56,954 research outputs found
PEM-West trajectory climatology and photochemical model sensitivity study prepared using retrospective meteorological data
Trajectory and photochemical model calculations based on retrospective meteorological data for the operations areas of the NASA Pacific Exploratory Mission (PEM)-West mission are summarized. The trajectory climatology discussed here is intended to provide guidance for flight planning and initial data interpretation during the field phase of the expedition by indicating the most probable path air parcels are likely to take to reach various points in the area. The photochemical model calculations which are discussed indicate the sensitivity of the chemical environment to various initial chemical concentrations and to conditions along the trajectory. In the post-expedition analysis these calculations will be used to provide a climatological context for the meteorological conditions which are encountered in the field
Faint emission lines in the Galactic H II regions M16, M20 and NGC 3603
We present deep echelle spectrophotometry of the Galactic {\hii} regions M16,
M20 and NGC 3603. The data have been taken with the Very Large Telescope
Ultraviolet-Visual Echelle Spectrograph in the 3100 to 10400 \AA range. We have
detected more than 200 emission lines in each region. Physical conditions have
been derived using different continuum and line intensity ratios. We have
derived He, C and O abundances from pure recombination
lines as well as abundances from collisionally excited lines for a large number
of ions of different elements. We have obtained consistent estimations of the
temperature fluctuation parameter, {\ts}, using different methods. We also
report the detection of deuterium Balmer lines up to D (M16) and to
D (M20) in the blue wings of the hydrogen lines, which excitation
mechanism seems to be continuum fluorescence. The temperature fluctuations
paradigm agree with the results obtained from optical CELs and the more
uncertain ones from far IR fine structure CELs in NGC 3603, although, more
observations covering the same volume of the nebula are necessary to obtain
solid conclusions.Comment: 22 pages, 13 Tables, 7 Figures. Accepted for publication by MNRA
Linear Response Theory and Optical Conductivity of Floquet Topological Insulators
Motivated by the quest for experimentally accessible dynamical probes of
Floquet topological insulators, we formulate the linear response theory of a
periodically driven system. We illustrate the applications of this formalism by
giving general expressions for optical conductivity of Floquet systems,
including its homodyne and heterodyne components and beyond. We obtain the
Floquet optical conductivity of specific driven models, including
two-dimensional Dirac material such as the surface of a topological insulator,
graphene, and the Haldane model irradiated with circularly or linearly
polarized laser, as well as semiconductor quantum well driven by an ac
potential. We obtain approximate analytical expressions and perform numerically
exact calculations of the Floquet optical conductivity in different scenarios
of the occupation of the Floquet bands, in particular, the diagonal Floquet
distribution and the distribution obtained after a quench. We comment on
experimental signatures and detection of Floquet topological phases using
optical probes.Comment: 16 pages, 10 figure
Fluctuating surface-current formulation of radiative heat transfer: theory and applications
We describe a novel fluctuating-surface current formulation of radiative heat
transfer between bodies of arbitrary shape that exploits efficient and
sophisticated techniques from the surface-integral-equation formulation of
classical electromagnetic scattering. Unlike previous approaches to
non-equilibrium fluctuations that involve scattering matrices---relating
"incoming" and "outgoing" waves from each body---our approach is formulated in
terms of "unknown" surface currents, laying at the surfaces of the bodies, that
need not satisfy any wave equation. We show that our formulation can be applied
as a spectral method to obtain fast-converging semi-analytical formulas in
high-symmetry geometries using specialized spectral bases that conform to the
surfaces of the bodies (e.g. Fourier series for planar bodies or spherical
harmonics for spherical bodies), and can also be employed as a numerical method
by exploiting the generality of surface meshes/grids to obtain results in more
complicated geometries (e.g. interleaved bodies as well as bodies with sharp
corners). In particular, our formalism allows direct application of the
boundary-element method, a robust and powerful numerical implementation of the
surface-integral formulation of classical electromagnetism, which we use to
obtain results in new geometries, including the heat transfer between finite
slabs, cylinders, and cones
Optimal Control Realizations of Lagrangian Systems with Symmetry
A new relation among a class of optimal control systems and Lagrangian
systems with symmetry is discussed. It will be shown that a family of solutions
of optimal control systems whose control equation are obtained by means of a
group action are in correspondence with the solutions of a mechanical
Lagrangian system with symmetry. This result also explains the equivalence of
the class of Lagrangian systems with symmetry and optimal control problems
discussed in \cite{Bl98}, \cite{Bl00}.
The explicit realization of this correspondence is obtained by a judicious
use of Clebsch variables and Lin constraints, a technique originally developed
to provide simple realizations of Lagrangian systems with symmetry. It is
noteworthy to point out that this correspondence exchanges the role of state
and control variables for control systems with the configuration and Clebsch
variables for the corresponding Lagrangian system.
These results are illustrated with various simple applications
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