Corotating Plasma Research Articles

On asymmetry of magnetic activity and plasma flow temperature in Jupiter’s magnetosphere

Discs of plasma around giant planets are natural laboratories that contain within mechanisms of transferring and keeping energy into the plasma and magnetic field system. Various missions to Jovian planets observed that expansion of plasmadiscs is not adiabatic and plasma temperature is increasing with radial distance. Magnetometer measurements from Juno mission were examined to determine plausibility of turbulent fluctuations as the plasma heating mechanism. Extensive azimuthal map of magnetic activity in Jupiter’s nightside plasmadisc is presented. Observations show that magnetic activity is distributed asymmetrically, with active and quiet regions. This is similar to the asymmetrical distribution of activity observed in Saturn’s magnetosphere. However, comprehensive study of temperature measurements showed that the only systematic change of temperature in magnetospheres of giant planets is in the radial direction. Observed breakfrequency in the magnetometer time series is systematically greater than the ion cyclotron frequency. Examination of the power spectrum points to that the kinetic energy of the corotating plasma as a source of increase of plasma temperature. This study shows that turbulent fluctuations themselves are not good candidates as a plasma heating mechanism. External pressure fluctuation however, can be used to convert kinetic energy of the plasma flow into thermal.

Radiation from axion star-neutron star binaries with a tilted rotation axis in the presence of plasma

We investigate the form of the radiation emitted by an axion star-neutron star binary using a f(r) = sech(r/R) profile for the axion star. Our analysis takes into account the co-rotating plasma of the neutron star. We find that there is significant enhancement to the radiated power if the neutron star’s spin is tilted towards the plane of the axion star-neutron star orbit, compared to the case where it is perpendicular. We also examine whether the neutron star’s co-rotating plasma can play a role in the emitted power and we find that even though dilute axion stars can in principle radiate more efficiently than dense axion stars, they will be pulled apart by the tidal forces of the neutron star.

Effect of the Magnetospheric Plasma Interaction and Solar Illumination on Ion Sputtering of Europa’s Surface Ice

AbstractFor the entire ion energy range observed at Europa, we calculate spatially resolved maps of the surface sputtering rates of H2O, O2, and H2 from impacts by magnetospheric ions. We use the perturbed electromagnetic fields from a hybrid model of Europa’s plasma interaction, along with a particle‐tracing tool, to calculate the trajectories of magnetospheric ions impinging onto the surface and their resultant sputtering yields. We examine how the distribution of the sputtering rates depends on the electromagnetic field perturbations, the angle between the solar radiation and the corotating plasma flow, and the thickness of the oxygen‐bearing layer within Europa’s surface. Our major findings are: (a) Magnetic field‐line draping partially diverts the impinging ions around Europa, reducing the sputtering rates on the upstream hemisphere, but allowing for substantial sputtering from the downstream hemisphere. In contrast, zero sputtering occurs in much of the downstream hemisphere with uniform electromagnetic fields. (b) If the oxygen‐bearing surface layer is thin compared to the penetration depth of magnetospheric ions, thermal ions dominate the O2 sputtering rates, and the region of strongest sputtering is persistently located near the upstream apex. However, if the oxygen‐bearing layer is thick compared to the penetration depth, energetic ions sputter the most O2, and the location of maximum sputtering follows the sub‐solar point as Europa orbits Jupiter. (c) The global production rate of O2 from Europa’s surface varies by a factor of 3 depending upon the Moon’s orbital position, with the maximum particle release occurring when Europa’s Sun‐lit and upstream hemispheres coincide.

The magnetic early B-type stars – III. A main-sequence magnetic, rotational, and magnetospheric biography

ABSTRACT Magnetic confinement of stellar winds leads to the formation of magnetospheres, which can be sculpted into centrifugal magnetospheres (CMs) by rotational support of the corotating plasma. The conditions required for the CMs of magnetic early B-type stars to yield detectable emission in H α – the principal diagnostic of these structures – are poorly constrained. A key reason is that no detailed study of the magnetic and rotational evolution of this population has yet been performed. Using newly determined rotational periods, modern magnetic measurements, and atmospheric parameters determined via spectroscopic modelling, we have derived fundamental parameters, dipolar oblique rotator models, and magnetospheric parameters for 56 early B-type stars. Comparison to magnetic A- and O-type stars shows that the range of surface magnetic field strength is essentially constant with stellar mass, but that the unsigned surface magnetic flux increases with mass. Both the surface magnetic dipole strength and the total magnetic flux decrease with stellar age, with the rate of flux decay apparently increasing with stellar mass. We find tentative evidence that multipolar magnetic fields may decay more rapidly than dipoles. Rotational periods increase with stellar age, as expected for a magnetic braking scenario. Without exception, all stars with H α emission originating in a CM are (1) rapid rotators, (2) strongly magnetic, and (3) young, with the latter property consistent with the observation that magnetic fields and rotation both decrease over time.

Energetic electron dynamics near Callisto

We examine the dynamics of energetic magnetospheric electrons exposed to the highly perturbed and asymmetric plasma environment of Jupiter’s moon Callisto. The interaction of the (nearly) corotating magnetospheric plasma with Callisto’s ionosphere and induced dipole locally generates intense electromagnetic pileup and draping signatures which vary as a function of the moon’s distance to the center of Jupiter’s magnetospheric current sheet. In our study, these field perturbations are represented using output from the AIKEF hybrid (kinetic ions, fluid electrons) model of Callisto’s interaction with the corotating plasma. In order to constrain the influence of Callisto’s variable electromagnetic environment on the dynamics of energetic electrons, we trace the trajectories of more than 6.7 million test particles as they travel through a distinct configuration of the locally perturbed fields. We present spatially resolved maps that display the accessibility of Callisto to electrons at energies E between 101 keV ≤E≤105 keV for multiple sets of these perturbed fields, corresponding to select distances of the moon to the center of the Jovian current sheet. The electromagnetic field perturbations near Callisto play a crucial role in generating inhomogeneous precipitation of energetic electrons onto the top of the moon’s atmosphere. In particular, Callisto’s Jupiter-facing and Jupiter-averted hemispheres are partially protected from energetic electron precipitation: when located far above or below the center of Jupiter’s current sheet, Callisto’s induced dipole shields the apices of these hemispheres, whereas near the center of the sheet, strong field line draping protects these regions. In contrast to this, the apex of Callisto’s trailing hemisphere is exposed to intense energetic electron precipitation at any distance to the center of the Jovian current sheet. We also present maps of energetic electron accessibility during the Galileo C3, C9, and C10 flybys, and calculate the intensity of energetic electron flux onto multiple locations at the top of Callisto’s atmosphere. These non-uniform electron fluxes likely cause inhomogeneous ionization of Callisto’s atmosphere and may even contribute to irregular erosion of the surface. This paper is a companion to Liuzzo et al. (2019), who studied the dynamics of energetic ions near Callisto.

Evidence for Auroral Emissions From Callisto's Footprint in HST UV Images

AbstractAuroral emissions are expected from the footprint of Callisto in Jupiter's upper atmosphere owing to the known interaction of its atmosphere with Jupiter's magnetosphere, and from the observed auroral emissions from the footprints of the other three Galilean satellites. The mapping of Callisto along modeled magnetic field lines at Jupiter, however, places the expected footprint at the same latitude as the main auroral emissions, making it difficult to detect. We analyzed ultraviolet images of Jupiter taken using the Hubble Space Telescope/Advanced Camera for Surveys instrument during a large observing campaign in 2007. Using a coaddition method similar to one used for Enceladus, we have identified a strong candidate for the footprint of Callisto on 24 May 2007. We tested this finding by applying the same coaddition technique to a nearly identical auroral configuration on 30 May 2007 when Callisto was behind Jupiter, not visible from Earth (central meridian longitude = 22°; sub‐Callisto system III longitude = 327°). By comparing the two coadded images, we can clearly see the presence of a strongly subcorotating spot close to the expected Callisto footprint location on 24 May and its absence on 30 May. On 24 May Callisto was located in the current sheet. We also found a probable candidate on 26 May 2007 during which time Callisto was positioned below the current sheet. The measured location and intensity of the auroral emission provide important information about the interaction of Callisto with Jupiter's magnetic field, the corotating plasma, and the neutral and ionized state of the thin atmosphere of Callisto.

Corotation Plasma Environment Model: An Empirical Probability Model of the Jovian Magnetosphere

We developed a new empirical model for corotating plasma in the Jovian magnetosphere. The model, named the corotation plasma environment model version 2 (CPEMv2), considers the charge density, velocity vector, and ion temperature based on Galileo/plasma system (PLS) ion data. In addition, we develop hot electron temperature and density models based on Galileo/PLS electron data. All of the models provide respective quantities in the magnetic equator plane of 9–30 $\mathrm {R}_{J}$ , while the charge density model can be extended to 3-D space. A characteristic feature of the CPEM is its support of the percentile as a user input. This feature enables us to model extreme conditions in addition to normal states. In this paper, we review the foundations of the new empirical model, present a general derivation algorithm, and offer a detailed formulation of each parameter of the CPEMv2. As all CPEM parameters are of the analytical form, their implementation is straightforward, and execution involves the use of a small number of computational resources. The CPEM is flexible; for example, it can be extended, as new data (from observations or simulation results) become available. The CPEM can be used for the mission operation of the European Space Agency’s mission to Jupiter, JUpiter ICy moons Explorer (JUICE), and for future data analyses.

THE INCLINATION ANGLE AND EVOLUTION OF THE BRAKING INDEX OF PULSARS WITH PLASMA-FILLED MAGNETOSPHERE: APPLICATION TO THE HIGH BRAKING INDEX OF PSR J1640–4631

ABSTRACT The recently discovered rotationally powered pulsar PSR J1640–4631 is the first to have a braking index measured, with high enough precision, that is greater than 3. An inclined magnetic rotator in vacuum or plasma would be subject not only to spin-down but also to an alignment torque. The vacuum model can address the braking index only for an almost orthogonal rotator, which is incompatible with the single-peaked pulse profile. The magnetic dipole model with the corotating plasma predicts braking indices between 3 and 3.25. We find that the braking index of 3.15 is consistent with two different inclination angles, 18.°5 ± 3° and 56° ± 4°. The smaller angle is preferred given that the pulse profile has a single peak and the radio output of the source is weak. We infer the change in the inclination angle to be at the rate −0.°23 per century, three times smaller in absolute value than the rate recently observed for the Crab pulsar.

Emission of energetic neutral atoms from water ice under Ganymede surface-like conditions

The co-rotating plasma around Jupiter precipitates on the surfaces of the jovian moons, where it is not hindered by a local magnetic field. Precipitating ions lead to the emission of energetic neutral atoms, which are produced via backscattering and sputtering processes, from the surface. The European Space Agency’s JUICE mission to Jupiter carries as part of the Particle Environment Package experiment an imaging energetic neutral atom spectrometer called the jovian Neutrals Analyzer (JNA). When it is in orbit around Ganymede, JNA will measure the energetic neutral atom flux emitted from the surface of Ganymede in the energy range from 10eV to 3300eV. The surface of Ganymede consists of a large fraction of water ice. To characterize the expected energetic neutral atom fluxes from water ice due to precipitating jovian plasma, we impacted protons and singly charged oxygen ions with energies up to 33keV on a salty water ice target kept at Ganymede surface conditions. Emitted energetic atoms were measured energy- and mass-resolved using the JNA prototype instrument. The data show high yields for energetic neutral atoms per incident ion in the JNA energy range. For incident protons, energetic neutral atom yields between 0.28 at 1keV and ∼40 at 33keV were observed. For incident singly charged oxygen ions, the observed energetic neutral atom yield ranged from 0.8 for at 3keV to ∼170 at 23keV.

Non-corotating models for pulsar magnetospheres

We reconsider pulsar electrodynamics for an obliquely rotating pulsar, and propose a way of synthesizing the vacuum dipole model (VDM) and the rotating magnetosphere model (RMM). We first modify the VDM by assuming that the parallel component of the inductive electric field is screened by charges. We refer to the resulting model as the minimal model. We calculate the screening charge density in the minimal model and compare it with the (Goldreich-Julian) charge density in the RMM. We identify the plasma velocity in the minimal model as the electric drift velocity due to the perpendicular component of the inductive electric field. We define a class of synthesized models as a linear combination of a fraction $y$ times the minimal model and $1-y$ times the RMM. These models require a gap (with $E_\parallel\ne0$) between the corotating stellar surface and the non-corotating magnetosphere. We present illustrative plots of the charge density, of the location of nulls (where the charge density is zero) and of the three components of the plasma velocity as a function of the angles ($\theta,\psi$) relative to the rotation axis, for specific values of the obliquity $\alpha$ and the parameter $y$. We discuss the question "Can any pulsar magnetosphere be corotating?" critically, pointing out difficulties associated with setting up corotation in the polar cap region. We speculate that the corotating plasma may flow across the last closed field line from the closed-field region. We suggest that abrupt changes in the spin-down rate in some pulsars may be due to jumps between the RMM and the minimal model.

Evidence from radial velocity measurements of a global electric field in Saturn's inner magnetosphere

The presence of a convection pattern in the Saturnian inner magnetosphere was first observed in moon signatures that were displaced from their expected locations. A noon to midnight directed electric field can account for these positional offsets, and also can explain asymmetries found in plasma temperatures between the day and night sides. By assuming magnetic field models and drift times, estimates of the electric field strength have been made. However the origin of the electric field remains a mystery. This paper, which presents new evidence for the convection pattern, uses measured in situ velocities with measured magnetic field values, at a range of radial distances without the restriction of requiring the presence of a moon. Advances in extracting plasma parameters with reduced uncertainties clearly show the convection pattern that was previously hidden when uncertainties were large. We confirm the earlier findings that there is a distinct drift velocity superposed on the corotating plasma that pushes the plasma toward ∼dawn that dominates over the general plasma outflow. We find that an electric field directed toward 01–02 h Saturnian local time best describes this drift, and that the value of the electric field strength is not constant with radial distance.

Flow stagnation at Enceladus: The effects of neutral gas and charged dust

Enceladus is one of Saturn's most active moons. It ejects neutral gas and dust particles from its southern plumes with velocities of hundreds of meters per second. The interaction between the ejected material and the corotating plasma in Saturn's magnetosphere leads to flow deceleration in ways that remain to be understood. The most effective mechanism for the interaction between the corotating plasma and the neutral gas is charge exchange which replaces the hotter corotating ions with nearly stationary cold ions that are subsequently accelerated by the motional electric field. Dust particles in the plume can become electrically charged through electron absorption and couple to the plasma through the motional electric field. The objective of this study is to determine the level of flow deceleration associated with each of these processes using Cassini RPWS dust impact rates, Cassini Plasma Spectrometer (CAPS) plasma data, and 3‐D electromagnetic hybrid (kinetic ions, fluid electrons) simulations. Hybrid simulations show that the degree of flow deceleration by charged dust varies considerably with the spatial distribution of dust particles. Based on the RPWS observations of dust impacts during the E7 Cassini flyby of Enceladus, we have constructed a dust model consisting of multiple plumes. Using this model in the hybrid simulation shows that when the dust density is high enough for complete absorption of electrons at the point of maximum dust density, the corotating flow is decelerated by only a few km/s. This is not sufficient to account for the CAPS observation of flow stagnation in the interaction region. On the other hand, charge exchange with neutral gas plumes similar to the modeled dust plumes but with base (plume opening) densities of ∼109 cm−3 result in flow deceleration similar to that observed by CAPS. The results indicate that charge exchange with neutral gas is the dominant mechanism for flow deceleration at Enceladus.

Influence of negatively charged plume grains and hemisphere coupling currents on the structure of Enceladus' Alfvén wings: Analytical modeling of Cassini magnetometer observations

[1] We present an analytical model of the Alfven wing system that is generated by the interaction between the plume of Enceladus and the corotating plasma in Saturn's inner magnetosphere. Our primary purpose is to explain the orientation of the magnetic field perturbations detected in Enceladus' Alfven wings by the Cassini magnetometer (MAG) instrument. Observational data from numerous close Enceladus flybys show both the Bx and By components (in Enceladus interaction coordinates: Bx, along corotation direction; By, toward or away from Saturn) in the center of the northern wing tube to possess a negative sign, whereas the opposite case of Bx and By being positive was observed within the southern wing. So far, none of the available models of Enceladus' magnetospheric interaction is able to reproduce this correlation between the directions of Bx and By. On the basis of the analytical calculations of Neubauer (1980, 1998) and Saur et al. (1999, 2007), we demonstrate that the observed orientation of the magnetic field may arise from the presence of negatively charged dust grains in the plume of Enceladus, serving as a sink for “free” magnetospheric electrons. Although the current carried by these particles does not make a noteworthy contribution to the magnetic field distortions in the interaction region, the negative charge accumulated by them needs to be accounted for in the quasi-neutrality condition of the plasma. The depletion of magnetospheric electrons within the plume is therefore far from causing only some localized perturbations of the magnetic field, but it drastically alters the nature of the interaction: we show that this process yields a reversal in the sign of the Hall conductivity, thereby giving rise to the observed field signatures. By applying a modified version of the Alfven wing model developed by Saur et al. (2007), we demonstrate that the magnetic field observations from Cassini's targeted Enceladus flybys can be understood by taking into account the influence of electron-absorbing dust grains. In contrast to what is claimed in recent literature, we therefore propose that magnetic field observations near Enceladus can be completely understood in terms of a local interaction model, i.e., that it is not necessary to consider the large-scale dynamics of the flux tubes in Saturn's magnetosphere. In addition, we provide first in situ evidence that the hemisphere coupling current system predicted by Saur et al. (2007) and the associated magnetic field discontinuities are indeed present at Enceladus. The field perturbations caused by these hemisphere coupling currents arise from the partial blockage of the Alfven wing at the nonconducting icy crust of Enceladus. This effect needs to be taken into account when interpreting Cassini MAG data from flybys that intersected the Enceladus flux tube and can only be reproduced by models that apply adequate boundary conditions to the surface of the icy moon.

Hybrid simulations of the plasma environment around Enceladus

The plasma environment around Enceladus is governed by the interaction between the corotating plasma with the body of the moon and the neutral gas associated with Saturn's extended cloud and plumes ejected from its southern polar region. To understand the nature of this interaction, we use 3‐D electromagnetic hybrid simulations that treat ions kinetically through particle‐in‐cell methods and treat the electrons as a charge‐neutralizing fluid. In these simulations, plasma interaction with the neutrals takes place through charge exchange. The results show that plasma absorption by Enceladus forms a tail‐like density cavity behind the moon and a depletion wake which is confined in the direction perpendicular to the magnetic field but extends many Enceladus radii along the magnetic field. Except in the cavity tail, the flow is not slowed by the interaction but is diverted toward the cavity in a symmetric fashion. Interaction of the corotating plasma with the extended neutrals results in the generation of ion cyclotron waves and plasma deceleration to velocities below the corotation speed. The results also show that the extent to which the presence of a plume in the southern pole affects the nature of the interaction depends on its density. For plumes with base densities of ∼106/cm3 (6.5 × 1029 molecules in the plume) or lower we find no significant impacts, while plumes with base densities of ∼107/cm3 and larger are found to greatly impact the interaction region. Specifically, the interaction is no longer symmetric with a density cavity tail in the Northern Hemisphere and a density enhancement tail in the Southern Hemisphere. At base densities of ∼108/cm3, a strong Alfven wing is also generated in the interaction. Preliminary comparisons of the simulations with the Cassini Plasma Spectrometer data during the 12 March 2008 encounter with Enceladus suggest plume base densities of ∼5 × 108/cm3 corresponding to production of ∼4 kg/s of new ions.

Interaction of Saturn's magnetosphere and its moons: 1. Interaction between corotating plasma and standard obstacles

The interaction of Saturn's inner magnetosphere with its moons ranges from the addition of significant quantities of gas, dust, and plasma, causing significant consequences for the dynamics and energetics of the entire Saturnian magnetosphere, to the simple absorption of plasma and energetic particles by the icy moons with non‐electrically conducting interiors. The interaction with these moons is complex with the contribution of many physical processes, depending on the geometry of any plume, the structure of the atmosphere, and its interaction with the surface and interior of the moon, the latter by induced fields. Our ultimate goal is to understand the complexities of this interaction and its temporal variations, especially at Enceladus. In this paper we use magnetohydrodynamics (MHD) code for addressing the flow around obstacles that are simpler than the Enceladus interaction. These simulations both help us understand the interaction with other icy moons and prepare us for the simulation of the flow around Enceladus. The processes involved include ordinary collisions, impact ionization, photoionization, and charge exchange. We examine a series of simple canonical interactions before we later apply our simulation where the multiple processes are occurring simultaneously with asymmetric geometries. We apply our 3‐D MHD model to simulate the interaction between the Saturnian corotational plasma flow for the following cases: an absorbing body having an insulating surface; ion pickup via photo and impact ionization from a spherically symmetric neutral cloud; charge exchange with such a neutral cloud; and ion pickup at an insulating, absorbing body with an atmosphere acted upon by the sum of the three ionization processes. In addition to validating the model and obtaining a deeper understanding of the consequences of each interaction, we can immediately make some conclusions about the Enceladus interaction. We find that the magnetometer data are most consistent with the surface of Enceladus being absorbing and insulating, rather than the surface being reflecting and electrically conducting. For the conditions in the corotating flow at Enceladus, the perturbation to the plasma flow produced by photo/impact ionization is an order of magnitude smaller than that produced by charge exchange. Moreover, the perturbation to the magnetic field Bz component by a spherically symmetric mass loading source alone is an order of magnitude smaller than that observed in the neighborhood of the plume. Thus, the perturbation observed in the magnetometer data is primarily due to the mass loading in the plume, which is primarily ion‐neutral charge exchange. The geometry and source strength of the plume are investigated in a following paper.

Modeling the Enceladus plume–plasma interaction

We investigate the chemical interaction between Saturn's corotating plasma and Enceladus' volcanic plumes. We evolve a parcel of ambient plasma as it passes through a prescribed H2O plume using a physical chemistry model adapted for water‐group reactions. The flow field is assumed to be that of a plasma around an electrically‐conducting obstacle centered on Enceladus and aligned with Saturn's magnetic field, consistent with Cassini magnetometer data. We explore the effects on the physical chemistry due to: (1) a small population of hot electrons, (2) a plasma flow decelerated in response to the pickup of fresh ions, and (3) the source rate of neutral H2O. The model confirms that charge exchange dominates the local chemistry and that H3O+ dominates the water‐group composition downstream of the Enceladus plumes. We also find that the amount of fresh pickup ions depends heavily on both the neutral source strength and on the presence of a persistent population of hot electrons.

Titan's induced magnetosphere under non-ideal upstream conditions: 3D multi-species hybrid simulations

A 3D, multi-species hybrid model (kinetic ions, fluid electrons) has been applied to the interaction between Saturn's largest satellite Titan and the plasma in the giant planet's outer magnetosphere. In contrast to the idealized picture deduced from Voyager 1 data, recent observations made by the Cassini magnetometer instrument suggest that the ambient magnetic field is not directed perpendicular to Titan's orbital plane. Therefore, our purpose is to investigate systematically how Titan's induced magnetosphere is affected by a tilt of the upstream magnetic field. In the first part of our study, the structure of Titan's induced magnetosphere is analyzed as a function of the angle between the ambient magnetic field B ̲ 0 and the bulk velocity u ̲ 0 of the corotating plasma flow. Our simulations show that introducing a flow-aligned magnetic field component goes along with an asymmetrization of Titan's magnetotail, in addition to the asymmetry that already arises from the large gyroradii of the ion species involved in the interaction. In the vicinity of Titan, the field lines become strongly twisted, permitting the wakeside magnetic lobe structure to even penetrate into the satellite's geometric plasma shadow. However, despite the increased complexity of Titan's magnetic environment, the overall characteristics of the pick-up tail remain practically the same as in the case of “ideal” magnetic field orientation ( B ̲ 0 ⊥ u ̲ 0 ) . In the second part of our study, we investigate in real-time the transition that Titan's plasma interaction undergoes during a change of the ambient magnetic field direction. In contrast to earlier analyses of Titan's plasma environment under non-stationary upstream conditions, the tilt of the ambient magnetic field is again taken into account. While in the case of B ̲ 0 being perpendicular to u ̲ 0 , the reconfiguration of Titan's induced magnetosphere is mainly governed by reconnection, our simulations suggest that when a flow-aligned field component is included, convection of the field lines around the obstacle's ionosphere plays the key role for the reconfiguration process.

Plasma wake of Tethys: Hybrid simulations versus Cassini MAG data

The interaction between Saturn's fifth largest satellite, Tethys, and the corotating plasma of the inner magnetosphere has been studied with a three‐dimensional hybrid simulation. Since Tethys possesses neither an intrinsic magnetic field nor a substantial ionosphere, the moon's surface is directly exposed to the impinging plasma. This leads to the formation of an extended density cavity in the downstream region, expanding above and below Tethys along the magnetic field lines. The resulting deficit of plasma pressure is compensated by a compression of the field lines at the wakeside. By confronting our simulation results with Cassini magnetometer data from the so far only targeted flyby in 2005, we demonstrate that these key features of Tethys' plasma interaction are quantitatively reproducible within the framework of the hybrid model. Besides, the simulation results illustrate the importance of ambipolar electric fields for ion dynamics in the wake of Tethys.

Three‐dimensional reconstructions of the early November 2004 Coordinated Data Analysis Workshop geomagnetic storms: Analyses of STELab IPS speed and SMEI density data

Combined interplanetary scintillation (IPS) and Solar Mass Ejection Imager (SMEI) remote‐sensing observations provide a view of the solar wind at almost all heliographic latitudes and covering distances from the Sun between 0.1 AU and 3.0 AU. They are used to study the development of the solar wind and coronal transients as they move out into interplanetary space, and also the inner heliospheric response to the passage of corotating solar structures and coronal mass ejections (CMEs). The observations take place in both radio scintillation level and speed for IPS, and in Thomson‐scattered white light brightness for SMEI. With colleagues at the Solar Terrestrial Environment Laboratory (STELab), Nagoya University, Japan, we have developed a data analysis system for the STELab IPS data which can also be applied to SMEI white light data. This employs a three‐dimensional (3‐D) reconstruction technique that obtains perspective views from solar corotating plasma and outward flowing solar wind as observed from the Earth by iterative fitting of a kinematic solar wind model to the data. This 3‐D modeling technique permits reconstructions of the density and speed of CMEs and other interplanetary transients at relatively coarse spatial and temporal resolutions. For the time‐dependent model (used here), these typically range from 5° to 20° in latitude and longitude, with a 1/2 to 1 day time cadence. For events during early November 2004 we compare these reconstructed structures with in situ measurements from the ACE and Wind (near‐Earth) spacecraft to validate the 3‐D tomographic reconstruction results and provide input to the ENLIL 3‐D magnetohydrodynamic (MHD) numerical model.

Ion-cyclotron wave generation by planetary ion pickup

Ion-cyclotron waves play important roles in planetary magnetospheres and are diagnostic of the processes operating in the magnetosphere and of the composition of the plasma producing the waves. At Jupiter, Io's exosphere interacts with the corotating magnetospheric plasma. At Saturn, the neutral torus around the E ring interacts with the corotating plasma. At the unmagnetized planets, Mars and Venus, the interaction is between the solar-wind flow and the planetary exosphere. A possible analog of these processes exists in the vicinity of the Earth's polar cusp where the shocked solar-wind plasma penetrates the Earth's exosphere.