6,210 research outputs found
Temporal evolution of oscillating coronal loops
Context. Transverse oscillations of coronal structures are currently
intensively studied to explore the associated magnetohydrodynamic wave physics
and perform seismology of the local medium. Aims. We make a first attempt to
measure the thermodynamic evolution of a sample of coronal loops that undergo
decaying kink oscillations in response to an eruption in the corresponding
active region. Methods. Using data from the six coronal wavelengths of SDO/AIA,
we performed a differential emission measure (DEM) analysis of 15 coronal loops
before, during, and after the eruption and oscillation. Results. We find that
the emission measure, temperature, and width of the DEM distribution undergo
significant variations on time scales relevant for the study of transverse
oscillations. There are no clear collective trends of increases or decreases
for the parameters we analysed. The strongest variations of the parameters
occur during the initial perturbation of the loops, and the influence of
background structures may also account for much of this variation. Conclusions.
The DEM analysis of oscillating coronal loops in erupting active regions shows
evidence of evolution on time scales important for the study of the
oscillations. Further work is needed to separate the various observational and
physical mechanisms that may be responsible for the variations in temperature,
DEM distribution width, and total emission measure.Comment: Accepted in A&
Preparation, Structure, and Reactivity of Nonstabilized Organoiron Compounds. Implications for Iron-Catalyzed Cross Coupling Reactions
A series of unprecedented organoiron complexes of the formal oxidation states −2, 0, +1, +2, and +3 is presented, which are largely devoid of stabilizing ligands and, in part, also electronically unsaturated (14-, 16-, 17- and 18-electron counts). Specifically, it is shown that nucleophiles unable to undergo β-hydride elimination, such as MeLi, PhLi, or PhMgBr, rapidly reduce Fe(3+) to Fe(2+) and then exhaustively alkylate the metal center. The resulting homoleptic organoferrate complexes [(Me4Fe)(MeLi)][Li(OEt2)]2 (3) and [Ph4Fe][Li(Et2O)2][Li(1,4-dioxane)] (5) could be characterized by X-ray crystal structure analysis. However, these exceptionally sensitive compounds turned out to be only moderately nucleophilic, transferring their organic ligands to activated electrophiles only, while being unable to alkylate (hetero)aryl halides unless they are very electron deficient. In striking contrast, Grignard reagents bearing alkyl residues amenable to β-hydride elimination reduce FeXn (n = 2, 3) to clusters of the formal composition [Fe(MgX)2]n. The behavior of these intermetallic species can be emulated by structurally well-defined lithium ferrate complexes of the type [Fe(C2H4)4][Li(tmeda)]2 (8), [Fe(cod)2][Li(dme)]2 (9), [CpFe(C2H4)2][Li(tmeda)] (7), [CpFe(cod)][Li(dme)] (11), or [Cp*Fe(C2H4)2][Li(tmeda)] (14). Such electron-rich complexes, which are distinguished by short intermetallic Fe−Li bonds, were shown to react with aryl chlorides and allyl halides; the structures and reactivity patterns of the resulting organoiron compounds provide first insights into the elementary steps of low valent iron-catalyzed cross coupling reactions of aryl, alkyl, allyl, benzyl, and propargyl halides with organomagnesium reagents. However, the acquired data suggest that such C−C bond formations can occur, a priori, along different catalytic cycles shuttling between metal centers of the formal oxidation states Fe(+1)/Fe(+3), Fe(0)/Fe(+2), and Fe(−2)/Fe(0). Since these different manifolds are likely interconnected, an unambiguous decision as to which redox cycle dominates in solution remains difficult, even though iron complexes of the lowest accessible formal oxidation states promote the reactions most effectively
Recent Important Decisions; Book Reviews
A collection of recent important court decisions and book reviews
Sex enhances adaptation by unlinking beneficial from detrimental mutations in experimental yeast populations
<p>Abstract</p> <p>Background</p> <p>The maintenance of sexuality is a classic problem in evolutionary biology because it is a less efficient mode of reproduction compared with asexuality; however, many organisms are sexual. Theoretical work suggests sex facilitates natural selection, and experimental data support this. However, there are fewer experimental studies that have attempted to determine the mechanisms underlying the advantage of sex. Two main classes of hypotheses have been proposed to explain its advantage: detrimental mutation clearance and beneficial mutation accumulation. Here we attempt to experimentally differentiate between these two classes by evolving <it>Saccharomyces cerevisiae </it>populations that differ only in their ability to undergo sex, and also manipulate mutation rate. We cannot manipulate the types of mutation that occur, but instead propagate populations in both stressful and permissive environments and assume that the extent of detrimental mutation clearance and beneficial mutation incorporation differs between them.</p> <p>Results</p> <p>After 300 mitotic generations interspersed with 11 rounds of sex we found there was no change or difference in fitness between sexuals and asexuals propagated in the permissive environment, regardless of mutation rate. Sex conferred a greater extent of adaptation in the stressful environment, and wild-type and elevated mutation rate sexual populations adapted equivalently. However, the asexual populations with an elevated mutation rate appeared more retarded in their extent of adaptation compared to asexual wild-type populations.</p> <p>Conclusions</p> <p>Sex provided no advantage in the permissive environment where beneficial mutations were rare. We could not evaluate if sex functioned to clear detrimental mutations more effectively or not here as no additional fitness load was observed in the mutator populations. However, in the stressful environment, where detrimental mutations were likely of more consequence, and where beneficial mutations were apparent, sex provided an advantage. In the stressful environment asexuals were increasingly constrained in their extent of adaptation with increasing mutation rate. Sex appeared to facilitate adaptation not just by more rapidly combining beneficial mutations, but also by unlinking beneficial from detrimental mutations: sex allowed selection to operate on both types of mutations more effectively compared to asexual populations.</p
Recent Legal Literature
Beale: The Law of Innkeepers and Hotels, including other Public Houses, Theatres, Sleeping Cars; Beale: The Law of Railroad Rate Regulation, with special reference to American Legislation; Moore: A Treatise on the Law of Carriers; Bernard: The First Year of Roman Law; Holland: The Elements of Jurisprudence; Wilcox: Foibles of the Bench; Wigmore (ed.): A Selection of Cases on Evidence for the Use of Students of Law; Fairlie: Local Government in Counties, Towns and Villages; Hirschl: Trial Tactics. A Treatment of the Methods of Conducting Litigatio
Coordination Chemistry of Ene-1,1-diamines and a Prototype "Carbodicarbene"
Carbophilic Lewis acids can polarize a coordinated π-bond by a slippage mechanism. A series of stable ylid- or enolate gold complexes of ene-1,1-diamines not only emulate this property, but also reveal the exceptional donor capacity of such electron-rich olefin ligands. Moreover, the first metal complex of a tetraaminoallene is reported, which features a prototype “carbodicarbene” ligand bound to a transition-metal template
Predicting frequency changes of global-scale solar Rossby modes due to solar cycle changes in internal rotation
Context. Large-scale equatorial Rossby modes have been observed on the Sun
over the last two solar cycles. Aims. We investigate the impact of the
time-varying zonal flows on the frequencies of Rossby modes. Methods. A
first-order perturbation theory approach is used to obtain an expression for
the expected shift in the mode frequencies due to perturbations in the internal
rotation rate. Results. Using the time-varying rotation from helioseismic
inversions we predict the changes in Rossby mode frequencies with azimuthal
orders from m = 1 to m = 15 over the last two solar cycles. The peak-to-peak
frequency change is less than 1 nHz for the m = 1 mode, grows with m, and
reaches 25 nHz for m = 15. Conclusions. Given the observational uncertainties
on mode frequencies due to the finite mode lifetimes, we find that the
predicted frequency shifts are near the limit of detectability
Vector Coherent State Realization of Representations of the Affine Lie Algebra
The method of vector coherent states is generalized to study representations
of the affine Lie algebra . A large class of highest weight irreps
is explicitly constructed, which contains the integrable highest weight irreps
as special cases.Comment: 8 pages plain latex. To appear in J. Phys.
Surfactant control of gas transfer velocity along an offshore coastal transect: results from a laboratory gas exchange tank
Understanding the physical and biogeochemical controls of air–sea gas
exchange is necessary for establishing biogeochemical models for predicting
regional- and global-scale trace gas fluxes and feedbacks. To this end we
report the results of experiments designed to constrain the effect of
surfactants in the sea surface microlayer (SML) on the gas transfer velocity
(<i>k</i><sub>w</sub>; cm h<sup>−1</sup>), seasonally (2012–2013) along a 20 km
coastal transect (North East UK). We measured total surfactant activity (SA),
chromophoric dissolved organic matter (CDOM) and chlorophyll <i>a</i> (Chl <i>a</i>) in
the SML and in sub-surface water (SSW) and we evaluated corresponding
<i>k</i><sub>w</sub> values using a custom-designed air–sea gas exchange tank.
Temporal SA variability exceeded its spatial variability. Overall, SA varied
5-fold between all samples (0.08 to 0.38 mg L<sup>−1</sup> T-X-100), being
highest in the SML during summer. SML SA enrichment factors (EFs) relative to
SSW were ∼ 1.0 to 1.9, except for two values (0.75; 0.89: February
2013). The range in corresponding <i>k</i><sub>660</sub> (<i>k</i><sub>w</sub> for CO<sub>2</sub> in
seawater at 20 °C) was 6.8 to 22.0 cm h<sup>−1</sup>. The film factor
<i>R</i><sub>660</sub> (the ratio of <i>k</i><sub>660</sub> for seawater to <i>k</i><sub>660</sub> for “clean”,
i.e. surfactant-free, laboratory water) was strongly correlated with SML SA
(<i>r</i> ≥ 0.70, <i>p</i> ≤ 0.002, each <i>n</i> = 16). High SML SA typically
corresponded to <i>k</i><sub>660</sub> suppressions ∼ 14 to 51 % relative to
clean laboratory water, highlighting strong spatiotemporal gradients in gas
exchange due to varying surfactant in these coastal waters. Such variability
should be taken account of when evaluating marine trace gas sources and
sinks. Total CDOM absorbance (250 to 450 nm), the CDOM spectral slope ratio
(<i>S</i><sub>R</sub> = <i>S</i><sub>275 − 295</sub>∕<i>S</i><sub>350 − 400</sub>), the 250 : 365 nm CDOM
absorption ratio (<i>E</i><sub>2</sub> : <i>E</i><sub>3</sub>), and Chl <i>a</i> all indicated spatial
and temporal signals in the quantity and composition of organic matter in the
SML and SSW. This prompts us to hypothesise that spatiotemporal variation in
<i>R</i><sub>660</sub> and its relationship with SA is a consequence of compositional
differences in the surfactant fraction of the SML DOM pool that warrants
further investigation
Shock compression and spallation of single crystal tantalum
We present molecular dynamics simulations of shock-induced plasticity and spall damage in single crystal Ta described by a recently developed embedded-atom-method (EAM) potential and a volumedependent qEAM potential. We use impact or Hugoniotstat simulations to investigate the Hugoniots, deformation and spallation. Both EAM and qEAM are accurate in predicting, e.g., the Hugoniots and γ - surfaces. Deformation and spall damage are anisotropic for Ta single crystals. Our preliminary results show that twinning is dominant for [100] and [110] shock loading, and dislocation, for [111]. Spallation initiates with void nucleation at defective sites from remnant compressional deformation or tensile plasticity. Spall strength decreases with increasing shock strength, while its rate dependence remains to be explored
- …