Bow shock specularly reflected ions in the presence of low-frequency electromagnetic waves: a case study (original) (raw)
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Planetary and Space Science, 2003
We present observations of E ¡ 40 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 ( B=B ∼ 1). When the waves are present the ion distributions appear as gyrophase-bunched gyrating distributions while ÿeld-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 re ection 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. ?
Journal of Geophysical Research, 2001
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 R E . 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 (I/SBI/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. by a narrow angular distribution collimated along interplanetary magnetic field lines, and the ion beams are observed upstream from quasi-perpendicular shocks [Bonifazi and Moreno, 1981]. The diffuse ions exhibit an almost isotropic pitch angle distribution and are found upstream from quasi-parallel shocks. Another category of ion distribution has been identified as intermediate ions; these distributions have a crescent-like shape in velocity space and are centered along the magnetic field direction. High time resolution observations have shown that many intermediate ion distributions have signatures of gyrating ions that are characterized by gyromotion around the magnetic field [Gurgiolo et al., 1981; Thomsen et al., 1985; Fuselier et al., 1986a]. These gyrating ions are gyrophase-bunched (nongyrotropic) or nearly gyrotropic. The gyrophase-bunched ions have been observed throughout the region probed by the ISEE 1 spacecraft (up to 10-15 R E from the bow shock), whereas the nearly gyrotropic distributions (or ring beams) are rarely observed beyond -4 R E from the shock [Fuselier et al., 1986a]. The gyrophase-bunched ions are caused either by the reflection of the solar wind at the shock [Gurgiolo et al., 1983] or by the disruption of an ion beam by waves generated by the beam-plasma instability [Hoshino and Teresawa, 1985; Thomsen et al., 1985]. In the first case a portion of the incoming solar wind ions are specularly reflected at the shock, leading to gyrating distributions [Gosling et al., 1982; Gurgiolo et al., 1983]. Furthermore, the bunched ions can undergo gyrophase mixing within a few Earth radii of the shock [Gurgiolo et al., 1993].
The dynamics and upstream distributions of ions reflected at the Earth's bow shock
Journal of Geophysical Research, 1984
Using test particle trajectories in their gyromotion, and nongyrotropic velocity a simplified model of a supercritical, oblique, distributions [Gurgiolo et al., 1981]. These are collisionless shock, we investigate the inter-found both in the foot of the quasi-perpendicular action of solar wind thermal ions with the bow shock [Paschmann et al., 1982] and upstream earth's bow shock. We present results for shocks of quasi-parallel shocks [Eastman et al., 1981; with shock angle (SBn) between 35" and 60" and Gosling et al., 1982]. show that their velocity space signatures are This taxonomy has been used in other studies consistent with observations of many upstream ion of upstream particles [e.g., Bonifazi and Moreno,
EPL (Europhysics Letters), 2013
We present measurements from the ESA/NASA Cluster mission that show in situ acceleration of ions to energies of 1 MeV outside the bow shock. The observed heating can be associated with the presence of electromagnetic structures with strong spatial gradients of the electric field that lead to ion gyro-phase breaking and to the onset of chaos in ion trajectories. It results in rapid, stochastic acceleration of ions in the direction perpendicular to the ambient magnetic field. The electric potential of the structures can be compared to a field of moguls on a ski slope, capable of accelerating and ejecting the fast running skiers out of piste. This mechanism may represent the universal mechanism for perpendicular acceleration and heating of ions in the magnetosphere, the solar corona and in astrophysical plasmas. This is also a basic mechanism that can limit steepening of nonlinear electromagnetic structures at shocks and foreshocks in collisionless plasmas.
GYROSURFING ACCELERATION OF IONS IN FRONT OF EARTH's QUASI-PARALLEL BOW SHOCK
The Astrophysical Journal, 2013
It is well known that shocks in space plasmas can accelerate particles to high energies. However, many details of the shock acceleration mechanism are still unknown. A critical element of shock acceleration is the injection problem; i.e., the presence of the so called seed particle population that is needed for the acceleration to work efficiently. In our case study, we present for the first time observational evidence of gyroresonant surfing acceleration in front of Earth's quasi-parallel bow shock resulting in the appearance of the long-suspected seed particle population. For our analysis, we use simultaneous multi-spacecraft measurements provided by the Cluster spacecraft ion (CIS), magnetic (FGM), and electric field and wave instrument (EFW) during a time period of large inter-spacecraft separation distance. The spacecraft were moving toward the bow shock and were situated in the foreshock region. The results show that the gyroresonance surfing acceleration takes place as a consequence of interaction between circularly polarized monochromatic (or quasi-monochromatic) transversal electromagnetic plasma waves and short large amplitude magnetic structures (SLAMSs). The magnetic mirror force of the SLAMS provides the resonant conditions for the ions trapped by the waves and results in the acceleration of ions. Since wave packets with circular polarization and different kinds of magnetic structures are very commonly observed in front of Earth's quasi-parallel bow shock, the gyroresonant surfing acceleration proves to be an important particle injection mechanism. We also show that seed ions are accelerated directly from the solar wind ion population.
Ion cyclotron wave emission at the quasi-perpendicular bow shock
Journal of Geophysical Research, 1993
The power spectra of magnetic fluctuations occurring close to the ramp of the quasiperpendicular, low-•3 bow shock indicate the presence of obliquely propagating electromagnetic waves with frequencies above the ion cyclotron frequency, fii. These waves appear to be associated with ion distributions consisting of a bi-Maxwellian core and an energetic, approximately gyrotropic ring. We investigate the generation of ion cyclotron waves by distributions of this type, using particle and wave data from the AMPTE/I]•M spacecraft. In the case of a monoenergetic ring, instability is possible over a broad range of frequencies w > fii, with the highest growth rates occurring at propagation angles of typically 50 ø -80 ø. As the velocity spread of the ring vr increases, the growth rate of perpendicular-propagating waves falls, complete stabilization occurring when vr is greater than about 20% of the mean ring speed u. The parallel-propagating Alfv•n ion cyclotron mode can be excited if the core is anisotropic, with T.i_ -• 3Ttl. The maximum growth rate is obtained when vr is comparable to the core ion parallel thermal speed. However, if vr << u, the growth rate is much smaller than f•i. Using these results, we show that certain qualitative features of the AMPTE/IRM wave data can be understood in terms of a nearly monoenergetic ion ring beam at the shock ramp, evolving into an extended ring beam, and then merging with a quasi-bi-Maxwellian ion core as it moves downstream.
2022
This study presents new observations of fine structure and motion of the bow shock formed in the solar wind, upstream of the Earth's magnetosphere. The NASA's MMS mission has recorded during 2 hours eleven encounters with an oscillatory shock, which moves with the speed of 4-17 km/s and has thickness of 130 km, or an ion gyroradius. The shock is formed by steepening of 1 mHz magnetosonic wave, creating compressional magnetic field and plasma density structures, which initiate a chain of cross-field current-driven instabilities that heat solar wind ions by the stochastic ExB wave energisation mechanism. The theoretical ion energisation limits are confirmed by observations. We have identified the ion acceleration mechanism operating at shocks and explained double beam structures in the velocity space. The nature of this mechanism has been revealed as a stochastic resonant acceleration (SRA). The results provide for the first time a consistent picture of a chain of plasma processes that generate collisionless shocks and are responsible for particles energisation.
1] In 2007 during the declining phase of the solar cycle the energetic upstream ion events occurred mainly after a corotating interaction region passed the Earth's magnetosphere. We study the relation between these upstream events observed from about 70 to 1750 R E away from the Earth and observations in the vicinity of the terrestrial bow shock (up to 30 R E ). For this purpose, simultaneous measurements of energetic ions from STEREO A and STEREO B (far upstream region) and from Cluster and Geotail (near the bow shock) are used. In all cases the energetic ions far upstream are associated with the upstream ion events near the bow shock. The upstream events are observed simultaneously mainly when the magnetic field is pointing along the line joining those satellites in the far upstream region with those near the terrestrial bow shock. The upstream events near the bow shock often coincide with sunward directed electron bursts, increased AE index (>200 nT), nonexponential proton spectra, and most important the presence of O + ions, all of which imply at least partly a magnetospheric origin. In ∼57% of cases the upstream ion events near the bow shock are associated with electron bursts and/or with the presence of O + , and ∼40% of the latter events are associated with electron bursts at STEREO A. Although we present strong evidence that the events are partially of magnetospheric origin, we do not exclude the presence of the ions accelerated at the bow shock.
Statistical analysis of diffuse ion events upstream of the Earth's bow shock
Journal of Geophysical Research, 1994
A statistical study of diffuse energetic ion events and their related waves upstream of the Earth's bow shock was performed using data from the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite over two 5-month periods in 1984 and 1985. The data set was used to test the assumption in the self-consistent model of the upstream wave and particle populations by Lee (1982) that the particle acceleration through hydromagnetic waves and the wave generation are directly coupled. The comparison between the observed wave power and the wave power predicted on the observed energetic particle energy density and solar wind parameters results in a high correlation coefficient of about 0.89. The intensity of diffuse ions falls off approximately exponentially with the distance upstream from the bow shock parallel to the magnetic field with e-folding distances which vary from • 3.3 RE to • 11.7 RE over the energy range from 10 keV/e to 67.3 keV/e for both protons and alpha particles. After normalizing the upstream particle densities to zero bow shock distance by using these exponential variations, a good correlation (0.7) of the density of the diffuse ions with the solar wind density was found. This supports the suggestion that the solar wind is the source of the diffuse ions. Furthermore, the spectral slope of the diffuse ions correlates well with the solar wind velocity component in the direction of the interplanetary magnetic field (0.68 and 0.66 for protons and alpha particles) which concurs with the notion that the solar wind plays an important role in the acceleration of the upstream. particles. shock-associated particles. Serris et el. [1976, 1978] and Krimigis et el. [1978] have shown that particles with energies _>300 keV are of magnetospheric origin. A1-1Max-Planck-Institut fdr extraterrestrische Physik, Garching,