3,944 research outputs found
Multifluid magnetohydrodynamic turbulent decay
It is generally believed that turbulence has a significant impact on the
dynamics and evolution of molecular clouds and the star formation which occurs
within them. Non-ideal magnetohydrodynamic effects are known to influence the
nature of this turbulence. We present the results of a suite of 512-cubed
resolution simulations of the decay of initially super-Alfvenic and supersonic
fully multifluid MHD turbulence. We find that ambipolar diffusion increases the
rate of decay of the turbulence while the Hall effect has virtually no impact.
The decay of the kinetic energy can be fitted as a power-law in time and the
exponent is found to be -1.34 for fully multifluid MHD turbulence. The power
spectra of density, velocity and magnetic field are all steepened significantly
by the inclusion of non-ideal terms. The dominant reason for this steepening is
ambipolar diffusion with the Hall effect again playing a minimal role except at
short length scales where it creates extra structure in the magnetic field.
Interestingly we find that, at least at these resolutions, the majority of the
physics of multifluid turbulence can be captured by simply introducing fixed
(in time and space) resistive terms into the induction equation without the
need for a full multifluid MHD treatment. The velocity dispersion is also
examined and, in common with previously published results, it is found not to
be power-law in nature.Comment: 16 pages, 15 figures, Accepted for publication in Ap
School Finance Reforms, Tax Limits, and Student Performance: Do Reforms Level Up or Dumb Down?
During the late 1970s and early 1980s, a majority of states substantially changed the ways in which schools were funded, either directly through court- or legislatively mandated school finance reform, or indirectly through tax and expenditure limits. To date, there have been few academic attempts to gauge the effects of these policy changes on actual outcomes of education. This paper is an attempt to fill this gap in the literature. We find compelling evidence that the imposition of tax or expenditure limits on local governments in a state results in a significant reduction in the mean for that state of student performance on standardized tests of mathematics skills. We also find that finance reforms in response to court mandates do not result in significant changes in either the mean level or the distribution of student performance on standardized tests of reading and mathematics. In addition, substantial finance reforms that are not legislative responses to explicit court mandates generally result in increases in mean student performance. Further, in those states that have implemented finance reforms of this type, the test performance of students residing in localities in which local revenues formed smaller shares of total revenue prior to the reforms improve relative to others after the reforms are implemented.
CVcat: an interactive database on cataclysmic variables
CVcat is a database that contains published data on cataclysmic variables and
related objects. Unlike in the existing online sources, the users are allowed
to add data to the catalogue. The concept of an ``open catalogue'' approach is
reviewed together with the experience from one year of public usage of CVcat.
New concepts to be included in the upcoming AstroCat framework and the next
CVcat implementation are presented. CVcat can be found at http://www.cvcat.org.Comment: 5 pages A&A Latex, 4 figures, accepted for publication in A&
Quantum Communication with an Accelerated Partner
An unsolved problem in relativistic quantum information research is how to
model efficient, directional quantum communication between localised parties in
a fully quantum field theoretical framework. We propose a tractable approach to
this problem based on solving the Heisenberg evolution of localized field
observables. We illustrate our approach by analysing, and obtaining approximate
analytical solutions to, the problem of communicating coherent states between
an inertial sender, Alice and an accelerated receiver, Rob. We use these
results to determine the efficiency with which continuous variable quantum key
distribution could be carried out over such a communication channel.Comment: Additional explanatory text and typo in Eq.17 correcte
Estimating the metric in curved spacetime with quantum fields
The geometry of space‐time is determined by physical measurements made with clocks and rulers. In so far as these are physical systems, the ultimate accuracy achievable is determined by quantum mechanics. In this paper we use methods from quantum parameter estimation theory to obtain uncertainty principles constraining how well we can estimate the components of a metric tensor using quantum field states propagating in curved space‐time, which is treated entirely classically
Non-ideal MHD turbulent decay in molecular clouds
It is well known that non-ideal magnetohydrodynamic effects are important in
the dynamics of molecular clouds: both ambipolar diffusion and possibly the
Hall effect have been identified as significant. We present the results of a
suite of simulations with a resolution of 512-cubed of turbulent decay in
molecular clouds incorporating a simplified form of both ambipolar diffusion
and the Hall effect simultaneously. The initial velocity field in the
turbulence is varied from being super-Alfv\'enic and hypersonic, through to
trans-Alfv\'enic but still supersonic.
We find that ambipolar diffusion increases the rate of decay of the
turbulence increasing the decay from to . The Hall effect
has virtually no impact in this regard. The power spectra of density, velocity
and the magnetic field are all affected by the non-ideal terms, being steepened
significantly when compared with ideal MHD turbulence with exponents. The
density power spectra components change from about 1.4 to about 2.1 for the
ideal and non-ideal simulations respectively, and power spectra of the other
variables all show similar modifications when non-ideal effects are considered.
Again, the dominant source of these changes is ambipolar diffusion rather than
the Hall effect. There is also a decoupling between the velocity field and the
magnetic field at short length scales. The Hall effect leads to enhanced
magnetic reconnection, and hence less power, at short length scales. The
dependence of the velocity dispersion on the characteristic length scale is
studied and found not to be power-law in nature.Comment: 16 pages, 17 figure
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