15,287 research outputs found
Voluntary Societies and Urban Elites in 19th Century Italy
Paper given at European History [E-seminars
Intermittency and Lifetime of the 625 Hz QPO in the 2004 Hyperflare from the Magnetar SGR 1806-20 as evidence for magnetic coupling between the crust and the core
Quasi-periodic oscillations (QPOs) detected in the 2004 giant flare from SGR
1806-20 are often interpreted as global magneto-elastic oscillations of the
neutron star. There is, however, a large discrepancy between theoretical
models, which predict that the highest frequency oscillations should die out
rapidly, and the observations, which suggested that the highest-frequency
signals persisted for ~100s in X-ray data from two different spacecraft. This
discrepancy is particularly important for the high-frequency QPO at ~625 Hz.
However, previous analyses did not systematically test whether the signal could
also be there in much shorter data segments, more consistent with the
theoretical predictions. Here, we test for the presence of the high-frequency
QPO at 625 Hz in data from both the Rossi X-ray Timing Explorer (RXTE) and the
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) systematically both in
individual rotational cycles of the neutron star, as well as averaged over
multiple successive rotational cycles at the same phase. We find that the QPO
in the RXTE data is consistent with being only present in a single cycle, for a
short duration of ~0.5s, whereas the RHESSI data are as consistent with a
short-lived signal that appears and disappears as with a long-lived QPO. Taken
together, this data provides evidence for strong magnetic interaction between
the crust and the core.Comment: Accepted for publication in ApJ. The data and simulations are
available at
http://figshare.com/articles/SGR_1806_20_Giant_Flare_Data_and_Simulations/1126082
, the code can be downloaded from
https://github.com/dhuppenkothen/giantflare-paper , some documentation is
under
http://nbviewer.ipython.org/github/dhuppenkothen/giantflare-paper/blob/master/documents/giantflare-analysis.ipyn
Formation of Plasmoid Chains in Fusion Relevant Plasmas
The formation of plasmoid chains is explored for the first time within the
context of the Taylor problem, in which magnetic reconnection is driven by a
small amplitude boundary perturbation in a tearing-stable slab plasma
equilibrium. Numerical simulations of a magnetohydrodynamical model of the
plasma show that for very small plasma resistivity and viscosity, the linear
inertial phase is followed by a nonlinear Sweet-Parker evolution, which gives
way to a faster reconnection regime characterized by a chain of plasmoids
instead of a slower Rutherford phase
Extended theory of the Taylor problem in the plasmoid-unstable regime
A fundamental problem of forced magnetic reconnection has been solved taking
into account the plasmoid instability of thin reconnecting current sheets. In
this problem, the reconnection is driven by a small amplitude boundary
perturbation in a tearing-stable slab plasma equilibrium. It is shown that the
evolution of the magnetic reconnection process depends on the external source
perturbation and the microscopic plasma parameters. Small perturbations lead to
a slow nonlinear Rutherford evolution, whereas larger perturbations can lead to
either a stable Sweet-Parker-like phase or a plasmoid phase. An expression for
the threshold perturbation amplitude required to trigger the plasmoid phase is
derived, as well as an analytical expression for the reconnection rate in the
plasmoid-dominated regime. Visco-resistive magnetohydrodynamic simulations
complement the analytical calculations. The plasmoid formation plays a crucial
role in allowing fast reconnection in a magnetohydrodynamical plasma, and the
presented results suggest that it may occur and have profound consequences even
if the plasma is tearing-stable.Comment: Accepted for publication in Physics of Plasma
Philofluid turbulent flow database
A set of velocity and passive scalar fields and their statistics coming from direct numerical simulations and large-eddy simulations. The database includes: shearless mixings in two a three dimensions, turbulent channel flow, cavity flow. Username and password to access the netdisks is provided upon request
Optimal path planning for surveillance with temporal-logic constraints
In this paper we present a method for automatically generating optimal robot paths satisfying high-level mission specifications. The motion of the robot in the environment is modeled as a weighted transition system. The mission is specified by an arbitrary linear temporal-logic (LTL) formula over propositions satisfied at the regions of a partitioned environment. The mission specification contains an optimizing proposition, which must be repeatedly satisfied. The cost function that we seek to minimize is the maximum time between satisfying instances of the optimizing proposition. For every environment model, and for every formula, our method computes a robot path that minimizes the cost function. The problem is motivated by applications in robotic monitoring and data-gathering. In this setting, the optimizing proposition is satisfied at all locations where data can be uploaded, and the LTL formula specifies a complex data-collection mission. Our method utilizes Büchi automata to produce an automaton (which can be thought of as a graph) whose runs satisfy the temporal-logic specification. We then present a graph algorithm that computes a run corresponding to the optimal robot path. We present an implementation for a robot performing data collection in a road-network platform.This material is based upon work supported in part by ONR-MURI (award N00014-09-1-1051), ARO (award W911NF-09-1-0088), and Masaryk University (grant numbers LH11065 and GD102/09/H042), and other funding sources (AFOSR YIP FA9550-09-1-0209, NSF CNS-1035588, NSF CNS-0834260). (N00014-09-1-1051 - ONR-MURI; W911NF-09-1-0088 - ARO; LH11065 - Masaryk University; GD102/09/H042 - Masaryk University; FA9550-09-1-0209 - AFOSR YIP; CNS-1035588 - NSF; CNS-0834260 - NSF
Gyro-induced acceleration of magnetic reconnection
The linear and nonlinear evolution of magnetic reconnection in collisionless
high-temperature plasmas with a strong guide field is analyzed on the basis of
a two-dimensional gyrofluid model. The linear growth rate of the reconnecting
instability is compared to analytical calculations over the whole spectrum of
linearly unstable wave numbers. In the strongly unstable regime (large \Delta
'), the nonlinear evolution of the reconnecting instability is found to undergo
two distinctive acceleration phases separated by a stall phase in which the
instantaneous growth rate decreases. The first acceleration phase is caused by
the formation of strong electric fields close to the X-point due to ion
gyration, while the second acceleration phase is driven by the development of
an open Petschek-like configuration due to both ion and electron temperature
effects. Furthermore, the maximum instantaneous growth rate is found to
increase dramatically over its linear value for decreasing diffusion layers.
This is a consequence of the fact that the peak instantaneous growth rate
becomes weakly dependent on the microscopic plasma parameters if the diffusion
region thickness is sufficiently smaller than the equilibrium magnetic field
scale length. When this condition is satisfied, the peak reconnection rate
asymptotes to a constant value.Comment: Accepted for publication on Physics of Plasma
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