33 research outputs found
Particle acceleration by turbulent magnetohydro-dynamic reconnection
Test particles in a two dimensional, turbulent MHD simulation are found to undergo significant acceleration. The magnetic field configuration is a periodic sheet pinch which undergoes reconnection. The test particles are trapped in the reconnection region for times of order an Alfven transit time in the large electric fields that characterize the turbulent reconnection process at the relatively large magnetic Reynolds number used in the simulation. The maximum speed attained by these particles is consistent with an analytic estimate which depends on the reconnection electric field, the Alfven speed, and the ratio of Larmor period to the Alfven transit time
Particle Acceleration in Multiple Dissipation Regions
The sharp magnetic discontinuities which naturally appear in solar magnetic
flux tubes driven by turbulent photospheric motions are associated with intense
currents. \citet{Par83} proposed that these currents can become unstable to a
variety of microscopic processes, with the net result of dramatically enhanced
resistivity and heating (nanoflares). The electric fields associated with such
``hot spots'' are also expected to enhance particle acceleration. We test this
hypothesis by exact relativistic orbit simulations in strong random phase
magnetohydrodynamic (MHD) turbulence which is forming localized super-Dreicer
Ohm electric fields ( = ) occurring in 2..15 % of
the volume. It is found that these fields indeed yield a large amplification of
acceleration of electrons and ions, and can effectively overcome the injection
problem. We suggest in this article that nanoflare heating will be associated
with sporadic particle acceleration.Comment: 12 pages, 5 figures, to appear in ApJ
Particle Acceleration in an Evolving Network of Unstable Current Sheets
We study the acceleration of electrons and protons interacting with
localized, multiple, small-scale dissipation regions inside an evolving,
turbulent active region. The dissipation regions are Unstable Current Sheets
(UCS), and in their ensemble they form a complex, fractal, evolving network of
acceleration centers. Acceleration and energy dissipation are thus assumed to
be fragmented. A large-scale magnetic topology provides the connectivity
between the UCS and determines in this way the degree of possible multiple
acceleration. The particles travel along the magnetic field freely without
loosing or gaining energy, till they reach a UCS. In a UCS, a variety of
acceleration mechanisms are active, with the end-result that the particles
depart with a new momentum. The stochastic acceleration process is represented
in the form of Continuous Time Random Walk (CTRW), which allows to estimate the
evolution of the energy distribution of the particles. It is found that under
certain conditions electrons are heated and accelerated to energies above 1 MeV
in much less than a second. Hard X-ray (HXR) and microwave spectra are
calculated from the electrons' energy distributions, and they are found to be
compatible with the observations. Ions (protons) are also heated and
accelerated, reaching energies up to 10 MeV almost simultaneously with the
electrons. The diffusion of the particles inside the active region is extremely
fast (anomalous super-diffusion). Although our approach does not provide
insight into the details of the specific acceleration mechanisms involved, its
benefits are that it relates acceleration to the energy release, and it well
describes the stochastic nature of the acceleration process.Comment: 37 pages, 10 figures, one of them in color; in press at ApJ (2004
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Development of a common data model for scientific simulations
The problem of sharing data among scientific simulation models is a difficult and persistent one. Computational scientists employ an enormous variety of discrete approximations in modeling physical processes on computers. Problems occur when models based on different representations are required to exchange data with one another, or with some other software package. Within the DOE`s Accelerated Strategic Computing Initiative (ASCI), a cross-disciplinary group called the Data Models and Formats (DMF) group, has been working to develop a common data model. The current model is comprised of several layers of increasing semantic complexity. One of these layers is an abstract model based on set theory and topology called the fiber bundle kernel (FBK). This layer provides the flexibility needed to describe a wide range of mesh-approximated functions as well as other entities. This paper briefly describes the ASCI common data model, its mathematical basis, and ASCI prototype development. These prototypes include an object-oriented data management library developed at Los Alamos called the Common Data Model Library or CDMlib, the Vector Bundle API from the Lawrence Livermore Laboratory, and the DMF API from Sandia National Laboratory
Understanding coronal heating and solar wind acceleration: Case for in situ near‐Sun measurements
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94903/1/rog1641.pd
La psicoterapia di gruppo a tempo limitato con pazienti affetti da sindrome fibromialgica
Il lavoro tratta della psicoterapia di gruppo a tempo limitato con pazienti affetti da sindrome fibromialgica. La scelta di un intervento attraverso la psicoterapia di gruppo per pazienti che lamentano sintomi somatici fornisce ai pazienti che soffrono della stessa patologia un contesto privilegiato di condivisione e di analisi delle difficolt\ue0 comuni e, favorendo il senso di universalit\ue0, permette una migliore convivenza con la malattia