1,898 research outputs found
Modelling ion populations in astrophysical plasmas: carbon in the solar transition region
The aim of this work is to improve the modelling of ion populations in higher
density, lower temperature astrophysical plasmas, of the type commonly found in
lower solar and stellar atmospheres. Ion population models for these regions
frequently employ the coronal approximation, which assumes conditions more
suitable to the upper solar atmosphere, where high temperatures and lower
densities prevail. Using the coronal approximation for modelling the solar
transition region gives theoretical lines intensities for the Li-like and
Na-like isoelectronic sequences which can be factors of 2-5 times lower than
observed. The works of Burgess & Summers (1969) and Nussbaumer & Storey (1975)
showed the important part ions in excited levels play when included in the
modelling. Their models, however, used approximations for the atomic rates to
determine the ion balance. Presented here is the first stage in updating these
earlier models of carbon by using rates from up-to-date atomic calculations and
more recent photo-ionising radiances for the quiet Sun. Where such atomic rates
are not readily available, in the case of electron-impact direct ionisation and
excitation--auto-ionisation, new calculations have been made and compared to
theoretical and experimental studies. The effects each atomic process has on
the ion populations as density changes is demonstrated, and final results from
the modelling are compared to the earlier works. Lastly, the new results for
ion populations are used to predict line intensities for the solar transition
region in the quiet Sun, and these are compared with predictions from
coronal-approximation modelling and with observations. Significant improvements
in the predicted line intensities are seen in comparison to those obtained from
zero-density modelling of carbon.Comment: Draft accepted by A&A, 13 pages, 15 figure
Diethyl 2,5-bisÂ[(1E)-(1H-pyrrol-2-ylÂmethylÂidene)amino]ÂthioÂphene-3,4-dicarboxylÂate
In the crystal structure of the title compound, C20H20N4O4S, the azomethine group adopt E conformations. The pyrrole units are twisted by 10.31 (4) and 18.90 (5)° with respect to the central thioÂphene ring. The three-dimensional network is close packed and involves N—H⋯O, N—H⋯N, C—H⋯N and C—H⋯O hydrogen bonding
Diethyl 2-[(1-methyl-1H-pyrrol-2-yl)methylÂeneÂamino]-5-(2-thienylmethylÂeneÂamino)thioÂphene-3,4-dicarboxylÂate
Both imine bonds of the title compound, C21H21N3O4S2, were found to be in the E configuration. The terminal pyrrole and thioÂphene rings are twisted by 2.5 (3) and 2.3 (2)°, respectively, from the mean plane of the central thioÂphene to which they are attached. The structure is disordered by exchange of the terminal heterocyclic rings; the site occupancy factors are ca 0.8 and 0.2. The crystal packing involves some π–π stacking [3.449 (4) Å between pyrrole and terminal thioÂphene rings]
Diethyl 2-amino-5-[(E)-(furan-2-ylÂmethylÂidene)amino]ÂthioÂphene-3,4-diÂcarboxylÂate
In the crystal structure of the title compound, C15H16N2O5S, the azomethine adopts the E configuration. The two heterocyclic rings adopt an antiÂperiplanar orientation. The mean planes of the thioÂphene and furan rings are twisted by 2.51 (4)°. The crystal structure exhibits interÂmolecular N—H⋯O hydrogen bonding. π–π stacking is also observed, the centroid-to-centroid distance being 3.770 (4) Å
Brownian Dynamics of a Sphere Between Parallel Walls
We describe direct imaging measurements of a colloidal sphere's diffusion
between two parallel surfaces. The dynamics of this deceptively simple
hydrodynamically coupled system have proved difficult to analyze. Comparison
with approximate formulations of a confined sphere's hydrodynamic mobility
reveals good agreement with both a leading-order superposition approximation as
well as a more general all-images stokeslet analysis.Comment: 4 pages, 3 figures, REVTeX with PostScript figure
Intrinsic Fluctuations and Driven Response of Insect Swarms
Animals of all sizes form groups, as acting together can convey advantages over acting alone; thus, collective animal behavior has been identified as a promising template for designing engineered systems. However, models and observations have focused predominantly on characterizing the overall group morphology, and often focus on highly ordered groups such as bird flocks. We instead study a disorganized aggregation (an insect mating swarm), and compare its natural fluctuations with the group-level response to an external stimulus. We quantify the swarm’s frequency-dependent linear response and its spectrum of intrinsic fluctuations, and show that the ratio of these two quantities has a simple scaling with frequency. Our results provide a new way of comparing models of collective behavior with experimental data
Microoptomechanical pumps assembled and driven by holographic optical vortex arrays
Beams of light with helical wavefronts can be focused into ring-like optical
traps known as optical vortices. The orbital angular momentum carried by
photons in helical modes can be transferred to trapped mesoscopic objects and
thereby coupled to a surrounding fluid. We demonstrate that arrays of optical
vortices created with the holographic optical tweezer technique can assemble
colloidal spheres into dynamically reconfigurable microoptomechanical pumps
assembled by optical gradient forces and actuated by photon orbital angular
momentum.Comment: 4 pages, 3 figures, submitted to Optics Expres
Diethyl 2,5-bisÂ[(E)-2-furylmethylÂeneÂamino]thioÂphene-3,4-dicarboxylÂate
The title compound, C20H18N2O6S, crystallizes as two independent molÂecules that are disposed about a pseudo-inversion center (1/2, 1/4, 1/8). The mean planes of the two terminal furyl rings are twisted with respect to the central thioÂphene ring by 7.33 (4) and 21.74 (5)° in one molÂecule, and by 6.91 (4) and 39.80 (6)° in the other
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