Theoretical analysis of resonance conditions in magnetized plasmas when the plasma/gyro frequency ratio is close to an integer

We study cylindrical oscillations of electrons in a plasma where the background magnetic field is parallel to the axis of a cylinder and the ion background is taken to have a constant positive density. Assuming self-similarity, we reduce the problem to two second order nonlinear differential equations, relating fluctuations of electron density and fluctuations of magnetic field. The system studied is shown to have two fundamental frequencies. In the linear domain, these frequencies correspond to the Z and X cold plasma wave cutoff frequencies (R = 0 and L = 0 conditions in the notation of Stix (1992), respectively). Thus they differ by the gyrofrequency. In the nonlinear domain, these frequencies change and resonances at harmonics and at the sum and difference frequencies appear. When the plasma/gyro frequency ratio is close to an integer, our results indicate special resonance conditions which are characterized by an increase of the amplitude and a narrowing of the fundamental resonances.     

Influence of the Pacific Decadal Oscillation on the climate of theSierra Nevada, California and Nevada

Mono Lake sediments have recorded five major oscillations in the hydrologic balance between A.D. 1700 and 1941. These oscillations can be correlated with tree-ring-based oscillations in Sierra Nevada snowpack. Comparison of a tree-ring-based reconstruction of the Pacific Decadal Oscillation (PDO) index (D’Arrigo et al., 2001) with a coral-based reconstruction of Subtropical South Pacific sea-surface temperature (Linsley et al., 2000) indicates a high degree of correlation between the two records during the past 300 yr. This suggests that the PDO has been a pan-Pacific phenomena for at least the past few hundred years. Major oscillations in the hydrologic balance of the Sierra Nevada correspond to changes in the sign of the PDO with extreme droughts occurring during PDO maxima. Four droughts centered on A.D. 1710, 1770, 1850, and 1930 indicate PDO-related drought reoccurrence intervals ranging from 60 to 80 yr.     

Paleoclimatic implications of the high stand of Lake Lahontan derived from models of evaporation and lake level

Based on previous climate model simulations of a split of the polar jet stream during the late Pleistocene, we hypothesize that (1) 20–13.5 ka BP, season-to-season variation in the latitudinal maximum of the jet stream core led to enhanced wetness in the Great Basin, and (2) after 13.5 ka BP, northward movement of the jet stream resulted in increased aridity similar to today. We suggest that the enhanced effective wetness was due to increased precipitation combined with an energy-limited reduction in evaporation rates that was caused by increased summer cloud cover. A physically based thermal evaporation model was used to simulate evaporation for Lake Lahontan under various hypothesized paleoclimates. The simulated evaporation rates, together with hypothetical rates of precipitation and discharge, were input to a water balance model of Lake Lahontan. A 42% reduction in evaporation rate, combined with maximum historical rates of precipitation (1.8 times the mean annual rate) and discharge (2.4 times the mean annual rate), were sufficient to maintain Lake Lahontan at its 20-15 ka BP level. When discharge was increased to 3.8 times the present-day, mean annual rate, the 13.5 ka BP maximum level of Lake Lahontan was attained within 1400 years. A 135-m drop from the maximum level to Holocene levels was simulated within 300 years under the imposition of the present-day hydrologic balance.     

Longevity overfishing

Overfishing is generally considered to be a reduction in biomass below some critical level such that the remaining fish are not able to replenish the population. We propose that the removal of large numbers of older age groups by fishing is also a form of overfishing, which we identify as longevity overfishing. Longevity overfishing is a potentially important consideration for the commercial fisheries off Canada’s Pacific coast that are dominated by species that have maximum ages of 30 years or longer. Sablefish is one of the key long-lived species that is managed for biomass and not longevity. An age structured model showed that if younger fish do not have the same productivity per unit biomass as older fish, the population depleted of older fishes would not recover after a shift of carrying capacity from a prolonged period of poor productivity to a more productive ocean ecosystem. Current management of long-lived species implicitly assumes that young fish will have the same productivity as older fishes, an assumption that is not supported by a sparse literature, and is thus not precautionary. We propose that the evolved age structure is an indication that long-lived species must be managed for longevity as well as biomass, which requires a management time frame that is decades and not annual.     

Temporal analysis of groundwater nitrate concentrations from wells in Prince Edward Island,Canada: application of a linear mixed effects model

Changes in nitrate concentration in groundwater from wells in Prince Edward Island, Canada were investigated over time using two datasets. Temporal trends in groundwater nitrate concentrations were assessed annually during 1981–1996 (1,299 observations), and both seasonally and monthly during 1988–1991 (1,868 observations). Data were analysed using linear mixed models with random effects and correlation structures. The average nitrate concentration in the monthly dataset was 3.99 mg/L as NO₃–N, with January, May, and November concentrations being higher (p?=?0.018). A seasonal effect was present when season was combined with land use type in an interaction term (p?=?0.004). Wells located in agricultural areas had greater nitrate concentrations than urban areas, which in turn, had greater values than low human-impact areas. Row-cropped areas had higher groundwater nitrate concentrations in the summer, whereas manure storage areas were higher in the spring and autumn. Nitrate in groundwater in areas with low human impact and with centralized sewage disposal infrastructure remained relatively low and stable throughout the seasons. There was no significant annual trend (p?=?0.954), but for individual sites, 9.6% significantly increased in nitrate concentration over time, and 6.6% significantly decreased over time.     

Anisotropic P-wave attenuation measured from a multi-azimuth surface seismic reflection survey

A system of aligned vertical fractures produces azimuthal variations in stacking velocity and amplitude variation with offset, characteristics often reported in seismic reflection data for hydrocarbon exploration. Studies of associated attenuation anisotropy have been mostly theoretical, laboratory or vertical seismic profiling based. We used an 11 common‐midpoint‐long portion of each of four marine surface‐seismic reflection profiles, intersecting each other at 45° within circa 100 m of a common location, to measure the azimuthal variation of effective attenuation, Q⁻¹_eff and stacking velocity, in a shallow interval, about 100 m thick, in which consistently orientated vertical fracturing was expected due to an underlying salt diapirism. We found qualitative and quantitative consistency between the azimuthal variation in the attenuation and stacking velocity, and published amplitude variation with offset results. The 135° azimuth line showed the least apparent attenuation (1000 Q⁻¹_eff= 16 ± 7) and the fastest stacking velocity, hence we infer it to be closest to the fracture trend: the orthogonal 45° line showed the most apparent attenuation (1000Q⁻¹_eff= 52 ± 15) and slowest stacking velocity. The variation of Q⁻¹_eff with azimuth φ is well fitted by 1000Q⁻¹_eff = 34 − 18cos[2(φ+40°)] giving a fracture direction of 140 ± 23° (±1SD, derived from ‘bootstrapping’ fits to all 11⁴ combinations of individual common‐midpoint/azimuth measurements), compared to 134 ± 47° from published amplitude variation with offset data. The effects of short‐window spectral estimation and choices of spectral ratio bandwidth and offset ranges used in attenuation analysis, individually give uncertainties of up to ±13° in fracture direction. This magnitude of azimuthal variation can be produced by credible crack geometries (e.g., dry cracks, radius 6.5 m, aspect ratio 3 × 10⁻⁵, crack density 0.2) but we do not claim these to be the actual properties of the interval studied, because of the lack of well control (and its consequences for the choice of theoretical model and host rock physical properties) and the small number of azimuths available here.     

Diffuse X-ray emission from late-type galaxy haloes

Current theories of galaxy formation predict that spiral galaxies are embedded in a reservoir of hot gas. This gas is able to cool on to the galaxy, replenishing cold gas that is consumed by star formation. Estimates of the X-ray luminosity emitted in the cooling region suggest a bolometric luminosity of the order of 10×10⁴¹ erg s⁻¹ in massive systems. We have used ROSAT PSPC data to search for extended X-ray emission from the haloes of three nearby, massive, late-type galaxies: NGC 2841, 4594 and 5529. We infer 95 per cent upper limits on the bolometric X-ray luminosities of the haloes of NGC 2841, 4594 and 5529 of 0.4, 1.2 and 3.8×10⁴¹ erg s⁻¹ respectively. Thus, the true luminosity lies well below the straightforward theoretical prediction. We discuss this discrepancy and suggest a number of ways in which the theoretical model might be brought into agreement with the observational results. A possible solution is that the gravitational potentials of the dark matter haloes of these galaxies are weaker than assumed in the current model. Alternatively, the present-day accretion may be substantially less than is required on average to build the disc over the Hubble time. Our results are, however, based on only three galaxies, none of which is ideal for this kind of study. A larger data set is required to explore this important problem further.     

Influence of exposure history on vitellogenin induction in medaka: a case of ‘estrogen memory’?

In this study, we exposed female medaka to aqueous solutions of o,p′-DDT until tissues residues reached approximately 100 g/g. Male offspring of exposed females and unexposed males that were raised to 6 months of age differed from untreated controls in their response to subsequent 4-day static exposure to 17-β-estradiol (E2; 12 μg/l as determined by steroid EIA). Homogenized liver samples from males from DDT treatments, females from DDT treatments, control males and control females produced two vitellogenin bands on a Western blot with no qualitative differences in band size and location between the groups. To quantify the relative level of vitellogenin, 5 μg of protein from each sample was analyzed by slot blot. A statistical comparison revealed a significant difference in optical density of the slots between control males and DDT exposed males (analysis of variance, n=18, P=0.021; Tukey test, n=9, P=0.047) indicating that previous transgenerational exposure to DDT enhanced the vitellogenic response to E2 exposure in males. These results indicate that exposure to estrogenic chemicals may produce an ‘estrogen memory’ which potentiates the response to a subsequent exposure event.     

Aspects of bridge-ground seismic response and liquefaction-induced deformations

Considerable bridge-ground interaction effects are involved in evaluating the consequences of liquefaction-induced deformations. Due to seismic excitation, liquefied soil layers may result in substantial accumulated permanent deformation of sloping ground near the abutments. Ultimately, global response is dictated by the bridge-ground interaction as an integral system. However, a holistic assessment of such response generally requires a highly demanding full three-dimensional (3D) model of the bridge and surrounding ground. As such, in order to capture a number of the salient involved mechanisms, this study focuses on the longitudinal seismic performance of a simpler idealized configuration, motivated by details of an existing bridge-ground configuration. In this model, a realistic multilayer soil profile is considered with interbedded liquefiable/nonliquefiable strata. The effect of the resulting liquefaction-induced ground deformation is explored. Attention is given to overall deformation of the bridge structure due to lateral spreading in the vicinity of the abutments. The derived insights indicate a need for such global analysis techniques, when addressing the potential hazard of liquefaction and its consequences.