4,024 research outputs found
Autonomous marine hyperspectral radiometers for determining solar irradiances and aerosol optical properties
We have developed two hyperspectral radiometer
systems which require no moving parts, shade rings or motorised
tracking, making them ideally suited for autonomous
use in the inhospitable remote marine environment. Both systems
are able to measure direct and diffuse hyperspectral irradiance
in the wavelength range 350–1050 nm at 6 nm (Spectrometer
1) or 3.5 nm (Spectrometer 2) resolution. Marine
field trials along a 100� transect (between 50� N and 50� S) of
the Atlantic Ocean resulted in close agreement with existing
commercially available instruments in measuring (1) photosynthetically
available radiation (PAR), with both spectrometers
giving regression slopes close to unity (Spectrometer 1:
0.960; Spectrometer 2: 1.006) and R2 �0.96; (2) irradiant
energy, with R2�0.98 and a regression slope of 0.75 which
can be accounted for by the difference in wavelength integration
range; and (3) hyperspectral irradiance where the agreement
on average was between 2 and 5 %. Two long duration
land-based field campaigns of up to 18 months allowed both
spectrometers to be well calibrated. This was also invaluable
for empirically correcting for the wider field of view
(FOV) of the spectrometers in comparison with the current
generation of sun photometers (�7.5� compared with �1�).
The need for this correction was also confirmed and independently quantified by atmospheric radiative transfer modelling
and found to be a function of aerosol optical depth
(AOD) and solar zenith angle. Once Spectrometer 2 was well
calibrated and the FOV effect corrected for, the RMSE in
retrievals of AOD when compared with a CIMEL sun photometer
were reduced to �0.02–0.03 with R2 > 0.95 at wavelengths
440, 500, 670 and 870 nm. Corrections for the FOV
as well as ship motion were applied to the data from the marine field trials. This resulted in AOD500 nm ranging between
0.05 in the clear background marine aerosol regions and
�0.5 within the Saharan dust plume. The RMSE between
the handheld Microtops sun photometer and Spectrometer 2
was between 0.047 and 0.057 with R2 > 0.94
On Certain Kähler Quotients of Quaternionic Kähler Manifolds
We prove that, given a certain isometric action of a two-dimensional Abelian group A on a quaternionic Kähler manifold M which preserves a submanifold N ⊂ M, the quotient M′ =N/A has a natural Kähler structure. We verify that the assumptions on the group action and on the submanifold N ⊂ M are satisfied for a large class of examples obtained from the supergravity c-map. In particular, we find that all quaternionic Kähler manifolds M in the image of the c-map admit an integrable complex structure compatible with the quaternionic structure, such that N ⊂ M is a complex submanifold. Finally, we discuss how the existence of the Kähler structure on M′ is required by the consistency of spontaneous to supersymmetry breakin
Comment on ``Conduction states in oxide perovskites: Three manifestations of Ti Jahn-Teller polarons in barium titanate''
In this comment to [S. Lenjer, O. F. Schirmer, H. Hesse, and Th. W. Kool,
Phys. Rev. B {\bf 66}, 165106 (2002)] we discuss the electronic structure of
oxygen vacancies in perovskites. First principles computations are in favour of
rather deep levels in these vacancies, and Lenjer et al suggest that the
electrons' interaction energy is negative, but data on electroconductivity are
against.Comment: 2 pages, no figure
Evaluation of the new ESR network software for the retrieval of direct sun products from CIMEL CE318 and PREDE POM01 sun-sky radiometers
The European Skynet Radiometers network (EuroSkyRad or ESR) has been recently established as a research network of European PREDE sun-sky radiometers. Moreover, ESR is federated with SKYNET, an international network of PREDE sun-sky radiometers mostly present in East Asia. In contrast to SKYNET, the European network also integrates users of the CIMEL CE318 sky–sun photometer. Keeping instrumental duality in mind, a set of open source algorithms has been developed consisting of two modules for (1) the retrieval of direct sun products (aerosol optical depth, wavelength exponent and water vapor) from the sun extinction measurements; and (2) the inversion of the sky radiance to derive other aerosol optical properties such as size distribution, single scattering albedo or refractive index. In this study we evaluate the ESR direct sun products in comparison with the AERosol RObotic NETwork (AERONET) products. Specifically, we have applied the ESR algorithm to a CIMEL CE318 and PREDE POM simultaneously for a 4-yr database measured at the Burjassot site (Valencia, Spain), and compared the resultant products with the AERONET direct sun measurements obtained with the same CIMEL CE318 sky–sun photometer. The comparison shows that aerosol optical depth differences are mostly within the nominal uncertainty of 0.003 for a standard calibration instrument, and fall within the nominal AERONET uncertainty of 0.01–0.02 for a field instrument in the spectral range 340 to 1020 nm. In the cases of the Ångström exponent and the columnar water vapor, the differences are lower than 0.02 and 0.15 cm, respectively. Therefore, we present an open source code program that can be used with both CIMEL and PREDE sky radiometers and whose results are equivalent to AERONET and SKYNET retrievals
Unsteady undular bores in fully nonlinear shallow-water theory
We consider unsteady undular bores for a pair of coupled equations of
Boussinesq-type which contain the familiar fully nonlinear dissipationless
shallow-water dynamics and the leading-order fully nonlinear dispersive terms.
This system contains one horizontal space dimension and time and can be
systematically derived from the full Euler equations for irrotational flows
with a free surface using a standard long-wave asymptotic expansion.
In this context the system was first derived by Su and Gardner. It coincides
with the one-dimensional flat-bottom reduction of the Green-Naghdi system and,
additionally, has recently found a number of fluid dynamics applications other
than the present context of shallow-water gravity waves. We then use the
Whitham modulation theory for a one-phase periodic travelling wave to obtain an
asymptotic analytical description of an undular bore in the Su-Gardner system
for a full range of "depth" ratios across the bore. The positions of the
leading and trailing edges of the undular bore and the amplitude of the leading
solitary wave of the bore are found as functions of this "depth ratio". The
formation of a partial undular bore with a rapidly-varying finite-amplitude
trailing wave front is predicted for ``depth ratios'' across the bore exceeding
1.43. The analytical results from the modulation theory are shown to be in
excellent agreement with full numerical solutions for the development of an
undular bore in the Su-Gardner system.Comment: Revised version accepted for publication in Phys. Fluids, 51 pages, 9
figure
Surface potential at a ferroelectric grain due to asymmetric screening of depolarization fields
Nonlinear screening of electric depolarization fields, generated by a stripe
domain structure in a ferroelectric grain of a polycrystalline material, is
studied within a semiconductor model of ferroelectrics. It is shown that the
maximum strength of local depolarization fields is rather determined by the
electronic band gap than by the spontaneous polarization magnitude.
Furthermore, field screening due to electronic band bending and due to presence
of intrinsic defects leads to asymmetric space charge regions near the grain
boundary, which produce an effective dipole layer at the surface of the grain.
This results in the formation of a potential difference between the grain
surface and its interior of the order of 1 V, which can be of either sign
depending on defect transition levels and concentrations. Exemplary acceptor
doping of BaTiO3 is shown to allow tuning of the said surface potential in the
region between 0.1 and 1.3 V.Comment: 14 pages, 11 figures, submitted to J. Appl. Phy
Magnetic properties of submicron Co islands and their use as artificial pinning centers
We report on the magnetic properties of elongated submicron magnetic islands
and their influence on a superconducting film. The magnetic properties were
studied by magnetization hysteresis loop measurements and scanning-force
microscopy. In the as-grown state, the islands have a magnetic structure
consisting of two antiparallel domains. This stable domain configuration has
been directly visualized as a 2x2-checkerboard pattern by magnetic-force
microscopy. In the remanent state, after magnetic saturation along the easy
axis, all islands have a single-domain structure with the magnetic moment
oriented along the magnetizing field direction. Periodic lattices of these Co
islands act as efficient artificial pinning arrays for the flux lines in a
superconducting Pb film deposited on top of the Co islands. The influence of
the magnetic state of the dots on their pinning efficiency is investigated in
these films, before and after the Co dots are magnetized.Comment: 6 pages including figure
Sustaining the Visitor Economy and Environment in the Isles of Scilly
This is the final version.This report summarises the key findings of a short project that took place from January to March 2021. The project examined ways to foster positive synergies between the visitor economy and the natural environment, and to generate additional support for protecting and restoring nature in the Isles of Scilly. The project identified the following ideas as worthy of further development: – A Scilly Pledge to raise awareness and appreciation of Scilly’s environment and culture, encouraging responsible behaviours. – A baseline assessment and monitoring programme to understand the changing ‘State of Nature’ in Scilly. – Raising charitable donations from visitors to help maintain Scilly’s natural environment including promotion of the Friends of Scilly Wildlife Scheme.UKRI Strategic Priorities Fund
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