4,645 research outputs found
Timing of sea ice retreat can alter phytoplankton community structure in the western Arctic Ocean
This study assesses the response of phytoplankton assemblages to recent
climate change, especially with regard to the shrinking of sea ice in the
northern Chukchi Sea of the western Arctic Ocean. Distribution patterns of
phytoplankton groups in the late summers of 2008â2010 were analysed based
on HPLC pigment signatures and, the following four major algal groups were
inferred via multiple regression and cluster analyses: prasinophytes,
diatoms, haptophytes and dinoflagellates. A remarkable interannual
difference in the distribution pattern of the groups was found in the
northern basin area. Haptophytes dominated and dispersed widely in warm
surface waters in 2008, whereas prasinophytes dominated in cold water in
2009 and 2010. A difference in the onset date of sea ice retreat was evident
among yearsâthe sea ice retreat in 2008 was 1â2 months earlier than in
2009 and 2010. The spatial distribution of early sea ice retreat matched the
areas in which a shift in algal community composition was observed.
Steel-Dwass's multiple comparison tests were used to assess the physical,
chemical and biological parameters of the four clusters. We found a
statistically significant difference in temperature between the
haptophyte-dominated cluster and the other clusters, suggesting that the
change in the phytoplankton communities was related to the earlier sea ice
retreat in 2008 and the corollary increase in sea surface temperatures.
Longer periods of open water during the summer, which are expected in the
future, may affect food webs and biogeochemical cycles in the western Arctic
due to shifts in phytoplankton community structure
Gauge Equivalence in Two--Dimensional Gravity
Two-dimensional quantum gravity is identified as a second-class system which
we convert into a first-class system via the Batalin-Fradkin (BF) procedure.
Using the extended phase space method, we then formulate the theory in most
general class of gauges. The conformal gauge action suggested by David, Distler
and Kawai is derived from a first principle. We find a local, light-cone gauge
action whose Becchi-Rouet-Stora-Tyutin invariance implies Polyakov's curvature
equation , revealing the origin of the
Kac-Moody symmetry. The BF degree of freedom turns out be dynamically
active as the Liouville mode in the conformal gauge, while in the light-cone
gauge the conformal degree of freedom plays that r{\^o}le. The inclusion of the
cosmological constant term in both gauges and the harmonic gauge-fixing are
also considered.Comment: 30 pages, KANAZAWA 93-
Magnetic properties of the Ag-In-rare-earth 1/1 approximants
We have performed magnetic susceptibility and neutron scattering measurements
on polycrystalline Ag-In-RE (RE: rare-earth) 1/1 approximants. In the magnetic
susceptibility measurements, for most of the RE elements, inverse
susceptibility shows linear behaviour in a wide temperature range, confirming
well localized isotropic moments for the RE ions. Exceptionally for the
light RE elements, such as Ce and Pr, non-linear behaviour was observed,
possibly due to significant crystalline field splitting or valence fluctuation.
For RE = Tb, the susceptibility measurement clearly shows a bifurcation of the
field-cooled and zero-field-cooled susceptibility at ~K,
suggesting a spin-glass-like freezing. On the other hand, neutron scattering
measurements detect significant development of short-range antiferromagnetic
spin correlations in elastic channel, which accompanied by a broad peak at
~meV in inelastic scattering spectrum. These features have
striking similarity to those in the Zn-Mg-Tb quasicrystals, suggesting that the
short-range spin freezing behaviour is due to local high symmetry clusters
commonly seen in both the systems.Comment: 14 pages, 12 figure
A Phase-Space Approach to Collisionless Stellar Systems Using a Particle Method
A particle method for reproducing the phase space of collisionless stellar
systems is described. The key idea originates in Liouville's theorem which
states that the distribution function (DF) at time t can be derived from
tracing necessary orbits back to t=0. To make this procedure feasible, a
self-consistent field (SCF) method for solving Poisson's equation is adopted to
compute the orbits of arbitrary stars. As an example, for the violent
relaxation of a uniform-density sphere, the phase-space evolution which the
current method generates is compared to that obtained with a phase-space method
for integrating the collisionless Boltzmann equation, on the assumption of
spherical symmetry. Then, excellent agreement is found between the two methods
if an optimal basis set for the SCF technique is chosen. Since this
reproduction method requires only the functional form of initial DFs but needs
no assumptions about symmetry of the system, the success in reproducing the
phase-space evolution implies that there would be no need of directly solving
the collisionless Boltzmann equation in order to access phase space even for
systems without any special symmetries. The effects of basis sets used in SCF
simulations on the reproduced phase space are also discussed.Comment: 16 pages w/4 embedded PS figures. Uses aaspp4.sty (AASLaTeX v4.0). To
be published in ApJ, Oct. 1, 1997. This preprint is also available at
http://www.sue.shiga-u.ac.jp/WWW/prof/hozumi/papers.htm
The Union and MĂ©decins Sans FrontiĂšres approach to operational research.
Operational research (OR) has become a hot topic at national meetings, international conferences and donor fora. The International Union Against Tuberculosis and Lung Disease (The Union) and MĂ©decins Sans FrontiĂšres (MSF) Operational Centre Brussels strongly promote and implement OR with colleagues in low- and middle-income countries. Here we describe how the two organisations define OR, and explain the guiding principles and methodology that underpin the strategy for developing and expanding OR in those countries. We articulate The Union's and MSF's approach to supporting OR, highlighting the main synergies and differences. Then, using the Malawi National Tuberculosis Control Programme as an example, we show how OR can be embedded within tuberculosis control activities, leading to changes in policy and practice at the national level. We discuss the difficult, yet vitally important, issue of capacity building, and share our vision of a new paradigm of product-related training and performance-based OR fellowships as two ways of developing the necessary skills at country level to ensure research is actually performed. Finally, we highlight the need to consider and incorporate into practice the ethical components of OR. This is a key moment to be involved in OR. We are confident that in partnership with interested stakeholders, including the World Health Organization, we can stimulate the implementation of quality, relevant OR as an integral part of health service delivery that in turn will lead to better health for people, particularly for those living in the poorer parts of the world
Nuclear Magnetic Relaxation in the Haldane-Gap Antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_)
A new theory is proposed to interpret nuclear spin-lattice relaxation-time
(T_1_) measurements on the spin-1 quasi-one-dimensional Heisenberg
antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_) (NENP). While Sagi and Affleck
pioneeringly discussed this subject in terms of field-theoretical languages,
there is no theoretical attempt yet to explicitly simulate the novel
observations of 1/T_1_ reported by Fujiwara et al.. By means of modified spin
waves, we solve the minimum of 1/T_1_ as a function of an applied field,
pending for the past decade.Comment: to be published in J. Phys. Soc. Jpn. 73, No. 4 (2004
Brueckner Rearrangement Effects in He and He
Rearrangement effects in light hypernuclei are investigated in the framework
of the Brueckner theory. We can estimate without detailed numerical
calculations that the energy of the -core is reduced by more than 2.5
MeV when the adheres to He to form He. Similar
assessment of rearrangement contributions is essential to deduce the strength
of interaction from experimentally observed . The recently observed experimental value of 1 MeV
for the of \hll suggests that the matrix element of
in \hll is around -2 MeV.Comment: 7 pages, to appear in Phys. Rev.
Measurement of the Xi-p Scattering Cross Sections at Low Energy
In this paper we report cross-section measurements for elastic and
inelastic scatterings at low energy using a scintillating fiber active target.
Upper limit on the total cross-section for the elastic scattering was found to
be 24 mb at 90% confidence level, and the total cross section for the
reaction was found to be mb. We
compare the results with currently competing theoretical estimates.Comment: 9 page
Electronic states and magnetic excitations in LiV2O4: Exact diagonalization study
Motivated by recent inelastic neutron scattering experiment we examine
magnetic properties of LiV2O4. We consider a model which describes the
half-filled localized A1g spins interacting via frustrated antiferromagnetic
Heisenberg exchange and coupled by local Hund's interaction with the 1/8-filled
itinerant Eg band, and study it within an exact diagonalization scheme. In the
present study we limited the analysis to the case of the cluster of two
isolated tetrahedrons. We obtained that both the ground state structure and
low-lying excitations depend strongly on the value of the Hund's coupling which
favors the triplet states. With increasing temperature the triplet states
become more and more populated which results in the formation of non-zero
residual magnetic moment. We present the temperature dependence of calculated
magnetic moment and of the spin-spin correlation functions at different values
of Hund's coupling and compare them with the experimental results.Comment: 7 pages. 6 eps figure
- âŠ