The Near-Earth Solar Wind

International audienc

Plasmasphere observations with Cluster completed by new data from an old mission, Dynamics Explorer-1

International audienceSince 2000 the four Cluster spacecraft have crossed the Earth's plasmasphere every 2.5 days, with various perigee altitudes (from 4 to 1.5 RE), different configurations (string of pearls, tetrahedron) and separations (from 10 to 100 000 km). The resulting data-set allows different types of inner magnetosphere studies and provides a huge amount of various plasmaspheric data, including plasmapause position information. It enables statistical analysis of the plasmapause or comparison with the position of the radiation belts. Moreover, plasmaspheric plumes can be studied in a statistical way or focusing on specific events, and in relation with wave activity (EMIC, electromagnetic rising tone, whistler). Recently, data from an old mission, Dynamics Explorer-1, have become available. In particular densities and temperatures for many ions (H+, He+, He++, O+, and O++) have been derived from the RIMS (Retarding Ion Mass Spectrometer) instrument and are available from October 1981 to January 1985. Those data, not available on-board the Cluster satellites, allow different studies and in particular the analyze of the distributions of those ions in the plasmasphere boundary layer, as a function of magnetic local time and geomagnetic activity

Plasmasphere observations with Cluster completed by new data from an old mission, Dynamics Explorer-1

International audienceSince 2000 the four Cluster spacecraft have crossed the Earth's plasmasphere every 2.5 days, with various perigee altitudes (from 4 to 1.5 RE), different configurations (string of pearls, tetrahedron) and separations (from 10 to 100 000 km). The resulting data-set allows different types of inner magnetosphere studies and provides a huge amount of various plasmaspheric data, including plasmapause position information. It enables statistical analysis of the plasmapause or comparison with the position of the radiation belts. Moreover, plasmaspheric plumes can be studied in a statistical way or focusing on specific events, and in relation with wave activity (EMIC, electromagnetic rising tone, whistler). Recently, data from an old mission, Dynamics Explorer-1, have become available. In particular densities and temperatures for many ions (H+, He+, He++, O+, and O++) have been derived from the RIMS (Retarding Ion Mass Spectrometer) instrument and are available from October 1981 to January 1985. Those data, not available on-board the Cluster satellites, allow different studies and in particular the analyze of the distributions of those ions in the plasmasphere boundary layer, as a function of magnetic local time and geomagnetic activity

Plasma waves associated with density holes upstream of Earth's bow shock

The regions upstream of Earth's bow shock provide a natural laboratory for a variety of fundamental plasma processes, which include the interaction between particle populations that stream away from the shock with the solar wind and low frequency waves. Recent observations from Cluster and Double Star have exhibited novel short-duration (~4-20 s) density holes. These density holes are accompanied by a variety of plasma waves at ~ 0.01 Hz to several tens kHz. Preliminary analysis shows that the gross-scale structures appear fast magnetosonic-like, while in the interior ion cyclotron waves, whistler mode waves, and strong electrostatic waves are observed. A broad spectrum of electrostatic turbulence (a few kHz to ~50 kHz)is observed at the upstream edges of density holes.These waves are likely to play important roles in determining the holes' properties, including heating of the suprathermal particles seen. Some of the wave activity, especially at the upstream edges of density holes, are similar to those observed at the Earth's bow shock. We have studied the wave characteristics in a number of density holes, and will discuss their possible mechanisms, influences on the plasma properties, and role in the structures' formation

Plasma waves observed in the high altitude cusp region by Cluster: an overview of three years of data.

The cusp regions of the Earth’s magnetosphere are possible direct entry regions for solar wind plasma into the magnetosphere. The high latitude cusps have been very few visited before Cluster mission; it is why the study of physical processes occurring in these regions constitute one of the Cluster prime objectives. One of the question is the role of waves in the dynamics of particles in the cusp. While different case studies have already been undertaken in the aim to establish what are the different wave modes present and what are their interactions with the particles, we present here the results of the systematic analysis of the occurrence and intensity of magnetic and electric components of the different waves present in the high altitude cusp. We use the different wave prime parameter data in different frequency bands covering the 1 Hz - 80 kHz frequency range and plot their intensity on the part of orbits that have been identified to contain cusp crossings, and this for three years of data. The method is similar to the one used by Lavraud et al., (2004) for ion plasma parameters in the high latitude cusp using the CIS experiment. Comparison between the different kind of waves, as characterised by their electrostatic or electromagnetic nature and their frequency range will be presented. This identification will be supported by some example of case study. The influence of the direction of the interplanetary magnetic field on the region of maximum intensity of the waves will be discussed, in term of possible link with a reconnection site. Wave results will be compared with ion results, in particular with the density study

Multiple harmonic ULF waves in the plasma sheet boundary layer: Instability analysis

International audience[1] Multiple‐harmonic electromagnetic waves in the ULF band have occasionally been observed in Earth's magnetosphere, both near the magnetic equator in the outer plasmasphere and in the plasma sheet boundary layer (PSBL) in Earth's magnetotail. Observations by the Cluster spacecraft of multiple‐harmonic electromagnetic waves with fundamental frequency near the local proton cyclotron frequency, W cp , were recently reported in the plasma sheet boundary layer by Broughton et al. (2008). A companion paper surveys the entire magnetotail passage of Cluster during 2003, and reports 35 such events, all in the PSBL, and all associated with elevated fluxes of counterstreaming ions and electrons. In this study we use observed pitch angle distributions of ions and electrons during a wave event observed by Cluster on 9 September 2003 to perform an instability analysis. We use a semiautomatic procedure for developing model distributions composed of bi‐Maxwellian components that minimizes the difference between modeled and observed distribution functions. Analysis of wave instability using the WHAMP electromagnetic plasma wave dispersion code and these model distributions reveals an instability near W cp and its harmonics. The observed and model ion distributions exhibit both beam‐like and ring‐like features which might lead to instability. Further instability analysis with simple beam‐like and ring‐like model distribution functions indicates that the instability is due to the ring‐like feature. Our analysis indicates that this instability persists over an enormous range in the effective ion beta (based on a best fit for the observed distribution function using a single Maxwellian distribution), b′, but that the character of the instability changes with b′. For b′ of order unity (for instance, the observed case with b′ ∼ 0.4), the instability is predominantly electromagnetic; the fluctuating magnetic field has components in both the perpendicular and parallel directions, but the perpendicular fluctuations are larger. If b′ is greatly decreased to about 5 × 10 −4 (by increasing the magnetic field), the instability becomes electrostatic. On the other hand, if b′ is increased (by decreasing the magnetic field), the instability remains electromagnetic, but becomes predominantly compressional (magnetic fluctuations predominantly parallel) at b′ ∼ 2. The b′ dependence we observe here may connect various waves at harmonics of the proton gyrofrequency found in different regions of space. (2010), Multiple harmonic ULF waves in the plasma sheet boundary layer: Instability analysis

Solitary Electromagnetic Pulses Detected with Super-Alfvénic flows in Earth's Geomagnetic Tail.

International audienceSolitary nonlinear (B=B 1) electromagnetic pulses have been detected in Earth's geomagnetic tail accompanying plasmas flowing at super-Alfvénic speeds. The pulses in the current sheet had durations of 5 s, were left-hand circularly polarized, and had phase speeds of approximately the Alfvén speed in the plasma frame. These pulses were associated with a field-aligned current J k and observed in low density (0:3 cm ÿ3), high temperature (T e T i 3 10 7 K), and 10 plasma that included electron and ion beams streaming along B. The wave activity was enhanced from below the ion cyclotron frequency to electron cyclotron and upper hybrid frequencies. The detailed properties suggest the pulses are non-linearly steepened ion cyclotron or Alfvén waves. We present first observations of solitary electromagnetic (EM) pulses detected in the Earth's geomagnetic tail during intervals of super-Alfvénic flows, where the perpendicular flow V ? exceeds the Alfvén speed V A. Most flow speeds are sub-Alfvénic and can be explained by recon-nection theories and models [1]. However, plasmas occasionally flow faster than V A [2]. The significance of V ? > V A flows and the possible nonlinear effects that might accompany them have not been discussed. Solitary waves are signatures of nonlinear interactions, and they can shed light on the elusive current sheet dynamics that drive the global aurora when Earth becomes an intense radio emitter and electrons are accelerated to hundreds of keV and ions to several MeV [3,4]. Solitary EM waves have been observed in comets and also in Earth's ionosphere [5,6], and Debye scale electro-static solitary waves are ubiquitous in the solar wind, bow shock, plasma sheet boundary layer (PSBL), and the auro-ral ionosphere [7-9]. Unipolar and bipolar wave packets have been detected in the PSBL, and magnetosonic soli-tons at the magnetopause [10,11]. However, there have been no reports of solitary pulses accompanying flows in which V ? > V A in the Earth's geomagnetic tail. A random sampling of cluster data of fast flowing plasmas indicates that V ? > V A flows occur when the geomag-netic tail is undergoing large fluctuations (but V ? > V A speeds are not always observed when and where large fluctuations occur). High-time resolution E and B fields during a number of events indicate that the EM pulses occur concurrently with V ? > V A flows in the current sheet (CS) while electrons and ions are streaming along B and wave activity is enhanced. The data in Fig. 1 are spin-averaged (4 s) from Cluster 1 (SC1) located at a tail distance of 19R E (R E Earth's radius). The ion bulk parameters are obtained by integrating the 3D distributions over the velocity space [12]. Measurements here were made in a thin CS (400 km) during an aurora [13]. SC1, initially in the plasma sheet PRL 98, 265001 (2007

Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions

International audienceSpacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma. Citation: Pedersen, A., et al. (2008), Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions

Density holes in the upstream solar wind

International audienceLarmor size transient structures with depletions as large as 99% of ambient solar wind density levels occur commonly upstream of Earth’s collisionless bow shock. These “density holes” have a mean duration of ∼17.9 ± 10.4s but holes as short as 4s have been observed. The average fractional density depletion (δn/n) inside the holes is ∼0.68 ± 0.14. The density of the upstream edge moving in the sunward direction can be enhanced by five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock, and measured local field geometries and Mach number support this view. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that back‐streaming particles interacting with the solar wind are important

Identification of natural plasma emissions observed close to the plasmapause by the Cluster-Whisper relaxation sounder

International audienceWe use the data collected by the Whisper instrument onboard the Cluster spacecraft for a first test of its capabilities in the identification of the natural plasma waves observed in the Earth's magnetosphere. The main signatures observed at the plasma frequency, upper hybrid frequency, and electron Bernstein modes were often difficult to be reliably recognized on previous missions. We use here the characteristic frequencies provided by the resonances triggered by the relaxation sounder of Whisper to identify with good confidence the various signatures detected in the complex wave spectra collected close to the plasmapause. Coupled with the good sensitivity, frequency and time resolution of Whisper, the resonances detected by the sounder allow one to precisely spot these natural emissions. This first analysis seems to confirm the interpretation of Geos observations: the natural emissions observed in Bernstein modes above the plasma frequency, now widely observed onboard Cluster, are not modeled by a single Maxwellian electrons distribution function. Therefore, multi-temperature electron distribution functions should be considered