and 20 R-S in Saturns magnetosphere have been derived using plasma and magnetic field data from 11 near-equatorial Cassini orbits spanning a 10 month interval. The current density generally shows only modest variations with local time and from pass to pass within this region, rising rapidly near similar to 5 R-S to peak at similar to 90 pAm(-2) at similar to 9 R-S and falling more gradually to below similar to 20 pA m(-2) at 20 R-S. The pressure gradient current is overall the most important component, the dominant inertia current in the inner region being significantly canceled by the oppositely directed pressure anisotropy current. These characteristics principally reflect the properties of the warm water plasma originating from the Enceladus torus to distances of similar to 10 R-S encompassing the usual current peak, inside of which distance the plasma properties are generally unvarying within factors of less than similar to 2. Increased variability is present at larger distances where the pressure of the hot magnetospheric plasma plays the more important role. In this region the dominant pressure gradient current is found to be strongest in the dusk to midnight sector and declines modestly, by factors of similar to 2 or less, in the midnight to dawn and dawn to noon sectors. Pass-to-pass temporal variability by factors of similar to 2-3 is also present in the outer region, particularly in the dawn to noon sector, probably reflecting both hot plasma injection events as well as solar wind-induced variations.

Plasma densities in the vicinity of Callisto from Galileo plasma wave observations

The Galileo spacecraft has made seven close flybys of Jupiters moon Callisto. During the closest of these (C22), which approached to within 535 km of the surface, the plasma wave instrument detected a very clear upper hybrid emission as the spacecraft passed near the moon. The peak electron density indicated by the upper hybrid resonance emission was 400 cm−3, almost one-thousand times the electron density in the magnetosphere of Jupiter at the orbit of Callisto. These observations indicate that Callisto is probably surrounded by a dense ionospheric-like plasma.

The polar cap environment of outflowing O(

Ion composition measurements by Dynamics Explorer 1 often show upward O+ beams at polar latitudes, with streaming energies of 1–20 eV or more. Here we utilize measurements of core (0–50 eV) and “energetic” (∼0–1 keV) ion composition, plasma waves, and auroral images from DE 1 and plasma ions and electrons from DE 2 to examine some of their properties in the context of the polar cap environment. It is found that two distinct populations of O+ beams are observed: “high-speed” (10–30 eV or higher streaming energies) and “low-speed” (generally <10-eV streaming energies). The “high-speed” polar beams show an “auroral” connection; i.e., they are observed on or near field lines threading auroral arcs seen in DE 1 images. The “low-speed” streams are on or near field lines threading the dark polar cap and may be convected from the cleft ion fountain. The low-speed streams are generally much more stable in energy and flux, while the high-speed streams tend to be bursty. In general, the streams are convecting antisunward, with velocities of 5–14 km/s in the orbital plane. We sought to obtain plasma density estimates from plasma wave measurements, through analysis of features of auroral hiss as well as upper hybrid emissions. Densities in the range 1–5 el/cm³ were indicated for one segment of a DE 1 polar cap pass; however, the measurements generally indicate little auroral hiss or upper hybrid emissions in the polar cap for the other cases considered here. Estimates of electrostatic potential drops above the DE 2 satellite have been made using energy-angle spectrograms of photoelectron data, under the assumption that the field lines of observation are “effectively” open. Potential drops often are in the 20- to 40-V range. At other times the potential falls below the ∼5-V instrument threshold, or there are insufficient photoelectron fluxes for estimation. These limited data suggest that the largest potential drops are just poleward of the cleft or near its poleward edge and there is a decline of the drop in the antisunward direction. No obvious correlation between the potential estimates and “nearby” O+ streaming energies is seen.

Electron densities near Io from Galileo plasma wave observations

This paper presents an overview of electron densities obtained near Io from the Galileo plasma wave instrument during the first four flybys of Io. These flybys were 10, which was a downstream wake pass that occurred on December 7, 1995; I24, which was an upstream pass that occurred on October 11, 1999; I25, which was a south polar pass that occurred on November 26, 1999; and I27, which was an upstream pass that occurred on February 22, 2000. Two methods were used to measure the electron density. The first was based on the frequency of upper hybrid resonance emissions, and the second was based on the low-frequency cutoff of electromagnetic radiation at the electron plasma frequency. For three of the flybys, 10, I25, and I27, large density enhancements were observed near the closest approach to Io. The peak electron densities ranged from 2.1 to 6.8 × 104 cm−3. These densities are consistent with previous radio occultation measurements of Ios ionosphere. No density enhancement was observed during the I24 flyby, most likely because the spacecraft trajectory passed too far upstream to penetrate Ios ionosphere. During two of the flybys, I25 and I27, abrupt step-like changes were observed at the outer boundaries of the region of enhanced electron density. Comparisons with magnetic field models and energetic particle measurements show that the abrupt density steps occur as the spacecraft penetrated the boundary of the Io flux tube, with the region of high plasma density on the inside of the flux tube. Most likely the enhanced electron density within the Io flux tube is associated with magnetic field lines that are frozen to Io by the high conductivity of Ios atmosphere, thereby enhancing the escape of plasma along the magnetic field lines that pass through Ios ionosphere.