22,485 research outputs found
Auroral magnetosphere-ionosphere coupling: A brief topical review
Auroral arcs result from the acceleration and precipitation of magnetospheric plasma in narrow regions characterized by strong electric fields both perpendicular and parallel to the earth's magnetic field. The various mechanisms that were proposed for the origin of such strong electric fields are often complementary Such mechanisms include: (1) electrostatic double layers; (2) double reverse shock; (3) anomalous resistivity; (4) magnetic mirroring of hot plasma; and (5) mapping of the magnetospheric-convection electric field through an auroral discontinuity
On the metal-insulator transition in the two-chain model of correlated fermions
The doping-induced metal-insulator transition in two-chain systems of
correlated fermions is studied using a solvable limit of the t-J model and the
fact that various strong- and weak-coupling limits of the two-chain model are
in the same phase, i.e. have the same low-energy properties. It is shown that
the Luttinger-liquid parameter K_\rho takes the universal value unity as the
insulating state (half-filling) is approached, implying dominant d-type
superconducting fluctuations, independently of the interaction strength. The
crossover to insulating behavior of correlations as the transition is
approached is discussed.Comment: 7 pages, 1 figur
Global dust model intercomparison in AeroCom phase I
This study presents the results of a broad intercomparison of a total of 15 global aerosol models within the AeroCom project. Each model is compared to observations related to desert dust aerosols, their direct radiative effect, and their impact on the biogeochemical cycle, i.e., aerosol optical depth (AOD) and dust deposition. Additional comparisons to Angström exponent (AE), coarse mode AOD and dust surface concentrations are included to extend the assessment of model performance and to identify common biases present in models. These data comprise a benchmark dataset that is proposed for model inspection and future dust model development. There are large differences among the global models that simulate the dust cycle and its impact on climate. In general, models simulate the climatology of vertically integrated parameters (AOD and AE) within a factor of two whereas the total deposition and surface concentration are reproduced within a factor of 10. In addition, smaller mean normalized bias and root mean square errors are obtained for the climatology of AOD and AE than for total deposition and surface concentration. Characteristics of the datasets used and their uncertainties may influence these differences. Large uncertainties still exist with respect to the deposition fluxes in the southern oceans. Further measurements and model studies are necessary to assess the general model performance to reproduce dust deposition in ocean regions sensible to iron contributions. Models overestimate the wet deposition in regions dominated by dry deposition. They generally simulate more realistic surface concentration at stations downwind of the main sources than at remote ones. Most models simulate the gradient in AOD and AE between the different dusty regions. However the seasonality and magnitude of both variables is better simulated at African stations than Middle East ones. The models simulate the offshore transport of West Africa throughout the year but they overestimate the AOD and they transport too fine particles. The models also reproduce the dust transport across the Atlantic in the summer in terms of both AOD and AE but not so well in winter-spring nor the southward displacement of the dust cloud that is responsible of the dust transport into South America. Based on the dependency of AOD on aerosol burden and size distribution we use model bias with respect to AOD and AE to infer the bias of the dust emissions in Africa and the Middle East. According to this analysis we suggest that a range of possible emissions for North Africa is 400 to 2200 Tg yr-1 and in the Middle East 26 to 526 Tg yr-1
A Chandra survey of fluorescence Fe lines in X-ray Binaries at high resolution
In this paper we present a comprehensive survey of 41 X-ray binaries (10
HMXBs and 31 LMXBs) with Chandra, with specific emphasis on the Fe K region and
the narrow Fe Kalpha line, at the highest resolution possible. We find that: a)
The Fe Kalpha line is always centered at 1.9387 +- 0.0016 Angstroms, compatible
with Fe I up to Fe X; we detect no shifts to higher ionization states nor any
difference between HMXBs and LMXBs. b) The line is very narrow, with FWHM < 5
mAngstroms, which means that the reprocessing material is not rotating at high
speeds. c) Fe Kalpha fluorescence is present in all the HMXB in the survey
while such emissions are very rare (~ 10% ) among LMXBs. d) The lack of Fe line
emission is always accompanied by the lack of any detectable K edge. e) We
obtain the empirical curve of growth of the equivalent width of the Fe Kalpha
line versus the density column of the reprocessing material, i.e. EW_{Kalpha}
vs N_{H}, and show that it is consistent with a reprocessing region spherically
distributed around the compact object. f) We show that fluorescence in X-ray
binaries follows the X-ray Baldwin effect. We interpret this finding as
evidence of decreasing neutral Fe abundance with increasing X-ray illumination
and use it to explain some spectral states of Cyg X-1 and as a possible cause
of the lack of narrow Fe line emission in LMXBs. g) Finally, we study anomalous
morphologies. We present the first evidence of a Compton shoulder in the HMXB
X1908+075. Also the Fe Kalpha lines of 4U1700-37 and LMC X-4 present asymmetric
wings suggesting the presence of highly structured stellar winds in these
systems.Comment: 12 pages, 11 figures, accepted for publication in Ap
Effect of edge transmission and elastic scattering on the resistance of magnetic barriers
Strong magnetic barriers are defined in two-dimensional electron gases by
magnetizing dysprosium ferromagnetic platelets on top of a Ga[Al]As
heterostructure. A small resistance across the barrier is observed even deep
inside the closed regime. We have used semiclassical simulations to explain
this behavior quantitatively in terms of a combined effect of elastic electron
scattering inside the barrier region and E x B drift at the intersection of the
magnetic barrier with the edge of the Hall bar.Comment: 7 pages 4 figure
On the Glauber model in a quantum representation
The Glauber model is reconsidered based on a quantum formulation of the
Master equation. Unlike the conventional approach the temperature and the Ising
energy are included from the beginning by introducing a Heisenberg-like picture
of the second quantized operators. This method enables us to get an exact
expression for the transition rate of a single flip-process
which is in accordance with the principle of detailed balance. The transition
rate differs significantly from the conventional one due to Glauber in the low
temperature regime. Here the behavior is controlled by the Ising energy and not
by the microscopic time scale.Comment: 8 page
Correlations in a two--chain Hubbard model
Equal time spin--spin and pair field correlation functions are calculated for
a two-chain Hubbard model using a density-matrix numerical renormalization
group approach. At half-filling, the antiferromagnetic and pair field
correlations both decay exponentially with the pair field having a much shorter
correlation length. This is consistent with a gapped spin-liquid ground state.
Below half--filling, the antiferromagnetic correlations become incommensurate
and the spin gap persists. The pair field correlations appear to follow a power
law decay which is similar to their non-interacting U=0 behavior.Comment: 9 pages and 5 postscript figures, RevTeX 3.0, UCI-CMTHE-94-01
(revised version
A Census of X-ray gas in NGC 1068: Results from 450ks of Chandra HETG Observation
We present models for the X-ray spectrum of the Seyfert 2 galaxy NGC 1068.
These are fitted to data obtained using the High Energy Transmission Grating
(HETG) on the Chandra X-ray observatory. The data show line and radiative
recombination continuum (RRC) emission from a broad range of ions and elements.
The models explore the importance of excitation processes for these lines
including photoionization followed by recombination, radiative excitation by
absorption of continuum radiation and inner shell fluorescence. The models show
that the relative importance of these processes depends on the conditions in
the emitting gas, and that no single emitting component can fit the entire
spectrum. In particular, the relative importance of radiative excitation and
photoionization/recombination differs according to the element and ion stage
emitting the line. This in turn implies a diversity of values for the
ionization parameter of the various components of gas responsible for the
emission, ranging from log(xi)=1 -- 3. Using this, we obtain an estimate for
the total amount of gas responsible for the observed emission. The mass flux
through the region included in the HETG extraction region is approximately 0.3
Msun/yr assuming ordered flow at the speed characterizing the line widths. This
can be compared with what is known about this object from other techniques.Comment: 39 pages, 12 figures, Ap. J. in pres
Mott-Superfluid transition in bosonic ladders
We show that in a commensurate bosonic ladder, a quantum phase transition
occurs between a Mott insulator and a superfluid when interchain hopping
increases. We analyse the properties of such a transition as well as the
physical properties of the two phases. We discuss the physical consequences for
experimental systems such as Josephson Junction arrays.Comment: 4 pages, 2 figures, revtex
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