149 research outputs found
Modeling the instantaneous response of glaciers after the collapse of the Larsen B Ice Shelf
Following the disintegration of the Larsen B Ice Shelf, Antarctic Peninsula, in 2002, regular surveillance of its âŒ20 tributary glaciers has revealed a response which is varied and complex in both space and time. The major outlets have accelerated and thinned, smaller glaciers have shown little or no change, and glaciers flowing into the remnant Scar Inlet Ice Shelf have responded with delay. In this study we present the first areawide numerical analysis of glacier dynamics before and immediately after the collapse of the ice shelf, combining new data sets and a stateâofâtheâart numerical ice flow model. We simulate the loss of buttressing at the grounding line and find a good qualitative agreement between modeled changes in glacier flow and observations. Through this study, we seek to improve confidence in our numerical models and their ability to capture the complex mechanical coupling between floating ice shelves and grounded ice
The internal structure of the Brunt Ice Shelf from ice-penetrating radar analysis and implications for ice shelf fracture
The rate and direction of rift propagation through ice shelves
depend on both the stress field and the heterogeneity (or otherwise) of the
physical properties of the ice. The Brunt Ice Shelf in Antarctica has
recently developed new rifts, which are being actively monitored as they
lengthen and interact with the internal structure of the ice shelf. Here we
present the results of a ground-penetrating radar survey of the Brunt Ice
Shelf aimed at understanding variations in the internal structure. We find
that there are flow bands composed mostly of thick (ca. 250 m) meteoric ice
interspersed with thinner (ca. 150 m) sections of ice shelf that have a large
proportion of sea ice and seawater-saturated firn. Therefore the ice shelf
is, in essence, a series of ice tongues cemented together with ice
mélange. The changes in structure are related both to the thickness and
flow speed of ice at the grounding line and to subsequent processes of firn
accumulation and brine infiltration as the ice shelf flows towards the
calving front. It is shown that rifts propagating through the Brunt Ice Shelf
preferentially skirt the edges of blocks of meteoric ice and slow their rate
of propagation when forced by the stress field to break through them, in
contrast to the situation on other ice shelves where rift propagation speeds
up in meteoric ice.</p
Generalized gaugings and the field-antifield formalism
We discuss the algebra of general gauge theories that are described by the
embedding tensor formalism. We compare the gauge transformations dependent and
independent of an invariant action, and argue that the generic transformations
lead to an infinitely reducible algebra. We connect the embedding tensor
formalism to the field-antifield (or Batalin-Vilkovisky) formalism, which is
the most general formulation known for general gauge theories and their
quantization. The structure equations of the embedding tensor formalism are
included in the master equation of the field-antifield formalism.Comment: 42 pages; v2: some clarifications and 1 reference added; version to
be published in JHE
Evolution of surface velocities and ice discharge of Larsen B outlet glaciers from 1995 to 2013
We use repeat-pass SAR data to produce detailed maps of surface motion covering the glaciers draining into the former Larsen B ice shelf, Antarctic Peninsula, for different epochs between 1995 and 2013. We combine the velocity maps with estimates of ice thickness to analyze fluctuations of ice discharge. The collapse of the central and northern sections of the ice shelf in 2002 led to a near-immediate acceleration of tributary glaciers as well as of the remnant ice shelf in Scar Inlet. Velocities of the glaciers discharging directly into the ocean remain to date well above the velocities of the pre-collapse period. The response of individual glaciers differs and velocities show significant temporal fluctuations, implying major variations in ice discharge and mass balance as well. Due to reduced velocity and ice thickness the ice discharge of Crane Glacier decreased from 5.02 Gt aâ1 in 2007 to 1.72 Gt aâ1 in 2013, whereas Hektoria and Green glaciers continue to show large temporal fluctuations in response to successive stages of frontal retreat. The velocity on Scar Inlet ice shelf increased two- to three fold since 1995, with the largest increase in the first years after the break-up of the main section of Larsen B. Flask and Leppard glaciers, the largest tributaries to Scar Inlet ice shelf, accelerated. In 2013 their discharge was 38% and 46%, higher than in 1995
Electric/magnetic duality for chiral gauge theories with anomaly cancellation
We show that 4D gauge theories with Green-Schwarz anomaly cancellation and
possible generalized Chern-Simons terms admit a formulation that is manifestly
covariant with respect to electric/magnetic duality transformations. This
generalizes previous work on the symplectically covariant formulation of
anomaly-free gauge theories as they typically occur in extended supergravity,
and now also includes general theories with (pseudo-)anomalous gauge
interactions as they may occur in global or local N=1 supersymmetry. This
generalization is achieved by relaxing the linear constraint on the embedding
tensor so as to allow for a symmetric 3-tensor related to electric and/or
magnetic quantum anomalies in these theories. Apart from electric and magnetic
gauge fields, the resulting Lagrangians also feature two-form fields and can
accommodate various unusual duality frames as they often appear, e.g., in
string compactifications with background fluxes.Comment: 37 pages; v2: typos corrected and 1 reference adde
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Snow densification and recent accumulation along the iSTAR traverse, Pine Island Glacier, Antarctica
Neutron probe measurements of snow density from 22 sites
in the Pine Island Glacier basin have been used to determine mean annual
accumulation using an automatic annual-layer identification routine. A mean
density profile which can be used to convert radar two-way-travel times to
depth has been derived, and the effect of annual fluctuations in density on
estimates of the depth of radar reflectors is shown to be insignificant, except
very near the surface. Vertical densification rates have been derived from the
neutron probe density profiles and from deeper firn core density profiles available
at 9 of the sites. These rates are consistent with the rates predicted by
the Herron and Langway model for stage 1 densification (by grain-boundary
sliding, grain growth and intracrystalline deformation) and stage 2 densification
(predominantly by sintering), except in a transition zone extending
from â8 to â13 m from the surface in which 10â14% of the compaction occurs.
Profiles of volumetric strain rate at each site show that in this transition
zone the rates are consistent with the Arthern densification model. Comparison
of the vertical densification rates and volumetric strain rates indicates
that the expected relation to mean annual accumulation breaks down
at high accumulation rates even when corrections are made for horizontal
ice velocity divergence
Symplectic structure of N=1 supergravity with anomalies and Chern-Simons terms
The general actions of matter-coupled N=1 supergravity have Peccei-Quinn
terms that may violate gauge and supersymmetry invariance. In addition, N=1
supergravity with vector multiplets may also contain generalized Chern-Simons
terms. These have often been neglected in the literature despite their
importance for gauge and supersymmetry invariance. We clarify the interplay of
Peccei-Quinn terms, generalized Chern-Simons terms and quantum anomalies in the
context of N=1 supergravity and exhibit conditions that have to be satisfied
for their mutual consistency. This extension of the previously known N=1
matter-coupled supergravity actions follows naturally from the embedding of the
gauge group into the group of symplectic duality transformations. Our results
regarding this extension provide the supersymmetric framework for studies of
string compactifications with axionic shift symmetries, generalized
Chern-Simons terms and quantum anomalies.Comment: 27 pages; v2: typos corrected; version to be published in
Class.Quantum Gra
Nuclear spins, magnetic moments and quadrupole moments of Cu isotopes from N = 28 to N = 46: probes for core polarization effects
Measurements of the ground-state nuclear spins, magnetic and quadrupole
moments of the copper isotopes from 61Cu up to 75Cu are reported. The
experiments were performed at the ISOLDE facility, using the technique of
collinear laser spectroscopy. The trend in the magnetic moments between the
N=28 and N=50 shell closures is reasonably reproduced by large-scale
shell-model calculations starting from a 56Ni core. The quadrupole moments
reveal a strong polarization of the underlying Ni core when the neutron shell
is opened, which is however strongly reduced at N=40 due to the parity change
between the and orbits. No enhanced core polarization is seen beyond
N=40. Deviations between measured and calculated moments are attributed to the
softness of the 56Ni core and weakening of the Z=28 and N=28 shell gaps.Comment: 13 pagers, 19 figures, accepted by Physical Review
Metastable supergravity vacua with F and D supersymmetry breaking
We study the conditions under which a generic supergravity model involving
chiral and vector multiplets can admit viable metastable vacua with
spontaneously broken supersymmetry and realistic cosmological constant. To do
so, we impose that on the vacuum the scalar potential and all its first
derivatives vanish, and derive a necessary condition for the matrix of its
second derivatives to be positive definite. We study then the constraints set
by the combination of the flatness condition needed for the tuning of the
cosmological constant and the stability condition that is necessary to avoid
unstable modes. We find that the existence of such a viable vacuum implies a
condition involving the curvature tensor for the scalar geometry and the charge
and mass matrices for the vector fields. Moreover, for given curvature, charges
and masses satisfying this constraint, the vector of F and D auxiliary fields
defining the Goldstino direction is constrained to lie within a certain domain.
The effect of vector multiplets relative to chiral multiplets is maximal when
the masses of the vector fields are comparable to the gravitino mass. When the
masses are instead much larger or much smaller than the gravitino mass, the
effect becomes small and translates into a correction to the effective
curvature. We finally apply our results to some simple classes of examples, to
illustrate their relevance.Comment: 40 pages; v2 some clarifications added in the introduction; v3 some
typos correcte
The supersymmetric tensor hierarchy of N=1,d=4 supergravity
In this paper we construct the supersymmetric tensor hierarchy of N=1, d=4
supergravity. We find some differences with the general bosonic construction of
4-dimensional gauged supergravities.
The global symmetry group of N=1,d=4 supergravity consists of three factors:
the scalar manifold isometry group, the invariance group of the complex vector
kinetic matrix and the U(1) R-symmetry group. In contrast to (half)-maximal
supergravities, the latter two symmetries are not embedded into the isometry
group of the scalar manifold. We identify some components of the embedding
tensor with Fayet-Iliopoulos terms and we find that supersymmetry implies that
the inclusion of R-symmetry as a factor of the global symmetry group requires a
non-trivial extension of the standard p-form hierarchy. This extension involves
additional 3- and 4-forms. One additional 3-form is dual to the superpotential
(seen as a deformation of the simplest theory).
We study the closure of the supersymmetry algebra on all the bosonic p-form
fields of the hierarchy up to duality relations. In order to close the
supersymmetry algebra without the use of duality relations one must construct
the hierarchy in terms of supermultiplets. Such a construction requires
fermionic duality relations among the hierarchy's fermions and these turn out
to be local.Comment: Latex2e, 42 pages, no figures Improved version to be published in
JEH
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