734 research outputs found
Sub-structure formation in starless cores
Motivated by recent observational searches of sub-structure in starless
molecular cloud cores, we investigate the evolution of density perturbations on
scales smaller than the Jeans length embedded in contracting isothermal clouds,
adopting the same formalism developed for the expanding Universe and the solar
wind. We find that initially small amplitude, Jeans-stable perturbations
(propagating as sound waves in the absence of a magnetic field), are amplified
adiabatically during the contraction, approximately conserving the wave action
density, until they either become nonlinear and steepen into shocks at a time
, or become gravitationally unstable when the Jeans length
decreases below the scale of the perturbations at a time . We
evaluate analytically the time at which the perturbations enter
the non-linear stage using a Burgers' equation approach, and we verify
numerically that this time marks the beginning of the phase of rapid
dissipation of the kinetic energy of the perturbations. We then show that for
typical values of the rms Mach number in molecular cloud cores, is
smaller than , and therefore density perturbations likely dissipate
before becoming gravitational unstable. Solenoidal modes grow at a faster rate
than compressible modes, and may eventually promote fragmentation through the
formation of vortical structures.Comment: 8 pages, 4 figure
EIT and TRACE responses to flare plasma
Aims: To understand the contribution of active region and flare plasmas to
the 195 channels of SOHO/EIT (Extreme-ultraviolet Imaging Telescope)
and TRACE (Transition Region and Coronal Explorer).
Methods: We have analysed an M8 flare simultaneously observed by the Coronal
Diagnostic Spectrometer (CDS), EIT, TRACE and RHESSI. We obtained synthetic
spectra for the flaring region and an outer region using the differential
emission measures (DEM) of emitting plasma based on CDS and RHESSI observations
and the CHIANTI atomic database. We then predicted the EIT and TRACE count
rates.
Results: For the flaring region, both EIT and TRACE images taken through the
195 filter are dominated by Fe (formed at about 20 MK).
However, in the outer region, the emission was primarily due to the Fe, with substantial contributions from other lines. The average count rate
for the outer region was within 25% the observed value for EIT, while for TRACE
it was a factor of two higher. For the flare region, the predicted count rate
was a factor of two (in case of EIT) and a factor of three (in case of TRACE)
higher than the actual count rate.
Conclusions: During a solar flare, both TRACE and EIT 195 channels
are found to be dominated by Fe emission. Reasonable agreement
between predictions and observations is found, however some discrepancies need
to be further investigated.Comment: 6 pages, 4 figure
Coronal Diagnostics from Narrowband Images around 30.4 nm
Images taken in the band centered at 30.4 nm are routinely used to map the
radiance of the He II Ly alpha line on the solar disk. That line is one of the
strongest, if not the strongest, line in the EUV observed in the solar
spectrum, and one of the few lines in that wavelength range providing
information on the upper chromosphere or lower transition region. However, when
observing the off-limb corona the contribution from the nearby Si XI 30.3 nm
line can become significant. In this work we aim at estimating the relative
contribution of those two lines in the solar corona around the minimum of solar
activity. We combine measurements from CDS taken in August 2008 with
temperature and density profiles from semiempirical models of the corona to
compute the radiances of the two lines, and of other representative coronal
lines (e.g., Mg X 62.5 nm, Si XII 52.1 nm). Considering both diagnosed
quantities from line ratios (temperatures and densities) and line radiances in
absolute units, we obtain a good overall match between observations and models.
We find that the Si XI line dominates the He II line from just above the limb
up to ~2 R_Sun in streamers, while its contribution to narrowband imaging in
the 30.4 nm band is expected to become smaller, even negligible in the corona
beyond ~2 - 3 R_Sun, the precise value being strongly dependent on the coronal
temperature profile.Comment: 26 pages, 11 figures; to be published in: Solar Physic
Polarization in the inner region of Pulsar Wind Nebulae
We present here the first effort to compute synthetic synchrotron
polarization maps of Pulsar Wind Nebulae (PWNe). Our goal is to highlight how
polarization can be used as an additional diagnostic tool for the flow
structure in the inner regions of these nebulae. Recent numerical simulations
suggest the presence of flow velocities ~0.5 c in the surroundings of the
termination shock, where most of the high energy emission comes from. We
construct polarization maps taking into account relativistic effects like
Doppler boosting and position angle swing. The effect of different bulk
velocities is clarified with the help of a toy-model consisting of a uniformly
emitting torus. We also present a map based on recent numerical simulations of
the entire nebula and compare it with presently available data. The comparison
with upcoming high resolution observations could provide new insight into the
inner structure of the nebula and put constraints on the geometrical properties
of the magnetic field.Comment: Accepted for publication on A&A, 6 pages, 2 figure
Multiwavelength study of 20 jets that emanate from the periphery of active regions
We present a multiwavelength analysis of 20 EUV jets which occurred at the
periphery of active regions close to sunspots. We discuss the physical
parameters of the jets and their relation with other phenomena such as H alpha
surges, nonthermal type III radio bursts and hard X-ray emission. Using AIA
wavelength channels sensitive to coronal temperatures, we studied the
temperature distribution in the jets using the line-of-sight Differential
Emission Measure technique. We also investigated the role of the photospheric
magnetic field using the LOS magnetogram data from the HMI. The lifetime of
jets range from 5 to 39 minutes with an average of 18 minutes and their
velocities range from 87 to 532 km/s with an average of 271 km/s. Most of the
jets are co-temporal with nonthermal type III radio bursts observed by the
Wind/WAVES spacecraft. We confirm the source region of these bursts using the
Potential Field Source Surface technique. 10 out of 20 events showed that the
jets originated in a region of flux cancellation and 6 jets in a region of flux
emergence. 4 events showed flux emergence and then cancellation during the jet
evolution. DEM analyses showed that for most of the spires of the jets, the DEM
peaked at around log T [K] = 6.2/6.3. In addition, we derived an emission
measure and a lower limit of electron density at the location of the spire and
the footpoint. These results are in agreement with those obtained earlier by
studying individual active region jets. The observation of flux cancellation,
the association with HXR emission and emission of nonthermal type III radio
bursts, suggest that the initiation and therefore, heating is taking place at
the base of the jet. This is also supported by the high temperature plasma
revealed by the DEM analysis in the jet footpoint. Our results provide
substantial constraints for theoretical modeling of the jets and their
thermodynamic nature.Part of the work was done when one of the authors (SMM) was a Junior Research Fellow at Inter-University Centre of Astronomy and Astrophysics (IUCAA), India. S.M.M. and D.T. acknowledge support from DST under the Fast Track Scheme (SERB/F/3369/2012/2013). S.M.M. also ackowledges support from the Cambridge Trust, University of Cambridge, UK. H.E.M. and G.D.Z. acknowledge the support of STFC. AIA data are courtesy of SDO (NASA) and the AIA consortium. RHESSI work is supported by NASA contract NAS 5-98033. The authors thank the open data policy of WIND/WAVES instrument team. This work utilizes data obtained by the Global Oscillation Network Group (GONG) Program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de AstrofÃŋsica de Canarias, and Cerro Tololo Interamerican Observatory.This is the final version of the article. It first appeared from EDP Sciences via http://dx.doi.org/10.1051/0004-6361/20152747
Simulated synchrotron emission from Pulsar Wind Nebulae
A complete set of diagnostic tools aimed at producing synthetic synchrotron
emissivity, polarization, and spectral index maps from relativistic MHD
simulations is presented. As a first application we consider here the case of
the emission from Pulsar Wind Nebulae (PWNe). The proposed method is based on
the addition, on top of the basic set of MHD equations, of an extra equation
describing the evolution of the maximum energy of the emitting particles. This
equation takes into account adiabatic and synchrotron losses along streamlines
for the distribution of emitting particles and its formulation is such that it
is easily implemented in any numerical scheme for relativistic MHD. Application
to the axisymmetric simulations of PWNe, analogous to those described by Del
Zanna et al. (2004, A&A, 421, 1063), allows direct comparison between the
numerical results and observations of the inner structure of the Crab Nebula,
and similar objects, in the optical and X-ray bands. We are able to match most
of the observed features typical of PWNe, like the equatorial torus and the
polar jets, with velocities in the correct range, as well as finer emission
details, like arcs, rings and the bright knot, that turn out to arise mainly
from Doppler boosting effects. Spectral properties appear to be well reproduced
too: detailed spectral index maps are produced for the first time and show
softening towards the PWN outer borders, whereas spectral breaks appear in
integrated spectra. The emission details are found to strongly depend on both
the average wind magnetization (here approximately 2%), and on the magnetic
field shape.Comment: 14 pages, submitted to A&
Cross-linguistic parallels in processing derivational morphology: evidence from Polish.
Neuroimaging evidence in English suggests that the neurocognitive processing of derivationally complex words primarily reflects their properties as whole forms. The current experiment provides a cross-linguistic examination of these proposals by investigating the processing of derivationally complex words in the rich morphological system of Polish. Within the framework of a dual language system approach, we asked whether there is evidence for decompositional processing of derivationally complex Polish stems - reflected in the activation of a linguistically specific decompositional system in the left hemisphere - or for increased competition between the derived stem and its embedded base stem in the bilateral system. The results showed activation in the bilateral system and no evidence for selective engagement of the left hemisphere decompositional system. This provides a cross-linguistic validation for the hypothesis that the neurocognitive processing of derived stems primarily reflects their properties as stored forms
High Order Upwind Schemes for Multidimensional Magnetohydrodynamics
A general method for constructing high order upwind schemes for
multidimensional magnetohydrodynamics (MHD), having as a main built-in
condition the divergence-free constraint \divb=0 for the magnetic field
vector \bb, is proposed. The suggested procedure is based on {\em
consistency} arguments, by taking into account the specific operator structure
of MHD equations with respect to the reference Euler equations of gas-dynamics.
This approach leads in a natural way to a staggered representation of the \bb
field numerical data where the divergence-free condition in the cell-averaged
form, corresponding to second order accurate numerical derivatives, is exactly
fulfilled. To extend this property to higher order schemes, we then give
general prescriptions to satisfy a order accurate \divb=0
relation for any numerical \bb field having a order interpolation
accuracy. Consistency arguments lead also to a proper formulation of the upwind
procedures needed to integrate the induction equations, assuring the exact
conservation in time of the divergence-free condition and the related
continuity properties for the \bb vector components. As an application, a
third order code to simulate multidimensional MHD flows of astrophysical
interest is developed using ENO-based reconstruction algorithms. Several test
problems to illustrate and validate the proposed approach are finally
presented.Comment: 34 pages, including 14 figure
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