K. Meziane

Low-frequency whistler waves and shocklets observed at quasi-perpendicular interplanetary shocks

[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.

Three‐dimensional observations of gyrating ion distributions far upstream from the Earth's bow shock and their association with low‐frequency waves

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 Earths bow shock.

Simultaneous observations of field‐aligned beams and gyrating ions in the terrestrial foreshock

[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

Self‐Reformation of the Quasi‐Perpendicular Shock: CLUSTER Observations

Among several mechanisms issued from simulation and theoretical studies proposed to account for the nonstationarity of quasi-perpendicular supercritical shocks, one process—the so-called self-reformation—driven by the accumulation of reflected ions in the foot has been intensively analyzed with simulations. Present results based on experimental CLUSTER mission clearly evidence signatures of this self-reformation process for the terrestrial bow shock. The study based on magnetic field measurements includes two parts: (i) a detailed analysis of one typical shock crossing for almost perpendicular shock directions where the risk of pollution by other nonstationarity mechanisms is minimal. A special attention is drawn on appropriate treatment of data to avoid any wrong interpretation. One key result is that the ramp width can reach a very narrow value covering a few electron inertial lengths only; (ii) a statistical analysis allows relating the signatures of this nonstationarity with different plasma conditions and shock regimes. Present results are discussed in comparison with previous simulation works.

Larmor radius size density holes discovered in the solar wind upstream of Earth’s bow shock

The Cluster and Double Star satellites recently observed plasma density holes upstream of Earth’s collisionless bow shock to apogee distances of ∼19 and 13 earth radii, respectively. A survey of 147 isolated density holes using 4s time resolution data shows they have a mean duration of ∼17.9±10.4s, but holes as short as 4s are observed. The average fractional density depletion (δn∕n) inside the holes is ∼0.68±0.14. The upstream edge of density holes can have enhanced densities that are five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock. Multispacecraft analyses show the density holes move with the solar wind, can have an ion gyroradius scale, and could be expanding. A small normal electric field points outward. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that backstreaming particles interacting with t...

Evidence for a high-energy tail associated with foreshock field-aligned beams

[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.

Proton cyclotron waves occurrence rate upstream from Mars observed by MAVEN: associated variability of the Martian upper atmosphere

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 MAVENs 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.

A review of field‐aligned beams observed upstream of the bow shock

For more than two decades the Earth’s bow shock and traveling interplanetary shocks have attracted much attention as researchers have attempted to understand the collisionless mechanisms that thermalize transmitted particles and accelerate those that are observed propagating away from the shock into the upstream. We are concerned here with the class of particles emerging from the shock that are field‐aligned and have energies of a few to several keV, and base our results on observations primarily from the Earth’s foreshock. While the basic empirical picture has been known for some time, fundamental questions about the underlying mechanisms producing them have resisted a comprehensive explanation. This review talk will begin with an overview of the observational framework, along with selected new results. The latter include recent refinements in the characterizations of upstream field‐aligned beams as a function of the shock geometry parameter θBn. Other observations from the Cluster spacecraft have shown ...

Field‐aligned and Gyrating Ion Beams in a Planetary Foreshock

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.

Characteristics of quasi‐monochromatic ULF waves in the Venusian foreshock

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.