1,206 research outputs found
A last glacial ice sheet on the Pacific Russian coast and catastrophic change arising from coupled iceâvolcanic interaction
Controversy exists over the extent of glaciation in Eastern Asia at the Last Glacial Maximum:
complete ice sheet cover vs. restricted mountain icefields (an area discrepancy equivalent to
3.7 Greenland Ice Sheets). Current arguments favour the latter. However, significant last
glacial ice-rafted debris (IRD) exists in NW Pacific ocean cores, which must have been
sourced from a major ice sheet somewhere bordering the North Pacific. The origin of this IRD
is addressed through a combination of marine core analysis, iceberg trajectory modelling and
remote sensing of glacial geomorphology. We find compelling evidence for two stages of
glaciation centred on the Kamchatka area of maritime southeast Russia during the last glacial,
with ice extent intermediate in size between previous maximum and minimum
reconstructions. Furthermore, a significant increase in iceberg flux precedes, and
accompanies, a substantial marine core ash deposit at around 40ka BP. We speculate that
rapid decay of the first stage of the ice sheet may have triggered substantial volcanic activity
The Effective Action For Brane Localized Gauge Fields
The low energy effective action including gauge field degrees of freedom on a
non-BPS p=2 brane embedded in a N=1, D=4 target superspace is obtained through
the method of nonlinear realizations of the associated super-Poincare
symmetries. The invariant interactions of the gauge fields and the brane
excitation modes corresponding to the Nambu-Goldstone degrees of freedom
resulting from the broken space translational symmetry and the target space
supersymmetries are determined. Brane localized matter field interactions with
the gauge fields are obtained through the construction of the combined gauge
and super-Poincare covariant derivatives for the matter fields.Comment: 12 pages, no figure
Effective theory for wall-antiwall system
We propose a useful method for deriving the effective theory for a system
where BPS and anti-BPS domain walls coexist. Our method respects an
approximately preserved SUSY near each wall. Due to the finite width of the
walls, SUSY breaking terms arise at tree-level, which are exponentially
suppressed. A practical approximation using the BPS wall solutions is also
discussed. We show that a tachyonic mode appears in the matter sector if the
corresponding mode function has a broader profile than the wall width.Comment: LaTeX file, 30 page, 5 eps figures, references adde
Very static enforcement of dynamic policies
Security policies are naturally dynamic. Reflecting this, there has been a growing interest in studying information-flow properties which change during program execution, including concepts such as declassification, revocation, and role-change.
A static verification of a dynamic information flow policy, from a semantic perspective, should only need to concern itself with two things: 1) the dependencies between data in a program, and 2) whether those dependencies are consistent with the intended flow policies as they change over time. In this paper we provide a formal ground for this intuition. We present a straightforward extension to the principal flow-sensitive type system introduced by Hunt and Sands (POPLâ06, ESOPâ11) to infer both end-to-end dependencies and dependencies at intermediate points in a program. This allows typings to be applied to verification of both static and dynamic policies. Our extension preserves the principal type systemâs distinguishing feature, that type inference is independent of the policy to be enforced: a single, generic dependency analysis (typing) can be used to verify many different dynamic policies of a given program, thus achieving a clean separation between (1) and (2).
We also make contributions to the foundations of dynamic information flow. Arguably, the most compelling semantic definitions for dynamic security conditions in the literature are phrased in the so-called knowledge-based style. We contribute a new definition of knowledge-based progress insensitive security for dynamic policies. We show that the new definition avoids anomalies of previous definitions and enjoys a simple and useful characterisation as a two-run style property
Exploring the extent to which fluctuations in iceârafted debris reflect mass changes in the source ice sheet : a modelâobservation comparison using the last BritishâIrish Ice Sheet
The British and Irish Ice Sheet (BIIS) was highly dynamic during the Late Quaternary, with considerable regional differences in the timing and extent of its change. This was reflected in equally variable offshore iceârafted debris (IRD) records. Here we reconcile these two records using the FRUGAL intermediate complexity icebergâclimate model, with varying BIIS catchmentâlevel iceberg fluxes, to simulate change in IRD origin and magnitude along the western European margin at 1000âyear time steps during the height of the last BIIS glaciation (31â6âkaâbp). This modelled IRD variability is compared with existing IRD records from the deep ocean at five cores along this margin. There is general agreement of the temporal and spatial IRD variability between observations and model through this period. The Porcupine Bank off northwestern Ireland was confirmed by the modelling as a major dividing line between sites possessing exclusively northern or southern source regions for offshore IRD. During Heinrich events 1 and 2, the cores show evidence of a proportion of North American IRD, more particularly to the south of the British Isles. Modelling supports this southern bias for likely Heinrich impact, but also suggests North American IRD will only reach the British margin in unusual circumstances
Consistent histories, the quantum Zeno effect, and time of arrival
We present a decomposition of the general quantum mechanical evolution
operator, that corresponds to the path decomposition expansion, and interpret
its constituents in terms of the quantum Zeno effect (QZE). This decomposition
is applied to a finite dimensional example and to the case of a free particle
in the real line, where the possibility of boundary conditions more general
than those hitherto considered in the literature is shown. We reinterpret the
assignment of consistent probabilities to different regions of spacetime in
terms of the QZE. The comparison of the approach of consistent histories to the
problem of time of arrival with the solution provided by the probability
distribution of Kijowski shows the strength of the latter point of view
General properties of the pion production reaction in nuclear matter
The pion production reaction on was
studied at incident pion energies of = 240, 260, 280, 300, and
320 MeV. The experiment was performed using the pion-channel at TRIUMF,
and multiparticle events, () and
(), were detected with the CHAOS spectrometer. Results
are reported in the form of both differential and total cross sections, and are
compared to theoretical predictions and the reaction phase space. The present
investigation of the T-dependence of the
reaction complements earlier examinations of the A-dependence of the reaction,
which was measured using , , , , , and
targets at 280 MeV. Some general properties of the
pion-induced pion production reaction in nuclear matter will be presented,
based on the combined results of the two studies.Comment: 23 pages, Latex, accepted for publication in Nucl. Phys.
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