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-bis­[(1E)-(1H-pyrrol-2-yl­methyl­idene)amino]­thio­phene-3,4-dicarboxyl­ate

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 thio­phene 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)methyl­ene­amino]-5-(2-thienylmethyl­ene­amino)thio­phene-3,4-dicarboxyl­ate

Both imine bonds of the title compound, C21H21N3O4S2, were found to be in the E configuration. The terminal pyrrole and thio­phene rings are twisted by 2.5 (3) and 2.3 (2)°, respectively, from the mean plane of the central thio­phene 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 thio­phene rings]

Diethyl 2-amino-5-[(E)-(furan-2-yl­methyl­idene)amino]­thio­phene-3,4-di­carboxyl­ate

In the crystal structure of the title compound, C15H16N2O5S, the azomethine adopts the E configuration. The two heterocyclic rings adopt an anti­periplanar orientation. The mean planes of the thio­phene and furan rings are twisted by 2.51 (4)°. The crystal structure exhibits inter­molecular 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-bis­[(E)-2-furylmethyl­ene­amino]thio­phene-3,4-dicarboxyl­ate

The title compound, C20H18N2O6S, crystallizes as two independent mol­ecules 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 thio­phene ring by 7.33 (4) and 21.74 (5)° in one mol­ecule, and by 6.91 (4) and 39.80 (6)° in the other