215 research outputs found
Observations of whistler mode waves with nonlinear parallel electric fields near the dayside magnetic reconnection separatrix by the Magnetospheric Multiscale mission
We show observations from the Magnetospheric Multiscale (MMS) mission of whistler mode waves in the Earth's low-latitude boundary layer (LLBL) during a magnetic reconnection event. The waves propagated obliquely to the magnetic field toward the X line and were confined to the edge of a southward jet in the LLBL. Bipolar parallel electric fields interpreted as electrostatic solitary waves (ESW) are observed intermittently and appear to be in phase with the parallel component of the whistler oscillations. The polarity of the ESWs suggests that if they propagate with the waves, they are electron enhancements as opposed to electron holes. The reduced electron distribution shows a shoulder in the distribution for parallel velocities between 17,000 and 22,000 km/s, which persisted during the interval when ESWs were observed, and is near the phase velocity of the whistlers. This shoulder can drive Langmuir waves, which were observed in the high-frequency parallel electric field data
Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath
Collisionless space plasma turbulence can generate reconnecting thin current
sheets as suggested by recent results of numerical magnetohydrodynamic
simulations. The MMS mission provides the first serious opportunity to check if
small ion-electron-scale reconnection, generated by turbulence, resembles the
reconnection events frequently observed in the magnetotail or at the
magnetopause. Here we investigate field and particle observations obtained by
the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel
bow shock geometry. We observe multiple small-scale current sheets during the
event and present a detailed look of one of the detected structures. The
emergence of thin current sheets can lead to electron scale structures where
ions are demagnetized. Within the selected structure we see signatures of ion
demagnetization, electron jets, electron heating and agyrotropy suggesting that
MMS spacecraft observe reconnection at these scales
Direct constraint on the distance of y2 Velorum from AMBER/VLTI observations
In this work, we present the first AMBER observations, of the Wolf-Rayet and
O (WR+O) star binary system y2 Velorum. The AMBER instrument was used with the
telescopes UT2, UT3, and UT4 on baselines ranging from 46m to 85m. It delivered
spectrally dispersed visibilities, as well as differential and closure phases,
with a resolution R = 1500 in the spectral band 1.95-2.17 micron. We interpret
these data in the context of a binary system with unresolved components,
neglecting in a first approximation the wind-wind collision zone flux
contribution. We show that the AMBER observables result primarily from the
contribution of the individual components of the WR+O binary system. We discuss
several interpretations of the residuals, and speculate on the detection of an
additional continuum component, originating from the free-free emission
associated with the wind-wind collision zone (WWCZ), and contributing at most
to the observed K-band flux at the 5% level. The expected absolute separation
and position angle at the time of observations were 5.1±0.9mas and
66±15° respectively. However, we infer a separation of
3.62+0.11-0.30 mas and a position angle of 73+9-11°. Our analysis thus
implies that the binary system lies at a distance of 368+38-13 pc, in agreement
with recent spectrophotometric estimates, but significantly larger than the
Hipparcos value of 258+41-31 pc
Two-Dimensional Velocity of the Magnetic Structure Observed on July 11, 2017 by the Magnetospheric Multiscale Spacecraft
In order to determine particle velocities and electric field in the frame of the magnetic structure, one first needs to determine the velocity of the magnetic structure in the frame of the spacecraft observations. Here, we demonstrate two methods to determine a two-dimensional magnetic structure velocity for the magnetic reconnection event observed in the magnetotail by the Magnetospheric Multiscale (MMS) spacecraft on July 11, 2017, Spatio-Temporal Difference (STD) and the recently developed polynomial reconstruction method. Both of these methods use the magnetic field measurements; the reconstruction technique also uses the current density measured by the particle instrument. We find rough agreement between the results of our methods and with other velocity determinations previously published. We also explain a number of features of STD and show that the polynomial reconstruction technique is most likely to be valid within a distance of 2 spacecraft spacings from the centroid of the MMS spacecraft. Both of these methods are susceptible to contamination by magnetometer calibration errors
On the deviation from Maxwellian of the ion velocity distribution functions in the turbulent magnetosheath
The degree of deviation from the thermodynamic equilibrium in the ion
velocity distribution functions (VDFs), measured by the Magnetospheric
Multiscale (MMS) mission in the Earth's turbulent magnetosheath, is
quantitatively investigated. Taking advantage of MMS ion data, having a
resolution never reached before in space missions, and of the comparison with
Vlasov-Maxwell simulations, this analysis aims at relating any deviation from
Maxwellian equilibrium to typical plasma parameters. Correlations of the
non-Maxwellian features with plasma quantities such as electric fields, ion
temperature, current density and ion vorticity are very similar in both
magnetosheath data and numerical experiments, and suggest that distortions in
the ion VDFs occur close to (but not exactly at) peaks in current density and
ion temperature. Similar results have also been found during a magnetopause
crossing by MMS. This work could help clarifying the origin of distortion of
the ion VDFs in space plasmas
Near-Infrared interferometry of Eta Carinae with high spatial and spectral resolution using the VLTI and the AMBER instrument
We present the first NIR spectro-interferometry of the LBV Eta Carinae. The K
band observations were performed with the AMBER instrument of the ESO Very
Large Telescope Interferometer using three 8.2m Unit Telescopes with baselines
from 42 to 89m. The aim of this work is to study the wavelength dependence of
Eta Car's optically thick wind region with a high spatial resolution of 5 mas
(11 AU) and high spectral resolution. The medium spectral resolution
observations (R=1,500) were performed in the wavelength range around both the
HeI 2.059 micron and the Br gamma 2.166 micron emission lines, the high
spectral resolution observations (R=12,000) only in the Br gamma line region.
In the K-band continuum, a diameter of 4.0 +/-0.2 mas (Gaussian FWHM, fit range
28-89m) was measured for Eta Car's optically thick wind region. If we fit
Hillier et al. (2001) model visibilities to the observed AMBER visibilities, we
obtain 50 % encircled-energy diameters of 4.2, 6.5 and 9.6mas in the 2.17
micron continuum, the HeI, and the Br gamma emission lines, respectively. In
the continuum near the Br gamma line, an elongation along a position angle of
120+/-15 degrees was found, consistent with previous VLTI/VINCI measurements by
van Boekel et al. (2003). 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 Br gamma 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.Comment: 22 pages, 14 figures, 2 tables; A&A in pres
Kinetic Interaction of Cold and Hot Protons With an Oblique EMIC Wave Near the Dayside Reconnecting Magnetopause
We report observations of the ion dynamics inside an Alfvén branch wave that propagates near the reconnecting dayside magnetopause. The measured frequency, wave normal angle and polarization are consistent with the predictions of a dispersion solver. The magnetospheric plasma contains hot protons (keV), cold protons (eV), plus some heavy ions. While the cold protons follow the magnetic field fluctuations and remain frozen-in, the hot protons are at the limit of magnetization. The cold protons exchange energy back and forth, adiabatically, with the wave fields. The cold proton velocity fluctuations contribute to balance the Hall term fluctuations in Ohm's law, and the wave E field has small ellipticity and right-handed polarization. The dispersion solver indicates that increasing the cold proton density facilitates propagation and amplification of these waves at oblique angles, as for the observed wave
Kepler observations of variability in B-type stars
The analysis of the light curves of 48 B-type stars observed by Kepler is
presented. Among these are 15 pulsating stars, all of which show low
frequencies characteristic of SPB stars. Seven of these stars also show a few
weak, isolated high frequencies and they could be considered as SPB/beta Cep
hybrids. In all cases the frequency spectra are quite different from what is
seen from ground-based observations. We suggest that this is because most of
the low frequencies are modes of high degree which are predicted to be unstable
in models of mid-B stars. We find that there are non-pulsating stars within the
beta Cep and SPB instability strips. Apart from the pulsating stars, we can
identify stars with frequency groupings similar to what is seen in Be stars but
which are not Be stars. The origin of the groupings is not clear, but may be
related to rotation. We find periodic variations in other stars which we
attribute to proximity effects in binary systems or possibly rotational
modulation. We find no evidence for pulsating stars between the cool edge of
the SPB and the hot edge of the delta Sct instability strips. None of the stars
show the broad features which can be attributed to stochastically-excited modes
as recently proposed. Among our sample of B stars are two chemically peculiar
stars, one of which is a HgMn star showing rotational modulation in the light
curve.Comment: 19 pages, 11 figures, 4 table
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