1,727 research outputs found
Simulating ice core 10Be on the glacial–interglacial timescale
10Be ice core measurements are an important tool for paleoclimate research, e.g., allowing for the reconstruction of past solar activity or changes in the geomagnetic dipole field. However, especially on multi-millennial timescales, the share of production and climate-induced variations of respective 10Be ice core records is still up for debate. Here we present the first quantitative climatological model of the 10Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10Be air–firn transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10Be snow-pit measurements are used for model calibration and validation. Being specifically configured for 10Be in polar ice, this tool thus allows for a straightforward investigation of production- and non-production-related modulation of this nuclide. We find that the polar 10Be ice concentration does not immediately record the globally mixed cosmogenic production signal. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input, we simulate the observed Greenland Summit (GRIP and GISP2) 10Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable of reproducing the largest portion of the observed 10Be changes. However, model–measurement differences exhibit multi-millennial trends (differences up to 87% in case of normalized to the Holocene records) which call for closer investigation. Focusing on the (12–37) b2k (before the year AD 2000) period, mean model–measurement differences of 30% cannot be attributed to production changes. However, unconsidered climate-induced changes could likely explain the model–measurement mismatch. In fact, the 10Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) b2k model–measurement differences
Self-Pulsating Semiconductor Lasers: Theory and Experiment
We report detailed measurements of the pump-current dependency of the
self-pulsating frequency of semiconductor CD lasers. A distinct kink in this
dependence is found and explained using rate-equation model. The kink denotes a
transition between a region where the self-pulsations are weakly sustained
relaxation oscillations and a region where Q-switching takes place. Simulations
show that spontaneous emission noise plays a crucial role for the cross-over.Comment: Revtex, 16 pages, 7 figure
The Spectral Dimension of Arctic Outgoing Longwave Radiation and Greenhouse Efficiency Trends From 2003 to 2016
Fourteen years of spectral fluxes derived from collocated Atmospheric Infrared Sounder (AIRS) and Clouds and the Earth’s Radiant Energy System (CERES) observations are used in conjunction with AIRS retrievals to examine the trends of zonal mean spectral outgoing longwave radiation (OLR) and greenhouse efficiency (GHE) in the Arctic. AIRS retrieved profiles are fed into a radiative transfer model to generate synthetic clear‐sky spectral OLR. Trends are derived from the simulated clear‐sky spectral OLR and GHE and then compared with their counterparts derived from collocated observations. Spectral trends in different seasons are distinctively different. March and September exhibit positive trends in spectral OLR over the far‐IR dirty window and mid‐IR window region for most of the Arctic. In contrast, spectral OLR trends in July are negative over the far‐IR dirty window and can be positive or negative in the mid‐IR window depending on the latitude. Sensitivity studies reveal that surface temperature contributes much more than atmospheric temperature and humidity to the spectral OLR and GHE trends, while the contributions from the latter two are also discernible over many spectral regions (e.g., trends in the far‐IR dirty window in March). The largest increase of spectral GHE is seen north of 80°N in March across the water vapor v2 band and far‐IR. When the secular fractional change of spectral OLR is less than that of surface spectral emission, an increase of spectral GHE can be expected. Spectral trend analyses reveal more information than broadband trend analyses alone.Key PointsObserved Arctic zonal mean trends of spectral flux and greenhouse efficiency are studied for the first timeSpectral trends are seasonally dependent and reveal more information than broadband trendsChanges in surface temperature contribute the most to overall spectral trends, but changes due to air temperature and humidity trends are discerniblePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151304/1/jgrd55648_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151304/2/jgrd55648.pd
A Conditional Yeast E1 Mutant Blocks the Ubiquitin–Proteasome Pathway and Reveals a Role for Ubiquitin Conjugates in Targeting Rad23 to the Proteasome
E1 ubiquitin activating enzyme catalyzes the initial step in all ubiquitin-dependent processes. We report the isolation of uba1-204, a temperature-sensitive allele of the essential Saccharomyces cerevisiae E1 gene, UBA1. Uba1-204 cells exhibit dramatic inhibition of the ubiquitin–proteasome system, resulting in rapid depletion of cellular ubiquitin conjugates and stabilization of multiple substrates. We have employed the tight phenotype of this mutant to investigate the role ubiquitin conjugates play in the dynamic interaction of the UbL/UBA adaptor proteins Rad23 and Dsk2 with the proteasome. Although proteasomes purified from mutant cells are intact and proteolytically active, they are depleted of ubiquitin conjugates, Rad23, and Dsk2. Binding of Rad23 to these proteasomes in vitro is enhanced by addition of either free or substrate-linked ubiquitin chains. Moreover, association of Rad23 with proteasomes in mutant and wild-type cells is improved upon stabilizing ubiquitin conjugates with proteasome inhibitor. We propose that recognition of polyubiquitin chains by Rad23 promotes its shuttling to the proteasome in vivo
Quantum oscillations of nitrogen atoms in uranium nitride
The vibrational excitations of crystalline solids corresponding to acoustic
or optic one phonon modes appear as sharp features in measurements such as
neutron spectroscopy. In contrast, many-phonon excitations generally produce a
complicated, weak, and featureless response. Here we present time-of-flight
neutron scattering measurements for the binary solid uranium nitride (UN),
showing well-defined, equally-spaced, high energy vibrational modes in addition
to the usual phonons. The spectrum is that of a single atom, isotropic quantum
harmonic oscillator and characterizes independent motions of light nitrogen
atoms, each found in an octahedral cage of heavy uranium atoms. This is an
unexpected and beautiful experimental realization of one of the fundamental,
exactly-solvable problems in quantum mechanics. There are also practical
implications, as the oscillator modes must be accounted for in the design of
generation IV nuclear reactors that plan to use UN as a fuel.Comment: 25 pages, 10 figures, submitted to Nature Communications,
supplementary information adde
Chandrasekhar-Kendall functions in astrophysical dynamos
Some of the contributions of Chandrasekhar to the field of
magnetohydrodynamics are highlighted. Particular emphasis is placed on the
Chandrasekhar-Kendall functions that allow a decomposition of a vector field
into right- and left-handed contributions. Magnetic energy spectra of both
contributions are shown for a new set of helically forced simulations at
resolutions higher than what has been available so far. For a forcing function
with positive helicity, these simulations show a forward cascade of the
right-handed contributions to the magnetic field and nonlocal inverse transfer
for the left-handed contributions. The speed of inverse transfer is shown to
decrease with increasing value of the magnetic Reynolds number.Comment: 10 pages, 5 figures, proceedings of the Chandrasekhar Centenary
Conference, to be published in PRAMANA - Journal of Physic
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Splanchnic metabolism of nutrients and hormones in steers fed alfalfa under conditions of increased absorption of ammonia and L-arginine supply across the portal-drained viscera
Effects of increased ammonia and/or arginine
absorption on net splanchnic (portal-drained viscera
[PDV] plus liver) metabolism of nonnitrogenous
nutrients and hormones in cattle were examined. Six
Hereford × Angus steers (501 ± 1 kg BW) prepared with
vascular catheters for measurements of net flux across
the splanchnic bed were fed a 75% alfalfa:25% (as-fed
basis) corn and soybean meal diet (0.523 MJ of ME/[kg
BW0.75.d]) every 2 h without (27.0 g of N/kg of DM) and
with 20 g of urea/kg of DM (35.7 g of N/kg of DM) in a
split-plot design. Net flux measurements were made
immediately before and after a 72-h mesenteric vein
infusion of L-arginine (15 mmol/h). There were no treatment
effects onPDVor hepaticO2 consumption. Dietary
urea had no effect on splanchnic metabolism of glucose
or L-lactate, but arginine infusion decreased net hepatic
removal of L-lactate when urea was fed (P < 0.01). Net PDV appearance of n-butyrate was increased by arginine
infusion (P < 0.07), and both dietary urea (P <
0.09) and arginine infusion (P < 0.05) increased net
hepatic removal of n-butyrate. Dietary urea also increased
total splanchnic acetate output (P < 0.06),
tended to increase arterial glucagon concentration (P
< 0.11), and decreased arterial ST concentration (P <
0.03). Arginine infusion increased arterial concentration
(P < 0.07) and net PDV release (P < 0.10) and
tended to increase hepatic removal (P < 0.11) of insulin,
as well as arterial concentration (P < 0.01) and total
splanchnic output (P < 0.01) of glucagon. Despite
changes in splanchnic N metabolism, increased ammonia
and arginine absorption had little measurable effect
on splanchnic metabolism of glucose and other nonnitrogenous
components of splanchnic energy metabolism
Internally Electrodynamic Particle Model: Its Experimental Basis and Its Predictions
The internally electrodynamic (IED) particle model was derived based on
overall experimental observations, with the IED process itself being built
directly on three experimental facts, a) electric charges present with all
material particles, b) an accelerated charge generates electromagnetic waves
according to Maxwell's equations and Planck energy equation and c) source
motion produces Doppler effect. A set of well-known basic particle equations
and properties become predictable based on first principles solutions for the
IED process; several key solutions achieved are outlined, including the de
Broglie phase wave, de Broglie relations, Schr\"odinger equation, mass,
Einstein mass-energy relation, Newton's law of gravity, single particle self
interference, and electromagnetic radiation and absorption; these equations and
properties have long been broadly experimentally validated or demonstrated. A
specific solution also predicts the Doebner-Goldin equation which emerges to
represent a form of long-sought quantum wave equation including gravity. A
critical review of the key experiments is given which suggests that the IED
process underlies the basic particle equations and properties not just
sufficiently but also necessarily.Comment: Presentation at the 27th Int Colloq on Group Theo Meth in Phys, 200
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