ESA Science & Technology - Publication Archive

In the equatorial region of the Earth's inner magnetosphere, the electromagnetic ion cyclotron (EMIC) triggered emissions are generated through interaction with energetic protons. We investigate the generation process of the EMIC triggered emissions in the He⁺ branch and associated precipitation of the energetic protons using a one-dimensional hybrid simulation with a cylindrical parabolic magnetic geometry. The simulation results show a good agreement with the nonlinear wave growth theory. As the electron density becomes higher as in the plasmasphere or the plasmaplume, the wave amplitude thresholds for both H⁺ and He⁺ band triggered emissions become lower and their nonlinear growth rates become higher. The higher hot proton density also makes the thresholds lower. While the H⁺ branch triggered emissions interact with a few keV protons, the He⁺ branch triggered emissions interact with more energetic protons of a few hundred keV with a larger nonlinear growth rate.

Recent observations of the solar wind have pointed out the existence of a cascade of magnetic energy from the scale of the proton Larmor radius rho_p down to the electron Larmor radius rho_e scale. In this Letter we study the spatial properties of magnetic field fluctuations in the solar wind and find that at small scales the magnetic field does not resemble a sea of homogeneous fluctuations, but rather a two-dimensional plane containing thin current sheets and discontinuities with spatial sizes ranging from l>=rho_p down to rho_e and below. These isolated structures may be manifestations of intermittency that localize sites of turbulent dissipation. Studying the relationship between turbulent dissipation, reconnection, and intermittency is crucial for understanding the dynamics of laboratory and astrophysical plasmas.

We report the first in situ observation of high-latitude magnetopause (near the northern duskward cusp) Kelvin-Helmholtz waves (KHW) by Cluster on January 12, 2003, under strongly dawnward interplanetary magnetic field (IMF) conditions. The fluctuations unstable to Kelvin-Helmholtz instability (KHI) are found to propagate mostly tailward, i.e., along the direction almost 90° to both the magnetosheath and geomagnetic fields, which lowers the threshold of the KHI. The magnetic configuration across the boundary layer near the northern duskward cusp region during dawnward IMF is similar to that in the low-latitude boundary layer under northward IMF, in that (1) both magnetosheath and magnetospheric fields across the local boundary layer constitute the lowest magnetic shear and (2) the tailward propagation of the KHW is perpendicular to both fields. Approximately 3-hour-long periods of the KHW during dawnward IMF are followed by the rapid expansion of the dayside magnetosphere associated with the passage of an IMF discontinuity that characterizes an abrupt change in IMF cone angle, phi = acos B x B , from -90° to -10°. Cluster, which was on its outbound trajectory, continued observing the boundary waves at the northern evening-side magnetopause during sunward IMF conditions following the passage of the IMF discontinuity. By comparing the signatures of boundary fluctuations before and after the IMF discontinuity, we report that the frequencies of the most unstable KH modes increased after the discontinuity passed. This result demonstrates that differences in IMF orientations (especially in phi) are associated with the properties of KHW at the high-latitude magnetopause due to variations in thickness of the boundary layer, and/or width of the KH-unstable band on the surface of the dayside magnetopause.

Lower hybrid drift waves (LHDW) are commonly observed at plasma boundaries in space and laboratory, often having the strongest measured electric fields within these regions. We use data from two of the Cluster satellites (C3 and C4) located in the Earth's magnetotail and separated by a distance of the order of the electron gyroscale. These conditions allow us, for the first time, to make cross-spacecraft correlations of the LHDW and to determine the phase velocity and wavelength of the LHDW. Our results are in good agreement with the theoretical prediction. We show that the electrostatic potential of LHDW is linearly related to fluctuations in the magnetic field magnitude, which allows us to determine the velocity vector through the relation: Integral_of delta_E dt v = phi_delta_B_par . The electrostatic potential fluctuations corresponds to ~10 per cent of the electron temperature, which suggests that the waves can strongly affect the electron dynamics.

Published online on 31 July 2012.

The loss of relativistic electrons from the Earth's radiation belts can be described in terms of the quasi-linear pitch angle diffusion by cyclotron-resonant waves, provided that their frequency spectrum is broad enough. Chorus waves at large wave-normal angles with respect to the magnetic field are often present in CLUSTER and THEMIS measurements in the outer belt at moderate to high latitudes. An approximate analytical formulation of diffusion coefficients has been derived in the low-frequency limit, leading to a simplified analytical expression of diffusion coefficients and lifetimes for energetic trapped electrons. Large values of the wave-normal angles between the Gendrin and resonance angles are shown to induce important increases in diffusion, thereby strongly reducing the particle lifetimes (by almost two orders of magnitude). The analytical diffusion coefficients and lifetimes obtained here are found to be in a good agreement with full numerical calculations based on CLUSTER chorus waves measurements in the outer belt for electron energies ranging from 100 keV to 2 MeV. Such very oblique chorus waves could contribute to a predominantly perpendicular anisotropy of the global equatorial electron population on the dayside and to a relative isotropization at low energy under disturbed conditions. It is also suggested that they might play a significant role in pulsating auroras.

Published online 16 June 2012, in Online First

The Earth's bow shock is the most studied example of a collisionless shock in the solar system. It is also widely used to model or predict the behaviour at other astrophysical shock systems. Spacecraft observations, theoretical modelling and numerical simulations have led to a detailed understanding of the bow shock structure, the spatial organization of the components making up the shock interaction system, as well as fundamental shock processes such as particle heating and acceleration. In this paper we review the observations of accelerated ions at and upstream of the terrestrial bow shock and discuss the models and theories used to explain them. We describe the global morphology of the quasi-perpendicular and quasi-parallel shock regions and the foreshock. The acceleration processes for field-aligned beams and diffuse ion distribution types are discussed with connection to foreshock morphology and shock structure. The different possible mechanisms for extracting solar wind ions into the acceleration processes are also described. Despite several decades of study, there still remain some unsolved problems concerning ion acceleration at the bow shock, and we summarize these challenges.

An analytical model of magnetosheath plasma flow is described and compared with a large dataset of magnetosheath ion flow velocity measurements from Cluster and THEMIS spacecraft. The model is based on previous works by Kobel and Flückiger (1994) and Génot et al. (2011) and has been modified to overcome the restrictions of these models on the shape of model magnetopause and bow shock. Our model is compatible with any parabolic bow shock model and arbitrary magnetopause model. The model is relatively simple to implement and computationally inexpensive, and its only inputs are upstream solar wind parameters. Comparison with observed data yields a good correspondence: median error in the direction of flow velocity is comparable with the instrumental error, and flow magnitude is predicted with a reasonable accuracy (relative error in flow speed was less than 25% for 86.5% of observations).

Published online on 1 October 2011. To appear in a Special Issue of the journal, in press. We have analyzed Cluster magnetic field and plasma data during a high-altitude cusp crossing in 2003. The Cluster separation was ~5000 km and provided unique measurements of high energy particle properties both inside the DiaMagnetic Cavity (DMC) and surrounding magnetosheath. Most of the high energy electrons and protons had pitch angles of ~90 degrees in the cavity and the high energy particle intensities dropped as a function of distance from the cavity boundary. By assuming conservation of the first adiabatic invariant for the electrons our analysis indicates that most of the high-energy electrons in the diamagnetic cavity cannot directly originate from the magnetosheath or from the magnetosphere. Our test particle simulations in a local 3-D high-resolution MHD cusp model show that particles can gain up to 40 keV and their pitch angles become nearly 90 degrees in the local cusp geometry due to gradients in reconnection 'quasi-potential' agreeing with the Cluster RAPID observations. These results strongly support a local acceleration of particles in the cusp diamagnetic cavities.

We present a local mesoscale model of the magnetospheric cusp region with high resolution (up to 300 km). We discuss the construction and implementation of the initial configuration and give a detailed description of the numerical simulation. An overview of simulation results for the case of strongly northward interplanetary magnetic field (IMF) is then presented and compared with data from Cluster 2 spacecraft from 14 February 2003. Results show a cusp diamagnetic cavity (CDC) with depth normal to the magnetospheric boundary on the order of 1-2 R_E and a much larger extent of ~5-9 R_E tangential to the boundary, bounded by a gradual inner boundary with the magnetospheric lobe and a more distinct exterior boundary with the magnetosheath. These results are qualitatively consistent with observational data.

The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with w < w_ci, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., w > w_ci) is more relevant. Here we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically b_i > b_e ~1 and for high oblique angles of propagation 80° d theta_kB d 90° as observed from Cluster spacecraft data. The linear properties of plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (theta_kB < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales k-rho_i e1 to frequencies either larger or smaller than w_ci, depending on the anisotropy k_para/k_perp. This extension into small scales is more readily called whistler (w > w_ci) or KAW (w < w_ci), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW.

Solar wind controls non-thermal escape of planetary atmospheric volatiles, regardless of the strength of planetary magnetic fields. For both Earth with a strong dipole and Mars with weak remnant fields, the oxygen ion (O⁺) outflow has been separately found to be enhanced during corotating interaction region (CIR) passage. Here we compared the enhancements of O⁺ outflow on Earth and Mars driven by a CIR in January, 2008 when Sun, Earth and Mars were approximately aligned. The CIR propagation was recorded by STEREO, ACE, Cluster and Mars Express (MEX). During the CIR passage, Cluster observed enhanced flux of upwelling oxygen ions above the Earth's polar region, while MEX detected an increased escape flux of oxygen ions in the Martian magnetosphere. We found that, (1) under a solar wind dynamic pressure increase by 2-3 nPa, the rate of increase in Martian O⁺ outflow flux was one order higher than those on Earth; (2) as response to the same part of the CIR body, the rate of increase in Martian O⁺ outflow flux was on the same order as for Earth. The comparison results imply that the dipole effectively prevents coupling of solar wind kinetic energy to planetary ions, and the distance to the Sun is also crucially important for planetary volatile loss in our inner solar system.

Magnetospheric substorms are elemental processes of solar wind energy storage and explosive release in Earth's magnetosphere. They encompass fundamental plasma physics questions, are ubiquitous during all types of geomagnetic conditions, contribute significantly to magnetic storms, and are a key element of Space Weather applications. This paper reviews recent major advances enabled by modern multi-point space-based and ground-based platforms. These datasets have also empowered a system-wide perspective and advanced modeling. We particularly highlight progress in two areas: (1) substorm onset timing and evidence for current sheet preconditioning and destabilization and (2) fast flows and dipolarizations, including the role of entropy in magnetotail plasma propagation.

The plasma density above the Earth's polar caps provide crucial information about the state of the magnetosphere. This region of space is known for its tenuous plasma and extremely low plasma densities, thus making traditional measurements with particle and plasma instruments extremely difficult. A new method based on spacecraft potential measurements from the electric field instrument onboard the Cluster satellites has shown that more reliable density measurements can be obtained. In this paper, we utilize this method and present a survey of the polar cap densities and the response to changes in the solar irradiation, solar wind parameters as well as processes internal to the magnetosphere. Our observations spans a time interval of almost 10 years, thus covering almost a full solar cycle. The observations seem to confirm that solar irradiance, and thus ionization through UV absorption in the atmosphere is the most important mechanism controlling the polar cap cold plasma density. We also find positive correlations between polar cap density and solar wind density and solar wind dynamic pressure, as well as geomagnetic activity levels.

It has recently been proposed that ripples inherent to the bow shock during radial interplanetary magnetic field (IMF) may produce local high speed flows in the magnetosheath. These jets can have a dynamic pressure much larger than the dynamic pressure of the solar wind. On 17 March 2007, several jets of this type were observed by the Cluster spacecraft. We study in detail these jets and their effects on the magnetopause, the magnetosphere, and the ionospheric convection. We find that (1) the jets could have a scale size of up to a few RE but less than ~6 RE transverse to the XGSE axis; (2) the jets caused significant local magnetopause perturbations due to their high dynamic pressure; (3) during the period when the jets were observed, irregular pulsations at the geostationary orbit and localised flow enhancements in the ionosphere were detected. We suggest that these inner magnetospheric phenomena were caused by the magnetosheath jets.

A pair of negative electric potential structures associated with inverted-V aurora is investigated using electric and magnetic field, ion and electron data from the Cluster spacecraft, crossing the auroral acceleration region (AAR) at different altitudes above the Northern hemisphere midnight auroral oval. The spatial and temporal development of the acceleration structures is studied, given the magnetic conjunction opportunity and the one minute difference between the Cluster spacecraft crossings. The configuration allowed for estimation of characteristic times of development for the two structures and of the parallel electric field and potential drop for the more stable one. The first potential structure had a width of <80 km (projected to the ionosphere) and was relatively short-lived, developing in less than 40 s and decaying in one minute. The parallel potential drop increased between altitudes of 1.13 RE and 1.3 RE, whereas the acceleration potential above 1.3 RE remained almost unchanged during that time. This intensification occurred mainly after the time when the associated upward current had reached its maximum value. The second structure had a width of <50 km and was subject to an increase by a factor of 3 of the parallel potential drop below 1.3 RE, during about 40 s, after which it remained rather stable for one minute or more. Similarly here, the acceleration potential above 1.3 RE remained roughly unchanged. For the more stable second structure, an average parallel electric field between 1.13 and 1.3 RE could be estimated (<0.56 mV/m). The conductance along the flux tube was also stable for one minute or more.

Shock waves are ubiquitous in space and astrophysics. They transform directed flow energy into thermal energy and accelerate energetic particles. The energy repartition is a multiscale process related to the spatial and temporal structure of the electromagnetic fields within the shock layer. While large scale features of ion heating are known, the electron heating and smaller scale fields remain poorly understood. We determine for the first time the scale of the electron temperature gradient via electron distributions measured in situ by the Cluster spacecraft. Half of the electron heating coincides with a narrow layer several electron inertial lengths (c/omega_pe) thick. Consequently, the nonlinear steepening is limited by wave dispersion. The dc electric field must also vary over these small scales, strongly influencing the efficiency of shocks as cosmic ray accelerators.

More than half a century after the discovery of Pi2 pulsations, Pi2 research is still vigorous and evolving. Especially in the last decade, new results have provided supporting evidence for some Pi2 models, challenged earlier interpretations, and led to entirely new models. We have gone beyond the inner magnetosphere and have explored the outer magnetosphere, where Pi2 pulsations have been observed in unexpected places. The new Pi2 models cover virtually all magnetotail regions and their coupling, from the reconnection site via the lobes and plasma sheet to the ionosphere. In addition to understanding the Pi2 phenomenon in itself, it has also been important to study Pi2 pulsations in their role as transient manifestations of the coupling between the magnetosphere and the ionosphere. The transient Pi2 is an integral part of the substorm phenomenon, especially during substorm onset. Key questions about the workings of magnetospheric substorms are still awaiting answers, and research on Pi2 pulsations can help with those answers. Furthermore, the role of Pi2 pulsations in association with other dynamic magnetospheric modes has been explored in the last decade. Thus, the application of Pi2 research has expanded over the years, assuring that Pi2 research will remain active in this decade and beyond. Here we review recent advances, which have given us a new understanding of Pi2 pulsations generated at various places in the magnetosphere during different magnetospheric modes. We review seven Pi2 models found in the literature and show how they are supported by observations from spacecraft and ground observatories as well as numerical simulations. The models have different degrees of maturity; while some enjoy wide acceptance, others are still speculative.

A refined cascade model for kinetic turbulence in weakly collisional astrophysical plasmas is presented that includes both the transition between weak and strong turbulence and the effect of nonlocal interactions on the nonlinear transfer of energy. The model describes the transition between weak and strong MHD turbulence and the complementary transition from strong kinetic Alfven wave (KAW) turbulence to weak dissipating KAW turbulence, a new regime of weak turbulence in which the effects of shearing by large scale motions and kinetic dissipation play an important role. The inclusion of the effect of nonlocal motions on the nonlinear energy cascade rate in the dissipation range, specifically the shearing by large-scale motions, is proposed to explain the nearly power-law energy spectra observed in the dissipation range of both kinetic numerical simulations and solar wind observations.

Recent progress in understanding the physics of magnetic reconnection is conveniently summarized in terms of a phase diagram which organizes the essential dynamics for a wide variety of applications in heliophysics, laboratory, and astrophysics. The two key dimensionless parameters are the Lundquist number and the macrosopic system size in units of the ion sound gyroradius. In addition to the conventional single X-line collisional and collisionless phases, multiple X-line reconnection phases arise due to the presence of the plasmoid instability either in collisional and collisionless current sheets. In particular, there exists a unique phase termed "multiple X-line hybrid phase" where a hierarchy of collisional islands or plasmoids is terminated by a collisionless current sheet, resulting in a rapid coupling between the macroscopic and kinetic scales and a mixture of collisional and collisionless dynamics. The new phases involving multiple X-lines and collisionless physics may be important for the emerging applications of magnetic reconnection to accelerate charged particles beyond their thermal speeds. A large number of heliophysical and astrophysical plasmas are surveyed and grouped in the phase diagram: Earths magnetosphere, solar plasmas (chromosphere, corona, wind, and tachocline), galactic plasmas (molecular clouds, interstellar media, accretion disks and their coronae, Crab nebula, Sgr A*, gamma ray bursts, and magnetars), and extragalactic plasmas (active galactic nuclei disks and their coronae, galaxy clusters, radio lobes, and extragalactic jets). Significance of laboratory experiments, including a next generation reconnection experiment, is also discussed.

Pc3 pulsations are observed in the magnetosphere with wave periods of 10-45 s. Two distinct populations have been observed; one exhibits a frequency dependence on the solar wind magnetic field strength, whereas the other does not. The first population is explained in terms of a model where the bow shock reflects ions which generate upstream foreshock ULF waves. These waves are convected through the shock to the dayside magnetopause and thus to the magnetosphere. The source of the second population is not well understood. In this paper we examine the generation of a transient patch of Pc3 wave activity due to a hot flow anomaly (HFA) using a unique spacecraft conjunction that occurred during the first Earth flyby of the Rosetta spacecraft. Cluster, upstream of the bow shock and close to the Sun-Earth line observed an HFA. At this time Rosetta was nearing closest approach and together with ground magnetometer stations, observed a transient interval of Pc3 wave activity. Analysis also shows that the Pc3 waves occurred in the absence of a ULF wavefield just upstream of the bow shock. This result shows that HFAs can be a source of Pc3 wave activity, and may explain in part the origin of the second population of Pc3 waves. It also demonstrates in new detail the manner in which kinetic physics at the bow shock, driven by structure in the solar wind, can influence magnetospheric dynamics.