34,808 research outputs found
Energy spectrum, dissipation and spatial structures in reduced Hall magnetohydrodynamic
We analyze the effect of the Hall term in the magnetohydrodynamic turbulence
under a strong externally supported magnetic field, seeing how this changes the
energy cascade, the characteristic scales of the flow and the dynamics of
global magnitudes, with particular interest in the dissipation.
Numerical simulations of freely evolving three-dimensional reduced
magnetohydrodynamics (RHMHD) are performed, for different values of the Hall
parameter (the ratio of the ion skin depth to the macroscopic scale of the
turbulence) controlling the impact of the Hall term. The Hall effect modifies
the transfer of energy across scales, slowing down the transfer of energy from
the large scales up to the Hall scale (ion skin depth) and carrying faster the
energy from the Hall scale to smaller scales. The final outcome is an effective
shift of the dissipation scale to larger scales but also a development of
smaller scales. Current sheets (fundamental structures for energy dissipation)
are affected in two ways by increasing the Hall effect, with a widening but at
the same time generating an internal structure within them. In the case where
the Hall term is sufficiently intense, the current sheet is fully delocalized.
The effect appears to reduce impulsive effects in the flow, making it less
intermittent.Comment: 17 pages, 10 figure
Modeling the variability of the BL Lacertae object PKS 2155-304
The bright X-ray selected BL Lacertae object PKS 2155-304 has been the target
of two intense multiwavelength campaigns, in November 1991 and in May 1994.
Although the spectral energy distributions at both epochs were quite similar,
the source exhibited two very distinct variability patterns that cannot be
easily reconciled with homogeneous, one-zone jet models. During the first epoch
the variability was almost achromatic in amplitude, with a time lag between
X-rays and UV of h, while during the second epoch the variability
amplitude increased as a function of wavelength, with the EUV flare peaking
day after the X-ray flare. We model the source using a
time-dependent inhomogeneous accelerating jet model. e reproduce the general
characteristics of the different variability signatures by assuming that plasma
disturbances with different physical properties propagate downstream in an
underlying jet characterized by the same set of physical parameters at both
epochs. A time delay of 1 day between the hardening of the UV
spectral index and the UV flux, present at both epochs, is modeled with
stochastic fluctuations in the particle acceleration manifested through small
variations of the maximum energy of the injected electrons. We predict that
similar time delays will be present in future observations, even in the absence
of strong variability events. We stress the importance of observations at
neighboring frequencies as a diagnostic tool for the structure of the quiescent
jet in blazars, especially in the seemingly dull case when strong variability
is absent.Comment: 8 pages, 5 figures. Accepted in ApJ Letter
Pilot3 D7.1 - Dissemination, communication and exploitation plan
This document is the Communication, Dissemination and Exploitation Plan (D7.1) of the Clean Sky 2 Innovation Action Pilot3. The document defines the communication and dissemination actions to be performed during the project, and the potential exploitation of the project results. A complete strategy of communication is presented, as well as the items and content already prepared for it
Ferromagnetic 0-pi Junctions as Classical Spins
The ground state of highly damped PdNi based 0-pi ferromagnetic Josephson
junctions shows a spontaneous half quantum vortex, sustained by a supercurrent
of undetermined sign. This supercurrent flows in the electrode of a Josephson
junction used as a detector and produces a phi(0)/4 shift in its magnetic
diffraction pattern. We have measured the statistics of the positive or
negative sign shift occurring at the superconducting transition of such a
junction. The randomness of the shift sign, the reproducibility of its
magnitude and the possibility of achieving exact flux compensation upon field
cooling: all these features show that 0-pi junctions behave as classical spins,
just as magnetic nanoparticles with uniaxial anisotropy.Comment: 4 pages, 4 figure
Pair Plasma Dominance in the Parsec-Scale Relativistic Jet of 3C345
We investigate whether a pc-scale jet of 3C345 is dominated by a normal
plasma or an electron-positron plasma. We present a general condition that a
jet component becomes optically thick for synchrotron self-absorption, by
extending the method originally developed by Reynolds et al. The general
condition gives a lower limit of the electron number density, with the aid of
the surface brightness condition, which enables us to compute the magnetic
field density. Comparing the lower limit with another independent constraint
for the electron density that is deduced from the kinetic luminosity, we can
distinguish the matter content. We apply the procedure to the five components
of 3C345 (C2, C3, C4, C5, and C7) of which angular diameters and radio fluxes
at the peak frequencies were obtainable from literature. Evaluating the
representative values of Doppler beaming factors by their equipartition values,
we find that all the five components are likely dominated by an
electron-positron plasma. The conclusion does not depend on the lower cutoff
energy of the power-law distribution of radiating particles.Comment: 17 page
Spinor Dynamics in an Antiferromagnetic Spin-1 Condensate
We observe coherent spin oscillations in an antiferromagnetic spin-1
Bose-Einstein condensate of sodium. The variation of the spin oscillations with
magnetic field shows a clear signature of nonlinearity, in agreement with
theory, which also predicts anharmonic oscillations near a critical magnetic
field. Measurements of the magnetic phase diagram agree with predictions made
in the approximation of a single spatial mode. The oscillation period yields
the best measurement to date of the sodium spin-dependent interaction
coefficient, determining that the difference between the sodium spin-dependent
s-wave scattering lengths is Bohr radii.Comment: 5 pages, 2 figures. Changes: added reference, minor correction
The Origin of Complex Behavior of Linearly Polarized Components in Parsec-Scale Jets
Evidence that the magnetic fields of extragalactic jets have a significant
fraction of their energy in a random component is briefly summarized, and a
detailed model of evolving, jet polarization structures is constructed, based
on this picture. The evolving magnetic field structure of an oblique shock
complex that forms in a relativistic jet simulation is explored by using
velocity data from the hydrodynamical simulation to advect an initially random
magnetic field distribution. Radiative transfer calculations reveal that
emission from a propagating region of magnetic field, `ordered' by the shock,
and lying approximately transverse to the flow direction, merges with that from
an evolving sheared region at the flow periphery. If such a flow were barely
resolved, observation would suggest evolution from a somewhat oblique, to a
more longitudinal, magnetic field structure with respect to the flow axis,
while higher resolution observations would infer a component following a
non-linear trajectory, and with a magnetic field orientation that rotates
during evolution. This result highlights the ambiguity in interpreting VLBP
data, and illustrates the importance of simulations in providing a framework
for proper interpretation of such data.Comment: Accepted for publication in ApJ (Part I). Low res. figures; originals
may be obtained from the "Relativistic Flows" section of
http://www.astro.lsa.umich.edu/users/hughes
Dispatcher3 D7.1 - Project communication, dissemination and exploitation plan
This document is the Communication, Dissemination and Exploitation Plan (D7.1) of the Clean Sky 2 Innovation Action Dispatcher3. The document defines the communication and dissemination actions to be performed during the project, and the potential exploitation of the project results. A complete strategy of communication is presented, as well as the items and content already prepared for it
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