2,191 research outputs found
Molecular gyroscopes and biological effects of weak ELF magnetic fields
Extremely-low-frequency magnetic fields are known to affect biological
systems. In many cases, biological effects display `windows' in biologically
effective parameters of the magnetic fields: most dramatic is the fact that
relatively intense magnetic fields sometimes do not cause appreciable effect,
while smaller fields of the order of 10--100 T do. Linear resonant
physical processes do not explain frequency windows in this case. Amplitude
window phenomena suggest a nonlinear physical mechanism. Such a nonlinear
mechanism has been proposed recently to explain those `windows'. It considers
quantum-interference effects on protein-bound substrate ions. Magnetic fields
cause an interference of ion quantum states and change the probability of
ion-protein dissociation. This ion-interference mechanism predicts specific
magnetic-field frequency and amplitude windows within which biological effects
occur. It agrees with a lot of experiments. However, according to the
mechanism, the lifetime of ion quantum states within a protein
cavity should be of unrealistic value, more than 0.01 s for frequency band
10--100 Hz. In this paper, a biophysical mechanism has been proposed that (i)
retains the attractive features of the ion interference mechanism and (ii) uses
the principles of gyroscopic motion and removes the necessity to postulate
large lifetimes. The mechanism considers dynamics of the density matrix of the
molecular groups, which are attached to the walls of protein cavities by two
covalent bonds, i.e., molecular gyroscopes. Numerical computations have shown
almost free rotations of the molecular gyros. The relaxation time due to van
der Waals forces was about 0.01 s for the cavity size of 28 angstr\"{o}ms.Comment: 10 pages, 7 figure
Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
Conversion of gravitational energy into radiation in accretion discs and the
origin of large scale magnetic fields in astrophysical rotators have often been
distinct topics of research. In semi-analytic work on both problems it has been
useful to presume large scale symmetries, necessarily resulting in mean field
theories. MHD turbulence makes the underlying systems locally asymmetric and
nonlinear. Synergy between theory and simulations should aim for the
development of practical mean field models that capture essential physics and
can be used for observational modeling. Mean field dynamo (MFD) theory and
alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century
MFD theory has more nonlinear predictive power compared to 20th century MFD
theory, whereas accretion theory is still in a 20th century state. In fact,
insights from MFD theory are applicable to accretion theory and the two are
artificially separated pieces of what should be a single theory. I discuss
pieces of progress that provide clues toward a unified theory. A key concept is
that large scale magnetic fields can be sustained via local or global magnetic
helicity fluxes or via relaxation of small scale magnetic fluctuations, without
the kinetic helicity driver of 20th century textbooks. These concepts may help
explain the formation of large scale fields that supply non-local angular
momentum transport via coronae and jets in a unified theory of accretion and
dynamos. In diagnosing the role of helicities and helicity fluxes in disk
simulations, each disk hemisphere should be studied separately to avoid being
misled by cancelation that occurs as a result of reflection asymmetry. The
fraction of helical field energy in disks is expected to be small compared to
the total field in each hemisphere as a result of shear, but can still be
essential for large scale dynamo action.Comment: For the Proceedings of the Third International Conference and
Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28
August 2011 at the Abdus Salam International Centre for Theoretical Physics,
Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica
Scripta (corrected small items to match version in print
Electron-Positron Jets from a Critically Magnetized Black Hole
The curved spacetime surrounding a rotating black hole dramatically alters
the structure of nearby electromagnetic fields. The Wald field which is an
asymptotically uniform magnetic field aligned with the angular momentum of the
hole provides a convenient starting point to analyze the effects of radiative
corrections on electrodynamics in curved spacetime. Since the curvature of the
spacetime is small on the scale of the electron's Compton wavelength, the tools
of quantum field theory in flat spacetime are reliable and show that a rotating
black hole immersed in a magnetic field approaching the quantum critical value
of ~G cm is unstable. Specifically, a maximally rotating
three-solar-mass black hole immersed in a magnetic field of ~G would be a copious producer of electron-positron pairs with a
luminosity of erg s.Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Mathematical stories: Why do more boys than girls choose to study mathematics at AS-level in England?
Copyright @ 2005 Taylor & FrancisIn this paper I address the question: How is it that people come to choose mathematics and in what ways is this process gendered? I draw on the findings of a qualitative research study involving interviews with 43 young people all studying mathematics in post-compulsory education in England. Working within a post-structuralist framework, I argue that gender is a project and one that is achieved in interaction with others. Through a detailed reading of Toni and Claudiaâs stories I explore the tensions for young women who are engaging in mathematics, something that is discursively inscribed as masculine, while (understandably) being invested in producing themselves as female. I conclude by arguing that seeing âdoing mathematicsâ as âdoing masculinityâ is a productive way of understanding why mathematics is so male dominated and by looking at the implications of this understanding for gender and mathematics reform work.This work is funded by the ESR
The Chandra X-ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions
We present an overview of the initial results from the Chandra Planetary
Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-ray
Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The
first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc
of Earth, yielding 4 detections of diffuse X-ray emission and 9 detections of
X-ray-luminous point sources at the central stars (CSPNe) of these objects.
Combining these results with those obtained from Chandra archival data for all
(14) other PNe within ~1.5 kpc that have been observed to date, we find an
overall X-ray detection rate of ~70%. Roughly 50% of the PNe observed by
Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing
shocks formed by energetic wind collisions is detected in ~30%; five objects
display both diffuse and point-like emission components. The presence of X-ray
sources appears correlated with PN density structure, in that molecule-poor,
elliptical nebulae are more likely to display X-ray emission (either point-like
or diffuse) than molecule-rich, bipolar or Ring-like nebulae. All but one of
the X-ray point sources detected at CSPNe display X-ray spectra that are harder
than expected from hot (~100 kK) central star photospheres, possibly indicating
a high frequency of binary companions to CSPNe. Other potential explanations
include self-shocking winds or PN mass fallback. Most PNe detected as diffuse
X-ray sources are elliptical nebulae that display a nested shell/halo structure
and bright ansae; the diffuse X-ray emission regions are confined within inner,
sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have
inner shell dynamical ages <~5x10^3 yr, placing firm constraints on the
timescale for strong shocks due to wind interactions in PNe.Comment: 41 pages, 6 figures; submitted to the Astronomical Journa
Absence of a metallic phase in random-bond Ising models in two dimensions: applications to disordered superconductors and paired quantum Hall states
When the two-dimensional random-bond Ising model is represented as a
noninteracting fermion problem, it has the same symmetries as an ensemble of
random matrices known as class D. A nonlinear sigma model analysis of the
latter in two dimensions has previously led to the prediction of a metallic
phase, in which the fermion eigenstates at zero energy are extended. In this
paper we argue that such behavior cannot occur in the random-bond Ising model,
by showing that the Ising spin correlations in the metallic phase violate the
bound on such correlations that results from the reality of the Ising
couplings. Some types of disorder in spinless or spin-polarized p-wave
superconductors and paired fractional quantum Hall states allow a mapping onto
an Ising model with real but correlated bonds, and hence a metallic phase is
not possible there either. It is further argued that vortex disorder, which is
generic in the fractional quantum Hall applications, destroys the ordered or
weak-pairing phase, in which nonabelian statistics is obtained in the pure
case.Comment: 13 pages; largely independent of cond-mat/0007254; V. 2: as publishe
Turbulent cross-helicity in the mean-field solar dynamo problem
We study the dynamical and statistical properties of turbulent cross-helicity
(correlation of the aligned fluctuating velocity and magnetic field
components). We derive an equation governing generation and evolution of the
turbulent cross-helicity and discuss its meaning for the dynamo. Using symmetry
properties of the problem we suggest a general expression for the turbulent
cross-helicity pseudo-scalar and compute the turbulent coefficients in this
expression. Effects of the density stratification, large-scale magnetic fields,
differential rotation and turbulent convection are taken into account. We
investigate the relative contribution of these effects to the cross-helicity
evolution for two kinds of dynamo models of the solar cycle including a
distributed mean-field model and a flux-transport dynamo model. We show that
the contribution from the density stratification follows the evolution of the
radial magnetic field, while large-scale electric currents produce a more
complicated pattern of the cross-helicity of the comparable magnitude. We
suggest that the results of observational analysis of the cross-helicity will
depend on the averaging scales. Our results show that the pattern of the
cross-helicity evolution strongly depends on details of the dynamo mechanism.
Thus, we anticipate that direct observations of the cross-helicity on the Sun
may serve for the diagnostic purpose of the solar dynamo process.Comment: 30 pages, 3 figures (accepted for ApJ
Detection of a Corrugated Velocity Pattern in the Spiral Galaxy NGC 5427
Here we report the detection, in Halpha emission, of a radial corrugation in
the velocity field of the spiral galaxy NGC 5427. The central velocity of the
Halpha line displays coherent, wavy-like variations in the vicinity of the
spiral arms. The spectra along three different arm segments show that the
maximum amplitude of the sinusoidal line variations are displaced some 500 pc
from the central part of the spiral arms. The peak blueshifted velocities
appear some 500 pc upstream the arm, whereas the peak redshifted velocities are
located some 500 pc downstream the arm. This kinematical behavior is similar to
the one expected in a galactic bore generated by the interaction of a spiral
density wave with a thick gaseous disk, as recently modeled by Martos & Cox
(1998).Comment: Accepted for publication in Ap
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