5,390 research outputs found
Simulations of Contrail Optical Properties and Radiative Forcing for Various Crystal Shapes
The aim of this study is to investigate the sensitivity of radiative-forcing computations to various contrail
crystal shape models. Contrail optical properties in the shortwave and longwave ranges are derived using
a ray-tracing geometric method and the discrete dipole approximation method, respectively. Both methods
present good correspondence of the single-scattering albedo and the asymmetry parameter in a transition
range (3–8 µm). There are substantial differences in single-scattering properties among 10 crystal models
investigated here (e.g., hexagonal columns and plates with different aspect ratios, and spherical particles). The
single-scattering albedo and the asymmetry parameter both vary by up to 0.1 among various crystal shapes.
The computed single-scattering properties are incorporated in the moderate-resolution atmospheric radiance
and transmittance model(MODTRAN) radiative transfer code to simulate solar and infrared fluxes at the top
of the atmosphere. Particle shapes have a strong impact on the contrail radiative forcing in both the shortwave
and longwave ranges. The differences in the net radiative forcing among optical models reach 50% with
respect to the mean model value. The hexagonal-column and hexagonal-plate particles show the smallest net
radiative forcing, and the largest forcing is obtained for the spheres. The balance between the shortwave
forcing and longwave forcing is highly sensitive with respect to the assumed crystal shape and may even
change the sign of the net forcing. The optical depth at which the mean diurnal radiative forcing changes sign
from positive to negative varies from 4.5 to 10 for a surface albedo of 0.2 and from 2 to 6.5 for a surface albedo
of 0.05. Contrails are probably never that optically thick (except for some aged contrail cirrus), however, and
so will not have a cooling effect on climate
Phonon-assisted relaxation between hole states in quantum dot molecules
We study theoretically phonon-assisted relaxation and inelastic tunneling of
holes in a double quantum dot. We derive hole states and relaxation rates from
kp Hamiltonians and show that there is a finite distance between the dots where
lifetimes of hole states are very long which is related to vanishing tunnel
coupling. We show also that the light hole admixture to hole states can
considerably affect the hole relaxation rates even though its magnitude is very
small
GASTOF: Ultra-fast ToF forward detector for exclusive processes at the LHC
GASTOF (Gas Time-of-Flight) detector is a Cherenkov detector proposed for
very precise (10--20 ps) arrival time measurements of forward protons at some
420 m from the central detectors of CMS and ATLAS. Such an excellent time
resolution will allow by z-by-timing technique for precise measurement of the
z-coordinate of the event vertex in exclusive production at the LHC, when two
colliding protons are scattered at very small angles. In the paper we present
first GASTOF prototype, simulations of its performance as well as first tests
using a cosmic muon telescope.Comment: 6 pages, 3 figures, presented at the conference ''Physics at LHC'',
Krakow, June 200
Calcium-independent inhibitory G-protein signaling induces persistent presynaptic muting of hippocampal synapses
Adaptive forms of synaptic plasticity that reduce excitatory synaptic transmission in response to prolonged increases in neuronal activity may prevent runaway positive feedback in neuronal circuits. In hippocampal neurons, for example, glutamatergic presynaptic terminals are selectively silenced, creating mute synapses, after periods of increased neuronal activity or sustained depolarization. Previous work suggests that cAMP-dependent and proteasome-dependent mechanisms participate in silencing induction by depolarization, but upstream activators are unknown. We, therefore, tested the role of calcium and G-protein signaling in silencing induction in cultured hippocampal neurons. We found that silencing induction by depolarization was not dependent on rises in intracellular calcium, from either extracellular or intracellular sources. Silencing was, however, pertussis toxin sensitive, which suggests that inhibitory G-proteins are recruited. Surprisingly, blocking four common inhibitory G-protein-coupled receptors (GPCRs) (adenosine A(1) receptors, GABA(B) receptors, metabotropic glutamate receptors, and CB(1) cannabinoid receptors) and one ionotropic receptor with metabotropic properties (kainate receptors) failed to prevent depolarization-induced silencing. Activating a subset of these GPCRs (A(1) and GABA(B)) with agonist application induced silencing, however, which supports the hypothesis that G-protein activation is a critical step in silencing. Overall, our results suggest that depolarization activates silencing through an atypical GPCR or through receptor-independent G-protein activation. GPCR agonist-induced silencing exhibited dependence on the ubiquitin-proteasome system, as was shown previously for depolarization-induced silencing, implicating the degradation of vital synaptic proteins in silencing by GPCR activation. These data suggest that presynaptic muting in hippocampal neurons uses a G-protein-dependent but calcium-independent mechanism to depress presynaptic vesicle release
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