835 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
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 figure
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 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
Organization of the respiratory supercomplexes in cells with defective complex III: Structural features and metabolic consequences
The mitochondrial respiratory chain encompasses four oligomeric enzymatic complexes (complex I, II, III and IV) which, together with the redox carrier ubiquinone and cytochrome c, catalyze electron transport coupled to proton extrusion from the inner membrane. The protonmotive force is utilized by complex V for ATP synthesis in the process of oxidative phosphorylation. Respiratory complexes are known to coexist in the membrane as single functional entities and as supramolecular aggregates or supercomplexes (SCs). Understanding the assembly features of SCs has relevant biomedical implications because defects in a single protein can derange the overall SC organization and compromise the energetic function, causing severe mitochondrial disorders. Here we describe in detail the main types of SCs, all characterized by the presence of complex III. We show that the genetic alterations that hinder the assembly of Complex III, not just the activity, cause a rearrangement of the architecture of the SC that can help to preserve a minimal energetic function. Finally, the major metabolic disturbances associated with severe SCs perturbation due to defective complex III are discussed along with interventions that may circumvent these deficiencies
Simulated synchrotron and Inverse Compton emission from Pulsar Wind Nebulae
We present a complete set of diagnostic tools aimed at reproducing synthetic
non-thermal (synchrotron and/or Inverse Compton, IC) emissivity, integrated
flux energy, polarization and spectral index simulated maps in comparison to
observations. The time dependent relativistic magnetohydrodynamic (RMHD)
equations are solved with a shock capturing code together with the evolution of
the maximum particles energy. Applications to Pulsar Wind Nebulae (PWNe) are
shown.Comment: 3 pages, 7 figures, proceeding of the conference "40 Years of Pulsars
", 12-17 August 2007, Montreal, Canada, submitted to AI
A solar active region loop compared with a 2D MHD model
We analyzed a coronal loop observed with the Normal Incidence Spectrometer
(NIS), which is part of the Coronal Diagnostic Spectrometer (CDS) on board the
Solar and Heliospheric Observatory (SOHO). The measured Doppler shifts and
proper motions along the selected loop strongly indicate unidirectional flows.
Analysing the Emission Measure Curves of the observed spectral lines, we
estimated that the temperature along the loop was about 380000 K. We adapted a
solution of the ideal MHD steady equations to our set of measurements. The
derived energy balance along the loop, as well as the advantages/disadvantages
of this MHD model for understanding the characteristics of solar coronal loops
are discussed.Comment: A&A in press, 10 pages, 6 figure
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
Primitive Variable Solvers for Conservative General Relativistic Magnetohydrodynamics
Conservative numerical schemes for general relativistic magnetohydrodynamics
(GRMHD) require a method for transforming between ``conserved'' variables such
as momentum and energy density and ``primitive'' variables such as rest-mass
density, internal energy, and components of the four-velocity. The forward
transformation (primitive to conserved) has a closed-form solution, but the
inverse transformation (conserved to primitive) requires the solution of a set
of five nonlinear equations. Here we discuss the mathematical properties of the
inverse transformation and present six numerical methods for performing the
inversion. The first method solves the full set of five nonlinear equations
directly using a Newton-Raphson scheme and a guess from the previous timestep.
The other methods reduce the five nonlinear equations to either one or two
nonlinear equations that are solved numerically. Comparisons between the
methods are made using a survey over phase space, a two-dimensional explosion
problem, and a general relativistic MHD accretion disk simulation. The run-time
of the methods is also examined. Code implementing the schemes is available for
download on the web.Comment: Accepted to ApJ, 33 pages, 8 figures (color and greyscale), 1
machine-readable table (tab2.txt), code available at
http://rainman.astro.uiuc.edu/codelib, a high-resolution and full-color PDF
version is located at
http://rainman.astro.uiuc.edu/codelib/codes/pvs_grmhd/ms.pd
Incorporating Uncertainties in Atomic Data Into the Analysis of Solar and Stellar Observations: A Case Study in Fe XIII
Information about the physical properties of astrophysical objects cannot be
measured directly but is inferred by interpreting spectroscopic observations in
the context of atomic physics calculations. Ratios of emission lines, for
example, can be used to infer the electron density of the emitting plasma.
Similarly, the relative intensities of emission lines formed over a wide range
of temperatures yield information on the temperature structure. A critical
component of this analysis is understanding how uncertainties in the underlying
atomic physics propagates to the uncertainties in the inferred plasma
parameters. At present, however, atomic physics databases do not include
uncertainties on the atomic parameters and there is no established methodology
for using them even if they did. In this paper we develop simple models for the
uncertainties in the collision strengths and decay rates for Fe XIII and apply
them to the interpretation of density sensitive lines observed with the EUV
Imagining spectrometer (EIS) on Hinode. We incorporate these uncertainties in a
Bayesian framework. We consider both a pragmatic Bayesian method where the
atomic physics information is unaffected by the observed data, and a fully
Bayesian method where the data can be used to probe the physics. The former
generally increases the uncertainty in the inferred density by about a factor
of 5 compared with models that incorporate only statistical uncertainties. The
latter reduces the uncertainties on the inferred densities, but identifies
areas of possible systematic problems with either the atomic physics or the
observed intensities.Comment: in press at Ap
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