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Observations from the MESSENGER spacecraft have considerably enhanced our understanding of the plasma environment at Mercury. In particular, measurements from the Fast Imaging Plasma Spectrometer (FIPS) provide evidences of a variety of ion species of planetary origin (He+, O+, Na+) in the northern dayside cusp and in the nightside plasma sheet. A more comprehensive view of Mercury's plasma environment will be provided by the Bepi Colombo mission that will be launched in 2018. Onboard the Bepi Colombo MMO spacecraft, the MPPE (Mercury Plasma/Particle Experiment) consortium gathers different sensors dedicated to particle measurements. Among these sensors, the Mass Spectrum Analyzer (MSA) is the instrument dedicated to plasma composition analysis. It consists of a top-hat for energy analysis followed by a Time-Of-Flight (TOF) chamber to derive the ion mass. Taking advantage of the spacecraft rotation, MSA will measure three-dimensional distribution functions in one spin (4 s), from energies characteristic of exospheric populations (in the eV range) up to plasma sheet energies (up to ~38 keV/q). A notable feature of the MSA instrument is that the TOF chamber is polarized with a linear electric field that leads to isochronous TOFs and enhanced mass resolution (typically, m/∆m ≈ 40 for ions with energies up to 13 keV/q). At Mercury, this capability is of paramount importance to thoroughly characterize the wide variety of ion species originating from the planet surface. It is thus anticipated that MSA will provide unprecedented information on ion populations in the Hermean environment and hence improve our understanding of the coupling processes at work.

In collisionless plasmas, turbulence is thought to play an important role in mass transport and energy dissipation. Magnetic fluctuations in the Jovian magnetosphere are essential in a turbulent state. Previous studies of that turbulence have focused on the large scales using low time resolution of magnetic field data. Here we extend those studies to cover a wider range of scales by combining both low and high-time-resolution data of Galileo magnetometer. We use particle data from the plasma instrument and include energetic particle contributions to estimate the local plasma parameters. We obtain 11 power spectra of magnetic field in the frequency range of 10(-4)-1Hz, which covers both magnetohydrodynamics and ion kinetic scales. The frequencies of the evidenced spectral breaks are found to be relatively well correlated with the characteristic scales of heavy ion. The spectral indices below and above the spectral breaks are found to be broad and cover the ranges of 0.6-1.9 and 1.7-2.5, respectively. An analysis of higher-order statistics shows an intermittent feature of the turbulence, found to be more prominent in the plasma sheet than in the lobe. Furthermore, a statistical survey of the power of the fluctuations using low-time-resolution data suggests a radially varying dawn-dusk asymmetry: the total power is larger in the duskside (dawnside) at \textless50 R-J (\textgreater80 R-J), which would reflect flow shear and global magnetospheric activity.

We provide evidence of the simultaneous occurrence of large-amplitude, quasi-parallel whistler mode waves and ion-scale magnetic structures, which have been observed by the Cluster spacecraft in the plasma sheet at 17 Earth radii, during a substorm event. It is shown that the magnetic structures are characterized by both a magnetic field strength minimum and a density hump and that they propagate in a direction quasi-perpendicular to the average magnetic field. The observed whistler mode waves are efficiently ducted by the inhomogeneity associated with such ion-scale magnetic structures. The large amplitude of the confined whistler waves suggests that electron precipitations could be enhanced locally via strong pitch angle scattering. Furthermore, electron distribution functions indicate that a strong parallel heating of electrons occurs within these ion-scale structures. This study provides new insights on the possible multiscale coupling of plasma dynamics during the substorm expansion, on the basis of the whistler mode wave trapping by coherent ion-scale structures.

[1] We investigate the excellent correlation between ionospheric upgoing electron beams and broadband electrostatic emissions (0-6 kHz) observed by Cluster, at ~5 to 9 Earth's radii above the polar cap. In the absence of detailed, high time resolution waveform data in that region, we precisely analyzed several electron beams to obtain information concerning wave-particle interactions. Our results indicate that these beams are extremely variable and occasionally show multiple components. The processes involved might then occur on very short time scales, of the order of or shorter than sampling rates, typically 100 ms. We suggest that non linearities are at the origin of the spread of the frequency range of the waves simultaneously observed, as well as of the beam variability. We conclude that these electron beams are likely to destabilize Langmuir waves and, by the non-linear evolution of the electron bump-on-tail instability, could be responsible for the appearance of electrostatic solitary waves above the polar cap.

Since January 2001, the multisatellite and multiinstrument CLUSTER mission gives a unique opportunity to study the structure and dynamics of the high-altitude polar cusp. On 17 March 2001, CLUSTER sampled the northern high-altitude cusp (J-9RE) around noon for more than 1 hour during very quiet interplanetary and magnetospheric conditions (P 0 driven patterns, including one or two lobe reconnection-cells in the dayside polar cap, according to the amplitude of the By component. The overall convection pattern responds in ∼3-5 min to abrupt changes in the IMF orientation. Successive electron and ion patches are interpreted as signatures of pulsed, enhanced reconnection in the high-latitude magnetopause, poleward of CLUSTER, at a distance estimated to be 8-12RE and, at times, less. A four-point boundary analysis demonstrates that reconnected flux tubes (Flux Transfer Events) convect with drift directions and velocities (6-15 km/s) in close agreement with the inferred convective patterns. Furthermore, CLUSTER demonstrates that boundary cusp motions, with a velocity up to ∼20 km/s, are immediately and directly induced by abrupt changes in the IMF orientation. Copyright 2003 by the American Geophysical Union.

Early results of a new self-consistent fluid model are presented for steady convection of the plasma from the geomagnetic tail through the Earth's inner magnetosphere below 10 Earth radii (RE), including its coupling with the ionosphere. This model computes the transport of both the ion and electron fluids and constitutes an important improvement of the fluid numerical model of Fontaine et al. (1985) (referred to as Paper 1), which simulated the convection of electrons only. The coupling with the ionospheric convection is effected by the precipitation of magnetospheric electrons, which enhances ionospheric auroral conductivities, and by the flow of region 2 field-aligned currents, which connect the magnetospheric and ionospheric circuits. This last coupling was not considered in Paper 1, because region 2 field-aligned currents are generated mainly by pressure gradients of the ion plasma that develop progressively during its motion. We also take into account the ion precipitation, which affects the generation of region 2 field-aligned currents, but we neglect their relatively small effect on the ionospheric conductivities. We devoted much effort to produce a self-consistent description of the transport of the ionospheric and magnetospheric plasmas, and their couplings. As a first step, in order to distinguish basic physical processes from effects of geometrical origin, we solved the equations for the case of a dipolar magnetic field as in Paper 1. As a consequence, we do not expect our results to reproduce very precisely the available observations but rather to provide only correct orders of magnitude. The numerical solution scheme of the hyperbolic equations governing the transport of both magnetospheric ions and electrons, of the elliptic equation of the ionospheric electrostatic potential, and of their coupling due to electron precipitation and region 2 field-aligned currents, is basically the same as Paper 1. It makes use of the finite element method. The numerical model is run to simulate the evolution of the system from an initial state in which the inner magnetosphere is free of plasma, to a steady state situation in which the plasma has penetrated into the inner magnetosphere from the tail. Stable magnetic conditions corresponding to a Kp between 2 and 3 are considered to allow a comparison with observations. The formation of a belt of electron and ion precipitation in the auroral zone is globally reproduced by the model, but the electron fluxes are underestimated and the auroral ion zone is located equatorward with regard to the DMSP satellite statistical observations. The field-aligned current region 2 computed by the model is comparable to the available statistical observations, apart from an underestimation of the amplitude. The convection structure is also found comparable to observations, thus improving the results of Paper 1. The amplitude and the direction of the major component of the electric field (the northward component being at least 4 times larger than the eastward component) is consistent with the observations, but the high latitude distribution of the eastward component is incorrectly predicted. The region 2 field-aligned currents prevent the electric field from penetrating to midlatitudes and induce an eastward rotation of the overall convection electric field distribution. It is suggested that the ion precipitation rate computed under the strong pitch angle scattering is overestimated and that ion and electron precipitation contributions to the ionospheric conductances are of the same order of magnitude in the dusk sector.

On the basis of theoretical calculations of electron diffusion coefficients and of OGO 5 data, Lyons [1974] suggested that electrostatic electron cyclotron harmonic waves had amplitudes large enough to cause the strong pitch angle diffusion of plasma sheet keV electrons and to be responsible for diffuse auroral precipitation. However, recent measurements of the wave location and amplitude performed aboard the GEOS spacecraft have brought new pieces of information challenging these conclusions. Our calculations are based on the theoretical tool developed by Lyons [1974] but take into account the recently observed wave confinement within a few degrees from the magnetic equator. Under these conditions, we avoid the numerical averaging of the pitch angle diffusion coefficient over the whole line of force, and we can derive an analytical expression of the minimum wave amplitude required to cause strong pitch angle diffusion for plasma sheet electrons. This expression has the advantage to be easily tractable in further calculations, and permits us to evaluate the dependence of the results on parameters that are not reported by observations, such as the wave number spectrum. Our results are found to differ from Lyons's [1974] predictions by a factor of about 2.5, and typically, a wave amplitude of more than 2 mV m−1 is required to put 1-keV electrons on strong diffusion. On the other hand, on the basis of a statistical analysis of electron cyclotron wave amplitudes measured in the nightside plasmasheet by the GEOS 2 spacecraft, we show that most of the time (>91 %), this typical value of 2 mV m−1 is not reached. This result appears inconsistent with the hypothesis that diffuse auroras, which are a permanent feature of the auroral zones, are due solely to electrostatic electron cyclotron waves. This leads us to the conclusion that these waves are not the only cause of diffuse electron precipitation. It is suggested that other mechanisms involving for instance the dynamics of the ions (such as field-aligned currents) could play an important role.

We present a new self-consistent model of the transport of hot electrons in the earth's magnetosphere and of their precipitation in the auroral ionosphere. In this model, the electron transport is described by the same fluid equations as in the work by Fontaine and Blanc (1983). But the electric field distribution which drives electron motions is computed from the equations of ionospheric currents, instead of being described by an empirical model as previously. The two basic equations to be solved, the hyperbolic equation governing electron transport and the elliptic equation governing the ionosphere current flow and the distribution of the electrostatic potential on the ionospheric conductor, are coupled via the ionospheric conductivities, which are functions of the electron precipitation flux at the top of the ionosphere. The numerical model is run to simulate the evolution of the system from an initial state in which there is no hot electron in the inner magnetosphere, to a steady state situation in which electrons penetrate into the inner magnetosphere from the tail and are transported onward until they precipitate into the ionosphere. To achieve this, the dawn-to-dusk electrostatic potential drop which is induced by the solar wind across the magnetosphere is turned on at t = 0 and maintained constant for the rest of the simulation. Using different values of this potential drop for each individual run, we simulate various levels of magnetic activity and compare our results with observations. The formation of a belt of electron precipitation in the auroral zones is very well reproduced by the model. Both its distribution in local time and the ionospheric location of its equatorward edge agree fairly well with statistical models derived from satellite observations of the auroral electron precipitation zones. On this latter point the agreement with observations is improved relative to our previous study. The formation of the electron precipitation belt modifies the convection pattern as it generates a localized enhancement of ionospheric conductivities in the auroral zone. This modification is essentially negligible on the sunlit side of the earth, where convection remains controlled by the distribution of conductivities due to the solar illumination. But it is very sensitive on the nightside, where the equipotentials are strongly distorted at the poleward and equatorward edges of the belt of enhanced conductivities, and where the evening vortex expands toward middle latitudes. In the equatorial plane of the magnetosphere, the shape of the plasmapause is also well reproduced; but its absolute size is underestimated for high levels of magnetic activity. A careful comparison of our computed electric fields with the fields measured at auroral and middle latitudes by several ionospheric incoherent scatter radars indicates that the shape of the local time variation of the fields is reproduced reasonably well, given the uncertainties inherent to the statistical averages derived from observations, and the intrinsic variability of the geophysical context. However, there is some indication that our model underestimates the magnitude of the meridional electric field at high latitudes, overestimates it at mid-latitudes, and predicts a position in local time of the convection vortices which is two or three hours westward of the observed ones. These discrepancies can receive the same explanation: the lack of a trapped population of energetic ions in our model, which other studies have shown to be responsible for increasing the high-latitude electric field and decreasing appreciably the mid-latitude electric field. Our next step in magnetospheric convection modeling will be to develop a two-fluid theory in which both electron and ion transport equations will be self-consistently coupled to the equation of the ionospheric electrical circuit.