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The Stac Fada “impact ejecta” layer: not what it seems
The Stac Fada Member (SFM) forms part of the Stoer Group of the Torridonian of NW Scotland. The SFM is unique in the Torridonian, being characterized by the presence of greenish altered glass clasts. Its origin has been debated for decades with several hypotheses being proposed but all invoking some connection with volcanic activity in the region. More recently, Amor et al. suggested that the SFM represents “a chord section through the continuous ejecta blanket surrounding an impact crater”. Here, we confirm the presence of shocked material within the SFM and then discuss its origin
Inflation-Produced Magnetic Fields in Nonlinear Electrodynamics
We study the generation of primeval magnetic fields during inflation era in
nonlinear theories of electrodynamics. Although the intensity of the produced
fields strongly depends on characteristics of inflation and on the form of
electromagnetic Lagrangian, our results do not exclude the possibility that
these fields could be astrophysically interesting.Comment: 6 page
Galactic interstellar filaments as probed by LOFAR and Planck
Recent Low Frequency Array (LOFAR) observations at 115-175 MHz of a field at
medium Galactic latitudes (centered at the bright quasar 3C196) have shown
striking filamentary structures in polarization that extend over more than 4
degrees across the sky. In addition, the Planck satellite has released full sky
maps of the dust emission in polarization at 353GHz. The LOFAR data resolve
Faraday structures along the line of sight, whereas the Planck dust
polarization maps probe the orientation of the sky projected magnetic field
component. Hence, no apparent correlation between the two is expected. Here we
report a surprising, yet clear, correlation between the filamentary structures,
detected with LOFAR, and the magnetic field orientation, probed by the Planck
satellite. This finding points to a common, yet unclear, physical origin of the
two measurements in this specific area in the sky. A number of follow-up multi-
frequency studies are proposed to shed light on this unexpected finding.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
The mesoproterozoic Stac Fada Member, NW Scotland : an impact origin confirmed but refined
Funding to GRO from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant program and the Canadian Space Agency (CSA) Canadian Analogue Research Network and Field Investigation programs is gratefully acknowledged. Part of LF’s work was supported by the Department of Foreign Affairs and International Trade (DFAIT), Government of Canada.The origin of the Stac Fada Member has been debated for decades with several early hypotheses being proposed, but all invoking some connection to volcanic activity. In 2008, the discovery of shocked quartz led to the hypothesis that the Stac Fada Member represents part the continuous ejecta blanket of a meteorite impact crater, the location of which was, and remains, unknown. In this paper, we confirm the presence of shock-metamorphosed and -melted material in the Stac Fada Member; however, we also show that its properties are unlike any other confirmed and well documented proximal impact ejecta deposits on Earth. Instead, the properties of the Stac Fada Member are most similar to the Onaping Formation of the Sudbury impact structure (Canada) and impact melt-bearing breccias from the Chicxulub impact structure (Mexico). We thus propose that, like the Sudbury and Chicxulub deposits, Melt Fuel Coolant Interactions – akin to what occur during phreatomagmatic volcanic eruptions – played a fundamental role in the origin of the Stac Fada Member. We conclude that these rocks are not primary impact ejecta but instead were deposited beyond the extent of the continuous ejecta blanket as high-energy ground-hugging sediment gravity flows.PostprintPeer reviewe
Observational constraints on models for the interstellar magnetic field in the Galactic disk
Our purpose is to place firm observational constraints on the three most
widely used theoretical models for the spatial configuration of the large-scale
interstellar magnetic field in the Galactic disk, namely, the ring, the
axisymmetric and the bisymmetric field models.
We use the rotation measures (RMs) of low-latitude Galactic pulsars and
combine them with their dispersion measures and estimated distances to map out
the line-of-sight component of the interstellar magnetic field in the near half
of the Galactic disk. We then fit our map of the line-of-sight field to the
three aforementioned theoretical field models and discuss the acceptability of
each fit, in order to determine whether the considered field model is allowed
by the pulsar data or not.
Strictly speaking, we find that all three field models are ruled out by the
pulsar data. Furthermore, none of them appears to perform significantly better
than the others. From this we conclude that the large-scale interstellar
magnetic field in the Galactic disk has a more complex pattern than just
circular, axisymmetric or bisymmetric.Comment: 11 pages, 8 figure
Pulsar motions in our Galaxy
Pulsar motions in our Galaxy from their birth until 2 Gyr are studied
statistically via Monte-Carlo simulation of pulsars with the
best available representation of the Galactic potential. We find that the
distribution of height above the Galactic plane for pulsars with characteristic
ages less than about 8 Myr could be well fitted by a Gaussian function. For
older pulsars, an extra exponential function is necessary to fit the
distribution. The scale-height of the Gaussian component increases linearly
with time until about 40 Myr. The height distribution becomes stabilized after
about 200 Myr. These results are not sensitive to initial height or radial
distributions. Taking the relationship between the initial velocity and height
distribution, we found from the latest pulsar catalog that the height
distribution of pulsars younger than 1 Myr directly implies the mean initial
velocity of km s. Comparison of simulated sample of pulsars
with the current available millisecond pulsars shows that their 1D initial
velocity dispersion should be most probably km s.Comment: 11 pages, 16 figures, accepted by MNRA
On the Origin of Cosmic Magnetic Fields
We review the literature concerning how the cosmic magnetic fields pervading
nearly all galaxies actually got started. some observational evidence involves
the chemical abundance of the light elements Be and B, while another one is
based on strong magnetic fields seen in high red shift galaxies. Seed fields,
whose strength is of order 10^{-20} gauss, easily sprung up in the era
preceding galaxy formation. Several mechanisms are proposed to amplify these
seed fields to microgauss strengths. The standard mechanism is the Alpha-Omega
dynamo theory. It has a major difficulty that makes unlikely to provide the
sole origin. The difficulty is rooted in the fact that the total flux is
constant. This implies that flux must be removed from the galactic discs. This
requires that the field and flux be separated, for otherwise interstellar mass
must be removed from the deep galactic gravitational and then their strength
increased by the alpha omega theory.Comment: 90 pages and 6 figures; accepted for publication in Reports of
Progress in Physics as an invited revie
The Origin of the Hot Gas in the Galactic Halo: Confronting Models with XMM-Newton Observations
We compare the predictions of three physical models for the origin of the hot
halo gas with the observed halo X-ray emission, derived from 26 high-latitude
XMM-Newton observations of the soft X-ray background between l=120\degr and
l=240\degr. These observations were chosen from a much larger set of
observations as they are expected to be the least contaminated by solar wind
charge exchange emission. We characterize the halo emission in the XMM-Newton
band with a single-temperature plasma model. We find that the observed halo
temperature is fairly constant across the sky (~1.8e6-2.3e6 K), whereas the
halo emission measure varies by an order of magnitude (~0.0005-0.006 cm^-6 pc).
When we compare our observations with the model predictions, we find that most
of the hot gas observed with XMM-Newton does not reside in isolated extraplanar
supernova remnants -- this model predicts emission an order of magnitude too
faint. A model of a supernova-driven interstellar medium, including the flow of
hot gas from the disk into the halo in a galactic fountain, gives good
agreement with the observed 0.4-2.0 keV surface brightness. This model
overpredicts the halo X-ray temperature by a factor of ~2, but there are a
several possible explanations for this discrepancy. We therefore conclude that
a major (possibly dominant) contributor to the halo X-ray emission observed
with XMM-Newton is a fountain of hot gas driven into the halo by disk
supernovae. However, we cannot rule out the possibility that the extended hot
halo of accreted material predicted by disk galaxy formation models also
contributes to the emission.Comment: 20 pages, 14 figures. New version accepted for publication in ApJ.
Changes include new section discussing systematic errors (Section 3.2),
improved method for characterizing our model spectra (4.2.2), changes to
discussion of other observations (5.1). Note that we can no longer rule out
possibility that extended hot halo of accreted material contributes to
observed halo emission (see 5.2.1
Seed fields for galactic dynamos by the magnetorotational instabilities
A linear but global numerical model for the magnetorotational instability
(MRI) in disk geometry is considered to estimate the instability parameters for
galaxies. Similarity rules suggested by a local analysis are applied to reveal
a universal behavior approached by the results of global calculations for large
magnetic Prandtl numbers. The findings are used to estimate the MRI
characteristics for galaxies with their very large magnetic Prandtl numbers
which cannot be attained in any numerical simulations. The resulting minimum
field for the instability, Bmin 10-25 G, is small compared to any seed fields
currently discussed. The growth times of MRI are estimated to be on the order
of the rotation period of the inner rigidly-rotating core, i.e. ~100 Myr.
Global MRI excites preferentially the magnetic field modes of quadrupolar
symmetry.Comment: 6 pages, 4 figures, Astron. Astrophys. (acc.
Rapid dissipation of magnetic fields due to Hall current
We propose a mechanism for the fast dissipation of magnetic fields which is
effective in a stratified medium where ion motions can be neglected. In such a
medium, the field is frozen into the electrons and Hall currents prevail.
Although Hall currents conserve magnetic energy, in the presence of density
gradients, they are able to create current sheets which can be the sites for
efficient dissipation of magnetic fields. We recover the frequency,
, for Hall oscillations modified by the presence of density
gradients. We show that these oscillations can lead to the exchange of energy
between different components of the field. We calculate the time evolution and
show that magnetic fields can dissipate on a timescale of order
. This mechanism can play an important role for magnetic
dissipation in systems with very steep density gradients where the ions are
static such as those found in the solid crust of neutron stars.Comment: 9 pages, changed fig.
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