STEREO and Wind observations of intense cyclotron harmonic waves at the Earth's bow shock and inside the magnetosheath (original) (raw)
Related papers
Electron Scattering by High-frequency Whistler Waves at Earth’s Bow Shock
The astrophysical journal, 2017
Electrons are accelerated to non-thermal energies at shocks in space and astrophysical environments. While different mechanisms of electron acceleration have been proposed, it remains unclear how non-thermal electrons are produced out of the thermal plasma pool. Here, we report in situ evidence of pitch-angle scattering of nonthermal electrons by whistler waves at Earth's bow shock. On 2015 November 4, the Magnetospheric Multiscale (MMS) mission crossed the bow shock with an Alfvén Mach number ∼11 and a shock angle ∼84°. In the ramp and overshoot regions, MMS revealed bursty enhancements of non-thermal (0.5-2 keV) electron flux, correlated with high-frequency (0.2-0.4 W ce , where W ce is the cyclotron frequency) parallel-propagating whistler waves. The electron velocity distribution (measured at 30 ms cadence) showed an enhanced gradient of phase-space density at and around the region where the electron velocity component parallel to the magnetic field matched the resonant energy inferred from the wave frequency range. The flux of 0.5 keV electrons (measured at 1 ms cadence) showed fluctuations with the same frequency. These features indicate that non-thermal electrons were pitch-angle scattered by cyclotron resonance with the high-frequency whistler waves. However, the precise role of the pitch-angle scattering by the higher-frequency whistler waves and possible nonlinear effects in the electron acceleration process remains unclear.
Electron Plasma Waves Upstream of the Earth's Bow Shock
Journal of Geophysical Research, 1985
Electrostatic waves are observed However, Gurnett et al. [1981] have noted that around the plasma frequency f in the electron this electrostatic noise is made up of an intense e foreshock, together with elec•ons backstreaming emission line, at f e' and of emissions above and from the bow shock Using data from the sounder below f that the• call the "sidebands". They ' pe aboard ISEE 1, we show that this noise, previ-have incerpreted the whole as Langmuir waves with ously understood as narrow band Langmuir waves a bandwidth due to Doppler shift and non-linear more or less widened by Doppler shift or non-effects. present observations of the associated solar wind burst noise by Harvey et al. [1979], are gene-not exactly equal to f-e' and If-f-^l may b• as rally considered as emitted by a beam-plasma large as 5 kHz. The shaPpe c •= of the spectrum depends instability at the electron plasma frequency f•e. on the connexion depth, and the wave number tends [see, for example, Fitzenreiter et al., 198 to increase with the observed frequency.
Large-amplitude electrostatic waves associated with magnetic ramp substructure at Earth's bow shock
Geophysical Research Letters, 2006
1] We present Polar observations of high frequency (100 Hz ] f ] 4000 Hz) electrostatic (ES) waves at Earth's bow shock under extreme solar wind conditions. Although solitary waves are observed, the most prevalent structures in the magnetic ramp are coherent, large-amplitude (up to 80 mV/m) ES wave packets, which last 10-30 cycles, and propagate at varied obliquities relative to the magnetic field. The ES wave power is well correlated with maxima in the magnetic ramp substructure, suggesting that these maxima are important source regions. Detailed interferometric based analysis of waveforms show that they have wavelengths of a few hundred meters (e.g., 20lD20l D 20lD 0.5r e ) and phase speeds at the acoustic speed, suggesting that they are ion acoustic waves (IAW)s. The IAWs, having potentials ]1 V with no net change, are not likely to affect bulk plasma energization, though they may scatter the plasma and thus affect plasma thermalization. Citation: Hull, A. (2006), Large-amplitude electrostatic waves associated with magnetic ramp substructure at Earth's bow shock, Geophys.
Journal of Geophysical Research, 1988
A theory is presented for the radiation at the third to fifth harmonics of the plasma frequency observed upstream from the Earth's bow shock: the radiation is produced by the process L + T • -• T in the foreshock, with the initial T • radiation being the frequently observed second harmonic radiation (generated by another process) and the L waves being products of the decay L • -• L + S of L • waves generated by a streaming instability. (Here L, S, and T denote Langmuir, ion acoustic, and 'transverse electromagnetic waves, respectively.) The theory can account for the observed radiation when unusually large levels (electric fields in excess of 10 mV/m) of suitable L waves are present. Such levels of L waves are possible, in principle, but have not been reported before; the radiation is observed quite infrequently, thereby implying a requirement for unusual foreshock conditions. Predictions for the characteristics of the source regions (one to each wing of the foreshock) and the bandwidth of the radiation are given. Potential problems for the theory, relating to the large levels of L waves required to account for the radiation, are discussed. 1. Introduction Radiation at the fundamental and second harmonic of the plasma frequency (fp = •Vp/2•r) is observed in solar radio bursts [e.g., McLean and Labrum, 1985, and references therein] and upstream from the Earth's bow shock [Dunckel, 1974; Gumett, 1975; Gumett et al., 1979; Harvey et al., 1979; Hoang et al.,
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.
Wave activity in front of high-β Earth bow shocks
Earth's bow shock in high β (ratio of thermal to magnetic pressure) solar wind environment is relatively rare phenomenon. However such a plasma object may be of interest for astrophysics. We survey statistics of high-β (β > 10) shock observations by near-Earth spacecraft since 1995. Typical solar wind parameters related with high β are: low speed, high density and very low IMF 1-2 nT. These conditions are usually quite transient and need to be verified immediately upstream of the observed shock crossings. About a hundred crossings were initially identified mostly with quazi-perpendicular geometry and high Mach number. In this report 22 Cluster project crossings are studied with spacecraft separation within 30-200 km. Observed shock front structure is different from that for quaziperpendicular supercritical shocks with β ∼1. There is no well defined ramp. Dominating magnetic waves have frequency 0.1-0.5 Hz (in some events 1-2 Hz). Polarization has no stable phase and is closer to linear. In some cases it is possible to determine wavelength at 0.1-0.5 Hz of the order of 200-900 km. Copyright statement. 1 Introduction Shocks are the primary dissipation mechanism in space plasmas with supersonic flows (Sagdeev, 1966; Kennel et al., 1985; Krasnoselskikh et al., 2013). A brand new branch of plasma science, theory of collisionless shocks, appeared in the sixties, in response to new observational data on solar flares and solar wind interaction with Earth magnetic field. In the solar system solar wind forms the bow shocks at planets and comets, the termination shock at the heliospheric interface. Interplanetary shocks develop, when large-scale transient structures propagate in solar wind after solar eruptions. In the distant space, shocks are associated with supernova explosions, stellar winds, collisions of galaxy clusters. Astrophysical shocks are believed to have a leading role in the acceleration process of cosmic rays (Krymskii, 1977; Axford et al., 1977). The review of space shock physics can be found in AGU Geophysical Monographs 34 and 35 (1985). The Earth bow shock has been most thoroughly studied since the launch of the first spacecraft and is the main source of our in-situ knowledge of collisionless shock structure and dynamics. Of particular interest to astrophysical applications are shocks in weak magnetic field environment (high-β shocks) (e.g., Markevitch and Vikhlinin, 2007). β is a dimensionless parameter, a ratio of plasma thermal to magnetic energy density. Unfortunately, observations of high β shocks near the Earth are quite rare, since the solar wind plasma usually has β ∼1. Very
Geophysical Research Letters
Electromagnetic whistler-mode chorus and electrostatic electron cyclotron harmonic (ECH) waves can contribute significantly to auroral electron precipitation and radiation belt electron acceleration. In the past, linear and nonlinear wave-particle interactions have been proposed to explain the occurrences of these magnetospheric waves. By analyzing Van Allen Probes data, we present here the first evidence for nonlinear coupling between chorus and ECH waves. The sum-frequency and difference-frequency interactions produced the ECH sidebands with discrete frequency sweeping structures exactly corresponding to the chorus rising tones. The newly generated weak sidebands did not satisfy the original electrostatic wave dispersion relation. After the generation of chorus and normal ECH waves by hot electron instabilities, the nonlinear wave-wave interactions could additionally redistribute energy among the resonant waves, potentially affecting to some extent the magnetospheric electron dynamics. Plain Language Summary Whistler-mode chorus and electron cyclotron harmonic emissions are two distinct magnetospheric waves responsible for auroral electron precipitation and radiation belt electron acceleration. How these magnetospheric waves are generated has remained an outstanding question. They were usually explained as a result of linear and nonlinear wave-particle interactions in early studies. By analyzing the high-resolution data of Van Allen Probes, we present here the first evidence for nonlinear coupling between chorus and electron cyclotron harmonic emissions. Such nonlinear wave-wave interactions could transfer energy among the resonant waves and affect the magnetospheric electron dynamics. This new finding will be of high interest to the communities of space plasma physics and magnetospheric physics.
Electron Scattering by Low-frequency Whistler Waves at Earth’s Bow Shock
The Astrophysical Journal, 2019
Electrons are accelerated to nonthermal energies at shocks in space and astrophysical environments. While shock drift acceleration (SDA) has been considered a key process of electron acceleration at Earth's bow shock, it has also been recognized that SDA needs to be combined with an additional stochastic process to explain the observed power-law energy spectra. Here, we show mildly energetic (∼0.5 keV) electrons are locally scattered (and accelerated while being confined) by magnetosonic-whistler waves within the shock transition layer, especially when the shock angle is large (q 70 Bn). When measured by the Magnetospheric Multiscale mission at a high cadence, ∼0.5 keV electron flux increased exponentially in the shock transition layer. However, the flux profile was not entirely smooth and the fluctuation showed temporal/spectral association with large-amplitude (dB B 0.3), low-frequency (W 0.1 ce where W ce is the cyclotron frequency), obliquely propagating (-q~ 30 60 kB , where q kB is the angle between the wave vector and background magnetic field) whistler waves, indicating that the particles were interacting with the waves. Particle simulations demonstrate that, although linear cyclotron resonances with ∼0.5 keV electrons are unlikely due to the obliquity and low frequencies of the waves, the electrons are still scattered beyond 90°pitch angle by (1) resonant mirroring (transit-time damping), (2) non-resonant mirroring, and (3) subharmonic cyclotron resonances. Such coupled nonlinear scattering processes are likely to provide the stochasticity needed to explain the power-law formation.
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].