7,752 research outputs found
Understanding and Enforcing Opacity
Abstract—This paper puts a spotlight on the specification and enforcement of opacity, a security policy for protecting sensitive properties of system behavior. We illustrate the fine granularity of the opacity policy by location privacy and privacy-preserving aggregation scenarios. We present a frame-work for opacity and explore its key differences and formal connections with such well-known information-flow models as noninterference, knowledge-based security, and declassifica-tion. Our results are machine-checked and parameterized in the observational power of the attacker, including progress-insensitive, progress-sensitive, and timing-sensitive attackers. We present two approaches to enforcing opacity: a whitebox monitor and a blackbox sampling-based enforcement. We report on experiments with prototypes that utilize state-of-the-art Satisfiability Modulo Theories (SMT) solvers and the random testing tool QuickCheck to establish opacity for the location and aggregation-based scenarios. I
Hidden by graphene -- towards effective screening of interface van der Waals interactions via monolayer coating
Recent atomic force microscopy (AFM) experiments~[ACS Nano {\bf 2014}, 8,
12410-12417] conducted on graphene-coated SiO demonstrated that monolayer
graphene (G) can effectively screen dispersion van der Waals (vdW) interactions
deriving from the underlying substrate: despite the single-atom thickness of G,
the AFM tip was almost insensitive to SiO, and the tip-substrate attraction
was essentially determined only by G. This G vdW {\it opacity} has far reaching
implications, encompassing stabilization of multilayer heterostructures,
micromechanical phenomena or even heterogeneous catalysis. Yet, detailed
experimental control and high-end applications of this phenomenon await sound
physical understanding of the underlying physical mechanism. By quantum
many-body analysis and ab-initio Density Functional Theory, here we address
this challenge providing theoretical rationalization of the observed G vdW {\it
opacity} for weakly interacting substrates. The non-local density response and
ultra slow decay of the G vdW interaction ensure compensation between standard
attractive terms and many-body repulsive contributions, enabling vdW {\it
opacity} over a broad range of adsorption distances. vdW {\it opacity} appears
most efficient in the low frequency limit and extends beyond London dispersion
including electrostatic Debye forces. By virtue of combined
theoretical/experimental validation, G hence emerges as a promising ultrathin
{\it shield} for modulation and switching of vdW interactions at interfaces and
complex nanoscale devices
Analytical Models of Exoplanetary Atmospheres. II. Radiative Transfer via the Two-stream Approximation
We present a comprehensive analytical study of radiative transfer using the
method of moments and include the effects of non-isotropic scattering in the
coherent limit. Within this unified formalism, we derive the governing
equations and solutions describing two-stream radiative transfer (which
approximates the passage of radiation as a pair of outgoing and incoming
fluxes), flux-limited diffusion (which describes radiative transfer in the deep
interior) and solutions for the temperature-pressure profiles. Generally, the
problem is mathematically under-determined unless a set of closures (Eddington
coefficients) is specified. We demonstrate that the hemispheric (or
hemi-isotropic) closure naturally derives from the radiative transfer equation
if energy conservation is obeyed, while the Eddington closure produces spurious
enhancements of both reflected light and thermal emission. We concoct recipes
for implementing two-stream radiative transfer in stand-alone numerical
calculations and general circulation models. We use our two-stream solutions to
construct toy models of the runaway greenhouse effect. We present a new
solution for temperature-pressure profiles with a non-constant optical opacity
and elucidate the effects of non-isotropic scattering in the optical and
infrared. We derive generalized expressions for the spherical and Bond albedos
and the photon deposition depth. We demonstrate that the value of the optical
depth corresponding to the photosphere is not always 2/3 (Milne's solution) and
depends on a combination of stellar irradiation, internal heat and the
properties of scattering both in optical and infrared. Finally, we derive
generalized expressions for the total, net, outgoing and incoming fluxes in the
convective regime.Comment: Accepted by ApJS. 23 pages, 11 figures, 3 tables, 158 equations. No
change from previous version except for title (to match ApJS convention
What do iris observations of Mg II k tell us about the solar plage chromosphere?
We analyze observations from the Interface Region Imaging Spectrograph of the
Mg II k line, the Mg II UV subordinate lines, and the O I 135.6 nm line to
better understand the solar plage chromosphere. We also make comparisons with
observations from the Swedish 1 m Solar Telescope of the H{\alpha} line, the Ca
II 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly
observations of the coronal 19.3 nm line. To understand the observed Mg II
profiles, we compare these observations to the results of numerical
experiments. The single-peaked or flat-topped Mg II k profiles found in plage
imply a transition region at a high column mass and a hot and dense
chromosphere of about 6500 K. This scenario is supported by the observed
large-scale correlation between moss brightness and filled-in profiles with
very little or absent self-reversal. The large wing width found in plage also
implies a hot and dense chromosphere with a steep chromospheric temperature
rise. The absence of emission in the Mg II subordinate lines constrain the
chromospheric temperature and the height of the temperature rise while the
width of the O I 135.6 nm line sets a limit to the non-thermal velocities to
around 7 km/s
Physics of SNeIa and Cosmology
We give an overview of the current understanding of Type Ia supernovae
relevant for their use as cosmological distance indicators. We present the
physical basis to understand their homogeneity of the observed light curves and
spectra and the observed correlations. This provides a robust method to
determine the Hubble constant, 67 +- 8 (2 sigma) km/Mpc/sec, independently from
primary distance indicators.
We discuss the uncertainties and tests which include SNe Ia based distance
determinations prior to delta-Ceph. measurements for the host galaxies. Based
on detailed models, we study the small variations from homogeneities and their
observable consequences. In combination with future data, this underlines the
suitability and promises the refinements needed to determine accurate relative
distances within 2 to 3 % and to use SNe Ia for high precision cosmology.Comment: to be published in "Stellar Candles", eds. Gieren et al. Lecture
Notes in Physics (http://link.springer.de/series/lnpp
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