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We present new results concerning electron temperature ( T e ) mapping in the inner magnetosphere of Saturn ( S ). The data are obtained by the Langmuir Probe (LP) instrument of the Radio and Plasma Wave Science (RPWS) investigation on board Cassini. The study is based on the fourteen first orbits around Saturn, and focus is on cold ( 5 cm −3 ) electron populations in the inner plasma torus (plasma disc). Apart from a general increase with distance from Saturn and away from the equatorial plane (latitude), we identify a possibly anisotropic electron temperature in the plasma disc, as well as the presence of photoelectrons from the spacecraft or E-ring dust particles. A polytropic index of γ ≈+0.4 is determined from the observations, which can be used in theoretical models of the plasma disc. We also infer local variations in T e near the equatorial plane, where the E-ring of dusty plasma and enhancements of water rich neutral gas exist, and determine constraints on the energy source required to heat the electrons in the plasma disc. We suggest that Joule heating or heating by low-frequency plasma waves as well as cooling by neutral gas and E-ring dust are involved in the energy balance, together with Coulomb collision heating by co-rotating ions.

We evaluate two prevailing substorm models with an event of plasma flow reversal from tailward to Earthward detected by Cluster at the downstream distance of ∼19 R E in the magnetotail during a substorm on August 22, 2001. We use the unique capability of Cluster measurements in determining gradients to examine the associated current density, Lorentz force, and current dissipation/dynamo term. In association with plasma flow reversal, it is found that (1) there was no clear quadrupole magnetic perturbation signature, (2) the x -component of the Lorentz force did not change sign, (3) the y -component of the product of the current density and the electric field was occasionally negative indicative of a dynamo effect, and (4) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source. These observations are consistent with the near-Earth initiation model for substorms with multiple current disruption sites moving progressively tailward near the late stage of substorm expansion.

The Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP) sensor observed the cold plasma environment around Titan during the first two flybys. The data show that conditions in Saturn's magnetosphere affect the structure and dynamics deep in the ionosphere of Titan. The maximum measured ionospheric electron number density reached 3800 per cubic centimeter near closest approach, and a complex chemistry was indicated. The electron temperature profiles are consistent with electron heat conduction from the hotter Titan wake. The ionospheric escape flux was estimated to be 10 25 ions per second.

It is of considerable interest to compile a model of the low frequency electromagnetic wave intensity across the polar caps, in and around the auroral zones, as well as at lower latitudes. Waves are playing a dynamic role in the auroral region and can be used to characterize the level of activity in the magnetosphere. At middle latitudes, Very Low Frequency (VLF) and Extremely Low Frequency (ELF) waves are known to scatter trapped energetic electrons, alter their angular and energy distributions, and eventually cause their precipitation into the Earth's atmosphere where it affects the propagation of radio signals. We report here on the development of a data-based model of the electromagnetic power spectral density in the VLF band (10–46 kHz) as surveyed onboard the Swedish Viking spacecraft in the high-latitude region in the northern hemisphere. The data have been sorted into bins according to spatial location and wave frequency. A preliminary statistical model is presented showing the mean electric power spectral density versus magnetic local time and versus invariant latitude at fixed height intervals and for fixed frequency bands within the VLF range. Our data base includes 1162 data intervals, each of 15 min length, during March–December 1986. As a future goal, an empirical model fitting these averages with simple analytical functions may also be developed. This work also aims to associate observed plasma wave characteristics in a particular region of the magnetosphere with geomagnetic conditions. Progress in these directions is reported.

Multi-spacecraft tracing of the high latitude magnetopause (MP) and boundary layers and Interball-1 statistics indicate that: 1. (a) The turbulent boundary layer (TBL) is a persistent feature in the region of the cusp and ‘sash’, a noticeable part of the disturbances weakly depends on the interplanetary magnetic field By component; TBL is a major site for magnetosheath (MSH) plasma penetration inside the magnetosphere through percolation and local reconnection. 2. (b) The TBL disturbances are mainly inherent with the characteristic kinked double-slope spectra and, most probably, 3-wave cascading. The bi-spectral phase coupling indicates self-organization of the TBL as the entire region with features of the non-equilibrium multi-scale and multi-phase system in the near-critical state. 3. (c) We've found the different outer cusp topologies in summer/winter periods: the summer cusp throat is open for the decelerated MSH flows, the winter one is closed by the distant MP with a large-scale (∼several Re) diamagnetic ‘plasma ball’ inside the MP; the ‘ball’ is filled from MSH through patchy merging rather than large-scale reconnection. 4. (d) A mechanism for the energy release and mass inflow is the local TBL reconnection, which operates at the larger scales for the average anti-parallel fields and at the smaller scales for the nonlinear fluctuating fields; the latter is operative throughout the TBL. The remote from TBL anti-parallel reconnection seems to happen independently.

We study the high latitude turbulent boundary layer (TBL), where the essential magnetosheath (MSH) ion heating (in 1.5–3 times) is seen in more than 80 % cases within ‘diamagnetic bubbles’ -structures with highly reduced magnetic field. The high- energy tails of the core particle distributions in TBL exceed 20keV for ions and 10keV for electrons on April 21, 1996. We find indications also for further acceleration of a small fraction of protons there up to 300 keV. More energetic particles could have maximum intensity just inside magnetopause that is consistent with their magnetospheric origin. Simultaneous Polar and Interball-1 data in TBL and MSH on May 29, 1996 demonstrate heating of the MSH core ions in cusp, TBL and stagnant MSH. In the TBL the O + ions might be heated as well (up to few keV) and even be backscattered towards the ionosphere together with the protons > 18 keV . The dominant incompressible Alfvenic waves in TBL constitute an intermediate chain for the transformation of the solar wind kinetic energy into the auroral particle acceleration/heating. The most intensive energy transformation occurs over cusps and along the boundary between the mantle and low latitude boundary layer down to X = − 6 R E . It provides primary heating for MSH ions in both cusp proper and boundary cusp.

We present a detailed analysis of magnetic bubbles observed by the Polar satellite in the high-latitude magnetopause boundary. The bubbles which represent holes or strong depressions (up to 98%) of the ambient magnetic field are filled with heated solar wind plasma elements and observed in the vicinity of strong magnetopause currents and possibly near the reconnection site. We analyze the wave modes in the frequency range 0-30 Hz at the magnetopause (bubble) layer and conclude that the broadband waves in this frequency range represent most likely spatial turbulence of kinetic Alfven waves (KAW), Doppler-shifted to higher frequencies (in the satellite frame) by convective plasma flows. We present also results of a numerical simulation which indicate that the bubbles are produced by a tearing mode reconnection process and the KAW fluctuations are related to the Hall instability created by macroscopic pressure and magnetic field gradients. The observed spatial spectrum of KAW extends from several ion gyroradii (∼ 500 km) down to the electron inertial length (∼ 5 km).

Multiresolution wavelet analysis of magnetic field, electric field, and plasma density records taken on Freja during strong auroral events shows evidence for cavity Alfven resonators in the topside ...

[1] The Cassini Langmuir probe, part of the Radio and Plasma Wave Science (RPWS) instrument, has provided a wealth of information about the cold and dense plasma in the Saturnian system. The analysis of the ion side current (current for negative potentials) measured by the probe from 2005 to 2008 reveals also a strong sensitivity to energetic electrons (250–450 eV). These electrons impact the surface of the probe, and generate a detectable current of secondary electrons. A broad secondary electrons current region is inferred from the observations in the dipole L Shell range of ∼6–10, with a peak full width at half maximum (FWHM) at L = 6.4–9.4 (near the Dione and Rhea magnetic dipole LShell values). This magnetospheric flux tube region, which displays a large day/night asymmetry, is related to the similar structure in the energetic electron fluxes as the one measured by the onboard Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer (CAPS). It corresponds spatially to both the outer electron radiation belt observed by the Magnetosphere Imaging Instrument (MIMI) at high energies and to the low-energy peak which has been observed since the Voyager era. Finally, a case study suggests that the mapping of the current measured by the Langmuir probe for negative potentials can allow to identify the plasmapause-like boundary recently identified at Saturn, and thus potentially identify the separation between the closed and open magnetic field lines regions.

We present an analysis of the wave environment observed inside an inverted-V electron event at an altitude of 1700 km, occurring in the upward directed current with intensity of a few µAm−2. During the analyzed event we observe that precipitating auroral electrons (∼ 2 keV) generate intense (40 mV/m) auroral hiss with peak power concentrated at the lower hybrid frequency flh ≈ fpi, and weak Langmuir waves at fpe. The low frequency turbulence observed within the event is found to be composed of a mixture of Alfven waves, O+ cyclotron waves, multi-ion resonances, and ion sound waves. The LH waves appear to interact modulationally with O+ and ion sound waves. From the observed values of flh and multi-ion resonance frequencies it is possible to estimate three-ion composition of plasma with representative values of [23, 19, 58] percent for normalized number densities of [H+, He+, O+], respectively.

[1] We investigate in detail the breakdown of the frozen-in condition detected by Cluster at the downstream distance of similar to 19 R-E in the midnight sector of the magnetotail during a substorm expansion on 22 August 2001. It is found that the breakdown occurred ( 1) in a low-density environment with moderate to large proton plasma flow and significant fluctuations in electric and magnetic fields, ( 2) in regions with predominantly dissipation but occasionally dynamo effect, and ( 3) at times simultaneously at two Cluster satellites separated by more than 1000 km in both X- and Z-directions. Evaluation of the terms in the generalized Ohm's law indicates that the anomalous resistivity contribution arising from field fluctuations during this event is the most significant, followed by the Hall, electron viscosity, and inertial contributions in descending order of importance. This result demonstrates for the first time from observations that anomalous resistivity from field fluctuations ( implying kinetic instabilities) can play a substantial role in the breakdown of the frozen-in condition in the magnetotail during substorm expansions. Consideration of several observed features in the breakdown regions indicates that the breakdown occurs in a turbulent site resembling observed features found in current disruption and dipolarization sites.

International audience; We present new results from the inner magnetosphere of Saturn obtained by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini around the period of the Saturn orbit injection (July 1, 2004). Plasma wave electric field emissions, voltage sweeps by the Langmuir probe (LP) and radio sounder data were used to infer the cold plasma (

Multi-spacecraft data from Cluster allow for a more detailed magnetopause and boundary layer structure determination than ever before. Reconstruction methods, in which the time variability observed during a pass is interpreted as being due to boundary motion and/or spatial structure, are particularly well suited for this task. Such methods rely on the availability of plasma and field data and adopt the tangential discontinuity hypothesis to determine the motion, acceleration, boundary structure, boundary curvature and surface wave speed over an extended time interval. In this paper one- and two-dimensional reconstruction methods are applied to multi-spacecraft data for the first time.

International audience; We present the results of a study based on several events of broadband ULF/ELF emissions observed in the auroral region by the CLUSTER multi-spacecraft at distances around 4-5 RE. These emissions, observed below the ion plasma frequency, are similar to the broadband emissions observed at lower altitudes (800-4000 km) by rockets (e.g. AMICIST) and satellites (e.g. FREJA and FAST). As successive passages of the four CLUSTER satellites through nearly the same regions show, the intensity of the emissions depend on the thermal properties of the plasma and gradients thereof. The total Poynting flux is downward and is comparable to energy fluxes observed at lower altitudes. We believe the broadband emissions are the result of dispersed Alfvén waves (DAW), which propagates down the magnetic field lines, and emits higher frequency ion plasma wave modes.

Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (waves and in-stabilities)

The Viking wave electric field and density fluctuation measurements together with simultaneous particle observations are used to study waves at frequencies below the local proton gyrofrequency. Such waves were observed during about 20% of nightside auroral field line crossings by Viking at altitudes between 2,000 and 10,000 km. The observations are different from earlier spacecraft observations of similar waves in such a way that the center frequency in about one out of four of the observed events was below the gyrofrequency of singly charged helium, which has not been reported previously. The waves were well correlated with precipitating electrons of energies of a few keV and with VLF auroral hiss. Detailed investigations of simultaneously observed wave emissions, particles, and total densities strongly suggest that secondary peaks at keV energies in the distributions of downgoing electrons can cause the emissions.

Simultaneous measurements of electric fields and upward moving ionospheric ions from the Viking satellite show that upward escape of ionospheric ions in the tens of eV to tens of keV energy range occurs together with low-frequency electric fluctuations, LEFs (frequencies less than ≈1 Hz). Upward flowing ions are observed when the electric field fluctuations are sufficiently strong in the altitude range ≈ 2000–13,500 km covered by Viking. Furthermore, the temperature of the upflowing ions is found to be well correlated with the power spectral density (PSD) of the LEFs. This suggests a conversion of electric field energy into ion thermal energy. A good correlation between the ion temperature and the wave power in the electrostatic ion cyclotron (EIC) frequency range is also found. However, the heating/acceleration and the corresponding escape of ionospheric ions is believed to be primarily caused by LEFs, first because of the much higher power in the LEFs than the power in the EIC frequency range and second because the PSD clearly peaks below ≈ 1 Hz while the frequency range of the EIC mostly appears as a monotonic tail or “background” of an overall PSD increase at low frequencies. Electric fluctuations, and the associated escape of ionospheric ions, are mainly observed inside or above the acceleration region. Below the acceleration region, predominantly downward accelerated electrons are observed. This can be understood as a short-circuiting of electric fluctuations by the plasma in the topside ionosphere where the plasma is gaining kinetic energy by the electric field. Thus the electric energy from the source/dynamo in the outer magnetosphere is dissipated in the topside layer of the ionosphere, causing an acceleration and escape of ionospheric plasma. Ion conies with an ion heating up to several keV are associated with the strongest electric fluctuations (up to ≈ 10³ (mV/m)²/Hz), while low-energy ion beams in general are connected with somewhat smaller electric fluctuations. Spatial or temporal electric fluctuations are expected to increase the magnetic moment of the local plasma via a transverse equal velocity gain acceleration process. This is consistent with the observed electric field power spectrum usually peaking below both the O+ and H+ local gyrofrequency.

During the first months of operation the Swedish magnetospheric satellite Viking has detected many examples of plasma waves with spectral features ordered by the proton gyro frequency. Even though the proton gyro frequency characterizes all such spectra, there are many variations from spectrum to spectrum. Different bands are excited and the emissions are located within different parts of the harmonic bands at different times; sometimes a little above the harmonics, sometimes slightly below, and sometimes in the middle of the bands. The amplitudes of the waves are small, on the order of 1 mV/m, often less. Some of these waves do not appear to have been previously reported and we present here a representative example that took place at an altitude of about 13000 km on geomagnetic field lines connected to morning side auroral oval. The generation mechanism is still uncertain but simultaneously measured particle data indicate that there is free energy available in the loss cone in the hot ion population. If the loss cone is the source of free energy, these waves are of importance for proton precipitation in diffuse aurora.

The Viking spacecraft carried a plasma wave interferometer consisting of two Langmuir probes separated by 80 m. The interferometer has measured density irregularities in the frequency range 1-200 Hz which can be interpreted as field aligned structures with phase velocities transverse to the geomagnetic field which are small compared to the spacecraft velocity relative to the plasma. The spatial structures which have nearly zero frequency in the plasma frame are Doppler shifted up to frequencies as large as 200 Hz in the spacecraft frame. The broadband amplitude of the density irregularities is typically .1% to .5%. The density irregularities appear to be a common feature of the high latitude ionosphere probed by Viking.

Data from particle detectors on board the satellite OGO-4 were used to study the precipitation of electrons in the energy range 0.7 to 24 keV. The latitude dependence of these particles in the local time region from midnight to dawn was investigated in detail. The analysis shows that the precipitation of particles of energies 2.3 to 24 keV is centered at an invariant latitude of about 68 deg at midnight with a clear shift in latitude with increasing local time and this shift is more pronounced for lower energies. The highest fluxes of particles in this energy interval are measured at midnight and they decrease rapidly with local time. The data in the energy range 2.3 to 24 keV support a theory where particles are injected in the midnight region from the tail gaining energy due to a betatron process and then drift eastwards in a combined electric and magnetic field. The main part of the electrons at 0.7 keV show a different behavior. They seem to undergo an acceleration process which is rather local, sometimes giving field aligned fluxes which may be super-imposed on the background precipitation.

Experimental evidence is provided for the existence of slow-mode magnetosonic solitons in the collisionless plasma at the magnetopause boundary layer. The solitons were detected by the fleet of Cluster spacecraft at the dusk flank of the magnetosphere as magnetic field depressions (up to 85%) accompanied with enhancement of the plasma density and temperature by a factor of 2. The solitons propagate 250 km=s with respect to the satellites and have perpendicular size of 1000–2000 km, which is a few ion inertial scale lengths. The comparison with numerical solutions of a theoretical model shows quantitative agreement between the model and observations. We present first observations of slow magnetosonic solitons detected in natural unbounded collisionless plasma by a fleet of four Cluster spacecraft [1]. Measurements were made at radial distance of 18RE at the magnetopause boundary layer, which is a current layer that separates the shocked and thermalized solar wind in the magnetosheath from the magnetic fields of the terrestrial origin. With multipoint measurements and complete plasma diagnostics of the Cluster mission we were able for the first time to provide all relevant parameters of the solitons and their environment.

Highlights are presented from studies of the electric field data from various regions along the Cluster orbit. They all point towards a very high coherence for phenomena recorded on four spacecraft that are separated by a few hundred kilometers for structures over the whole range of apparent frequencies from 1 mHz to 9 kHz. This presents completely new opportunities to study spatial-temporal plasma phenomena from the magnetosphere out to the solar wind. A new probe environment was constructed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.Key words. Magnetospheric physics (electric fields) – Space plasma physics (electrostatic structures; turbulence)