Dynamic Ly alpha jets

The solar chromosphere and transition region are highly structured and complex regimes. A recent breakthrough has been the identification of dynamic fibrils observed in H alpha as caused by field-aligned magnetoacoustic shocks. We seek to find whether such dynamic fibrils are also observed in Ly alpha. We used a brief sequence of four high-resolution Ly alpha images of the solar limb taken by the Very high Angular resolution ULtraviolet Telescope (VAULT), which displays many extending and retracting Ly alpha jets. We measured their top trajectories and fitted parabolas to the 30 best-defined ones. Most jet tops move supersonically. Half of them decelerate, sometimes superballistically, the others accelerate. This bifurcation may arise from incomplete sampling of recurrent jets. The similarities between dynamic Ly alpha jets and H alpha fibrils suggest that the magnetoacoustic shocks causing dynamic H alpha fibrils also affect dynamic Ly alpha jets.Comment: 5 pages, 7 figures; changed title and content; accepted in Astronomy and Astrophysics; eps figures in full resolution are available at http://www.astro.sk/~koza/publications/vault/figs

Dynamics of isolated magnetic bright points derived from Hinode/SOT G-band observations

Small-scale magnetic fields in the solar photosphere can be identified in high-resolution magnetograms or in the G-band as magnetic bright points (MBPs). Rapid motions of these fields can cause magneto-hydrodynamical waves and can also lead to nanoflares by magnetic field braiding and twisting. The MBP velocity distribution is a crucial parameter for estimating the amplitudes of those waves and the amount of energy they can contribute to coronal heating. The velocity and lifetime distributions of MBPs are derived from solar G-band images of a quiet sun region acquired by the Hinode/SOT instrument with different temporal and spatial sampling rates. We developed an automatic segmentation, identification and tracking algorithm to analyse G-Band image sequences to obtain the lifetime and velocity distributions of MBPs. The influence of temporal/spatial sampling rates on these distributions is studied and used to correct the obtained lifetimes and velocity distributions for these digitalisation effects. After the correction of algorithm effects, we obtained a mean MBP lifetime of (2.50 +- 0.05) min and mean MBP velocities, depending on smoothing processes, in the range of (1 - 2) km/s. Corrected for temporal sampling effects, we obtained for the effective velocity distribution a Rayleigh function with a coefficient of (1.62 +- 0.05) km/s. The x- and y- components of the velocity distributions are Gaussians. The lifetime distribution can be fitted by an exponential function.Comment: Astronomy and Astrophysics (in press

Relating chaos to deterministic diffusion of a molecule adsorbed on a surface

Chaotic internal degrees of freedom of a molecule can act as noise and affect the diffusion of the molecule on a substrate. A separation of time scales between the fast internal dynamics and the slow motion of the centre of mass on the substrate makes it possible to directly link chaos to diffusion. We discuss the conditions under which this is possible, and show that in simple atomistic models with pair-wise harmonic potentials, strong chaos can arise through the geometry. Using molecular-dynamics simulations, we demonstrate that a realistic model of benzene is indeed chaotic, and that the internal chaos affects the diffusion on a graphite substrate

A Comparative Study of the Interactions of Two Calcium Phosphates, PEO/PBT Copolymer (Polyactive) and a Silicone Rubber with Bone and Fibrous Tissue

In this study, hydroxyapatite, tetracalcium phosphate, HPEO/PBT 55145 copolymer, PEO/PBT 55!45 copolymer (Polyactive) and silicone rubber were implanted as dense blocks, subcutaneously and. into the tibia of rats. Biocompatibility and degradation were investigated but most attention was directed to .the bone/biomaterial interactions. None of the materials showed any significant adverse tissue reactions. With exception of the silicone rubber, all materials sho~ed bone bonding phenomena based on both morphological and mechanical evaluations. (H)PEO/PBT 55145 copolymer is the first polymer reported to be bonded by bone and thus widens the spectrum of bone bonding materials with a low modulus, degradable, elastomer in contrast to the high modulus glasses and ceramics that are available to date. The possible associated bone-bonding mechanism is briefly discussed

Dynamics of the solar chromosphere. V. High-frequency modulation in ultraviolet image sequences from TRACE

We search for signatures of high-frequency oscillations in the upper solar photosphere and low chromosphere in the context of acoustic heating of outer stellar atmospheres. We use ultraviolet image sequences of a quiet center-disk area from the Transition Region and Coronal Explorer (TRACE) mission which were taken with strict cadence regularity. The latter permits more reliable high-frequency diagnosis than in earlier work. Spatial Fourier power maps, spatially averaged coherence and phase-difference spectra, and spatio-temporal k-f decompositions all contain high-frequency features that at first sight seem of considerable intrinsic interest but actually are more likely to represent artifacts of different nature. Spatially averaged phase difference measurement provides the most sensitive diagnostic and indicates the presence of acoustic modulation up to f=20 mHz (periods down to 50 seconds) in internetwork areas.Comment: 9 pages, 8 figure

Radius of curvature approach to the Kolmogorov-Sinai entropy of dilute hard particles in equilibrium

We consider the Kolmogorov-Sinai entropy for dilute gases of $N$ hard disks or spheres. This can be expanded in density as $h_{\mathrm{KS}} \propto n N [\ln n a^d+ B + O(n a^d)+O(1/N)]$, with $a$ the diameter of the sphere or disk, $n$ the density, and $d$ the dimensionality of the system. We estimate the constant $B$ by solving a linear differential equation for the approximate distribution of eigenvalues of the inverse radius of curvature tensor. We compare the resulting values of $B$ both to previous estimates and to existing simulation results, finding very good agreement with the latter. Also, we compare the distribution of eigenvalues of the inverse radius of curvature tensor resulting from our calculations to new simulation results. For most of the spectrum the agreement between our calculations and the simulations again is very good.Comment: 12 pages, 4 figure

Multiscale magnetic underdense regions on the solar surface: Granular and Mesogranular scales

The Sun is a non-equilibrium dissipative system subjected to an energy flow which originates in its core. Convective overshooting motions create temperature and velocity structures which show a temporal and spatial evolution. As a result, photospheric structures are generally considered to be the direct manifestation of convective plasma motions. The plasma flows on the photosphere govern the motion of single magnetic elements. These elements are arranged in typical patterns which are observed as a variety of multiscale magnetic patterns. High resolution magnetograms of quiet solar surface revealed the presence of magnetic underdense regions in the solar photosphere, commonly called voids, which may be considered a signature of the underlying convective structure. The analysis of such patterns paves the way for the investigation of all turbulent convective scales from granular to global. In order to address the question of magnetic structures driven by turbulent convection at granular and mesogranular scales we used a "voids" detection method. The computed voids distribution shows an exponential behavior at scales between 2 and 10 Mm and the absence of features at 5-10 Mm mesogranular scales. The absence of preferred scales of organization in the 2-10 Mm range supports the multiscale nature of flows on the solar surface and the absence of a mesogranular convective scale

Expected resolution limits of x-ray free-electron laser single-particle imaging for realistic source and detector properties

The unprecedented intensity of x-ray free-electron laser sources has enabled single-particle x-ray diffraction imaging (SPI) of various biological specimens in both two-dimensional projection and three dimensions (3D). The potential of studying protein dynamics in their native conditions, without crystallization or chemical staining, has encouraged researchers to aim for increasingly higher resolutions with this technique. The currently achievable resolution of SPI is limited to the sub-10 nanometer range, mainly due to background effects, such as instrumental noise and parasitic scattering from the carrier gas used for sample delivery. Recent theoretical studies have quantified the effects of x-ray pulse parameters, as well as the required number of diffraction patterns to achieve a certain resolution, in a 3D reconstruction, although the effects of detector noise and the random particle orientation in each diffraction snapshot were not taken into account. In this work, we show these shortcomings and address limitations on achievable image resolution imposed by the adaptive gain integrating pixel detector noise