683 research outputs found
Relative drifts and temperature anisotropies of protons and particles in the expanding solar wind -- 2.5D hybrid simulations
We perform 2.5D hybrid simulations to investigate the origin and evolution of
relative drift speeds between protons and particles in the
collisionless turbulent low- solar wind plasma. We study the generation
of differential streaming by wave-particle interactions and absorption of
turbulent wave spectra. Next we focus on the role of the relative drifts for
the turbulent heating and acceleration of ions in the collisionless fast solar
wind streams. The energy source is given by an initial broad-band spectrum of
parallel propagating Alfv\'en-cyclotron waves, which co-exists with the plasma
and is self-consistently coupled to the perpendicular ion bulk velocities. We
include the effect of a gradual solar wind expansion, which cools and
decelerates the minor ions. This paper for the first time considers the
combined effect of self-consistently initialized dispersive turbulent
Alfv\'enic spectra with differentially streaming protons and particles
in the expanding solar wind outflows within a 2.5D hybrid simulation study. In
the non-expanding wind, we find a threshold value of the differential streaming
, for which the relative drift speed remains
nearly steady. For ions, streaming below the threshold value, the waves act to
increase the magnitude of the relative drift speed. Ions, which stream faster
than the threshold value become subject to nonlinear streaming instability and
as the system evolves their bulk velocities decrease. We find that the solar
wind expansion strongly affects the relative drift speeds and significantly
slows down both ion species for all values of the relative drift speeds
considered in this study.Comment: 11 pages, 13 figures, submitted to A&
The Connectivity of Boolean Satisfiability: Computational and Structural Dichotomies
Boolean satisfiability problems are an important benchmark for questions
about complexity, algorithms, heuristics and threshold phenomena. Recent work
on heuristics, and the satisfiability threshold has centered around the
structure and connectivity of the solution space. Motivated by this work, we
study structural and connectivity-related properties of the space of solutions
of Boolean satisfiability problems and establish various dichotomies in
Schaefer's framework.
On the structural side, we obtain dichotomies for the kinds of subgraphs of
the hypercube that can be induced by the solutions of Boolean formulas, as well
as for the diameter of the connected components of the solution space. On the
computational side, we establish dichotomy theorems for the complexity of the
connectivity and st-connectivity questions for the graph of solutions of
Boolean formulas. Our results assert that the intractable side of the
computational dichotomies is PSPACE-complete, while the tractable side - which
includes but is not limited to all problems with polynomial time algorithms for
satisfiability - is in P for the st-connectivity question, and in coNP for the
connectivity question. The diameter of components can be exponential for the
PSPACE-complete cases, whereas in all other cases it is linear; thus, small
diameter and tractability of the connectivity problems are remarkably aligned.
The crux of our results is an expressibility theorem showing that in the
tractable cases, the subgraphs induced by the solution space possess certain
good structural properties, whereas in the intractable cases, the subgraphs can
be arbitrary
Nonlinear Terms of MHD Equations for Homogeneous Magnetized Shear Flow
We have derived the full set of MHD equations for incompressible shear flow
of a magnetized fluid and considered their solution in the wave-vector space.
The linearized equations give the famous amplification of slow magnetosonic
waves and describe the magnetorotational instability. The nonlinear terms in
our analysis are responsible for the creation of turbulence and self-sustained
spectral density of the MHD (Alfven and pseudo-Alfven) waves. Perspectives for
numerical simulations of weak turbulence and calculation of the effective
viscosity of accretion disks are shortly discussed in k-space.Comment: 13 pages, no figures; AIP Conference Proceedings 1356, Proceedings of
the School and Workshop on Space Plasma Physics (1--12 September 2010, Kiten,
Bulgaria), American Institute of Physics, Melville, NY, 201
THE RADIOPROTECTIVE EFFECT OF PHASEOLUS VULGARIS PHYTOHEMAGGLUTININS EXERTED ON VITIA FABA GERMS
No abstract
ANTIGENIC UNITY BETWEEN PHYTOHEMAGGLUTININ OF PHASEOLUS VULGARIS AND SOME BACTERIA AND VIRA
No abstract
Reconstruction of a Broadband Spectrum of Alfvenic Fluctuations
Alfvenic fluctuations in the solar wind exhibit a high degree of velocities and magnetic field correlations consistent with Alfven waves propagating away and toward the Sun. Two remarkable properties of these fluctuations are the tendencies to have either positive or negative magnetic helicity (-1 less than or equal to sigma(sub m) less than or equal to +1) associated with either left- or right- topological handedness of the fluctuations and to have a constant magnetic field magnitude. This paper provides, for the first time, a theoretical framework for reconstructing both the magnetic and velocity field fluctuations with a divergence-free magnetic field, with any specified power spectral index and normalized magnetic- and cross-helicity spectrum field fluctuations for any plasma species. The spectrum is constructed in the Fourier domain by imposing two conditions-a divergence-free magnetic field and the preservation of the sense of magnetic helicity in both spaces-as well as using Parseval's theorem for the conservation of energy between configuration and Fourier spaces. Applications to the one-dimensional spatial Alfvenic propagation are presented. The theoretical construction is in agreement with typical time series and power spectra properties observed in the solar wind. The theoretical ideas presented in this spectral reconstruction provide a foundation for more realistic simulations of plasma waves, solar wind turbulence, and the propagation of energetic particles in such fluctuating fields
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