1,889 research outputs found
Numerical Studies of Weakly Stochastic Magnetic Reconnection
We study the effects of turbulence on magnetic reconnection using
three-dimensional numerical simulations. This is the first attempt to test a
model of fast magnetic reconnection proposed by Lazarian & Vishniac (1999),
which assumes the presence of weak, small-scale magnetic field structure near
the current sheet. This affects the rate of reconnection by reducing the
transverse scale for reconnection flows and by allowing many independent flux
reconnection events to occur simultaneously. We performed a number of
simulations to test the dependencies of the reconnection speed, defined as the
ratio of the inflow velocity to the Alfven speed, on the turbulence power, the
injection scale and resistivity. Our results show that turbulence significantly
affects the topology of magnetic field near the diffusion region and increases
the thickness of the outflow region. We confirm the predictions of the Lazarian
& Vishniac model. In particular, we report the growth of the reconnection speed
proportional to ~ V^2, where V is the amplitude of velocity at the injection
scale. It depends on the injection scale l as ~ (l/L)^(2/3), where L is the
size of the system, which is somewhat faster but still roughly consistent with
the theoretical expectations. We also show that for 3D reconnection the Ohmic
resistivity is important in the local reconnection events only, and the global
reconnection rate in the presence of turbulence does not depend on it.Comment: 8 pages, 8 figure
Black Holes and Vacuum Cleaners: Using Metaphor, Relevance, and Inquiry in Labels for Space Images
This study extended research on the development of explanatory labels for
astronomical images for the non-expert lay public. The research questions
addressed how labels with leading questions/metaphors and relevance to everyday
life affect comprehension of the intended message for deep space images, the
desire to learn more, and the aesthetic appreciation of images. Participants
were a convenience sample of 1,921 respondents solicited from a variety of
websites and through social media who completed an online survey that used four
high-resolution images as stimuli: Sagittarius A*, Solar Flare, Cassiopeia A,
and the Pinwheel Galaxy (M101). Participants were randomly assigned initially
to 1 of 3 label conditions: the standard label originally written for the
image, a label with a leading question containing a metaphor related to the
information for the image, or a label that contained a fact about the image
relevant to everyday life. Participants were randomly assigned to 1 image and
compared all labels for that image. Open-ended items at various points asked
participants to pose questions to a hypothetical astronomer. Main findings were
that the relevance condition was significantly more likely to increase wanting
to learn more; the original label was most likely to increase overall
appreciation; and, smart phone users were more likely to want to learn more and
report increased levels of appreciation. Results are discussed in terms of the
need to examine individual viewer characteristics and goals in creating
different labels for different audiences.Comment: 50 pages, 7 tables, 2 figures, accepted by the journal "Psychology of
Aesthetics, Creativity, and the Arts
The Fusion-by-Diffusion model as a tool to calculate cross sections for the production of superheavy nuclei
This article summarizes recent progress in our understanding of the reaction
mechanisms leading to the formation of superheavy nuclei in cold and hot fusion
reactions. Calculations are done within the Fusion-by-Diffusion (FBD) model
using the new nuclear data tables by Jachimowicz et al. [At. Data Nucl. Data
Tables 138, 101393 (2021)]. The synthesis reaction is treated in a standard way
as a three-step process (i.e., capture, fusion, and survival). Each reaction
step is analyzed separately. Model calculations are compared with selected
experimental data on capture, fissionlike and fusion cross sections, fusion
probabilities, and evaporation residue excitation functions. The role of the
angular momentum in the fusion step is discussed in detail. A set of fusion
excitation functions with corresponding fusion probabilities is provided for
cold and hot synthesis reactions.Comment: submitted to EPJ A Topical Issue: Heavy and Super-Heavy Nuclei and
Elements: Production and Propertie
Cosmic-ray driven dynamo in galaxies
We present recent developments of global galactic-scale numerical models of
the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker
(1992). We conduct a series of direct CR+MHD numerical simulations of the
dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and
CR components. We take into account CRs accelerated in randomly distributed
supernova (SN) remnants, and assume that SNe deposit small-scale, randomly
oriented, dipolar magnetic fields into the ISM. The amplification timescale of
the large-scale magnetic field resulting from the CR-driven dynamo is
comparable to the galactic rotation period. The process efficiently converts
small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields.
The resulting magnetic field structure resembles the X-shaped magnetic fields
observed in edge-on galaxies.Comment: 6 pages, 4 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche
Accuracy of computerized tomography in determining hepatic tumor size in patients receiving liver transplantation or resection
Computerized tomography (CT) of liver is used in oncologic practice for staging tumors, evaluating response to treatment, and screening patients for hepatic resection. Because of the impact of CT liver scan on major treatment decisions, it is important to assess its accuracy. Patients undergoing liver transplantation or resection provide a unique opportunity to test the accuracy of hepatic-imaging techniques by comparison of finding of preoperative CT scan with those at gross pathologic examination of resected specimens. Forty-one patients who had partial hepatic resection (34 patients) or liver transplantation (eight patients) for malignant (30 patients) or benign (11 patients) tumors were evaluable. Eight (47%) of 17 patients with primary malignant liver tumors, four (31%) of 13 patients with metastatic liver tumors, and two (20%) of 10 patients with benign liver tumors had tumor nodules in resected specimens that were not apparent on preoperative CT studies. These nodules varied in size from 0.1 to 1.6 cm. While 11 of 14 of these nodules were 1.0 cm. These results suggest that conventional CT alone may be insufficient to accurately determine the presence or absence of liver metastases, extent of liver involvement, or response of hepatic metastases to treatment
Intermittency of Magnetohydrodynamic Turbulence: Astrophysical Perspective
Intermittency is an essential property of astrophysical fluids, which
demonstrate an extended inertial range. As intermittency violates
self-similarity of motions, it gets impossible to naively extrapolate the
properties of fluid obtained computationally with relatively low resolution to
the actual astrophysical situations. In terms of Astrophysics, intermittency
affects turbulent heating, momentum transfer, interaction with cosmic rays,
radio waves and many more essential processes. Because of its significance,
studies of intermittency call for coordinated efforts from both theorists and
observers. In terms of theoretical understanding we are still just scratching a
surface of a very rich subject. We have some theoretically well justified
models that are poorly supported by experiments, we also have She-Leveque
model, which could be vulnerable on theoretical grounds, but, nevertheless, is
well supported by experimental and laboratory data. I briefly discuss a rather
mysterious property of turbulence called ``extended self-similarity'' and the
possibilities that it opens for the intermittency research. Then I analyze
simulations of MHD intermittency performed by different groups and show that
their results do not contradict to each other. Finally, I discuss the
intermittency of density, intermittency of turbulence in the
viscosity-dominated regime as well as the intermittency of polarization of
Alfvenic modes. The latter provides an attractive solution to account for a
slower cascading rate that is observed in some of the numerical experiments. I
conclude by claiming that a substantial progress in the field may be achieved
by studies of the turbulence intermittency via observations.Comment: 14 pages, 7 figures, invited lecture at Trieste, published
International Journal of Modern Physics D, July 2
Discussion of the Electromotive Force Terms in the Model of Parker-unstable Galactic Disks with Cosmic Rays and Shear
We analyze the electromotive force (EMF) terms and basic assumptions of the
linear and nonlinear dynamo theories in our three-dimensional (3D) numerical
model of the Parker instability with cosmic rays and shear in a galactic disk.
We also apply the well known prescriptions of the EMF obtained by the nonlinear
dynamo theory (Blackman & Field 2002 and Kleeorin et al. 2003) to check if the
EMF reconstructed from their prescriptions corresponds to the EMF obtained
directly from our numerical models. We show that our modeled EMF is fully
nonlinear and it is not possible to apply any of the considered nonlinear
dynamo approximations due to the fact that the conditions for the scale
separation are not fulfilled.Comment: 15 pages, 12 figure
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