Transpolar arc evolution and associated potential patterns

International audienceWe present two event studies encompassing detailed relationships between plasma convection, field-aligned current, auroral emission, and particle precipitation boundaries. We illustrate the influence of the Interplanetary Magnetic Field By component on theta aurora development by showing two events during which the theta originates on both the dawn and dusk sides of the auroral oval. Both theta then move across the entire polar region and become part of the opposite side of the auroral oval. Electric and magnetic field and precipitating particle data are provided by DMSP, while the Polar UVI instrument provides measurements of auroral emissions. Utilizing satellite data as inputs, the Royal Institute of Technology model provides the high-latitude ionospheric electrostatic potential pattern calculated at different times during the evolution of the theta aurora, resulting from a variety of field-aligned current configurations associated with the changing global aurora. Key words. Ionosphere (auroral ionosphere; electric fields and currents). Magnetospheric physics (magnetosphereionosphere interactions

Effect of tensile stress on the in-plane resistivity anisotropy in BaFe2As2

The effect of uniaxial tensile stress and the resultant strain on the structural/magnetic transition in the parent compound of the iron arsenide superconductor, BaFe$_2$As$_2$, is characterized by temperature-dependent electrical resistivity, x-ray diffraction and quantitative polarized light imaging. We show that strain induces a measurable uniaxial structural distortion above the first-order magnetic transition and significantly smears the structural transition. This response is different from that found in another parent compound, SrFe$_2$As$_2$, where the coupled structural and magnetic transitions are strongly first order. This difference in the structural responses explains the in-plain resistivity anisotropy above the transition in BaFe$_2$As$_2$. This conclusion is supported by the Ginzburg-Landau - type phenomenological model for the effect of the uniaxial strain on the resistivity anisotropy

Iterative approach to computational enzyme design

A general approach for the computational design of enzymes to catalyze arbitrary reactions is a goal at the forefront of the field of protein design. Recently, computationally designed enzymes have been produced for three chemical reactions through the synthesis and screening of a large number of variants. Here, we present an iterative approach that has led to the development of the most catalytically efficient computationally designed enzyme for the Kemp elimination to date. Previously established computational techniques were used to generate an initial design, HG-1, which was catalytically inactive. Analysis of HG-1 with molecular dynamics simulations (MD) and X-ray crystallography indicated that the inactivity might be due to bound waters and high flexibility of residues within the active site. This analysis guided changes to our design procedure, moved the design deeper into the interior of the protein, and resulted in an active Kemp eliminase, HG-2. The cocrystal structure of this enzyme with a transition state analog (TSA) revealed that the TSA was bound in the active site, interacted with the intended catalytic base in a catalytically relevant manner, but was flipped relative to the design model. MD analysis of HG-2 led to an additional point mutation, HG-3, that produced a further threefold improvement in activity. This iterative approach to computational enzyme design, including detailed MD and structural analysis of both active and inactive designs, promises a more complete understanding of the underlying principles of enzymatic catalysis and furthers progress toward reliably producing active enzymes

Uniaxial strain detwinning of CaFe2As2 and BaFe2As2: optical and transport study

TThe parent compounds of iron-arsenide superconductors, $A$Fe$_{2}$As$_{2}$ ($A$=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature $T_{\mathrm{TO}}$ in the range 135 to 205K depending on the alkaline earth element. Below $T_{\mathrm{TO}}$ the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2}$. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron $X$-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, $\rho$, decreases along the orthorhombic $a_{o}$-axis but increases along the orthorhombic $b_{o}$-axis in both compounds. Immediately below $T_{\mathrm{TO}}$ the ratio $\rho_{bo}/ \rho_{ao}$ = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe$_{2}$As$_{2}$, BaFe$_{2}$As$_{2}$ reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above $T_{\mathrm{TO}}$ in BaFe$_{2}$As$_{2}$ than in CaFe$_{2}$As$_{2}$, or that there is a higher temperature crossover or phase transition.Comment: extended versio

Twenty Years of Embryonic Stem Cell Research in Farm Animals

Contents Notable distinctions between an embryonic stem cell (ESC) and somatic cell are that an ESC can maintain an undifferentiated state indefinitely, self-renew, and is pluripotent, meaning that the ESC can potentially generate cells representing all the three primordial germ layers and contribute to the terminally differentiated cells of a conceptus. These attributes make the ESC an ideal source for genome editing for both agricultural and biomedical applications. Although, ESC lines have been successfully established from rodents and primates, authentic ungulate stem cell lines on the contrary are still not available. Outstanding issues including but not limited to differences in pluripotency characteristics among the existing ESC lines, pre-implantation embryo development, pluripotency pathways, and culture conditions plague our efforts to establish authentic ESC lines from farm animals. In this review, we highlight some of these issues and discuss how the recent derivation of induced pluripotent stem cells (iPSCs) might augur the establishment of robust authentic ESC lines from farm animals

Statistics of a parallel Poynting vector in the auroral zone as a function of altitude using Polar EFI and MFE data and Astrid-2 EMMA data

We study the wave-related (AC) and static (DC) parallel Poynting vector (Poynting energy flux) as a function of altitude in auroral field lines using Polar EFI and MFE data. The study is statistical and contains 5 years of data in the altitude range 5000–30000 km. We verify the low altitude part of the results by comparison with earlier Astrid-2 EMMA Poynting vector statistics at 1000 km altitude. The EMMA data are also used to statistically compensate the Polar results for the missing zonal electric field component. We compare the Poynting vector with previous statistical DMSP satellite data concerning the electron precipitation power. We find that the AC Poynting vector (Alfvén-wave related Poynting vector) is statistically not sufficient to power auroral electron precipitation, although it may, for <i>K<sub>p</sub></i>>2, power 25–50% of it. The statistical AC Poynting vector also has a stepwise transition at <i>R</i>=4 <i>R<sub>E</sub></i>, so that its amplitude increases with increasing altitude. We suggest that this corresponds to Alfvén waves being in Landau resonance with electrons, so that wave-induced electron acceleration takes place at this altitude range, which was earlier named the Alfvén Resonosphere (ARS). The DC Poynting vector is ~3 times larger than electron precipitation and corresponds mainly to ionospheric Joule heating. In the morning sector (02:00–06:00 MLT) we find that the DC Poynting vector has a nontrivial altitude profile such that it decreases by a factor of ~2 when moving upward from 3 to 4 <i>R<sub>E</sub></i> radial distance. In other nightside MLT sectors the altitude profile is more uniform. The morning sector nontrivial altitude profile may be due to divergence of the perpendicular Poynting vector field at <i>R</i>=3–4 <i>R<sub>E</sub></i>. <p><b>Keywords.</b> Magnetospheric physics (Auroral phenomena; Magnetosphere-ionosphere interactions) – Space plasma physics (Wave-particle interactions

Chromatographic Properties of Different Methyl—Phenyl (1:1) Substituted Silicone Stationary Phases for Open-Tubular Gas Chromatography

The influence of different configurations of silicones having 50% methyl and 50% phenyl substitution on chromatographic properties, such as polarity and thermal stability, has been systematically investigated. Polysiloxanes composed of dimethyl and diphenyl units show very low levels of column bleed at temperatures up to 370°C, while polymers having methyl—phenyl substitution show severe bleeding at this temperature. The polarity of the latter polymers, as reflected by Kováts indices, is higher than for the polymers composed by dimethyl—diphenyl unit

Poloidal ULF oscillations in the dayside magnetosphere: a Cluster study

Three ULF wave events, all occurring in the dayside magnetopshere during magnetically quiet times, are studied using the Cluster satellites. The multi-point measurements obtained from Cluster are used to determine the azimuthal wave number for the events by means of the phase shift and the azimuthal separation between the satellites. Also, the polarisation of the electric and magnetic fields is examined in a field-aligned coordinate system, which, in turn, gives the mode of the oscillations. The large-inclination orbits of Cluster allow us to examine the phase relationship between the electric and magnetic fields along the field lines. The events studied have large azimuthal wave numbers (<i>m</i>~100), two of them have eastward propagation and all are in the poloidal mode, consistent with the large wave numbers. We also use particle data from geosynchronous satellites to look for signatures of proton injections, but none of the events show any sign of enhanced proton flux. Thus, the drift-bounce resonance instability seems unlikely to have played any part in the excitation of these pulsations. As for the drift-mirror instability we conclude that it would require an unreasonably high plasma pressure for the instability criterion to be satisfied.<br><br><b>Keywords.</b> Ionosphere (Wave propagation) – Magnetospheric physics (Plasma waves and instabilities; Instruments and techniques