627 research outputs found
Bi-Directional Energy Cascades and the Origin of Kinetic Alfv\'enic and Whistler Turbulence in the Solar Wind
The observed sub-proton scale turbulence spectrum in the solar wind raises
the question of how that turbulence originates. Observations of keV energetic
electrons during solar quite-time suggest them as possible source of free
energy to drive the turbulence. Using particle-in-cell simulations, we explore
how free energy in energetic electrons, released by an electron two-stream
instability drives Weibel-like electromagnetic waves that excite wave-wave
interactions. Consequently, both kinetic Alfv\'enic and whistler waves are
excited that evolve through inverse and forward magnetic energy cascades.Comment: 12 pages, 5 figures, Submitted to Physical Review Letter
A unified modulo scheduling and register allocation technique for clustered processors
This work presents a modulo scheduling framework for clustered ILP processors that integrates the cluster assignment, instruction scheduling and register allocation steps in a single phase. This unified approach is more effective than traditional approaches based on sequentially performing some (or all) of the three steps, since it allows optimizing the global code generation problem instead of searching for optimal solutions to each individual step. Besides, it avoids the iterative nature of traditional approaches, which require repeated applications of the three steps until a valid solution is found. The proposed framework includes a mechanism to insert spill code on-the-fly and heuristics to evaluate the quality of partial schedules considering simultaneously inter-cluster communications, memory pressure and register pressure. Transformations that allow trading pressure on a type of resource for another resource are also included. We show that the proposed technique outperforms previously proposed techniques. For instance, the average speed-up for the SPECfp95 is 36% for a 4-cluster configuration.Peer ReviewedPostprint (published version
Boundaries in the Moyal plane
We study the oscillations of a scalar field on a noncommutative disc
implementing the boundary as the limit case of an interaction with an
appropriately chosen confining background. The space of quantum fluctuations of
the field is finite dimensional and displays the rotational and parity symmetry
of the disc. We perform a numerical evaluation of the (finite) Casimir energy
and obtain similar results as for the fuzzy sphere and torus.Comment: 19 pages, 6 figures. Replaced by published versio
Kinetic Theory and Fast Wind Observations of the Electron Strahl
We develop a model for the strahl population in the solar wind -- a narrow,
low-density and high-energy electron beam centered on the magnetic field
direction. Our model is based on the solution of the electron drift-kinetic
equation at heliospheric distances where the plasma density, temperature, and
the magnetic field strength decline as power-laws of the distance along a
magnetic flux tube. Our solution for the strahl depends on a number of
parameters that, in the absence of the analytic solution for the full electron
velocity distribution function (eVDF), cannot be derived from the theory. We
however demonstrate that these parameters can be efficiently found from
matching our solution with observations of the eVDF made by the Wind
satellite's SWE strahl detector. The model is successful at predicting the
angular width (FWHM) of the strahl for the Wind data at 1 AU, in particular by
predicting how this width scales with particle energy and background density.
We find the strahl distribution is largely determined by the local temperature
Knudsen number , which parametrizes solar wind
collisionality. We compute averaged strahl distributions for typical Knudsen
numbers observed in the solar wind, and fit our model to these data. The model
can be matched quite closely to the eVDFs at 1 AU, however, it then
overestimates the strahl amplitude at larger heliocentric distances. This
indicates that our model may be improved through the inclusion of additional
physics, possibly through the introduction of "anomalous diffusion" of the
strahl electrons
Electron scattering in isotonic chains as a probe of the proton shell structure of unstable nuclei
Electron scattering on unstable nuclei is planned in future facilities of the
GSI and RIKEN upgrades. Motivated by this fact, we study theoretical
predictions for elastic electron scattering in the N=82, N=50, and N=14
isotonic chains from very proton-deficient to very proton-rich isotones. We
compute the scattering observables by performing Dirac partial-wave
calculations. The charge density of the nucleus is obtained with a covariant
nuclear mean-field model that accounts for the low-energy electromagnetic
structure of the nucleon. For the discussion of the dependence of scattering
observables at low-momentum transfer on the gross properties of the charge
density, we fit Helm model distributions to the self-consistent mean-field
densities. We find that the changes shown by the electric charge form factor
along each isotonic chain are strongly correlated with the underlying proton
shell structure of the isotones. We conclude that elastic electron scattering
experiments in isotones can provide valuable information about the filling
order and occupation of the single-particle levels of protons.Comment: 13 pages; 19 figure
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