2,846 research outputs found
Circularly polarized waves in a plasma with vacuum polarization effects
The theory for large amplitude circularly polarized waves propagating along
an external magnetic field is extended in order to include also vacuum
polarization effects. A general dispersion relation, which unites previous
results, is derived.Comment: 5 pages (To appear in Physics of Plasmas
Measurement of Diffusible Hydrogen Content and Hydrogen Effects on the Cracking Potential of Duplex Stainless Steel Weldments (Part I)(Materials, Metallurgy & Weldability)
Are Southern Resident killer whales on a path to extinction?
Numerous extinctions reveal species tipping points where multiple pressures converge to cause a cascade to demise. This paper examines how multiple pressures impacting Southern Resident Killer Whales (SRKW) could lead to their rapid downward spiral to extinction. The most commonly cited impacts to this population are historically low abundance of chinook salmon prey, vessel disturbance from ships and boats, toxicant loads above established health effects thresholds, and risk of oil spills from increased vessel and rail oil transport. Inbreeding and a heavily skewed male biased sex ratio at birth compound these problems. Lack of prey increases nutritional stress and dependence on fat reserves, while decreasing concentrations of bioactive thyroid hormone (T3). Fat metabolism increases circulating lipophilic toxins, which can further decrease T3 concentrations. Collectively, these impacts appear to increase rates of spontaneous abortion and neonatal loss. They may also lead to immunosuppression with a likely rise in incidence of disease and mortality. Associated population decline inherently increases inbreeding, which can increase male sex ratio bias at birth. Reduced T3 may also affect sex ratio at birth. SRKW seasonal presence in the Salish Sea is declining, attributed to declining prey and increased whale-watch vessels. Breeding events within and between pods are commonly observed during these visits to the inland waters. Their decline in visits may limit opportunities for breeding with a more diverse gene-pool, which could further increase inbreeding and associated potential for male-biased sex ratio bias at birth. Chinook remain at the epicenter of all this, indicating an increase in Chinook salmon abundance to be the single most effective mitigation we can do to recover SRKWs. Salmon are sentinels of ecosystem imbalance spanning from toxic contaminants to habitat destruction. Creative and immediate solutions are essential to expedite the process of recovery and ensure the survival of this iconic species
Performance test of QU-fitting in cosmic magnetism study
QU-fitting is a standard model-fitting method to reconstruct distribution of
magnetic fields and polarized intensity along a line of sight (LOS) from an
observed polarization spectrum. In this paper, we examine the performance of
QU-fitting by simulating observations of two polarized sources located along
the same LOS, varying the widths of the sources and the gap between them in
Faraday depth space, systematically. Markov Chain Monte Carlo (MCMC) approach
is used to obtain the best-fit parameters for a fitting model, and Akaike and
Bayesian Information Criteria (AIC and BIC, respectively) are adopted to select
the best model from four fitting models. We find that the combination of MCMC
and AIC/BIC works fairly well in model selection and estimation of model
parameters in the cases where two sources have relatively small widths and a
larger gap in Faraday depth space. On the other hand, when two sources have
large width in Faraday depth space, MCMC chain tends to be trapped in a local
maximum so that AIC/BIC cannot select a correct model. We discuss the causes
and the tendency of the failure of QU-fitting and suggest a way to improve it.Comment: 8 pages, 9 figures, submitted to MNRA
Nonlinear wave interaction and spin models in the MHD regime
Here we consider the influence on the electron spin in the MHD regime.
Recently developed models which include spin-velocity correlations are taken as
a starting point. A theoretical argument is presented, suggesting that in the
MHD regime a single fluid electron model with spin correlations is equivalent
to a model with spin-up and spin-down electrons constituting different fluids,
but where the spin-velocity correlations are omitted. Three wave interaction of
2 shear Alfven waves and a compressional Alfven wave is then taken as a model
problem to evaluate the asserted equivalence. The theoretical argument turns
out to be supported, as the predictions of the two models agree completely.
Furthermore, the three wave coupling coefficients obey the Manley-Rowe
relations, which give further support to the soundness of the models and the
validity of the assumptions made in the derivation. Finally we point out that
the proposed two-fluid model can be incorporated in standard Particle-In-Cell
schemes with only minor modifications.Comment: 8 page
Methyl methanesulfonate (MMS) produces heat-labile DNA damage but no detectable in vivo DNA double-strand breaks
Homologous recombination (HR) deficient cells are sensitive to methyl methanesulfonate (MMS). HR is usually involved in the repair of DNA double-strand breaks (DSBs) in Saccharomyces cerevisiae implying that MMS somehow induces DSBs in vivo. Indeed there is evidence, based on pulsed-field gel electrophoresis (PFGE), that MMS causes DNA fragmentation. However, the mechanism through which MMS induces DSBs has not been demonstrated. Here, we show that DNA fragmentation following MMS treatment, and detected by PFGE is not the consequence of production of cellular DSBs. Instead, DSBs seen following MMS treatment are produced during sample preparation where heat-labile methylated DNA is converted into DSBs. Furthermore, we show that the repair of MMS-induced heat-labile damage requires the base excision repair protein XRCC1, and is independent of HR in both S.cerevisiae and mammalian cells. We speculate that the reason for recombination-deficient cells being sensitive to MMS is due to the role of HR in repair of MMS-induced stalled replication forks, rather than for repair of cellular DSBs or heat-labile damage
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