Mode coupling due to twisting of magnetic field lines

A new approach to the theory of mode coupling in an inhomogeneous, birefringent medium is used to treat mode coupling in a magnetized plasma with a twisted magnetic field. The twist introduces a resonance, corresponding to the rate of twisting being equal to the rate of generalized Faraday rotation. When this resonance occurs it introduces a new regime of strong mode coupling. The implications of this regime are discussed in connection with the long-standing problem in solar radiophysics that mode coupling appears to be stronger than theory implies, but no obvious resolution of the problem is found.     

Proximity to ice fields and lake depth as modulators of paleoclimate records: a regional study from southwest Yukon,Canada

Pronounced climate warming during the past century has been well documented in high-latitude regions. Nonetheless, considerable heterogeneity exists in northern climate trends. We examined the roles of cryospheric landscape and lake depth in modulating the rate and magnitude of local climate responses through a paleolimnological study of lakes from southwest Yukon, Canada. By sampling lakes at varying distances from the Wrangell-St. Elias ice fields, we hypothesized that, for lakes of similar maximum depth, sites closest to the ice fields would be relatively complacent in terms of their chironomid and diatom assemblage changes over the past ~200 years. This hypothesis is based on the moderating effect of the glaciers on local climate, which would be most pronounced in the lakes nearest to the ice fields. However, given the known ecological differences between deep and shallow lakes, we further predicted that, for a given distance from the ice fields, a sediment record from a shallower lake would show the greatest change in stratigraphic subfossil assemblages. Because of the complicated shape of the ice fields, we applied the longitude for each site (which decreases from west to east) to approximate the proximity of our study lakes to the ice fields. Consistent with our predictions, we observed a space-transgressive pattern in the chironomid assemblage turnover that was associated with their proximity to the ice fields (r = ?0.75, P = 0.034, n = 8) across lakes of similar depth (mean maximum depth ± 1, SE = 18.1 ± 2.6 m). Considering a broader network of lakes that represented a greater range in maximum depth (4.9–29 m), we found that differences in subfossil chironomid assemblages between the modern and ca. AD 1800 sediment layers were strongly related to lake depth (r = ?0.77, P < 0.001, n = 15), but failed to detect a significant relationship with latitude or longitude (i.e. our proxy for proximity to the ice fields). Similarly, our comparative high-resolution analyses of two lakes with distinct lake morphometries, but similar proximities to the ice fields, demonstrated the predicted contrasting pattern: we observed pronounced post-1880 changes in the biotic assemblages in the shallow lake and a muted and delayed response (i.e. ~1970s) in the deeper lake. Our findings confirm that cryospheric landscape features can strongly modulate regional climate. Furthermore, our work shows that investigators need to be conscious of how climate change affects the structure and functioning of lakes of different typologies, which influences the way in which paleoclimate signals are recorded and interpreted.     

Impact cratering and the Oort Cloud

We calculate the expected flux profile of comets into the planetary system from the Oort Cloud arising from Galactic tides and encounters with molecular clouds. We find that both periodic and sporadic bombardment episodes, with amplitudes an order of magnitude above background, occur on characteristic time-scales ∼25–35 Myr. Bombardment episodes occurring preferentially during spiral arm crossings may be responsible both for mass extinctions of life and the transfer of viable microorganisms from the bombarded Earth into the disturbing nebulae. Good agreement is found between the theoretical expectations and the age distribution of large, well-dated terrestrial impact craters of the past 250 Myr. A weak periodicity of ∼36 Myr in the cratering record is consistent with the Sun's recent passage through the Galactic plane, and implies a central plane density  ∼0.15 M_⊙ pc⁻³  . This leaves little room for a significant dark matter component in the disc.     

Behaviour of Tl relative to K,Rb, Sr and Ba in mineralized and unmineralized metavolcanics from the Dir area,northern Pakistan

Copper mineralization in the Dir area of northern Pakistan is confined to the quartz veins and associated with hydrothermally altered metavolcanics. Chalcopyrite is the main copper-bearing phase with subordinate amounts of bornite, chalcocite, covellite, malachite and azurite. Both mineralized quartz veins and associated unmineralized (least altered) and mineralized (strongly altered) metavolcanics have been analyzed for Cu, Au, Ag, Tl, K, Rb, Ba and Sr. An increase of Cu, Au, Ag, Rb/Sr and Tl/Sr, and a decrease of Sr and K/Rb is observed in both mineralized metavolcanics and mineralized quartz veins. Thallium shows lithophile behaviour in the Dir metavolcanics and no chalcophile behaviour was observed. The Tl/Sr ratio might be an indirect guide for the exploration of volcanic-hosted hydrothermal copper deposits.     

A rationale for beach selection by the public on the coast of Wales, UK

Questionnaires were completed by 2306 beach users at 19 Welsh beaches with respect to beach selection parameters. The modal group of respondents was female aged 30–39. Beach choice was primarily determined by clean litter-free sand and seawater, followed by safety. Refreshment facilities and beach awards were deemed minor considerations by the public when choosing a beach to visit. Approximately 58 per cent of respondents were aware of beach award and rating schemes. Of coastal visitors interviewed for this paper, 67 per cent rated a beach as 'important' or 'very important' to their holiday, with just 2 per cent replying that they were unimportant.     

Desertification in southern Europe

The threat of desertification appears in southern Europe when periods of prolonged drought are followed by torrential rains falling on steep, unstable terrain. The plant cover of many regions has been degraded since very ancient times. Some are now recovering as a result of emigration and reduce pressure on the land. Tourist and commercial agricultural developments threaten habitat values and water supplies, especially in certain coastal areas. Relatively abundant archeological and literary resources offer excellent opportunities for tracing land use histories and throwing more light on the nature and direction of environmental change. Such longitudinal studies should be integrated with remote sensing and nested sampling on the ground of current landscape conditions and monitoring of current processes.