P. M. E. Décréau

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

Spacecraft 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,

Plasmasphere dynamics in the duskside bulge region: A new look at an old topic

Data acquired during several multiday periods in 1982 at ground stations Siple, Halley, and Kerguelen and on satellites DE 1, ISEE 1, and GEOS 2 have been used to investigate thermal plasma structure and dynamics in the duskside plasmasphere bulge region of the Earth. The distribution of thermal plasma in the dusk bulge sector is difficult to describe realistically, in part because of the time integral manner in which the thermal plasma distribution depends upon the effects of bulk cross-B flow and interchange plasma flows along B. While relatively simple MHD models can be useful for qualitatively predicting certain effects of enhanced convection on a quiet plasmasphere, such as an initial sunward entrainment of the outer regions, they are of limited value in predicting the duskside thermal plasma structures that are observed. Furthermore, use of such models can be misleading if one fails to realize that they do not address the question of the formation of the steep plasmapause profile or provide for a possible role of instabilities or other irreversible processes in plasmapause formation. Our specific findings, which are based both upon the present case studies and upon earlier work, include the following: (1) during active periods the plasmasphere appears to become divided into two entities, a main plasmasphere and a duskside bulge region. The latter consists of outlying or outward extending plasmas that are the products of erosion of the main plasmasphere; (2) in the aftermath of an increase in convection activity, the main plasmasphere tends (from a statistical point of view) to become roughly circular in equatorial cross section, with only a slight bulge at dusk; (3) the abrupt westward edge of the duskside bulge observed from whistlers represents a state in the evolution of sunward extending streamers; (4) in the aftermath of a weak magnetic storm, 10 to 30% of the plasma removed from the outer plasmasphere appears to remain in the afternoon-dusk sector beyond the main plasmasphere. This suggests that plasma flow from the afternoon-dusk magnetosphere into the boundary layers is to some extent impeded, possibly through a mechanism that partially decouples the high altitude and ionospheric-level flow regimes; (5) outlying dense plasma structures may circulate in the outer duskside magnetosphere for many days following an increase in convection, unless there is extremely deep quieting; (6) a day-night plasmatrough boundary may be identified in equatorial satellite data; (7) factor-of-2-to-10 density irregularities appear near the plasmapause in the postdusk sector in the aftermath of weak magnetic storms; (8) during the refilling of the plasmatrough from the ionosphere at L = 4.6, predominantly bidirectional field aligned and equatorially trapped light ion pitch angle distributions give way to a predominantly isotropic distribution (as seen by DE 1) when the plasma density reaches a level a factor of about 3 below the saturated plasmasphere level; (9) some outlying dense plasma structures are effectively detached from the main plasmasphere, while others appear to be connected to that body.

Wave-particle interactions in the equatorial source region of whistler-mode emissions

Wave-particle interactions can play a key role in the process of transfer of energy between different electron populations in the outer Van Allen radiation belt. We present a case study of wave-particle interactions in the equatorial source region of whistler-mode emissions. We select measurements of the Cluster spacecraft when these emissions are observed in the form of random hiss with only occasional discrete chorus wave packets, and where the wave propagation properties are very similar to previously analyzed cases of whistler-mode chorus. We observe a positive divergence of the Poynting flux at minima of the magnetic field modulus along the magnetic field lines, indicating the central position of the source. In this region we perform a linear stability analysis based on the locally measured electron phase space densities. We find two unstable electron populations. The first of them consists of energy-dispersed and highly anisotropic injected electrons at energies of a few hundreds eV to a few keV, with the perpendicular temperature more than 10 times higher than the parallel temperature with respect to the magnetic field line. Another unstable population is formed by trapped electrons at energies above 10 keV. We show that the injected electrons at lower energies can be responsible for a part of the waves that propagate obliquely at frequencies above one half of the electron cyclotron frequency. Our model of the trapped electrons at higher energies gives insufficient growth of the waves below one half of the electron cyclotron frequency and a nonlinear generation mechanism might be necessary to explain their presence even in this simple case.

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

[1] Multiple‐harmonic electromagnetic waves in the ULF band have occasionally been observed in Earths magnetosphere, both near the magnetic equator in the outer plasmasphere and in the plasma sheet boundary layer (PSBL) in Earths 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,

Cluster observations of finite amplitude Alfven waves and small-scale magnetic filaments downstream of a quasi-perpendicular shock

The Cluster satellites crossed the Earths bow shock several times on 31 March 2001. For all these crossings the bow shock was supercritical and quasi-perpendicular. We present here the results of a detailed analysis of the magnetic field fluctuations observed downstream of the shock. We use data from the four Cluster spacecraft to determine the behavior and the geometry of these fluctuations with good accuracy. Shortly after the ramp crossing, we observed a large-amplitude nonlinear Alfven wave, propagating along the downstream average magnetic field with a spectrum peaking at two frequencies below the proton and the alpha ion cyclotron frequencies. Farther downstream in the magnetosheath the magnetic field fluctuations took the form of three-dimensional structures which can be interpreted as cylindrical field-aligned current tubes. It is the first time that such current tubes have been observed downstream of a quasi-perpendicular shock, and they are closely associated with a quasi-monochromatic, finite amplitude Alfven wave. We suggest that a close relation exists between the nonlinear Alfven wave and the current tubes as a result of a filamentation instability which is expected to occur at β ≥ 1 and for frequencies comparable to the ion cyclotron frequencies.

A case study of plasma structure in the dusk sector associated with enhanced magnetospheric convection

In a case study from June 8–9, 1982, data from ground whistler stations Siple and Halley, Antarctica, located at L ∼4.3 and spaced by ∼2 hours in MLT, and from satellites DE 1 and GEOS 2, have provided confirming evidence that the bulge region of the magnetosphere can exhibit an abrupt westward “edge,” as reported earlier from whistlers. The present data and previous MHD modeling work suggest that this distinctive feature develops during periods of steady or declining substorm activity, when dense plasma previously carried sunward under the influence of enhanced convection activity begins to rotate with the Earth at angular velocities that decrease with increasing L value and becomes spirallike in form. For the first time, whistler data have been used to identify a narrow dense plasma feature, separated from the main plasmasphere and extending sunward into the late afternoon sector at L values near the outer observed limits of the main plasmasphere bulge. The westward edge of the main bulge, found by both whistler stations to be at ∼1800 MLT, appeared to be quasi-stationary in Sun-Earth coordinates during the prevailing conditions of gradually declining geomagnetic agitation. It is possible that outlying dense plasma features such as the one observed develop as part of the process leading to the occurrence of the more readily detectable abrupt westward edge of the bulge. It was not possible in this case to determine the extent to which the outlying feature was smoothly attached to or isolated from the main bulge region.

Dynamics and waves near multiple magnetic null points in reconnection diffusion region

Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three-dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler-mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for the whistler wave regime of reconnection.

Modeling of Cluster's electric antennas in space: Application to plasma diagnostics

[1]xa0The main characteristics of the long-boom electric antennas installed on board the Cluster satellites are derived from finite element modeling in a kinetic and isotropic space plasma, in the frequency range of about 1–100 kHz. The model is based on the surface charge distribution method in quasi-static conditions. The impedances of both types of antenna, i.e., the double-wire and the double-probe, are computed versus the frequency normalized with respect to the local plasma frequency and for several different Debye lengths. Most of the code outputs are checked using analytic estimations for better understanding of the involved physical mechanisms. As a by-product, the effective length of the double-probe antenna and the mutual impedance between the two antennas are computed by the code. It is shown that if it had been possible to implement such measurements on board, one would have been able not only to determine accurately the electric characteristics of the antennas but also to estimate the local plasma parameters. Nevertheless, an interesting feature predicted by the model has been checked recently in orbit by running a special mode of operation for testing the mutual impedance measurement. The preliminary results are globally consistent with the predictions, except that they suggest that our Maxwellian model for the electron distribution should be revised in order to explain the unexpected low-frequency response. After analysis of the electron flux measurements obtained simultaneously, it appears that a rough adjustment of the electron distribution with a two-component distribution allows us to account for the observations.

Crater flux transfer events: Highroad to the X line?

We examine Cluster observations of a so‐called magnetosphere crater FTE, employing data from five instruments (FGM, CIS, EDI, EFW, and WHISPER), some at the highest resolution. The aim of doing this is to deepen our understanding of the reconnection nature of these events by applying recent advances in the theory of collisionless reconnection and in detailed observational work. Our data support the hypothesis of a stratified structure with regions which we show to be spatial structures. We support the bulge‐like topology of the core region (R3) made up of plasma jetting transverse to reconnected field lines. We document encounters with a magnetic separatrix as a thin layer embedded in the region (R2) just outside the bulge, where the speed of the protons flowing approximately parallel to the field maximizes: (1) short (fraction of a sec) bursts of enhanced electric field strengths (up to ∼30 mV/m) and (2) electrons flowing against the field toward the X line at approximately the same time as the bursts of intense electric fields. R2 also contains a density decrease concomitant with an enhanced magnetic field strength. At its interface with the core region, R3, electric field activity ceases abruptly. The accelerated plasma flow profile has a catenary shape consisting of beams parallel to the field in R2 close to the R2/R3 boundary and slower jets moving across the magnetic field within the bulge region. We detail commonalities our observations of crater FTEs have with reconnection structures in other scenarios. We suggest that in view of these properties and their frequency of occurrence, crater FTEs are ideal places to study processes at the separatrices, key regions in magnetic reconnection. This is a good preparation for the MMS mission.

Cluster observations of fast shocks in the magnetosheath launched as a tangential discontinuity with a pressure increase crossed the bow shock

[1]xa0The interaction of a tangential discontinuity (TD) and accompanying dynamic pressure increase with the Earths bow shock launches a fast shock that travels ahead of the TD in the magnetosheath and carries a significant portion of the pressure change. In this event study, we use observations from the Cluster spacecraft and magnetohydrodynamic simulations to identify the fast shock and its properties and to track the TD in the magnetosheath. Velocities of the fast shock and the TD were determined by triangulation using the four distant Cluster spacecraft. The fast shock is a planar structure, traveling nearly perpendicular to B at the magnetosonic speed in the plasma rest frame. Changes in density and ∣B∣ are correlated, with about a 20% increase in each. A current was observed tangential to the plane of the fast shock, and the positive E •J there provided an electromagnetic energy source for the observed heating of the ions. The fast shock is generated by the pressure change and determines the timing of the initial response of the magnetopause to that change. The TD was moving nearly in the −XGSE direction and was being compressed as it moved inward. The passage of the TD ushered in large-scale compressive structure in the magnetosheath magnetic field, which satisfied the mirror mode instability criterion. Velocities of a fast rarefaction wave, reflected from the magnetopause, and an additional slow-mode structure, which was not a product of the initial interaction with the bow shock, were determined by triangulation.