396 research outputs found
Reconnection Studies Under Different Types of Turbulence Driving
We study a model of fast magnetic reconnection in the presence of weak
turbulence proposed by Lazarian and Vishniac (1999) using three-dimensional
direct numerical simulations. The model has been already successfully tested in
Kowal et al. (2009) confirming the dependencies of the reconnection speed
on the turbulence injection power and the injection scale
expressed by a constraint
and no observed dependency on Ohmic resistivity. In Kowal et al. (2009), in
order to drive turbulence, we injected velocity fluctuations in Fourier space
with frequencies concentrated around , as described in
Alvelius (1999). In this paper we extend our previous studies by comparing fast
magnetic reconnection under different mechanisms of turbulence injection by
introducing a new way of turbulence driving. The new method injects velocity or
magnetic eddies with a specified amplitude and scale in random locations
directly in real space. We provide exact relations between the eddy parameters
and turbulent power and injection scale. We performed simulations with new
forcing in order to study turbulent power and injection scale dependencies. The
results show no discrepancy between models with two different methods of
turbulence driving exposing the same scalings in both cases. This is in
agreement with the Lazarian and Vishniac (1999) predictions. In addition, we
performed a series of models with varying viscosity . Although Lazarian
and Vishniac (1999) do not provide any prediction for this dependence, we
report a weak relation between the reconnection speed with viscosity,
.Comment: 19 pages, 9 figures. arXiv admin note: text overlap with
arXiv:0903.205
Modification of Projected Velocity Power Spectra by Density Inhomogeneities in Compressible Supersonic Turbulence
(Modified) The scaling of velocity fluctuation, dv, as a function of spatial
scale L in molecular clouds can be measured from size-linewidth relations,
principal component analysis, or line centroid variation. Differing values of
the power law index of the scaling relation dv = L^(g3D) in 3D are given by
these different methods: the first two give g3D=0.5, while line centroid
analysis gives g3D=0. This discrepancy has previously not been fully
appreciated, as the variation of projected velocity line centroid fluctuations
(dv_{lc} = L^(g2D)) is indeed described, in 2D, by g2D=0.5. However, if
projection smoothing is accounted for, this implies that g3D=0. We suggest that
a resolution of this discrepancy can be achieved by accounting for the effect
of density inhomogeneity on the observed g2D obtained from velocity line
centroid analysis. Numerical simulations of compressible turbulence are used to
show that the effect of density inhomogeneity statistically reverses the effect
of projection smoothing in the case of driven turbulence so that velocity line
centroid analysis does indeed predict that g2D=g3D=0.5. Using our numerical
results we can restore consistency between line centroid analysis, principal
component analysis and size-linewidth relations, and we derive g3D=0.5,
corresponding to shock-dominated (Burgers) turbulence. We find that this
consistency requires that molecular clouds are continually driven on large
scales or are only recently formed.Comment: 28 pages total, 20 figures, accepted for publication in Ap
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