545 research outputs found
Thermal structure of hot non-flaring corona from Hinode/EIS
In previous studies a very hot plasma component has been diagnosed in solar
active regions through the images in three different narrow-band channels of
SDO/AIA. This diagnostic from EUV imaging data has also been supported by the
matching morphology of the emission in the hot Ca XVII line, as observed with
Hinode/EIS. This evidence is debated because of unknown distribution of the
emission measure along the line of sight. Here we investigate in detail the
thermal distribution of one of such regions using EUV spectroscopic data. In an
active region observed with SDO/AIA, Hinode/EIS and XRT, we select a subregion
with a very hot plasma component and another cooler one for comparison. The
average spectrum is extracted for both, and 14 intense lines are selected for
analysis, that probe the 5.5 < log T < 7 temperature range uniformly. From
these lines the emission measure distributions are reconstructed with the MCMC
method. Results are cross-checked with comparison of the two subregions, with a
different inversion method, with the morphology of the images, and with the
addition of fluxes measured with from narrow and broad-band imagers. We find
that, whereas the cool region has a flat and featureless distribution that
drops at temperature log T >= 6.3, the distribution of the hot region shows a
well-defined peak at log T = 6.6 and gradually decreasing trends on both sides,
thus supporting the very hot nature of the hot component diagnosed with
imagers. The other cross-checks are consistent with this result. This study
provides a completion of the analysis of active region components, and the
resulting scenario supports the presence of a minor very hot plasma component
in the core, with temperatures log T > 6.6.Comment: 12 pages, 8 figures, accepted for publicatio
Activation of MHD reconnection on ideal timescales
Magnetic reconnection in laboratory, space and astrophysical plasmas is often
invoked to explain explosive energy release and particle acceleration. However,
the timescales involved in classical models within the macroscopic MHD regime
are far too slow to match the observations. Here we revisit the tearing
instability by performing visco-resistive two-dimensional numerical simulations
of the evolution of thin current sheets, for a variety of initial
configurations and of values of the Lunquist number , up to . Results
confirm that when the critical aspect ratio of is reached in the
reconnecting current sheets, the instability proceeds on ideal (Alfv\'enic)
macroscopic timescales, as required to explain observations. Moreover, the same
scaling is seen to apply also to the local, secondary reconnection events
triggered during the nonlinear phase of the tearing instability, thus
accelerating the cascading process to increasingly smaller spatial and temporal
scales. The process appears to be robust, as the predicted scaling is measured
both in inviscid simulations and when using a Prandtl number in the
viscous regime.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
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
On the importance of background subtraction in the analysis of coronal loops observed with TRACE
In the framework of TRACE coronal observations, we compare the analysis and
diagnostics of a loop after subtracting the background with two different and
independent methods. The dataset includes sequences of images in the 171 A, 195
A filter bands of TRACE. One background subtraction method consists in taking
as background values those obtained from interpolation between concentric
strips around the analyzed loop. The other method is a pixel-to-pixel
subtraction of the final image when the loop had completely faded out, already
used by Reale & Ciaravella 2006. We compare the emission distributions along
the loop obtained with the two methods and find that they are considerably
different. We find differences as well in the related derive filter ratio and
temperature profiles. In particular, the pixel-to-pixel subtraction leads to
coherent diagnostics of a cooling loop. With the other subtraction the
diagnostics are much less clear. The background subtraction is a delicate issue
in the analysis of a loop. The pixel-to-pixel subtraction appears to be more
reliable, but its application is not always possible. Subtraction from
interpolation between surrounding regions can produce higher systematic errors,
because of intersecting structures and of the large amount of subtracted
emission in TRACE observations.Comment: 9 pages, 9 figure
Implications of Food Subsistence for Monetary Policy and Inflation
The chapter introduces subsistence requirements in food consumption into a simple New Keynesian model with flexible food and sticky non-food prices. It shows how the endogenous structural transformation that results from subsistence affects the dynamics of the economy, the design of monetary policy, and the properties of inflation at different levels of development. A calibrated version of the model encompasses both rich and poor countries and broadly replicates the properties of inflation across the development spectrum, including the dominant role played by changes in the relative price of food in poor countries. The authors derive a welfare-based loss function for the monetary authority and show that optimal policy calls for complete (in some cases near-complete) stabilization of sticky-price non-food inflation, despite the presence of a food-subsistence threshold. Subsistence amplifies the welfare losses of policy mistakes, however, raising the stakes for monetary policy at earlier stages of development.</p
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
Quantum control theory for coupled 2-electron dynamics in quantum dots
We investigate optimal control strategies for state to state transitions in a
model of a quantum dot molecule containing two active strongly interacting
electrons. The Schrodinger equation is solved nonperturbatively in conjunction
with several quantum control strategies. This results in optimized electric
pulses in the THz regime which can populate combinations of states with very
short transition times. The speedup compared to intuitively constructed pulses
is an order of magnitude. We furthermore make use of optimized pulse control in
the simulation of an experimental preparation of the molecular quantum dot
system. It is shown that exclusive population of certain excited states leads
to a complete suppression of spin dephasing, as was indicated in Nepstad et al.
[Phys. Rev. B 77, 125315 (2008)].Comment: 24 pages, 9 figure
Relativistic MHD Simulations of Jets with Toroidal Magnetic Fields
This paper presents an application of the recent relativistic HLLC
approximate Riemann solver by Mignone & Bodo to magnetized flows with vanishing
normal component of the magnetic field.
The numerical scheme is validated in two dimensions by investigating the
propagation of axisymmetric jets with toroidal magnetic fields.
The selected jet models show that the HLLC solver yields sharper resolution
of contact and shear waves and better convergence properties over the
traditional HLL approach.Comment: 12 pages, 5 figure
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