845 research outputs found
Does the Sun Shrink with Increasing Magnetic Activity?
We have analyzed the full set of SOHO/MDI f- and p-mode oscillation
frequencies from 1996 to date in a search for evidence of solar radius
evolution during the rising phase of the current activity cycle. Like Antia et
al. (2000), we find that a significant fraction of the f-mode frequency changes
scale with frequency; and that if these are interpreted in terms of a radius
change, it implies a shrinking sun. Our inferred rate of shrinkage is about 1.5
km/y, which is somewhat smaller than found by Antia et al. We argue that this
rate does not refer to the surface, but rather to a layer extending roughly
from 4 to 8 Mm beneath the visible surface. The rate of shrinking may be
accounted for by an increasing radial component of the rms random magnetic
field at a rate that depends on its radial distribution. If it were uniform,
the required field would be ~7 kG. However, if it were inwardly increasing,
then a 1 kG field at 8 Mm would suffice.
To assess contribution to the solar radius change arising above 4Mm, we
analyzed the p-mode data. The evolution of the p-mode frequencies may be
explained by a magnetic^M field growing with activity. The implications of the
near-surface magnetic field changes depend on the anisotropy of the random
magnetic field. If the field change is predominantly radial, then we infer an
additional shrinking at a rate between 1.1-1.3 km/y at the photosphere. If on
the other hand the increase is isotropic, we find a competing expansion at a
rate of 2.3 km/y. In any case, variations in the sun's radius in the activity
cycle are at the level of 10^{-5} or less, hence have a negligible contribution
to the irradiance variations.Comment: 10 pages (ApJ preprint style), 4 figures; accepted for publication in
Ap
Physical routes for the synthesis of kesterite
This paper provides an overview of the physical vapor technologies used to synthesize Cu2ZnSn(S,Se)4
thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films
have been fabricated using sequential stacking or co-sputtering of precursors as well as using
sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition,
pulsed laser deposition of composite targets and monograin growth by the molten salt method were
developed as alternative methods for kesterite layers deposition. This review presents the growing
increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical
description of the main issues limiting this efficiency and of the experimental pathways designed to
prevent or limit these issues is provided and discussed as well. Afinal section is dedicated to the
description of promising process steps aiming at further improvements of solar cell efficiency, such as
alkali doping and bandgap grading1. R Caballero and M León acknowledge financial support via the Spanish Ministry of Science, Innovation and Universities project (WINCOST, ENE2016-80788-C5-2-R) and thank H2020 EU Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968).
2. S Canulescu and J Schou acknowledge the support from Innovation Fund Denmark.
3. D-H Kim acknowledges financial support via the DGIST R&D Program of the Ministry of Science and ICT, KOREA (18-BD-05).
4.C. Malerba acknowledges the support from the Italian Ministry of Economic development in the framework of the Operating Agreement with ENEA for the Research on the Electric System.
5.A Redinger acknowledges financial support via the FNR Attract program, Project : SUNSPOT, Nr.11244141.
6. E Saucedo thanks H2020 EU Programme under the projects STARCELL (H2020-NMBP-03-2016-720907) and INFINITE-CELL (H2020-MSCA-RISE-2017-777968), the Spanish Ministry of Science, Innovation and Universities for the IGNITE project (ENE2017-87671-C3-1-R), and the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). IREC belong to
the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the ‘Generalitat de Catalunya’ (Ref. 2017 SGR 862).
7. Taltech acknowledges financial support via the Estonian Ministry of Education and Research funding project IUT19-28 and the European Union Regional Development Fund, Project TK141.
8. B Vermang has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (Grant Agreement No 715027
Autonomous on-board data processing and instrument calibration software for the SO/PHI
The extension of on-board data processing capabilities is an attractive
option to reduce telemetry for scientific instruments on deep space missions.
The challenges that this presents, however, require a comprehensive software
system, which operates on the limited resources a data processing unit in space
allows. We implemented such a system for the Polarimetric and Helioseismic
Imager (PHI) on-board the Solar Orbiter (SO) spacecraft. It ensures autonomous
operation to handle long command-response times, easy changing of the processes
after new lessons have been learned and meticulous book-keeping of all
operations to ensure scientific accuracy. This contribution presents the
requirements and main aspects of the software implementation, followed by an
example of a task implemented in the software frame, and results from running
it on SO/PHI. The presented example shows that the different parts of the
software framework work well together, and that the system processes data as we
expect. The flexibility of the framework makes it possible to use it as a
baseline for future applications with similar needs and limitations as SO/PHI.Comment: Conference: SPIE Astronomical Telescopes + Instrumentatio, Software
and Cyberinfrastructure for Astronomy
Detecting the direction of a signal on high-dimensional spheres: Non-null and Le Cam optimality results
We consider one of the most important problems in directional statistics,
namely the problem of testing the null hypothesis that the spike direction
of a Fisher-von Mises-Langevin distribution on the -dimensional
unit hypersphere is equal to a given direction . After a reduction
through invariance arguments, we derive local asymptotic normality (LAN)
results in a general high-dimensional framework where the dimension goes
to infinity at an arbitrary rate with the sample size , and where the
concentration behaves in a completely free way with , which
offers a spectrum of problems ranging from arbitrarily easy to arbitrarily
challenging ones. We identify various asymptotic regimes, depending on the
convergence/divergence properties of , that yield different
contiguity rates and different limiting experiments. In each regime, we derive
Le Cam optimal tests under specified and we compute, from the Le Cam
third lemma, asymptotic powers of the classical Watson test under contiguous
alternatives. We further establish LAN results with respect to both spike
direction and concentration, which allows us to discuss optimality also under
unspecified . To investigate the non-null behavior of the Watson test
outside the parametric framework above, we derive its local asymptotic powers
through martingale CLTs in the broader, semiparametric, model of rotationally
symmetric distributions. A Monte Carlo study shows that the finite-sample
behaviors of the various tests remarkably agree with our asymptotic results.Comment: 47 pages, 4 figure
Digitalizing the welfare state: citizenship discourses in Danish digitalization strategies from 2002 to 2015
Deflection and Rotation of CMEs from Active Region 11158
Between the 13 and 16 of February 2011 a series of coronal mass ejections
(CMEs) erupted from multiple polarity inversion lines within active region
11158. For seven of these CMEs we use the Graduated Cylindrical Shell (GCS)
flux rope model to determine the CME trajectory using both Solar Terrestrial
Relations Observatory (STEREO) extreme ultraviolet (EUV) and coronagraph
images. We then use the Forecasting a CME's Altered Trajectory (ForeCAT) model
for nonradial CME dynamics driven by magnetic forces, to simulate the
deflection and rotation of the seven CMEs. We find good agreement between the
ForeCAT results and the reconstructed CME positions and orientations. The CME
deflections range in magnitude between 10 degrees and 30 degrees. All CMEs
deflect to the north but we find variations in the direction of the
longitudinal deflection. The rotations range between 5\mydeg and 50\mydeg with
both clockwise and counterclockwise rotations occurring. Three of the CMEs
begin with initial positions within 2 degrees of one another. These three CMEs
all deflect primarily northward, with some minor eastward deflection, and
rotate counterclockwise. Their final positions and orientations, however,
respectively differ by 20 degrees and 30 degrees. This variation in deflection
and rotation results from differences in the CME expansion and radial
propagation close to the Sun, as well as the CME mass. Ultimately, only one of
these seven CMEs yielded discernible in situ signatures near Earth, despite the
active region facing near Earth throughout the eruptions. We suggest that the
differences in the deflection and rotation of the CMEs can explain whether each
CME impacted or missed the Earth.Comment: 18 pages, 6 figures, accepted in Solar Physic
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