45,365 research outputs found
Statistical study of free magnetic energy and flare productivity of solar active regions
Photospheric vector magnetograms from Helioseismic and Magnetic Imager on
board the Solar Dynamic Observatory are utilized as the boundary conditions to
extrapolate both non-linear force-free and potential magnetic fields in solar
corona. Based on the extrapolations, we are able to determine the free magnetic
energy (FME) stored in active regions (ARs). Over 3000 vector magnetograms in
61 ARs were analyzed. We compare FME with ARs' flare index (FI) and find that
there is a weak correlation () between FME and FI. FME shows slightly
improved flare predictability relative to total unsigned magnetic flux of ARs
in the following two aspects: (1) the flare productivity predicted by FME is
higher than that predicted by magnetic flux and (2) the correlation between FI
and FME is higher than that between FI and magnetic flux. However, this
improvement is not significant enough to make a substantial difference in
time-accumulated FI, rather than individual flare, predictions.Comment: The paper was submitted to ApJ and it is accepted no
Does the 2D Hubbard Model Really Show d-Wave Superconductivity?
Some issues concerning the question if the two-dimensional Hubbard model
really show d-wave superconductivity are briefly discussed.Comment: Revtex, no figure
Whole-brain patterns of 1H-magnetic resonance spectroscopy imaging in Alzheimer's disease and dementia with Lewy bodies
Acknowledgements We thank Craig Lambert for his help in processing the MRS data. The study was funded by the Sir Jules Thorn Charitable Trust (grant ref: 05/JTA) and was supported by the National Institute for Health Research (NIHR) Newcastle Biomedical Research Centre and the Biomedical Research Unit in Lewy Body Dementia based at Newcastle upon Tyne Hospitals National Health Service (NHS) Foundation Trust and Newcastle University and the NIHR Biomedical Research Centre and Biomedical Research Unit in Dementia based at Cambridge University Hospitals NHS Foundation Trust and the University of Cambridge.Peer reviewedPublisher PD
Autonomous Integrated Receive System (AIRS) requirements definition. Volume 3: Performance and simulation
The autonomous and integrated aspects of the operation of the AIRS (Autonomous Integrated Receive System) are discussed from a system operation point of view. The advantages of AIRS compared to the existing SSA receive chain equipment are highlighted. The three modes of AIRS operation are addressed in detail. The configurations of the AIRS are defined as a function of the operating modes and the user signal characteristics. Each AIRS configuration selection is made up of three components: the hardware, the software algorithms and the parameters used by these algorithms. A comparison between AIRS and the wide dynamics demodulation (WDD) is provided. The organization of the AIRS analytical/simulation software is described. The modeling and analysis is for simulating the performance of the PN subsystem is documented. The frequence acquisition technique using a frequency-locked loop is also documented. Doppler compensation implementation is described. The technological aspects of employing CCD's for PN acquisition are addressed
Thermodynamics of spin-1/2 tetrameric Heisenberg antiferromagnetic chain
The thermodynamic properties of a spin S=1/2 tetrameric Heisenberg
antiferromagnetic chain with alternating interactions AF1-AF2-AF1-F (AF and F
denote the antiferromagnetic and ferromagnetic couplings, respectively) are
studied by means of the transfer-matrix renormalization group method and
Jordan-Wigner transformation. It is found that in the absence of magnetic
field, the thermodynamic behaviors are closely related to the gapped low-lying
excitations, and a novel structure with three peaks in the temperature
dependence of specific heat is unveiled. In a magnetic field, a phase diagram
in the temperature-field plane for the couplings satisfying JAF1=JAF2=JF is
obtained, in which various phases are identified. The temperature dependence of
thermodynamic quantities including the magnetization, susceptibility and
specific heat are studied to characterize the corresponding phases. It is
disclosed that the magnetization has a crossover behavior at low temperature in
the Luttinger liquid phase, which is shown falling into the same class as that
in the S=1 Haldane chain. In the plateau regime, the thermodynamic behaviors
alter at a certain field, which results from the crossing of two excitation
spectra. By means of the fermion mapping, it is uncovered that the system has
four spectra from fermion and hole excitations that are responsible for the
observed thermodynamic behaviors.Comment: 10 pages, 10 figures, accepted by Phys. Rev.
Hydrological long-term dry and wet periods in the Xijiang River basin, South China
In this study, hydrological long-term dry and wet periods are analyzed for the Xijiang River basin in South China. Daily precipitation data of 118 stations and data on daily discharge at Gaoyao hydrological station at the mouth of the Xijiang River for the period 1961–2007 are used. At a 24-month timescale, the standardized precipitation index (SPI-24) for the six sub-basins of the Xijiang River and the standardized discharge index (SDI-24) for Gaoyao station are applied. The monthly values of the SPI-24 averaged for the Xijiang River basin correlate highly with the monthly values of the SDI-24. Distinct long-term dry and wet sequences can be detected. <br><br> The principal component analysis is applied and shows spatial disparities in dry and wet periods for the six sub-basins. The correlation between the SPI-24 of the six sub-basins and the first principal component score shows that 67% of the variability within the sub-basins can be explained by dry and wet periods in the east of the Xijiang River basin. The spatial dipole conditions (second and third principal component) explain spatiotemporal disparities in the variability of dry and wet periods. All sub-basins contribute to hydrological dry periods, while mainly the northeastern sub-basins cause wet periods in the Xijiang River. We can also conclude that long-term dry events are larger in spatial extent and cover all sub-basins while long-term wet events are regional phenomena. <br><br> A spectral analysis is applied for the SPI-24 and the SDI-24. The results show significant peaks in periodicities of 11–14.7 yr, 2.8 yr, 3.4–3.7 yr, and 6.3–7.3 yr. The same periodic cycles can be found in the SPI-24 of the six sub-basins but with some variability in the mean magnitude. A wavelet analysis shows that significant periodicities have been stable over time since the 1980s. Extrapolations of the reconstructed SPI-24 and SDI-24 represent the continuation of observed significant periodicities at given magnitudes until 2030. The projected hydrological long-term dry and wet periods can be used for planning purposes in water resources management. The applied methodologies prove to be able to identify spatial disparities, and to detect significant periodicities in hydrological long-term dry and wet periods in the Xijiang River basin
Magnetic order in CaFe1-xCoxAsF (x = 0, 0.06, 0.12) superconductor compounds
A Neutron Powder Diffraction (NPD) experiment has been performed to
investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF
superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF
undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the
magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The
antiferromagnetic structure of the parent compound has been determined with a
unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB
aligned along the long a-axis. With increasing Co doping, the long range
antiferromagnetic order has been observed to coexist with superconductivity in
the orthorhombic phase of the underdoped CaFe0.94Co0.06AsF with a reduced Fe
moment (0.15(5)uB). Magnetic order is completely suppressed in optimally doped
CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity
might be related to mesoscopic phase separation.Comment: 4pages, 4figure
- …