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 ( ). 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 ( ), low-frequency ( where is the cyclotron frequency), obliquely propagating ( , where 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.
Coulomb collisions provide plasma resistivity and diffusion but in many low-density astrophysical plasmas such collisions between particles are extremely rare. Scattering of particles by ...electromagnetic waves can lower the plasma conductivity. Such anomalous resistivity due to wave-particle interactions could be crucial to many processes, including magnetic reconnection. It has been suggested that waves provide both diffusion and resistivity, which can support the reconnection electric field, but this requires direct observation to confirm. Here, we directly quantify anomalous resistivity, viscosity, and cross-field electron diffusion associated with lower hybrid waves using measurements from the four Magnetospheric Multiscale (MMS) spacecraft. We show that anomalous resistivity is approximately balanced by anomalous viscosity, and thus the waves do not contribute to the reconnection electric field. However, the waves do produce an anomalous electron drift and diffusion across the current layer associated with magnetic reconnection. This leads to relaxation of density gradients at timescales of order the ion cyclotron period, and hence modifies the reconnection process.
We present spectroscopic and photometric observations of the star HD 173977. It appears that the star is part of a double line binary system, with a period of 1.801 d, corresponding to twice the ...period of the photometric variations. Hence the star is an ellipsoidal variable. The system is probably synchronized. The physical parameters of both components were derived through two independant methods, one based on evolutionary tracks, the other being the result of the behaviour of light curves in a close binary system. After removing the ellipsoidal variations, 3 frequencies are detected in the photometric data: 8.56, 14.51 and 16.42 d-1, while 2 additional frequencies are also possible: 10.96 and 12.11 d-1. In accordance with its position in the HR diagram, the primary component of HD 173977 should be considered as a δ Scuti star and no longer as a γ Doradus star. In addition, HD 173844, used as a check star, is discovered variable with a 15.79 d-1 frequency and is classified as a δ Scuti star.
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Chorus emissions are a striking feature of the electromagnetic wave environment in the Earth's magnetosphere. These bursts of whistler‐mode waves exhibit characteristic frequency sweeps (chirps) ...believed to result from wave‐particle trapping of cyclotron‐resonant particles. Based on the theory of Omura et al. (2008), we predict the sweep rates of chorus elements observed by the THEMIS satellites. The predictions use independent observations of the electron distribution functions and have no free parameters. The predicted chirp rates are a function of wave amplitude, and this relation is clearly observed. The predictive success of the theory lends strong support to its underlying physical mechanism: cyclotron‐resonant wave‐particle trapping.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
High‐resolution multispacecraft magnetic field measurements from the Magnetospheric Multiscale mission's flux‐gate magnetometer are employed to examine statistical properties of plasma turbulence in ...the terrestrial magnetosheath and in the solar wind. Quantities examined include wave number spectra; structure functions of order two, four, and six; probability density functions of increments; and scale‐dependent kurtoses of the magnetic field. We evaluate the Taylor frozen‐in approximation by comparing single‐spacecraft time series analysis with direct multispacecraft measurements, including evidence based on comparison of probability distribution functions. The statistics studied span spatial scales from the inertial range down to proton and electron scales. We find agreement of spectral estimates using three different methods, and evidence of intermittent turbulence in both magnetosheath and solar wind; however, evidence for subproton‐scale coherent structures, seen in the magnetosheath, is not found in the solar wind.
Plain Language Summary
The unique measurement capabilities of the Magnetospheric Multiscale mission are employed to investigate turbulence in the terrestrial magnetosheath and in the solar wind. These statistical analyses extend our knowledge of the turbulent environment in these near‐Earth space plasmas.
Key Points
Structure functions up to 6th order are computed in magnetosheath and solar wind; differences are noted in the statistics in the two cases
Agreement between single‐ and two‐spacecraft results confirms Taylor “frozen‐in” hypothesis at subproton scales
Strong subproton‐scale intermittency is obtained for magnetosheath, but not for solar wind
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In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by ...bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 di, where di is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma..
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Aims. We present the first NIR spectro-interferometry of the LBV η Carinae. The observations were performed with the AMBER instrument of the ESO Very Large Telescope Interferometer (VLTI) using ...baselines from 42 to 89 m. The aim of this work is to study the wavelength dependence of η Car's optically thick wind region with a high spatial resolution of 5 mas (11 AU) and high spectral resolution. Methods. The observations were carried out with three 8.2 m Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1500 (MR-K mode) and 12 000 (HR-K mode). The MR-K observations were performed in the wavelength range around both the $\ion{He}{i}$ 2.059 μm and the Brγ 2.166 μm emission lines, the HR-K observations only in the Brγ line region. Results. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of η Car. In the K-band continuum, a diameter of $4.0\pm0.2$ mas (Gaussian FWHM, fit range 28–89 m baseline length) was measured for η Car's optically thick wind region. If we fit Hillier et al. (2001, ApJ, 553, 837) model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17$\,\mu$m continuum, the $\ion{He}{i}$, and the Brγ emission lines, respectively. In the continuum near the Brγ line, an elongation along a position angle of $120\degr\pm15\degr$ was found, consistent with previous VINCI/VLTI measurements by van Boekel et al. (2003, A&A, 410, L37). We compare the measured visibilities with predictions of the radiative transfer model of Hillier et al. (2001), finding good agreement. Furthermore, we discuss the detectability of the hypothetical hot binary companion. For the interpretation of the non-zero differential and closure phases measured within the Brγ line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.
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We present simultaneous measurements of dayside diffuse aurora and whistler‐mode waves made by the South Pole all‐sky imager and two of the THEMIS spacecraft. We found a high correlation between the ...diffuse aurora intensity at 557.7 nm near the footprint latitudes of THEMIS and whistler‐mode wave intensity measured on board. The power in other wave modes was negligibly small in most cases, indicating that the dayside diffuse aurora is driven by precipitating energetic electrons resonating with whistler‐mode waves. The high correlation over a wide L* range (6 < L* < 11) further allowed us to magnetically link the wave and magnetospheric plasma distributions with the auroral patterns. Two distinct regions of whistler‐mode waves and ambient plasma density were found outside the plasmasphere near the equator: (1) intense waves in a smooth, low density and (2) moderate waves with enhanced and fluctuating density. The whistler‐mode wave intensity in the fluctuating plasma density region is positively correlated with the ambient density variations. The corresponding auroral images show an azimuthally elongated diffuse auroral band on the field lines connected to the low density region, as opposed to a structured diffuse aurora on the fluctuating density field lines. Each structured diffuse auroral patch was stable for a few tens of minutes and slowly drifted azimuthally. The high correlation of waves and auroras indicates that the structured diffuse auroral pattern reflects the spatial distribution of whistler‐mode waves and ambient plasma density in space. The enhanced density measured by the spacecraft is quasi‐spatial and contributes to enhanced growth of whistler‐mode waves.
Key Points
Simultaneous dayside diffuse aurora and whistler measurements are presented
Diffuse aurora and whistler intensities are highly correlated
Structure and motion of low energy plasma are inferred from auroral patches
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The young stellar object MWC 297 is an embedded B1.5Ve star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has been observed with the VLT ...interferometer equipped with the AMBER instrument during its first commissioning run. AMBER/VLTI is currently the only near infrared interferometer that can observe spectrally dispersed visibilities. MWC 297 has been spatially resolved in the continuum with a visibility of $0.50^{+0.08}_{-0.10}$ as well as in the Brγ emission line where the visibility decreases to $0.33\pm0.06$. This change in the visibility with wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by the Brγ emission line and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. We also reproduce the shape of the visibilities in the Brγ line as well as the profile of this line obtained at an higher spectral resolution with the VLT/ISAAC spectrograph, and those of the Hα and Hβ lines. The disk and wind models yield a consistent inclination of the system of approximately 20°. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind that has a much higher velocity in the polar region than at the equator. The AMBER/VLTI unique capability of measuring spectral visibilities therefore allows us for the first time to compare the apparent geometry of a wind with the disk structure in a young stellar system.
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The first-order Fermi acceleration of electrons requires an injection of electrons into a mildly relativistic energy range. However, the mechanism of injection has remained a puzzle both in theory ...and observation. We present direct evidence for a novel stochastic shock drift acceleration theory for the injection obtained with Magnetospheric Multiscale observations at the Earth's bow shock. The theoretical model can explain electron acceleration to mildly relativistic energies at high-speed astrophysical shocks, which may provide a solution to the long-standing issue of electron injection.
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