8,083 research outputs found
Three-Dimensional Simulations of Solar and Stellar Dynamos: The Influence of a Tachocline
We review recent advances in modeling global-scale convection and dynamo
processes with the Anelastic Spherical Harmonic (ASH) code. In particular, we
have recently achieved the first global-scale solar convection simulations that
exhibit turbulent pumping of magnetic flux into a simulated tachocline and the
subsequent organization and amplification of toroidal field structures by
rotational shear. The presence of a tachocline not only promotes the generation
of mean toroidal flux, but it also enhances and stabilizes the mean poloidal
field throughout the convection zone, promoting dipolar structure with less
frequent polarity reversals. The magnetic field generated by a convective
dynamo with a tachocline and overshoot region is also more helical overall,
with a sign reversal in the northern and southern hemispheres. Toroidal
tachocline fields exhibit little indication of magnetic buoyancy instabilities
but may be undergoing magneto-shear instabilities.Comment: 14 pages, 5 color figures, to appear in Proc. GONG 2008/SOHO XXI
Meeting on Solar-Stellar Dynamos as Revealed by Helio and Asteroseismology,
held August 15-18, 2008, Boulder, CO, Astronomical Soc. Pac. Conf. Series,
volume TB
Quantum State Diffusion and Time Correlation Functions
In computing the spectra of quantum mechanical systems one encounters the
Fourier transforms of time correlation functions, as given by the quantum
regression theorem for systems described by master equations. Quantum state
diffusion (QSD) gives a useful method of solving these problems by unraveling
the master equation into stochastic trajectories; but there is no generally
accepted definition of a time correlation function for a single QSD trajectory.
In this paper we show how QSD can be used to calculate these spectra directly;
by formally solving the equations which arise, we arrive at a natural
definition for a two-time correlation function in QSD, which depends explicitly
on both the stochastic noise of the particular trajectory and the time of
measurement, and which agrees in the mean with the ensemble average definition
of correlation functions.Comment: 16 pages standard LaTeX + 1 figure (uuencoded postscript) Numerous
minor revisions and clarifications. To appear in J. Mod. Optic
Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields
We present the first results from three-dimensional spherical shell
simulations of magnetic dynamo action realized by turbulent convection
penetrating downward into a tachocline of rotational shear. This permits us to
assess several dynamical elements believed to be crucial to the operation of
the solar global dynamo, variously involving differential rotation resulting
from convection, magnetic pumping, and amplification of fields by stretching
within the tachocline. The simulations reveal that strong axisymmetric toroidal
magnetic fields (about 3000 G in strength) are realized within the lower stable
layer, unlike in the convection zone where fluctuating fields are predominant.
The toroidal fields in the stable layer possess a striking persistent
antisymmetric parity, with fields in the northern hemisphere largely of
opposite polarity to those in the southern hemisphere. The associated mean
poloidal magnetic fields there have a clear dipolar geometry, but we have not
yet observed any distinctive reversals or latitudinal propagation. The presence
of these deep magnetic fields appears to stabilize the sense of mean fields
produced by vigorous dynamo action in the bulk of the convection zone.Comment: 4 pages, 3 color figures (compressed), in press at ApJ
Preparation of pure and mixed polarization qubits and the direct measurement of figures of merit
Non-classical joint measurements can hugely improve the efficiency with which
certain figures of merit of quantum systems are measured. We use such a
measurement to determine a particular figure of merit, the purity, for a
polarization qubit. In the process we highlight some of subtleties involved in
common methods for generating decoherence in quantum optics.Comment: 5 pages, 3 figures, 1 tabl
Weak measurements are universal
It is well known that any projective measurement can be decomposed into a
sequence of weak measurements, which cause only small changes to the state.
Similar constructions for generalized measurements, however, have relied on the
use of an ancilla system. We show that any generalized measurement can be
decomposed into a sequence of weak measurements without the use of an ancilla,
and give an explicit construction for these weak measurements. The measurement
procedure has the structure of a random walk along a curve in state space, with
the measurement ending when one of the end points is reached. This shows that
any measurement can be generated by weak measurements, and hence that weak
measurements are universal. This may have important applications to the theory
of entanglement.Comment: 4 pages, RevTeX format, essentially the published version, reference
update
Recommended from our members
Country and Time Variation in Exchange Rate Pass Through: What Drives it?
A large sample of developed and emerging economies is utilized to investigate import exchange rate pass-through. Panel models reveal that various economic aspects of the destination country can explain about one third of the total variation in pass-through elasticities and the remaining variation comes largely in the form of unobserved country-specific effects. Inflation, exchange rate volatility, openness and relative wealth play a clear role as drivers of emerging marketsâ pass-through whereas the output gap and protectionism appear influential more generally. Nonlinearity regarding large-versus-small changes in the exchange rate is quite pervasive. Our evidence challenges the widely-held view that pass-through has been universally falling in developed markets and that it is higher for emerging markets. The economic drivers are shown to play a role as out-of-sample predictors of pass-through. The findings confirm pricing-to-market theories and have implications for the optimal conduct of monetary policy
The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems. III. Hubble Space Telescope Imaging of \lya-Absorbing Galaxies at z < 1
We present initial results of a program to obtain and analyze HST WFPC2
images of galaxies identified in an imaging and spectroscopic survey of faint
galaxies in fields of HST spectroscopic target QSOs. We measure properties of
87 galaxies, of which 33 are associated with corresponding \lya absorption
systems and 24 do not produce corresponding \lya absorption lines to within
sensitive upper limits. Considering only galaxy and absorber pairs that are
likely to be physically associated and excluding galaxy and absorber pairs
within 3000 \kms of the background QSOs leaves 26 galaxy and absorber pairs and
seven galaxies that do not produce corresponding \lya absorption lines to
within sensitive upper limits. Redshifts of the galaxy and absorber pairs range
from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations
of the galaxy and absorber pairs range from 12.4 to kpc with a
median of kpc. The primary result of the analysis is that the
amount of gas encountered along the line of sight depends on the galaxy impact
parameter and B-band luminosity but does not depend strongly on the galaxy
average surface brightness, disk-to-bulge ratio, or redshift. This result
confirms and improves upon the anti-correlation between \lya absorption
equivalent width and galaxy impact parameter found previously by Lanzetta et
al. (1995). There is no evidence that galaxy interactions play an important
role in distributing tenuous gas around galaxies in most cases. Galaxies might
account for all \lya absorption systems with \AA, but this depends on
the unknown luminosity function and gaseous cross sections of low-luminosity
galaxies as well as on the uncertainties of the observed number density of \lya
absorption systems.Comment: Minor changes. Figure 1 stays intact and is available at
ftp://ftp.ess.sunysb.edu/pub/lanzetta/wfpc
High Metallicity Mg II Absorbers in the z < 1 Lyman alpha Forest of PKS 0454+039: Giant LSB Galaxies?
We report the discovery of two iron-group enhanced high-metallicity Mg II
absorbers in a search through 28 Lyman Alpha forest clouds along the PKS
0454+039 sight line. Based upon our survey and the measured redshift number
densities of W_r(MgII) <= 0.3 A absorbers and Lyman Alpha absorbers at z ~ 1,
we suggest that roughly 5% of Lyman Alpha absorbers at z < 1 will exhibit
"weak" Mg II absorption to a 5-sigma W_r(2796) detection limit of 0.02 A. The
two discovered absorbers, at redshifts z = 0.6248 and z = 0.9315, have W_r(Lya)
= 0.33 and 0.15 A, respectively. Based upon photoionization modeling, the H I
column densities are inferred to be in the range 15.8 <= log N(HI) <= 16.8
cm^-2. For the z = 0.6428 absorber, if the abundance pattern is solar, then the
cloud has [Fe/H] > -1; if its gas-phase abundance follows that of depleted
clouds in our Galaxy, then [Fe/H] > 0 is inferred. For the z = 0.9315 absorber,
the metallicity is [Fe/H] > 0, whether the abundance pattern is solar or
suffers depletion. Imaging and spectroscopic studies of the PKS 0454+039 field
reveal no candidate luminous objects at these redshifts. We discuss the
possibility that these Mg II absorbers may arise in the class of "giant" low
surface brightness galaxies, which have [Fe/H] >= -1, and even [Fe/H] >= 0, in
their extended disks. We tentatively suggest that a substantial fraction of
these "weak" Mg II absorbers may select low surface brightness galaxies out to
z ~ 1.Comment: Accepted The Astrophysical Journal; 25 pages; 6 encapsulated figure
- âŠ