Search

The analysis of the structure function of a GNSS signal amplitude measured on the ground has revealed that ionospheric scintillation could be considered a proxy for ionospheric turbulence. More precisely, and in a recent report, the existence of a linear range with respect to the time lag in the structure function has been highlighted. In this context, the inertial-range analog has been determined from the analysis of a large set of scintillation events collected over several days from Pond Inlet located in the northern polar region and from Sao Paolo located at 23.2 degrees South of the Equator. At high latitude, we found that the mean value of the first-order scaling exponent is H = 0.55 ± 0.07, while a low altitude H is typically larger with H = 0.84 ±.11. This result clearly indicates that the long-time lag positive correlation remains persistent in the low latitude region. At high latitude however, both negative and positive long time lag correlation can occur. In addition, the obtained results clearly show that the inertial range analog is significantly smaller at high latitude, particularly the upper bound time lags at which the structure function deviates from linearity. This distinction may pinpoint to a difference in the ionospheric irregularity drift speed.

International audience; The Martian bow shock is a rich example of a supercritical, mass-loaded collisionless shock that coexists with ultra-low frequency upstream waves that are generated by the pick-up of exospheric ions. The small size of the bow shock stand-off distance (comparable with the solar wind ion convective gyroradius) raises questions about the nature of the particle acceleration and energy dissipation mechanism at work. The study of the Martian shock structure is crucial to understand its microphysics and is of special interest to understand the solar wind—planet interaction with a virtually unmagnetized body. We report on a complete identification and first characterization of the supercritical substructures of the Martian quasi-perpendicular shock, under the assumption of a moving shock layer, using MAVEN magnetic field and solar wind plasma observations for two examples of shock crossings. We obtained substructures length-scales comparable to those of the Terrestrial shock, with a narrow shock ramp of the order of a few electron inertial lengths. We also observed a well defined foot (smaller than the proton convected gyroradius) and overshoot that confirm the importance of ion dynamics for dissipative effects.

International audience; Electron velocity distributions in Mars's magnetosheath show a systematic erosion of the energy spectrum with distance downstream from the bow shock. Previous attempts to model this erosion invoked assumptions to promote electron ionization impact collisions with Mars's neutral hydrogen exosphere. We show that the near collision-free magnetosheath requires a kinetic description; the population of electrons at any location is a convolution of electrons arriving from more distant regions that ultimately map directly to the solar wind. We construct a simple model that captures all the essential physics. The model demonstrates how the erosion of the electron distributions is the result of the trapping, escape, and replacement of electrons that traverse the global bow shock; some are temporarily confined to the expanding cavity formed by the cross-shock electrostatic potential. The model also has implications for the ability of solar wind electrons to reach altitudes below the pileup boundary.

In this work we perform the first statistical analysis of the main properties of waves observed in the 0.05–0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the '30 s' waves observed at the Earth's foreshock, the normalized wave amplitude (∼0.2) and occurrence rate (∼0.5%) are much smaller. This suggests significant lower backstreaming proton fluxes, due to the relatively low solar wind Alfvenic Mach number around Mercury. These differences could also be related to the relatively smaller foreshock size and/or more variable solar wind conditions. Furthermore, we estimate that the speed of resonant backstreaming protons in the solar wind reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the solar wind speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.

Using MAVEN observations, we report variations of the amplitude of electromagnetic waves observed at the local proton cyclotron frequency upstream from the bow shock on short (plasma) time/length-scales: 1) a sharp sudden increase of the amplitude when crossing the electron foreshock boundary and 2) a decrease of this amplitude clearly correlated with the increasing distance from the shock along the magnetic field inside the foreshock without any simple relation to the planetary radial distance. These waves are excited by unstable ring-beam velocity distributions of newborn protons produced by ionization of exospheric hydrogen atoms. The amplitude of these waves is generally expected to depend only on different drivers including the observed large seasonality of the hydrogen exosphere, the EUV solar flux, the solar wind density and velocity or the IMF cone angle at different levels of importance. No noticeable wave amplitude change is expected when crossing the electron foreshock boundary and inside the pure electron foreshock. Surprisingly, we found that that these waves also display the two same aforementioned properties as the foreshock electrons fluxes at Mars though the wave origin is related to the ions only. We investigate the possibility that the extra free energy necessary to increase the wave amplitude could be due to supplementary ionization of hydrogen atoms by electron impact ionization inside the foreshock. Therefore, the electron foreshock also plays a role in the production of pickup protons which contribute to the planetary escape from high altitude.

International audience; Backstreaming electrons emanating from the bow shock of Mars reported from the Mars Atmosphere and Volatile EvolutioN/Solar Wind Electron Analyzer observations show a flux fall off with the distance from the shock. This feature is not observed at the terrestrial foreshock. The flux decay is observed only for electron energy E ≥ 29 eV. A reported recent study indicates that Mars foreshock electrons are produced at the shock in a mirror reflection of a portion of the solar wind electrons. In this context, and given that the electrons are sufficiently energetic to not be affected by the interplanetary magnetic field fluctuations, the observed flux decrease appears problematic. We investigate the possibility that the flux fall off with distance results from the impact of backstreaming electrons with Mars exospheric neutral hydrogen. We demonstrate that the flux fall off is consistent with the electron-atomic hydrogen impact cross section for a large range of energy. A better agreement is obtained for energy where the impact cross section is the highest. One important consequence is that foreshock electrons can play an important role in the production of pickup ions at Mars far exosphere.

Seen from the solar wind (SW) reference frame, the presence of newborn planetary protons upstream from the Martian and Venusian bow shocks and SW protons reflected from each of them constitute two sources of non-thermal proton populations. In both cases, the resulting proton velocity distribution function is highly unstable and capable of giving rise to ultra-low frequency quasi-monochromatic electromagnetic plasma waves. When these instabilities take place, the resulting non-linear waves are convected by the SW and interact with non-thermal protons located downstream from the wave generation region (upstream from the bow shock), playing a predominant role in their dynamics. To improve our understanding of these phenomena, we study the interaction between a charged particle and a large amplitude monochromatic circularly-polarized electromagnetic wave propagating parallel to a background magnetic field, from first principles. We determine the number of fix points in velocity space, their stability and their dependence on different wave-particle parameters. Particularly, we determine the temporal evolution of a charged particle in the pitch angle-gyrophase velocity plane under nominal conditions expected for backstreaming protons in planetary foreshocks and for newborn planetary protons in the upstream regions of Venus and Mars. In addition, the inclusion of wave ellipticity effects provides an explanation for pitch-angle distributions of supra-thermal protons observed at the Earth's foreshock, reported in previous studies. These analyses constitute a mean to evaluate if non-thermal proton velocity distribution functions observed at these plasma environments present signatures that can be understood in terms of non-linear wave-particle processes.

International audience; The location of ultra-low frequency (ULF) quasi-monochromatic wave onset upstream of Venus bow shock is explored using Venus Express magnetic field data. We report the existence of a spatial foreshock boundary behind which ULF waves are present. We have found that the ULF wave boundary at Venus is sensitive to the interplanetary magnetic field (IMF) direction like the terrestrial one and appears well defined for a cone angle larger than 30o. In the Venusian foreshock, the inclination angle of the wave boundary with respect to the Sun-Venus direction increases with the IMF cone angle. We also found that for the IMF nominal direction (θBX = 36°) at Venus’ orbit, the value of this inclination angle is 70o. Moreover, we have found that the inferred velocity of an ion traveling along the ULF boundary is in a qualitative agreement with a quasi-adiabatic reflection of a portion of the solar wind at the bow shock. For an IMF nominal direction at Venus, the inferred bulk speed of ions traveling along this boundary is 1.07 VSW, sufficiently enough to overcome the solar wind convection. This strongly suggests that the backstreaming ions upstream of the Venusian bow shock provide the main energy source for the ULF waves.

International audience; We report, for the first time, strong evidences that a fast Fermi mechanism is taking place at the Mars bow shock. The MAVEN spacecraft observations from the Solar Wind Electron Analyzer instrument show electron flux spikes with energies up to ∼1.5 keV. These spikes are associated with sunward propagating electrons and appear when the interplanetary field line threading the spacecraft is connected near the Martian bow shock tangency point. The observed loss cone distribution is a salient feature of these backstreaming electrons as the phase space density peaks on a ring centered along the magnetic field direction. Moreover, the data show no evidence of any effect due to a hypothetical cross-shock electric potential on the observed angular distributions. Although similar distributions are seen at the terrestrial bow shock, the quantitative analysis of the measurements strongly indicates that the electrons are produced at the shock foot and escape upstream before exploring the entire shock structure.

International audience; The statistical properties of ULF waves observed upstream of Venus foreshock are investigated. The study is restricted to waves which are observed well below the local proton cyclotron frequency. Using the magnetic field observations from Venus Express between May 2006 and February 2012, 115 quasi-monochromatic ULF wave trains have been identified. Statistical results show that the wave periods are mainly from 20 to 30 s in the spacecraft frame, which is about 2-3 times of the local proton cyclotron period. The transverse power dominates the power spectrum, and most of the waves display nearly circular or slightly elliptical polarization in the spacecraft frame. Moreover, these ULF waves mainly have small relative amplitudes with respect to the ambient field magnitude B0 for parallel component (δB||/B0 less than 0.3), while the range of relative amplitudes for perpendicular component δB⊥/B0 is from ~0.1 to ~1.0. Wave propagation angles are mainly less than 30° with respect to the mean magnetic field direction. The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock, which suggests that backstreaming ions in the Venusian foreshock form an important energy source for the generation of the waves.

We analyse energy-time dispersed ion signatures that have been observed by CLUSTER in the dayside magnetosheath. These events are characterized by sudden increases in the ion flux at energies larger than 10 keV. The high energy ions (30 keV) are first detected, with the transition to the low energy ions (5 keV) lasting about 100 s. These injections are often associated with transient plasma structures of a few minutes in duration, characterized by a hotter, less dense plasma and a diverted flow velocity, thus presenting similarities with "hot flow anomalies". They also involve modifications of the magnetic field direction, suggesting that the shock interacts with a solar wind discontinuity at the time of the event. The injections can originate from the magnetosphere or the shock region. Studying in detail a particular event, we discuss this last hypothesis. We show that the observed energy/time dispersion can be explained by combining a time-of-flight effect with a drift of the source of energetic particles along the shock. We propose that the acceleration results from a Fermi process linked to the interaction of the discontinuity with a quasi-perpendicular shock. This model explains the observed pitch-angle selection of the accelerated particles. The Fermi process acting on the beam of ions reflected from the shock appears to be sufficiently efficient to accelerate over short time scales (less than 30 s) particles at energies above 30 keV.Key words. Magnetospheric physics (solar-wind-magnetosphere interaction; magnetosheath) – Space plasma physics (shock waves)

We present case examples of foreshock density holes and results from a statistical survey, which provide additional characterizations of these recently-described structures. Specific effort is made to place these objects into context with well-studied foreshock phenomena, such as hot flow anomalies (HFAs) and large-amplitude magnetic pulsations (SLAMS). Density holes are observed during higher-than-average solar wind speeds (~620 km s−1), have well-correlated density and magnetic field intensities, and anti-correlated density and temperature variations. Like HFAs, these structures occur over a wide range of foreshock geometries, suggesting that this is not a determining factor. They are embedded within IMF current sheets, but their cross-structure magnetic shears are considerably lower than for HFAs. When the Cluster spacecraft are widely separated, they are able to measure structure time development, with substantial changes occurring over 10s of seconds, confirming an earlier case study, and possibly indicating short lifetimes as well. We find that density holes can occur in the absence of strong upstream magnetic pulsations and/or density enhancements, which rules out a "wake effect" as the sole explanation for their formation. Most important is the observation that the observed solar wind motional electric fields tend to have components pointing away from the embedding IMF current sheets. Density holes have no connection with magnetic holes and foreshock cavities, and appear not to be early-stage or weakly-formed HFAs.

International audience; Observations of the Mars upper atmosphere made from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft have been used to determine the loss rates of gas from the upper atmosphere to space for a complete Mars year (16 Nov 2014 – 3 Oct 2016). Loss rates for H and O are sufficient to remove ∼2-3 kg/s to space. By itself, this loss would be significant over the history of the planet. In addition, loss rates would have been greater early in history due to the enhanced solar EUV and more-active Sun. Integrated loss, based on current processes whose escape rates in the past are adjusted according to expected solar evolution, would have been as much as 0.8 bar CO2 or 23 m global equivalent layer of H2O; these losses are likely to be lower limits due to the nature of the extrapolation of loss rates to the earliest times. Combined with the lack of surface or subsurface reservoirs for CO2 that could hold remnants of an early, thick atmosphere, these results suggest that loss of gas to space has been the dominant process responsible for changing the climate of Mars from an early, warmer environment to the cold, dry one that we see today.

It is well established that interplanetary disturbances cause the Earth's bow shock to move sunward or earthward. Since the launch of the Cluster mission, precise determination of the shock velocity was carried out allowing for the possibility to perform sophisticated investigations about the shock dynamics. The shock normal and velocity are determined for 381 Earth's bow shock crossings using data from the Spatio Temporal Analysis of Field Fluctuations - Spectrum Analyzer experiment on board the Cluster spacecraft. We have found that the observed radial shock velocity is well fitted by a Maxwellian for speeds less than 80 km s−1. The Maxwellian fit provides a standard deviation σVobs=24 km s−1. Assuming that the shock motion is exclusively controlled by the time rate of change of solar wind ram pressure, a probability density for the shock velocity near the nose region is constructed using the basic elements of probability theory. The obtained results are in a very good agreement with the observations when typical conditions of the solar wind are considered. The present study indicates that the ram pressure is a predominant parameter in shock dynamics and the relevance of the Mach number is not a determinant.

Multipoint-measurements by the magnetic field Cluster-FGM (Flux Gate Magnetometer) are used to determine the local shock normal, and in turn allow the study of shock location shape and the velocity of the Earth's bow shock. The shock crossings cover orbits in which the spacecraft separation is of the order of ~ 600 km or less. A data selection of 133 bow shock crossings, ranging from quasi-steady perpendicular to moderately noisy oblique geometries, have been analyzed using a standard timing analysis. Prior to applying the timing technique, the magnetic field fluctuations, when present, are suppressed using low band-pass filtering. The present study contributes to similar studies conducted in the past and available in the literature through the inclusion of a larger data set. The shock standoff distance is determined conjointly with a paraboloid model and the results from a timing analysis. A statistical study reveals a standoff distance well in agreement with the standard gas dynamics model prediction for high Mach number MA. We have also found that for about half the crossings, the timing shock normals agree, within 11°, with a conic-based shock model. Our results strongly indicate that the motion of the shock is predominantly along the Sun–Earth direction; a departure from this direction is not related to the shock-crossing location. Shock velocities below ~ 80 km / s satisfactorily follow a nearly Gaussian distribution with zero mean and a standard deviation of ~ 42 km / s . Finally, we show that high speed motions are correlated with sharp increases in the solar wind upstream ram pressure, and are consistent with gas dynamics model predictions.

In this work we perform a statistical analysis of 92 foreshock cavitons observed with the Cluster spacecraft 1 during the period 2001–2006. We analyze time intervals during which the spacecraft was located in the Earth's foreshock with durations longer than 10 min. Together these amount to ~ 50 days. The cavitons are transient structures in the Earth's foreshock. Their main signatures in the data include simultaneous depletions of the magnetic field intensity and plasma density, which are surrounded by a rim of enhanced values of these two quantities. Cavitons form due to nonlinear interaction of transverse and compressive ultra-low frequency (ULF) waves and are therefore always surrounded by intense compressive ULF fluctuations. They are carried by the solar wind towards the bow shock. This work represents the first systematic study of a large sample of foreshock cavitons. We find that cavitons appear for a wide range of solar wind and interplanetary magnetic field conditions and are therefore a common feature upstream of Earth's quasi-parallel bow shock with an average occurrence rate of ~ 2 events per day. We also discuss their observational properties in the context of other known upstream phenomena and show that the cavitons are a distinct structure in the foreshock.

[1] The parallel and perpendicular reduced distribution functions of field-aligned beams (FABs) observed upstream of the Earth's bow shock using the Cluster spacecrafts are examined. A previous study revealed that FABs, observed in oblique shock geometries, exhibit reduced distribution functions with high-energy tails. A selection of FABs with weak-energy tails are considered, and the associated reduced distributions are fit with Maxwellians. First, we have found that the FABs full width at half maximum (FWHM), σ∥ and σ⊥ derived from the fit, are linearly correlated with the solar wind speed (or equivalently to solar wind temperature). Moreover, the parallel beam σ∥ has a very weak dependence upon the beam parallel speed which reflects the shock geometry; we have found that σ∥∼0.23Vsw. In contrast, we have found that the perpendicular beam σ⊥, in the range of beam speeds investigated, depends on the shock geometry. These new results indicate that the parallel σ∥ is essentially controlled by the solar wind while the shock geometry plays, along with the solar wind, a role in the perpendicular σ⊥. These results also put some strong constraints on theoretical models as far as field-aligned beam production mechanisms are concerned. One potential explanation for the significant perpendicular broadening of the FAB distribution reported in this study could be the presence of kinetic Alfven (or/and whistler) turbulence at the shock.

We report occasional observations of two simultaneously distinct ion foreshock components recorded by the Cluster spacecraft upstream of the Earth's bow shock. In most occurrences, the lower-energy population originates as a field-aligned beam (FAB) associated with quasi-perpendicular regions, which loses energy as the IMF rotates into oblique geometries. A second beam, with energies in excess of ~10 keV, appears sometimes in association with the onset of ultra-low frequency (ULF) waves, and sometimes ahead of the appearance of the latter. Measurements from the mass spectrometer indicate that both beams consist of protons. While the lower-speed beam is well-accounted for by a known reflection mechanism, the non-radial IMF orientations as well as other arguments seem to rule out magnetosheath or magnetospheric sources for the higher energy component. The wave characteristics are typical of the oblique foreshock and we have found that they are in cyclotron-resonance with the low speed beam (FAB). These observations constitute a theoretical challenge since conventional mechanisms described in the literature cannot account for the production of beams at two different energies.

Analytic treatments of a particle encountering a collisionless shock have commonly been based on the assumption that the shock surface is quasi-planar with length scales larger than the particle gyroradius. Within this framework, the particle distribution function width is supposed to be conserved in any shock reflection process. It is well known, however, that the thermal energy associated with backstreaming ions upstream of Earth's bow shock is significantly larger than the incident solar wind thermal energy. In a previous study, we found that non-thermal features of ions reflected quasi-adiabatically can be accounted for by considering the effect of small, normally distributed fluctuations of the shock normal over short temporal or spatial scales. The strong dependence of the particle acceleration on shock geometry leads to an increase in the temperature and to a non-thermal tail. Here, we conduct a similar analysis to investigate the effects of small, normally distributed fluctuations in the shock normal direction for specularly reflected ions. This later mechanism is considered of first importance in the dissipation process occurring at quasi-perpendicular shocks. We have derived the probability distribution functions f ( v ∥ ) and f ( v ⊥ ) of ions issued from a specular reflection of incident solar wind in the presence of normal direction fluctuations. These distributions deviate weakly from a Maxwellian, in agreement with the observations. In particular, a qualitative agreement with the ion thermal energy is obtained for fluctuations of the normal orientation in the 5–8° range about the nominal direction. Also, we have found that the shock θ B n has a weak effect on the shape of the distribution. While, not a strong determinant of the reflected distribution characteristics, the dynamical shock structure at ion scales cannot be ignored when accounting for the shock-accelerated particle thermal energy.

Measurements provided by the Magnetometer and the Extreme Ultraviolet Monitor (EUVM) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft together with atomic H exospheric densities derived from numerical simulations are studied for the time interval from October 2014 up to March 2016. We determine the proton cyclotron waves (PCWs) occurrence rate observed upstream from Mars at different times. We also study the relationship with temporal variabilities of the high altitude Martian hydrogen exosphere and the solar EUV flux reaching the Martian environment. We find that the abundance of PCWs is higher when Mars is close to perihelion, and decreases to lower and approximately constant values after the Martian Northern Spring Equinox. We also conclude that these variabilities cannot be associated with biases in MAVEN's spatial coverage or changes in the background magnetic field orientation. Higher H exospheric densities on the Martian day side are also found when Mars is closer to perihelion, as a result of changes in the thermospheric response to variability in the ultraviolet flux reaching Mars at different orbital distances. A consistent behavior is also observed in the analyzed daily irradiances measured by the MAVEN EUVM. The latter trends point towards an increase in the planetary proton densities upstream from the Martian bow shock near perihelion. These results then suggest a method to indirectly monitor the variability of the H exosphere up to very high altitudes during large time intervals (compared to direct measurements of neutral particles), based on the observed abundance of PCWs.

We present observations of E keV/e backstreaming ions measured by the Cluster Ion Spectrometry experiment at 1–2RE upstream of the bow shock. The ions are simultaneously observed at all three spacecrafts for which CIS measurements are available. The proton distributions are analyzed using 4 s time resolution. They are observed in association with low-frequency quasi-monochromatic waves with substantial amplitudes (δ B /B∼1) . When the waves are present the ion distributions appear as gyrophase-bunched gyrating distributions while field-aligned beams are also observed just adjacent to the interval of wave occurrence. The gyrating distributions are observed at distances larger than an ion Larmor radius and they have pitch-angles inconsistent with a specular reflection mechanism at the bow shock. The properties of the ULF waves reveal that they are in cyclotron resonance with the ion parallel beams that could drive a right-hand ion/ion instability responsible for the wave occurrence. Moreover, the observed pitch-angles for the gyrating ion distributions are consistent with the theoretical value expected if they are produced by a coherent nonlinear wave–particle interaction.

This report discusses the nature of gyrating ion distributions observed on board the Wind spacecraft by the three-dimensional ion electrostatic analyzer with high geometrical factor (3DP PESA-High). The gyrating ion distributions are observed near the inner ion beam foreshock boundary at distances between ∼9 and ∼83 RE. Our upstream measurements confirm several features previously reported using two-dimensional measurements. These distributions are observed in association with low-frequency waves with substantial amplitude (|δB|/B > 0.2). The analysis of the waves shows that they propagate in the right-hand mode roughly along the background magnetic field. The ions are bunched in gyrophase angle when the associated waves are quasi-monochromatic and high in amplitude. The peak of the ion distribution function rotates in the gyrophase plane. If the wave train is nonmonochromatic, the particle phase angle distribution is extended over a larger range, suggesting the occurrence of a phase mixing effect or a source at the shock. The phase angle distribution also seems to be energy-dependent, and no gyrophase rotation is observed in this case. Furthermore, we have characterized the ion distributions by computing their densities as well as parallel and perpendicular velocities. The results clearly indicate that the waves are cyclotron-resonant with the field-aligned beams observed just upstream. The resonance condition strongly suggests the local production of these gyrating ions in a field-aligned-beam disruption. Such a resonant wave-particle interaction may be a dominant characteristic of the back-streaming ion population in the foreshock at large distances from the Earth's bow shock.

Well-defined ring-like backstreaming ion distributions have been recently reported from observations made by the 3DP/PESA-High analyzer onboard the WIND spacecraft in the Earth's foreshock at large distances from the bow shock, which suggests a local production mechanism. The maximum phase space density for these distributions remains localized at a nearly constant pitch-angle value for a large number of gyroperiods while the shape of the distribution remains very steady. These distributions are also observed in association with quasi-monochromatic low frequency (~ 50 mHz) waves with substantial amplitude (δB/B>0.2). The analysis of the magnetic field data has shown that the waves are propagating parallel to the background field in the right-hand mode. Parallel ion beams are also often observed in the same region before the observation of both the ring-like distributions and the waves. The waves appear in cyclotron resonance with the ion parallel beams. We investigate first the possibility that the ion beams could provide the free energy source for driving an ion/ion instability responsible for the ULF wave occurrence. For that, we solve the wave dispersion relation with the observed parameters. Second, we show that the ring-like distributions could then be produced by a coherent nonlinear wave-particle interaction. It tends to trap the ions into narrow cells in velocity space centered on a well-defined pitch-angle, directly related to the saturation wave amplitude in the analytical theory. The theoretical predictions are in good quantitative agreement with the observations

On December 6, 1994, during a CIR (Corotating Interaction Region) event, the WIND 3D-Plasma and energetic particle experiment observed a burst (≤ 24 s) of 238-676 keV protons close to the electron foreshock boundary, followed by a 156-236 keV, 101-156 keV and 33-101 keV proton bursts about ∼40 s, 65 s and 85 s later, respectively. Similar dispersed bursts of helium with energies between -50 keV and ∼ 1 MeV followed the proton bursts. During this time, the IMF direction varied slowly with an almost monotonic decrease in θ Bn . The proton energy spectrum is initially peaked at -350 keV, and progresses to lower energies with time. The proton 3D angular distributions are peaked at ∼30° pitch-angle, propagating away from the shock, but they are non-gyrotropic. Finally, we show that the proton energy-spectra as well as the pitch angle distribution agree quantitatively with a model of a single adiabatic reflection of the incident energetic interplanetary ions by the quasi-perpendicular shock.

A statistical investigation of the location of onset of intermediate and gyrating ion populations in the Earth's foreshock is presented based on Fixed Voltage Analyzer data from ISEE 1. This study reveals the existence of a spatial boundary for intermediate and gyrating ion populations that coincides with the reported ULF wave boundary. This boundary position in the Earth's foreshock depends strongly upon the magnetic cone angle θBX and appears well defined for relatively large cone angles, though not for small cone angles. As reported in a previous study of the ULF wave boundary, the position of the intermediate-gyrating ion boundary is not compatible with a fixed growth rate of the waves resulting from the interaction between a uniform beam and the ambient plasma. The present work examines the momentum associated with protons which travel along this boundary, and we show that the variation of the boundary position (or equivalently, the associated particle momentum) with the cone angle is related to classical acceleration mechanisms at the bow shock surface. The same functional behavior as a function of the cone angle is obtained for the momentum predicted by an acceleration model and for the particle momentum associated with the boundary. However, the model predicts systematically larger values of the momentum than the observation related values by a constant amount; we suggest that this difference may be due to some momentum exchange between the incident solar-wind population and the backstreaming particles through a wave-particle interaction resulting from a beam plasma instability.Key words. Intermediate ion boundary · Statistical investigation · Earth's foreshock · ISEE 1 spacecraft

Several upstream ion distributions having a gyrating signature have been identified with the 3DP/PESA-High analyser on board the WIND spacecraft. These distributions are observed at distances greater than 20 R E from the Earth's bow shock. The distributions are observed in association with low frequency waves propagating quasi-parallel to the background magnetic field. By estimating the bulk velocity of the gyrating ions, we have found that the waves resonate with the particles. The observation of gyrating ions at large distances from the shock suggests their local production, probably from field-aligned beam disruption.

[1] Using ionospheric total electron content (TEC) measured by Global Positioning System (GPS) receivers of the Canadian High Arctic Ionospheric Network (CHAIN) we provide clear evidence for a systematic and propagating temporary TEC enhancement produced by compression of the magnetosphere due to a sudden increase in solar wind dynamic pressure. The magnetospheric compression is evident in THEMIS/GOES satellite data. Application of a GPS triangulation technique revealed that the TEC changes propagated with a speed of 3–6 km/s in the antisunward direction near noon and ∼8 km/s in the sunward direction in the pre-noon lower latitude sector. Characteristics of these TEC changes along with riometer absorption measurements seems to indicate that the TEC change is due to electron density enhancement in the F region and is possibly due to particle precipitation associated with sudden magnetospheric compression.

The Earth's ionosphere is embedded in the “magnetosphere” a cavity carved by the interaction of the high-speed solar wind and its “frozen-in” magnetic field with the terrestrial magnetic field. The solar wind is inherently non-steady, with its magnetic field, density, and flow speed varying on a range of time and amplitude scales. Variations in the solar wind and its magnetic field are known to be the major driver of variations in the high-latitude ionosphere. Using ionospheric total electron content (TEC) measured by Global Positioning System (GPS) receivers of the Canadian High Arctic Network (CHAIN), we provide clear evidence for a systematic and propagating TEC enhancement produced by the compression of the magnetosphere due to a sudden increase in the solar wind dynamic pressure. The magnetospheric compression is evident in the THEMIS/GOES data. Application of a GPS triangulation technique revealed that the TEC chnages propagated with a speed of ∼ 6 km/s in the antisunward direction near noon and ∼ 7 km/s in the sunward direction in the pre-noon sector. This is consistent with the scenario of increased ionospheric convection due to the magnetospheric compression. The characteristics of the TEC changes seems to indicate that they are due to the particle precipitation associated with the sudden magnetospheric compression.

[1] Over the past few decades, satellite investigations have revealed that the solar wind contains a plethora of distribution function signatures (proton beam, anisotropic cores) requiring models departing from the conventional fluid theory and necessitating the inclusion of wave-particle interactions. Numerous theoretical models and explanations have been proposed, but several questions concerning time and heliocentric evolution as well as the formation of beam components and anisotropic cores remain unanswered. We propose a simple solution for the generation of beam components, as a result of physical trapping due to obliquely propagating Alfven modes, based on a previously reported dynamical system describing the wave interaction with a single ion in the absence of dissipation mechanisms. We have found that beams with significant densities (7%–8%) and with velocities of the order of the Alfven speed can be generated. The beams' properties, including density, velocity, and temperature, are dependent on the wave propagation angle. The results are in qualitative agreement with the observations and could provide an explanation for the evolution of nonthermal distribution functions observed in fast solar wind and other space plasmas regimes.

Previous work has shown that solar wind suprathermal electrons can display a number of features in terms of their anisotropy. Of importance is the occurrence of counter-streaming electron patterns, i.e., with "beams" both parallel and anti-parallel to the local magnetic field, which is believed to shed light on the heliospheric magnetic field topology. In the present study, we use STEREO data to obtain the statistical properties of counter-streaming suprathermal electrons (CSEs) in the vicinity of corotating interaction regions (CIRs) during the period March–December 2007. Because this period corresponds to a minimum of solar activity, the results are unrelated to the sampling of large-scale coronal mass ejections, which can lead to CSE owing to their closed magnetic field topology. The present study statistically confirms that CSEs are primarily the result of suprathermal electron leakage from the compressed CIR into the upstream regions with the combined occurrence of halo depletion at 90° pitch angle. The occurrence rate of CSE is found to be about 15–20% on average during the period analyzed (depending on the criteria used), but superposed epoch analysis demonstrates that CSEs are preferentially observed both before and after the passage of the stream interface (with peak occurrence rate >35% in the trailing high speed stream), as well as both inside and outside CIRs. The results quantitatively show that CSEs are common in the solar wind during solar minimum, but yet they suggest that such distributions would be much more common if pitch angle scattering were absent. We further argue that (1) the formation of shocks contributes to the occurrence of enhanced counter-streaming sunward-directed fluxes, but does not appear to be a necessary condition, and (2) that the presence of small-scale transients with closed-field topologies likely also contributes to the occurrence of counter-streaming patterns, but only in the slow solar wind prior to CIRs.

We address the unsolved question of how foreshock field-aligned beam (FAB) parallel temperatures are produced. Studies including numerical simulations and recent observations have indicated that shocks can be nonstationary and include embedded spatial structures with varied scales. As a first step towards assessing the impact of such variability on backstreaming ions, we examine how a randomly distributed shock normal direction will affect FAB parallel velocity (v ∥) distributions. Assuming that the FABs are produced in a quasi-adiabatic reflection process at the shock, we derive a probability distribution function for v ∥. These derived distributions exhibit second, third and fourth order moments that agree well with the observations for a large range of reflection efficiencies δ, and depend strongly upon the average angle between the magnetic field and the shock normal θ Bn0. Best agreement is obtained for fluctuations of the normal orientation of a few degrees about a nominal direction. The derived model predicts a strong correlation between the shock geometry (θ Bn0) and the moments of the parallel velocity distribution, but with stronger tails extending to higher values of θ Bn0, a trend opposite to the observations.

[1] We present observations of low-frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular interplanetary (IP) shocks observed by the Wind spacecraft. Four of the five IP shocks had oblique precursor whistler waves propagating at angles with respect to the magnetic field of 20–50 and large propagation angles with respect to the shock normal; thus they do not appear to be phase standing. One event, the strongest in our study and likely supercritical, had low-frequency waves consistent with steepened magnetosonic waves called shocklets. The shocklets are seen in association with diffuse ion distributions. Both the shocklets and precursor whistlers are often seen simultaneously with anisotropic electron distributions unstable to the whistler heat flux instability. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. These results may offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

[1] The reduced particle distributions of field-aligned beams observed upstream of the bow shock are examined in detail using Cluster spacecraft. We find that the reduced parallel and perpendicular distribution forms can be strongly geometry-dependent. Above a certain critical value of the angle between the local shock normal and the direction of the magnetic field, qBn, the reduced distributions are remarkably well fit by Maxwellians. We have not found any significant changes to the spread in energies for beams at higher values of qBn. When the angle qBn decreases, leading to smaller beam velocities, a highenergy tail in the distribution appears. When the tail is present, the bulk of the distribution remains Maxwellian. The development of the high-energy tail is well correlated with decreases in the beam speed (or equivalently qBn). Moreover, detailed examination of the angular distributions indicates that particles in the tails of the distributions propagate at significant pitch angles with respect to the magnetic field (are not field-aligned, as are those within the bulk of the distribution) and that these pitch angles are energy-dependent. These new observations do not fit any production mechanism expected at the shock or result from known wave-particle interactions upstream of or within the shock layer.

Different types of backstreaming ion distributions have been reported in the region upstream from the Earth’s bow shock and magnetically connected to it (ion foreshock): field‐aligned beams (FABs), gyrating ion and diffuse ion distributions. Contrary to the first type, the two others are always associated with ULF waves. Among them, gyrating ions with well‐defined pitch‐angle and gyrophase organization around the local magnetic field have been frequently observed in association with large amplitude quasi‐monochromatic right‐hand mode waves. These waves reveal the existence of coherent wave‐particle interaction which is an efficient process to dissipate the energy of the particles reflected at the collisionless bow shock. It has been shown recently from a large data set from multi‐spacecraft observations by Cluster that the gyrophase‐bunched ion distributions are mainly produced by such a process from cyclotron‐resonant FABs observed just both at the edge of the gyrating ions region and the boundary of ULF waves.

[1] The wave-particle interaction plays a fundamental role in plasma physics; it is an energy momentum exchange mechanism between particles and waves. The most fundamental examples treated in the literature address the interaction of a single wave propagating along a background magnetic field with a single ion or electron. In the present paper we propose to investigate this specific problem, but this time we take into account the fact that the electromagnetic wave propagates obliquely with respect to the background field. The obliqueness manifests itself in the appearance of a parallel component in the electric field which in turn impacts the dynamics of the charged particle it interacts with and provides a mechanism of acceleration. This parallel component of the electric field can trap or untrap particles. The parallel propagation case is recovered automatically by setting the angle of propagation with respect to the background magnetic field to zero. A simple, yet complex, dynamical system is derived and limiting cases are treated analytically while numerical integration is used to investigate the general cases. We find that physical trapping occurs for a class of initial conditions and that phase space trapping (pitch angle versus gyrophase for example) remains in some cases a signature of the dynamical system.

[1] We have investigated the kinetic signatures within, and at the edges of, a foreshock cavity. Such cavities are believed to be formed when an isolated collection of interplanetary magnetic field lines connect to quasi-parallel regions of the Earth's bow shock, allowing energetic ions to flow upstream and excavate a local cavity. Observations by the Cluster spacecraft show precisely this configuration. The suprathermal ions can be seen just outside the edges of the cavity within a restricted range of gyrophases, consistent with their gyromotion tangential to the layer containing the cavity. Foreshock cavities, if sufficiently common, may play significant roles in triggering magnetospheric events. Thus our confirmation of their relatively simple formation mechanism lends support to their inferred frequency.

The foreshock region is the first signature of the interaction of the solar wind with a planet’s plasma environment when approaching its collisionless bow shock. Part of its structure and dynamic is determined by instabilities, which are created by the interaction of the solar wind with backstreaming ion populations. The interaction of the reflected ions with the solar wind drives ion/ion beam instabilities, which generate waves that are then convected towards the shock by the solar wind. Subsequently they may mediate the shock structure and its reflection properties. The most well‐know examples are the field aligned ion beams (FABs), produced by reflection processes in the quasi‐perpendicular and oblique regions of the shock. Other prominent examples are the gyrating ions with well‐defined pitch‐angle and gyrophase organization around the local magnetic field observed downstream of the FABs region. These gyrophase‐bunched ions are always associated with large amplitude quasi‐monochromatic right‐hand mode low‐frequency waves. Different mechanisms have been put forward to explain these ion features. This paper will discuss recent advances on this topic from multi‐spacecraft observations (Cluster) as well as theoretical considerations.

An energetic ion (E≤40) event observed by the CLUSTER/CIS experiment upstream of the Earth's bow shock is studied in detail. The ion event is observed in association with quasi-monochromatic ULF MHD-like waves, which we show modulate the ion fluxes. According to three statistical bow shock position models, the Cluster spacecrafts are located at ~0.5 Re from the shock and the averaged bow shock θBn0 is about ~30°. The analysis of the three-dimensional angular distribution indicates that ions propagating roughly along the magnetic field direction are observed at the onset of the event. Later on, the angular distribution is gyrophase-bunched and the pitch-angle distribution is peaked at α0~θBn0, consistent with the specular reflection production mechanism. The analysis of the waves shows that they are left-handed in the spacecraft frame of reference (right-handed in the solar wind frame) and propagate roughly along the ambient magnetic field; we have found that they are in cyclotron-resonance with the field-aligned beam observed just upstream. Using properties of the waves and particles, we explain the observed particle flux-modulation in the context of θBn changes at the shock caused by the convected ULF waves. We have found that the high count rates coincide with particles leaving the shock when θBn angles are less than ~40°, consistent with the specular reflection hypothesis as the production mechanism of ions.

[1] We examine an energetic (2–30 keV) upstream ion event presenting a clear doublepeak spectrum observed � 1 RE upstream from the bow shock. The lower-energy (E � 3.5 keV) peak is associated with an ion beam propagating along the magnetic field direction, while the higher-energy (E � 13 keV) peak is associated with gyrating ions having pitch angles � 30� . The latter population progressively extends to lower energies over the span of the event. During times when the field-aligned beams were observed, the interplanetary magnetic field was remarkably steady, while the appearance of the 30� pitch angle gyrating ions was accompanied by the onset of large-amplitude ultralow frequency fluctuations of the magnetic field. Our analysis indicates that the gyrating ions had guiding centers on field lines downstream of the field-aligned component but that both populations could be sampled simultaneously because of the orbits of the former. We find that the downstream limit of the field-aligned beams is populated with protons having a speed 1.68 times the solar wind velocity, which is inconsistent with any known shockrelated emission mechanisms. This boundary makes an angle of 77� with respect to the Sun-Earth line in agreement with theoretical predictions. Just downstream of this rapid transition, gyrating ions having a flow speed of 1.52 times the solar wind speed are observed in association with ULF waves. Like the field-aligned beams, the gyrating ions reported here have streaming speeds inconsistent with any known shock emission mechanisms. While the simultaneous observation of field-aligned and gyrating components is possible because of the large gyration orbits of the latter, the observational sequence is consistent with a very sharp (]1 gyroradius) boundary separating the guiding centers of each. Explicit observations of such a sharp demarcation between these populations have not been reported before, and they place a significant constraint on the production mechanisms of the two populations. Our interpretation of these observations provides a refinement of the usual framework for foreshock morphology. INDEX TERMS: 2116 Interplanetary Physics: Energetic particles, planetary; 2164 Interplanetary Physics: Solar wind plasma; 2134 Interplanetary Physics: Interplanetary magnetic fields; 2154 Interplanetary Physics: Planetary bow shocks; 7851 Space Plasma Physics: Shock waves; KEYWORDS: foreshock boundary, ultralow frequency waves, bow shock, field-aligned beam, magnetic moment, shock emission mechanism

[1] We present Cluster ion observations obtained at 18 RE in the magnetotail on 1 October 2001. According to a recent analysis, the quartet encountered a reconnection region and a tailward-moving neutral line. We examine in detail selected 3-D ion distributions, which through much of the hour following 0925 UT were non-gyrotropic. B-perpendicular slices of velocity space showed crescent-shaped regions. Occupied gyrophases were consistent over a wide range of parallel velocities, stable over time, and occurred unaccompanied by strong ion gyrofrequency waves. We interpret these observations as signatures of remote sensing near sharp particle gradients. In this view, distributions obtained simultaneously while Cluster straddled the current sheet are simply explained. Additionally, the computed first moments can have large transverse ( × B) components (, a unit boundary normal), without net plasma transport. We infer separate O+ layers above and below the current sheet.

[1] We report on gyrophase-restricted ion beams with energies extending from ∼100 keV up to ∼2 MeV, observed by Wind in the Earth's distant (∼65 RE) foreshock. The ion gyrophases seen were nearly constant during periods when the distance to the shock contact point could be expected to vary by several RE, when there was no significant wave activity. At times the distributions had two peaks ∼180° apart in gyrophase. These were consistent with a remotely-sensed energetic ion foreshock region having a thickness

[1] We present a statistical study of 216 energetic ion events observed just upstream of Earth's bow shock by the Wind spacecraft. The ions that compose this database range in energy from 27 keV to 2 MeV. Of particular interest is the effect of shock geometry on the properties of the energetic ions. Our approach is different from previous studies in that we only include upstream ion events associated with a local bow shock crossing. In this way we determine the shock geometry locally, rather than from extrapolations of remote upstream observations to a model shock source location, providing a better quantitative determination of the properties of the energetic ions in relation to shock geometry. Under typical interplanetary conditions in the absence of a preexisting population of ambient energetic ions (E ≥ 50 keV), we find that the ion energy spectrum is characterized by an energy cutoff at ∼200–330 keV. Neither the energy cutoff nor the energetic ion spectrum show any dependence on shock geometry as measured by the angle between the shock normal and magnetic field θBn. However, when an ambient population of energetic particles is present in the interplanetary medium, ion flux levels measured just upstream of the terrestrial shock reach a minimum near θBn ∼ 45° irrespective of energy. The energy spectrum spans up to ∼2 MeV. Moreover, ions with energies E ≥ 550 keV are observed only at θBn ≥ 45°. The flux levels of these more energetic ions increase, in an average sense, with increasing values of θBn. Lee's [1982] self-consistent theory of ion diffusive shock acceleration appears to explain the energetic ion flux levels observed at θBn ≤ 40°. Upstream energetic ions with energies E ≥ 550 keV appear to be the result of shock drift acceleration.