5,999 research outputs found
Quantifying the Effect of Non-Larmor Motion of Electrons on the Pressure Tensor
In space plasma, various effects of magnetic reconnection and turbulence
cause the electron motion to significantly deviate from their Larmor orbits.
Collectively these orbits affect the electron velocity distribution function
and lead to the appearance of the "non-gyrotropic" elements in the pressure
tensor. Quantification of this effect has important applications in space and
laboratory plasma, one of which is tracing the electron diffusion region (EDR)
of magnetic reconnection in space observations. Three different measures of
agyrotropy of pressure tensor have previously been proposed, namely,
, and . The multitude of contradictory measures has
caused confusion within the community. We revisit the problem by considering
the basic properties an agyrotropy measure should have. We show that
, and are all defined based on the sum of the
principle minors (i.e. the rotation invariant ) of the pressure tensor. We
discuss in detail the problems of -based measures and explain why they may
produce ambiguous and biased results. We introduce a new measure
constructed based on the determinant of the pressure tensor (i.e. the rotation
invariant ) which does not suffer from the problems of -based
measures. We compare with other measures in 2 and 3-dimension
particle-in-cell magnetic reconnection simulations, and show that can
effectively trace the EDR of reconnection in both Harris and force-free current
sheets. On the other hand, does not show prominent peaks in
the EDR and part of the separatrix in the force-free reconnection simulations,
demonstrating that does not measure all the non-gyrotropic
effects in this case, and is not suitable for studying magnetic reconnection in
more general situations other than Harris sheet reconnection.Comment: accepted by Phys. of Plasm
Quantification of effects of climate variations and human activities on runoff by a monthly water balance model: A case study of the Chaobai River basin in northern China
The Chaobai River basin in northern China consists of two major tributaries, the Chao River and Bai River. Monthly observations of precipitation, streamfiow, and panevaporation data are available for 35 years (1961-1966 and 1973-2001). Using the annual time series of the observed streamfiow, one break point at 1979 is detected and is adopted to divide the data set into two study periods, the "before" and "after" periods marking the onset of significant anthropogenic alteration of the flow (reservoirs and silt retention dams, five times increase in population) and significant changes in land use (conversion to terraced fields versus sloping fields). The distributed time-variant gain model (DTVGM) was used to evaluate the water resources of the area. Furthermore, the Bayesian method used by Engeland et al. (2005) was used in this paper to evaluate two uncertainty sources (i.e., the model parameter and model structure) and for assessing the DTVGM's performance over the Chaobai River basin. Comparing the annual precipitation means over 13 years (1961-1966 and 1973-1979), the means of the second period (1980-2001) decreased by 5.4% and 4.9% in the Chao River and Bai River basins, respectively. However, the related annual runoff decreased by 40.3% and 52.8%, respectively, a much greater decline than exhibited by precipitation. Through the monthly model simulation and the fixing-changing method, it is determined that decreases in runoff between the two periods can be attributed to 35% (31%) from climate variations and 68% (70%) from human activities in the Chao River (Bai River). Thus, human impact exerts a dominant influence upon runoff decline in the Chaobai River basin compared to climate. This study enhances our understanding of the relative roles of climate variations and human activities on runoff. © 2009 by the American Geophysical Union.published_or_final_versio
Optical Interferometry of early-type stars with PAVO@CHARA. I. Fundamental stellar properties
We present interferometric observations of 7 main-sequence and 3 giant stars
with spectral types from B2 to F6 using the PAVO beam combiner at the CHARA
array. We have directly determined the angular diameters for these objects with
an average precision of 2.3%. We have also computed bolometric fluxes using
available photometry in the visible and infrared wavelengths, as well as
space-based ultraviolet spectroscopy. Combined with precise \textit{Hipparcos}
parallaxes, we have derived a set of fundamental stellar properties including
linear radius, luminosity and effective temperature. Fitting the latter to
computed isochrone models, we have inferred masses and ages of the stars. The
effective temperatures obtained are in good agreement (at a 3% level) with
nearly-independent temperature estimations from spectroscopy. They validate
recent sixth-order polynomial (B-V)- empirical relations
\citep{Boyajian2012a}, but suggest that a more conservative third-order
solution \citep{vanBelle2009} could adequately describe the
(V-K)- relation for main-sequence stars of spectral type A0 and
later. Finally, we have compared mass values obtained combining surface gravity
with inferred stellar radius (\textit{gravity mass}) and as a result of the
comparison of computed luminosity and temperature values with stellar
evolutionary models (\textit{isochrone mass}). The strong discrepancy between
isochrone and gravity mass obtained for one of the observed stars,
\,Lyr, suggests that determination of the stellar atmosphere parameters
should be revised.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Extraction of Principle Knowledge from Process Patents for Manufacturing Process Innovation
Process patents contain substantial knowledge of the principles behind manufacturing process problems-solving; however, this knowledge is implicit in lengthy texts and cannot be directly reused in innovation design. To effectively support systematic manufacturing process innovation, this paper presents an approach to extracting principle innovation knowledge from process patents. The proposed approach consists of (1) classifying process patents by taking process method, manufacturing object and manufacturing feature as the references; (2) extracting generalized process contradiction parameters and the principles behind solving such process contradictions based on patent mining and technology abstraction of TRIZ (the theory of inventive problem solving); and (3) constructing a domain process contradiction matrix and mapping the relationship between the matrix and the corresponding process patents. Finally, a case study is presented to illustrate the applicability of the proposed approach
Effects of nano-void density, size, and spatial population on thermal conductivity: a case study of GaN crystal
The thermal conductivity of a crystal is sensitive to the presence of
surfaces and nanoscale defects. While this opens tremendous opportunities to
tailor thermal conductivity, a true "phonon engineering" of nanocrystals for a
specific electronic or thermoelectric application can only be achieved when the
dependence of thermal conductivity on the defect density, size, and spatial
population is understood and quantified. Unfortunately, experimental studies of
effects of nanoscale defects are quite challenging. While molecular dynamics
simulations are effective in calculating thermal conductivity, the defect
density range that can be explored with feasible computing resources is
unrealistically high. As a result, previous work has not generated a fully
detailed understanding of the dependence of thermal conductivity on nanoscale
defects. Using GaN as an example, we have combined physically-motivated
analytical model and highly-converged large scale molecular dynamics
simulations to study effects of defects on thermal conductivity. An analytical
expression for thermal conductivity as a function of void density, size, and
population has been derived and corroborated with the model, simulations, and
experiments
Coulomb scattering inducing time lag in strong-field tunneling ionization
We study ionization of atoms in strong elliptically-polarized laser fields.
We focus on the physical origin of the offset angle in the photoelectron
momentum distribution and its possible relation to a specific time. By
developing a model which is based on strong-field approximation and considers
the classical Coulomb scattering, we are able to quantitatively explain recent
attoclock experiments in a wide region of laser and atomic parameters. The
offset angle can be understood as arising from the scattering of the electron
by the ionic potential when the electron exits the laser-Coulomb-formed barrier
through tunneling. The scattering time is manifested as the Coulomb-induced
ionization time lag and is encoded in the offset angle.Comment: 5 pages, 4 figure
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