4,275 research outputs found
Magnetic Reconnection with Radiative Cooling. I. Optically-Thin Regime
Magnetic reconnection, a fundamental plasma process associated with a rapid
dissipation of magnetic energy, is believed to power many disruptive phenomena
in laboratory plasma devices, the Earth magnetosphere, and the solar corona.
Traditional reconnection research, geared towards these rather tenuous
environments, has justifiably ignored the effects of radiation on the
reconnection process. However, in many reconnecting systems in high-energy
astrophysics (e.g., accretion-disk coronae, relativistic jets, magnetar flares)
and, potentially, in powerful laser plasma and z-pinch experiments, the energy
density is so high that radiation, in particular radiative cooling, may start
to play an important role. This observation motivates the development of a
theory of high-energy-density radiative magnetic reconnection. As a first step
towards this goal, we present in this paper a simple Sweet--Parker-like theory
of non-relativistic resistive-MHD reconnection with strong radiative cooling.
First, we show how, in the absence of a guide magnetic field, intense cooling
leads to a strong compression of the plasma in the reconnection layer,
resulting in a higher reconnection rate. The compression ratio and the layer
temperature are determined by the balance between ohmic heating and radiative
cooling. The lower temperature in the radiatively-cooled layer leads to a
higher Spitzer resistivity and hence to an extra enhancement of the
reconnection rate. We then apply our general theory to several specific
astrophysically important radiative processes (bremsstrahlung, cyclotron, and
inverse-Compton) in the optically thin regime, for both the zero- and
strong-guide-field cases. We derive specific expressions for key reconnection
parameters, including the reconnection rate. We also discuss the limitations
and conditions for applicability of our theory.Comment: 31 pages, 1 figur
Reducing dependence on big brother: Higher education looks for innovative funding opportunities
This paper presents innovative programs that business schools can utilize to reduce dependence on public funds. A review of the literature shows the theoretical and empirical foundation of higher education funding dilemmas. While higher education is moving towards a global ambition, scarcity hinders governments to fully support programs long-term; thus, cost-sharing and cost-shifting measures must occur for higher education to support current programs. In this study, we examine two universities (one U.S. and one UK.) and provide practical summaries of programs that have provided additional funds. We show that diversity of funding sources is essential for survival of higher education institutions. Market forces require competition to reduce higher education operational costs while providing students and corporate clients an a la carte educational experience
Inferring hidden Markov models from noisy time sequences: a method to alleviate degeneracy in molecular dynamics
We present a new method for inferring hidden Markov models from noisy time
sequences without the necessity of assuming a model architecture, thus allowing
for the detection of degenerate states. This is based on the statistical
prediction techniques developed by Crutchfield et al., and generates so called
causal state models, equivalent to hidden Markov models. This method is
applicable to any continuous data which clusters around discrete values and
exhibits multiple transitions between these values such as tethered particle
motion data or Fluorescence Resonance Energy Transfer (FRET) spectra. The
algorithms developed have been shown to perform well on simulated data,
demonstrating the ability to recover the model used to generate the data under
high noise, sparse data conditions and the ability to infer the existence of
degenerate states. They have also been applied to new experimental FRET data of
Holliday Junction dynamics, extracting the expected two state model and
providing values for the transition rates in good agreement with previous
results and with results obtained using existing maximum likelihood based
methods.Comment: 19 pages, 9 figure
Current Status of Simulations
As the title suggests, the purpose of this chapter is to review the current
status of numerical simulations of black hole accretion disks. This chapter
focuses exclusively on global simulations of the accretion process within a few
tens of gravitational radii of the black hole. Most of the simulations
discussed are performed using general relativistic magnetohydrodynamic (MHD)
schemes, although some mention is made of Newtonian radiation MHD simulations
and smoothed particle hydrodynamics. The goal is to convey some of the exciting
work that has been going on in the past few years and provide some speculation
on future directions.Comment: 15 pages, 14 figures, to appear in the proceedings of the ISSI-Bern
workshop on "The Physics of Accretion onto Black Holes" (8-12 October 2012
Force-free magnetosphere of an aligned rotator with differential rotation of open magnetic field lines
Here we briefly report on results of self-consistent numerical modeling of a
differentially rotating force-free magnetosphere of an aligned rotator. We show
that differential rotation of the open field line zone is significant for
adjusting of the global structure of the magnetosphere to the current density
flowing through the polar cap cascades. We argue that for most pulsars
stationary cascades in the polar cap can not support stationary force-free
configurations of the magnetosphere.Comment: 5 pages, 4 figures. Presented at the conference "Isolated Neutron
Stars: from the Interior to the Surface", London, April 24-28, 2006; to
appear in Astrophysics and Space Science. Significantly revised version, a
mistake found by ourselfs in the numerical code was corrected, all presented
results are obtained with the correct version of the cod
Universal stowage module for future space exploration
The design effort to develop, design, and fabricate a prototype Universal Stowage Module with universal restraints that are readily adaptable for most sizes and shapes of items that would be launched into space and returned aboard shuttle payloads is presented
- …