7,655 research outputs found
EMPIRE: A robust empirical reconstruction of solar irradiance variability
We present a new empirical model of total and spectral solar irradiance (TSI
and SSI) variability entitled EMPirical Irradiance REconstruction (EMPIRE). As
with existing empirical models, TSI and SSI variability is given by the linear
combination of solar activity indices. In empirical models, UV SSI variability
is usually determined by fitting the rotational variability in activity indices
to that in measurements. Such models have to date relied on ordinary least
squares regression, which ignores the uncertainty in the activity indices. In
an advance from earlier efforts, the uncertainty in the activity indices is
accounted for in EMPIRE by the application of an error-in-variables regression
scheme, making the resultant UV SSI variability more robust. The result is
consistent with observations and unprecedentedly, with that from other
modelling approaches, resolving the long-standing controversy between existing
empirical models and other types of models. We demonstrate that earlier
empirical models, by neglecting the uncertainty in activity indices,
underestimate UV SSI variability. The reconstruction of TSI and visible and IR
SSI from EMPIRE is also shown to be consistent with observations. The EMPIRE
reconstruction is of utility to climate studies as a more robust alternative to
earlier empirical reconstructions.Comment: J. Geophys. Res. (2017
Solar cycle variation in solar irradiance
The correlation between solar irradiance and the 11-year solar activity cycle
is evident in the body of measurements made from space, which extend over the
past four decades. Models relating variation in solar irradiance to
photospheric magnetism have made significant progress in explaining most of the
apparent trends in these observations. There are, however, persistent
discrepancies between different measurements and models in terms of the
absolute radiometry, secular variation and the spectral dependence of the solar
cycle variability. We present an overview of solar irradiance measurements and
models, and discuss the key challenges in reconciling the divergence between
the two
No evidence of a significant role for CTLA-4 in multiple sclerosis
Variation in the cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) gene plays a significant role in determining susceptibility to autoimmune thyroid disease and type 1 diabetes. Its role in multiple sclerosis is more controversial. In order to explore this logical candidate more thoroughly, we genotyped 771 multiple sclerosis trio families from the United Kingdom for the 3? untranslated region variable number tandem repeat, the CT60 single nucleotide polymorphism (SNP) and five haplotype-tagging SNPs. No individual marker or common haplotype showed evidence of association with disease. These data suggest that any effect of CTLA-4 on multiple sclerosis susceptibility is likely to be very small
A mechanistic model of connector hubs, modularity, and cognition
The human brain network is modular--comprised of communities of tightly
interconnected nodes. This network contains local hubs, which have many
connections within their own communities, and connector hubs, which have
connections diversely distributed across communities. A mechanistic
understanding of these hubs and how they support cognition has not been
demonstrated. Here, we leveraged individual differences in hub connectivity and
cognition. We show that a model of hub connectivity accurately predicts the
cognitive performance of 476 individuals in four distinct tasks. Moreover,
there is a general optimal network structure for cognitive
performance--individuals with diversely connected hubs and consequent modular
brain networks exhibit increased cognitive performance, regardless of the task.
Critically, we find evidence consistent with a mechanistic model in which
connector hubs tune the connectivity of their neighbors to be more modular
while allowing for task appropriate information integration across communities,
which increases global modularity and cognitive performance
Efficient out-coupling of high-purity single photons from a coherent quantum dot in a photonic-crystal cavity
We demonstrate a single-photon collection efficiency of from
a quantum dot in a low-Q mode of a photonic-crystal cavity with a single-photon
purity of recorded above the saturation power. The high
efficiency is directly confirmed by detecting up to kilocounts per
second on a single-photon detector on another quantum dot coupled to the cavity
mode. The high collection efficiency is found to be broadband, as is explained
by detailed numerical simulations. Cavity-enhanced efficient excitation of
quantum dots is obtained through phonon-mediated excitation and under these
conditions, single-photon indistinguishability measurements reveal long
coherence times reaching ns in a weak-excitation regime. Our work
demonstrates that photonic crystals provide a very promising platform for
highly integrated generation of coherent single photons including the efficient
out-coupling of the photons from the photonic chip.Comment: 13 pages, 8 figures, submitte
Non-commutative field theory approach to two-dimensional vortex liquid system
We investigate the non-commutative (NC) field theory approach to the vortex
liquid system restricted to the lowest Landau level (LLL) approximation. NC
field theory effectively takes care of the phase space reduction of the LLL
physics in a -product form and introduces a new gauge invariant form of
a quartic potential of the order parameter in the Ginzburg-Landau (GL) free
energy. This new quartic interaction coupling term has a non-trivial
equivalence relation with that obtained by Br\'ezin, Nelson and Thiaville in
the usual GL framework. The consequence of the equivalence is discussed.Comment: Add vortex lattice formation, more references, and one autho
The nature of solar brightness variations
The solar brightness varies on timescales from minutes to decades.
Determining the sources of such variations, often referred to as solar noise,
is of importance for multiple reasons: a) it is the background that limits the
detection of solar oscillations, b) variability in solar brightness is one of
the drivers of the Earth's climate system, c) it is a prototype of stellar
variability which is an important limiting factor for the detection of
extra-solar planets. Here we show that recent progress in simulations and
observations of the Sun makes it finally possible to pinpoint the source of the
solar noise. We utilise high-cadence observations from the Solar Dynamic
Observatory and the SATIRE model to calculate the magnetically-driven
variations of solar brightness. The brightness variations caused by the
constantly evolving cellular granulation pattern on the solar surface are
computed with the MURAM code. We find that surface magnetic field and
granulation can together precisely explain solar noise on timescales from
minutes to decades, i.e. ranging over more than six orders of magnitude in the
period. This accounts for all timescales that have so far been resolved or
covered by irradiance measurements. We demonstrate that no other sources of
variability are required to explain the data. Recent measurements of Sun-like
stars by CoRoT and Kepler uncovered brightness variations similar to that of
the Sun but with much wider variety of patterns. Our finding that solar
brightness variations can be replicated in detail with just two well-known
sources will greatly simplify future modelling of existing CoRoT and Kepler as
well as anticipated TESS and PLATO data.Comment: This is the submitted version of the paper published in Nature
Astronom
Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface
The variation in the radiative output of the Sun, described in terms of solar
irradiance, is important to climatology. A common assumption is that solar
irradiance variability is driven by its surface magnetism. Verifying this
assumption has, however, been hampered by the fact that models of solar
irradiance variability based on solar surface magnetism have to be calibrated
to observed variability. Making use of realistic three-dimensional
magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art
solar magnetograms from the Solar Dynamics Observatory, we present a model of
total solar irradiance (TSI) that does not require any such calibration. In
doing so, the modeled irradiance variability is entirely independent of the
observational record. (The absolute level is calibrated to the TSI record from
the Total Irradiance Monitor.) The model replicates 95% of the observed
variability between April 2010 and July 2016, leaving little scope for
alternative drivers of solar irradiance variability at least over the time
scales examined (days to years).Comment: Supplementary Materials;
https://journals.aps.org/prl/supplemental/10.1103/PhysRevLett.119.091102/supplementary_material_170801.pd
Reconstruction of spectral solar irradiance since 1700 from simulated magnetograms
We present a reconstruction of the spectral solar irradiance since 1700 using
the SATIRE-T2 (Spectral And Total Irradiance REconstructions for the Telescope
era version 2) model. This model uses as input magnetograms simulated with a
surface flux transport model fed with semi-synthetic records of emerging
sunspot groups. We used statistical relationships between the properties of
sunspot group emergence, such as the latitude, area, and tilt angle, and the
sunspot cycle strength and phase to produce semi-synthetic sunspot group
records starting in the year 1700. The semisynthetic records are fed into a
surface flux transport model to obtain daily simulated magnetograms that map
the distribution of the magnetic flux in active regions (sunspots and faculae)
and their decay products on the solar surface. The magnetic flux emerging in
ephemeral regions is accounted for separately based on the concept of extended
cycles whose length and amplitude are linked to those of the sunspot cycles
through the sunspot number. The magnetic flux in each surface component
(sunspots, faculae and network, and ephemeral regions) was used to compute the
spectral and total solar irradiance between the years 1700 and 2009. This
reconstruction is aimed at timescales of months or longer although the model
returns daily values. We found that SATIRE-T2, besides reproducing other
relevant observations such as the total magnetic flux, reconstructs the total
solar irradiance (TSI) on timescales of months or longer in good agreement with
the PMOD composite of observations, as well as with the reconstruction starting
in 1878 based on the RGO-SOON data. The model predicts an increase in the TSI
of 1.2[+0.2, -0.3] Wm-2 between 1700 and the present. The spectral irradiance
reconstruction is in good agreement with the UARS/SUSIM measurements as well as
the Lyman-alpha composite.Comment: 13 pages, 10 figure
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