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Several generalizations of the well-known fluid model of Braginskii (Rev. of Plasma Phys., 1965) are considered. We use the Landau collisional operator and the moment method of Grad. We focus on the 21-moment model that is analogous to the Braginskii model, and we also consider a 22-moment model. Both models are formulated for general multi-species plasmas with arbitrary masses and temperatures, where all the fluid moments are described by their evolution equations. The 21-moment model contains two "heat flux vectors" (3rd and 5th-order moments) and two "viscosity-tensors" (2nd and 4th-order moments). The Braginskii model is then obtained as a particular case of a one ion-electron plasma with similar temperatures, with de-coupled heat fluxes and viscosity-tensors expressed in a quasi-static approximation. We provide all the numerical values of the Braginskii model in a fully analytic form (together with the 4th and 5th-order moments). For multi-species plasmas, the model makes calculation of transport coefficients straightforward. Formulation in fluid moments (instead of Hermite moments) is also suitable for implementation into existing numerical codes. It is emphasized that it is the quasi-static approximation which makes some Braginskii coefficients divergent in a weakly-collisional regime. Importantly, we show that the heat fluxes and viscosity-tensors are coupled even in the linear approximation, and that the fully contracted (scalar) perturbations of the 4th-order moment, which are accounted for in the 22-moment model, modify the energy exchange rates. We also provide several Appendices, which can be useful as a guide for deriving the Braginskii model with the moment method of Grad., Comment: Updated version. Title change from "Braginskii-type" to "Braginskii Type" to match the published version (accepted to ApJ Supplements, 29 Jan 2022). Bookmarks included, TOC is now at the beginning, numbering in Appendix I is fixed, few typos corrected

Observations of a continual erosion of the strahl and build up of the halo with distance from the sun suggests that, at least in part, the halo may be formed as a result of scattering of the strahl. This hypothesis is supported in this paper by observation of intense scattering of strahl electrons, which gives rise to a proto-halo electron population. This population eventually merges into, or becomes the halo. The fact that observations of intense scattering of the strahl are not common implies that the formation of the halo may not be a continuous process, but one that occurs, in part, in bursts in regions where the conditions responsible for the scattering are optimum.

We present here the results of a study of interacting magnetic fields that involves a force normal to the reconnection layer. In the presence of such force, the reconnection layer becomes unstable to interchange disturbances. The interchange instability results in formation of tongues of heated plasma that leaves the reconnection layer through its wide surface rather than through its narrow ends, as is the case in traditional magnetic reconnection models. This plasma flow out of the reconnection layer facilitates the removal of plasma from the layer and leads to fast reconnection. The proposed mechanism provides fast reconnection of interacting magnetic fields and does not depend on the thickness of the reconnection layer. This instability explains the strong turbulence and bidirectional streaming of plasma that is directed toward and away from the reconnection layer that is observed frequently above reconnection layers. The force normal to the reconnection layer also accelerates the removal of plasma islands appearing in the reconnection layer during turbulent reconnection. In the presence of this force normal to the reconnection layer, these islands are removed from the reconnection layer by the "buoyancy force", as happens in the case of interchange instability that arises due to the polarization electric field generated at the boundaries of the islands.

International audience; We report Magnetospheric Multiscale (MMS) observations of a reconnecting current sheet in the presence of a weak density asymmetry with large guide field at the dayside magnetopause. An ion diffusion region (IDR) was detected associated with this current sheet. Parallel current dominated over the perpendicular current in the IDR, as found in previous studies of component reconnection. Electrons were preferentially heated parallel to the magnetic field within the IDR. The heating was manifested as a flattop distribution below 400eV. Two types of electromagnetic electron whistler waves were observed within the regions where electrons were heated. One type of whistler wave was associated with nonlinear structures in E-|| with amplitudes up to 20mV/m. The other type was not associated with any structures in E-||. Poynting fluxes of these two types of whistler waves were directed away from the X-line. We suggest that the nonlinear evolution of the oblique whistler waves gave rise to the solitary structures in E-||. There was a perpendicular super-Alfvenic outflow jet that was carried by magnetized electrons. Intense electrostatic lower hybrid drift waves were localized in the current sheet center and were probably driven by the super-Alfvenic electron jet, the velocity of which was approximately equal to the diamagnetic drift of demagnetized ions. Our observations suggest that the guide field significantly modified the structures (Hall electromagnetic fields and current system) and wave properties in the IDR.

The Cluster mission has been operated successfully for 14 years. During this time period, the evolution of the orbit has enabled Cluster to sample many more magnetospheric regions than was initially anticipated. So far, the separation of the Cluster spacecraft has been changed more than 30 times and has ranged from a few kilometres up to 36 000 km. These orbital changes have enabled the science team to address a wide variety of scientific objectives in key regions of Earth's geospace environment: the solar wind and bow shock, the magnetopause, polar cusps, magnetotail, plasmasphere and the auroral acceleration region. Recent results have shed new light on solar wind turbulence. They showed that the magnetosheath can be asymmetric under low Mach number and that it can contain density enhancement that may affect the magnetosphere. The magnetopause was found to be thinner and to have a higher current density on the duskside than on the dawnside. New methods have been used to obtain characteristic of the magnetotail current sheet and high-temporal-resolution measurements of electron pitch angle within flux transfer events (FTEs). Plasmaspheric wind has been discovered, and the refilling of the plasmasphere was observed for the first time over a very wide range of L shells. New models of global electric and magnetic fields of the magnetosphere have been obtained where Cluster, due to its polar orbit, has been essential. Finally, magnetic reconnection was viewed for the first time with high-resolution wave and electron measurements and acceleration of plasma was observed during times of varying rate of magnetic reconnection. The analysis of Cluster data was facilitated by the creation of the Cluster Science Data System (CSDS) and the Cluster Science Archive (CSA). Those systems were implemented to provide, for the first time for a plasma physics mission, a long-term public archive of all calibrated high-resolution data from all instruments.

We report unambiguous in situ observation of the coalescence of macroscopic flux ropes by the magnetospheric multiscale (MMS) mission. Two coalescing flux ropes with sizes of ∼1 RE were identified at the subsolar magnetopause by the occurrence of an asymmetric quadrupolar signature in the normal component of the magnetic field measured by the MMS spacecraft. An electron diffusion region (EDR) with a width of four local electron inertial lengths was embedded within the merging current sheet. The EDR was characterized by an intense parallel electric field, significant energy dissipation, and suprathermal electrons. Although the electrons were organized by a large guide field, the small observed electron pressure nongyrotropy may be sufficient to support a significant fraction of the parallel electric field within the EDR. Since the flux ropes are observed in the exhaust region, we suggest that secondary EDRs are formed further downstream of the primary reconnection line between the magnetosheath and magnetospheric fields.

A case study is presented using measurements from the Cluster spacecraft and ground-based magnetometers that show a substorm onset propagating from the inner to outer plasma sheet. On 3 October 2005, Cluster, traversing an ion-scale current sheet at the near-Earth plasma sheet, detected a sudden enhancement of Bz, which was immediately followed by a series of flux rope structures. Both the local Bz enhancement and flux ropes propagated tailward. Approximately 5 min later, another Bz enhancement, followed by a large density decrease, was observed to rapidly propagate earthward. Between the two Bz enhancements, a significant removal of magnetic flux occurred, possibly resulting from the tailward moving Bz enhancement and flux ropes. In our scenario, this flux removal caused the magnetotail to be globally stretched so that the thinnest sheet formed tailward of Cluster. The thinned current sheet facilitated magnetic reconnection that quickly evolved from plasma sheet to lobe and generated the later earthward moving dipolarization front (DF) followed by a reduction in density and entropy. Ground magnetograms located near the meridian of Cluster's magnetic foot points show two-step bay enhancements. The positive bay associated with the first Bz enhancement indicates that the substorm onset signatures propagated from the inner to the outer plasma sheet, consistent with the Cluster observation. The more intense bay features associated with the later DF are consistent with the earthward motion of the front. The event suggests that current disruption signatures that originated in the near-Earth current sheet propagated tailward, triggering or facilitating midtail reconnection, thereby preconditioning the magnetosphere for a later strong substorm enhancement.

We present Cluster observations of wave-particle interactions during an earthward propagating dipolarization front (DF) and associated fast plasma bulk flows detected at the central current sheet in Earth's magnetotail. During this period, flux tubes behind the DF frequently contain more energetic or hotter ions than did the preexisting flux tubes ahead of the DF. On the other hand, electrons within the DF flux tubes heat less, or are even colder, than were the preexisting populations and are often accompanied by superposed isolated beams. At the same time, electrostatic emissions are strongly enhanced over a wide range of frequencies (up to several times the electron cyclotron frequency) behind the DFs. This low-frequency electrostatic wave power is well correlated with ion energization. From linear theory, we find two wave modes: a high-frequency beam mode and a low-frequency whistler mode that are associated with the electron beam component. We attribute the generation of whistlers to electron beams that persist for a while before undergoing rapid thermalization. The existence of isolated beam components behind DFs detected during the 4 s Cluster spin period indicates that DFs either provide a continuous source of electron beams or facilitate a physical process that maintains the beams against rapid thermalization. Our analysis suggests that the earthward motion of the DF flux tube, via Fermi acceleration as the magnetic field lines behind the DF shorten, can lead to the persistent electron beams that generate whistler mode waves, which in turn can heat ions. This scenario, by which free energy in electron beams generates waves that then heat ions, accounts for the Cluster observations of different energization behaviors between electrons and ions behind DFs.

The Taylor microscale is one of the fundamental turbulence scales. Not easily estimated in the interplanetary medium employing single spacecraft data, it has generally been studied through two point correlations. In this paper we present an alternative, albeit mathematically equivalent, method for estimating the Taylor microscale (λT). We make two independent determinations employing multi-spacecraft data sets from the Cluster mission, one using magnetic field data and a second using electron velocity data. Our results using the magnetic field data set yields a scale length of 1538 ± 550 km, slightly less than, but within the same range as, values found in previous magnetic-field-based studies. During time periods where both magnetic field and electron velocity data can be used, the two values can be compared. Relative comparisons show λT computed from the velocity is often significantly smaller than that from the magnetic field data. Due to a lack of events where both measurements are available, the absolute λT based on the electron fluid velocity is not able to be determined.

[1] Using a global magnetohydrodynamic (MHD) simulation of the magnetosphere during a disturbed interval on 14 September 2004, we have investigated fluctuations in plasma properties of the magnetotail in the Pi2 range and their relationship to dipolarization fronts (DFs). Results from the MHD simulation indicate that this event is a very active interval with variable convection and disorder in the tail on a range of scales as small as ∼1 RE. DFs are observed in the simulation at the leading edge of fast earthward flows that originate from reconnection regions that form between ∼−15 and −30 RE in the tail. Pi2 period fluctuations are identified in pressure, magnetic field, and velocity components inside −13 REfollowing each burst of DFs in the midnight sector. The fluctuations observed in the pressure appear to be generated by the successive DFs as they approach the interface between stretched tail field lines and dipolar field lines. Fluctuations in the velocity may be the result of interactions between successive DFs and are amplified directly following the passage of the DFs as they propagate earthward. Although the limited azimuthal extent of the pulsations near the plasma sheet, just inside of the braking region, makes it difficult to draw a direct comparison between the ground-based measurements and the pulsations at −6 RE, the temporal evolution of the simulated DFs and Pi2 pulsations approximately reproduces the timing of the variations observed by satellites and ground-based instruments. Therefore, we have been able to use the global simulation to track the bursty flows, dipolarization fronts, and associated Pi2 period fluctuations throughout the entire magnetosphere in order to understand the sources of the changes measured in the near-Earth region.

[1] Magnetic reconnection is an efficient way to convert magnetic energy into particle energy. In this paper, we use Cluster thermal electron and ion measurements in the vicinity of a reconnection X line to delineate the structure of the reconnection current sheet. Multispacecraft observations made by Cluster on 18 August 2002 indicate that an X line drifted close to the spacecraft, about 3.4 RE earthward of the position where another X line had been observed earlier. Comparison of the Hall magnetic and electric field geometry and the observed properties of energetic electron beams streaming along the separatrix between the Cluster spacecraft indicates that the second X line formed within 20 s of the observation of the first X line. Repeated flow reversals and Hall field geometry together with the presence of a magnetic island embedded in the outflow region downstream of the first X line suggest that the initial current sheet was unstable, perhaps to the tearing mode. We identify a region with a thickness of 0.72 ion inertial lengths (29 electron inertial lengths, de) of super-Alfvenic electron outflow (greater than the ion in-flow Alfven speed) during the period when the spacecraft was in the vicinity of the neutral sheet. Slightly below the neutral sheet, Cluster observed asymmetric counter-streaming electrons with a loss of axisymmetry in the electron (V⟂1,V⟂2) distribution functions over a thin boundary with a thickness of several de. This electron-scale transition layer was embedded in a much wider region where both the ion and electron Walen tests failed, and the electron super-Alfvenic bulk outflow jets with high-energy electron beams were detected. Those phenomena provide details of the substructure of the reconnection current sheet and suggest that the spacecraft traversed or skimmed the tailward edge of an elongated electron current layer. We also note that this event differs from a previously reported reconnection event in that strong electron temperature anisotropy (T∥>T⟂) is observed both in the inflow region and in the exhaust, where the anisotropy appears to be associated with the elongated electron outflow jets.

After 12 years of operations, the Cluster mission continues to successfully fulfil its scientific objectives. The main goal of the Cluster mission, comprised of four identical spacecraft, is to study in three dimensions small-scale plasma structures in key plasma regions of the Earth's environment: solar wind and bow shock, magnetopause, polar cusps, magnetotail, plasmasphere and auroral zone. During the course of the mission, the relative distance between the four spacecraft has been varied from 20 km to 36 000 km to study the scientific regions of interest at different scales. Since summer 2005, new multi-scale constellations have been implemented, wherein three spacecraft (C1, C2, C3) are separated by 10 000 km, while the fourth one (C4) is at a variable distance ranging between 20 km and 10 000 km from C3. Recent observations were conducted in the auroral acceleration region with the spacecraft separated by 1000s km. We present highlights of the results obtained during the last 12 years on collisionless shocks, magnetopause waves, magnetotail dynamics, plasmaspheric structures, and the auroral acceleration region. In addition, we highlight Cluster results on understanding the impact of Coronal Mass Ejections (CME) on the Earth environment. We will also present Cluster data accessibility through the Cluster Science Data System (CSDS), and the Cluster Active Archive (CAA), which was implemented to provide a permanent and public archive of high resolution Cluster data from all instruments.

International audience; We have investigated the dependence of mass, energy, and charge of solar wind (SW) transport across Earth?s bow shock. An examination of 111 crossings during quiet SW in both quasi-perpendicular and quasi-parallel shock regions shows that 64 crossings had various degrees of heating and thermalization of SW. We found 22 crossings where the SW speed was

International audience; We report Magnetospheric Multiscale observations of high-frequency electrostatic waves in the vicinity of the reconnection ion diffusion region on the dayside magnetopause. The ion diffusion region is identified during two magnetopause crossings by the Hall electromagnetic fields, the slippage of ions with respect to the magnetic field, and magnetic energy dissipation. In addition to electron beam modes that have been previously detected at the separatrix on the magnetospheric side of the magnetopause, we report, for the first time, the existence of electron cyclotron harmonic waves at the magnetosheath separatrix. Broadband waves between the electron cyclotron and electron plasma frequencies, which were probably generated by electron beams, were found within the magnetopause current sheet. Contributions by these high-frequency waves to the magnetic energy dissipation were negligible in the diffusion regions as compared to those of lower-frequency waves.

The Universe is permeated by hot, turbulent, magnetized plasmas. Turbulent plasma is a major constituent of active galactic nuclei, supernova remnants, the intergalactic and interstellar medium, the solar corona, the solar wind and the Earth's magnetosphere, just to mention a few examples. Energy dissipation of turbulent fluctuations plays a key role in plasma heating and energization, yet we still do not understand the underlying physical mechanisms involved. THOR is a mission designed to answer the questions of how turbulent plasma is heated and particles accelerated, how the dissipated energy is partitioned and how dissipation operates in different regimes of turbulence. THOR is a single-spacecraft mission with an orbit tuned to maximize data return from regions in near-Earth space - magnetosheath, shock, foreshock and pristine solar wind - featuring different kinds of turbulence. Here we summarize the THOR proposal submitted on 15 January 2015 to the 'Call for a Medium-size mission opportunity in ESAs Science Programme for a launch in 2025 (M4)'. THOR has been selected by European Space Agency (ESA) for the study phase. Funding: The THOR science team thanks the Swedish National Space Board for support to carry out a technical assessment phase study before the proposal submission. We acknowledge the useful discussion and comments from the THOR team (http://thor.irfu.se/team) and particularly D. Delcourt, D. Fontaine, A. Kis, G. Lapenta, M. Maksimovic, M. Opher, G. Paschmann, A. Petrukovic, S. Schwartz. We acknowledge: the support of the UK Space Agency through grant ST/N003322/1 to ICL; the support of Agenzia Spaziale Italiana through contract ASI-INAF 2015-039-R.O to University of Calabria, Italy and at IAPS/INAF, Rome; the support of the Belgian Science Policy Office through PRODEX PEA 4000116805 to BIRA-IASB; the support of the Czech Science Foundation through project 16-04956S to Charles University, Prague; the support of ESA PRODEX to IAP Prague; the support of CNRS and CNES to IRAP, LPP, LP2CE and LESIA; the support of the German Space Agency through grant 50 00 1603 to CAU; the support of Swedish National Space Board through grants 232/15 and 257/15 to IRF, Uppsala; the support of Academy of Finland through grant 267144 and European Research Council Consolidator through grant 682068-PRESTISSIMO to FMI; the support of ISSI team 'Kinetic Turbulence and Heating in the Solar wind'; the support of FP7 projects STORM and SHOCK. Vlasiator (http://vlasiator.fmi.fi) has been developed with the support of Academy of Finland and European Research Council Starting grant 200141-QuESpace.

International audience; NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.