On a dynamo driven topographically by longitudinal libration

Variation in the angular velocity Ω of a planetary body is called libration or longitudinal libration when the Ω-axis is fixed in direction. This motion of the body's solid mantle drives motions in its fluid core, either by viscous coupling across the core-mantle interface S, or topographically when S is asymmetric with respect to the Ω-axis, the only case considered in this article. A significant topographically-driven flow is identified having uniform vorticity within S and no component parallel to the Ω-axis. Its dynamic stability depends on the amplitude, Ω ₁, of the sinusoidally varying part of Ω and on the ratio, b/a, of the lengths of the principal axes of S, assumed spheroidal. In (Ω ₁/Ω ₀, b/a) parameter space where Ω ₀ is the average Ω, islands are shown to exist where the constant vorticity states are dynamically unstable. These are surrounded by a sea in which they are stable. When the fluid is slightly viscous, a state in the stable sea retains its uniform vorticity structure except in a viscous boundary layer on S in which the flow acquires a component parallel to the Ω-axis. For (Ω ₁/Ω ₀, b/a) on an island where the uniform vorticity state is unstable, an “alternative flow” exists, which is three-dimensional and is examined here. Assuming that the core is electrically conducting, kinematic dynamos are sought. Uniform vorticity flow appears to be non-regenerative but, when it is stable and viscosity acts to create a sufficiently strong boundary layer flow, dynamo action may occur. It is shown that the alternative flow that exists on an instability island in (Ω ₁/Ω ₀,?b/a) space can be vigorously regenerative.     

Spatial management of small-scale fisheries on the west coast of Baja California Sur, Mexico

To understand small-scale fisheries in Mexico, the spatial and temporal distribution of the landed catch of several species was analyzed, including abalone, clams, snails, octopus, squids, lobsters, shrimps, blue crabs, sharks, rays and finfish along the 349 km of the Gulf of Ulloa coast. Data reported by fishers from 1998 to 2009 on catch volume and value per group of species and per fishing locality, together with the number of landing events, were used to define fishing zones and the relative importance of each fishery. Finfish turned out the primary fishery in the region, but the other fisheries showed relevant spatial changes that should be considered when establishing management goals and fishing regulations according to resource availability, fleet efficiency, processing capacity, commercialization of fishing products and profit distribution.     

Numerical simulation of Martian historical dynamo: Impact of the Rayleigh number on the dynamo state

The observed Mars remnant magnetism suggests that there was an active dynamo in the Martian core. We use the MoSST core dynamics model to simulate the Martian historical dynamo, focusing on the variation of the dynamo states with the Rayleigh number Ra (a non-dimensional parameter describing the buoyancy force in the core). Our numerical results show that the mean field length scale does not vary monotonically with the Rayleigh number, and the field morphology at the core mantle boundary changes with Rayleigh number. In particular, it drifts westward with a speed decreasing with Rayleigh number. Supported by National Natural Science Foundation of China (Grant No. 40328006)     

The end of shark finning? Impacts of declining catches and fin demand on coastal community livelihoods

For several decades, fishing sharks for their fins has provided important livelihoods for eastern Indonesian coastal communities that fish the Halmahera, Arafura and Timor Seas. Fishery and interview data collected in 2012-13 from three case studies on the islands of Seram, Aru and Rote were used to examine changes in shark fishers’ livelihoods over the preceding 20 years. While recent declines in catches and shark fin prices have had a substantial impact on fishers’ livelihoods, the fishery's low visibility in some areas of its geographic range and its political complexity in general have meant that government and international development agencies have largely been unaware of this impact. Many respondents remembered the Asian Financial Crisis in 1997-98 and the turn of the millennium as a time when sharks were still abundant and shark fin prices high, but were concerned about the on-going fall of shark fin prices since March 2012. High-value species, particularly guitarfish, hammerhead and sandbar sharks were most affected, losing up to 40% of their pre-2012 value. These changes, combined with the loss of fishing grounds, few attractive options for alternative income and restrictive debt relationships with shark fin bosses, have led some fishers to resort to high-risk activities such as blast fishing, illegal transboundary fishing, and people smuggling. This paper examines the multi-layered causes and consequences of fishers’ decision-making in response to adverse changes in their fishery, and explores options and obstacles to pursuing livelihoods that carry lower environmental, financial and personal risks.     

Kinematic dynamo induced by helical waves

We investigate numerically the kinematic dynamo induced by the superposition of two helical waves in a periodic box as a simplified model to understand the dynamo action in astronomical bodies. The effects of magnetic Reynolds number, wavenumber and wave frequency on the dynamo action are studied. It is found that this helical-wave dynamo is a slow dynamo. There exists an optimal wavenumber for the dynamo growth rate. A lower wave frequency facilitates the dynamo action and the oscillations of magnetic energy emerge at some particular wave frequencies.     

Erosion and runoff reduction potential of vetiver grass for hill slopes: A physical model study

Severe erosion is caused by intense rainfall in tropical regions. The erodible soil of steep hill slopes, accompanied by destruction of vegetation due to human interventions results in accelerated erosion. A sustainable and cost-effective solution such as vetiver grass (Chrysopogon zizanioides) is, thus, required to control the erosion process. In the current study, 6 small-scale glass models: 1 bare and 5 with vetiver grass, having a slope angle of 37° have been constructed. One year after planting, artificial rainfall of extremely high intensity was applied to all 6 small models and the role of vetiver canopy and roots in erosion and runoff control was observed. To see the effect of soil texture, one among these 5 models was made with silty sand and others contained sandy silt. The results demonstrated that, for sandy silt, the inclusion of vetiver reduced the soil loss by 94%–97%, and soil detachment rates were lowered by 95%. The average runoff also was reduced by 21%. The canopy cover showed a positive impact on reducing both quantities. An increase in average root diameter from 1.6 to 2.5 mm increases the soil loss due to its negative impact on added cohesion. The added cohesion showed a linearly negative correlation with soil loss. A composite system of vetiver and jute geotextile was most effective in erosion reduction among 4 vegetated models with sandy silt. Under same vetiver planting layout, the grass covered model of silty sand yielded 84% lower erosion and 62.5% lower runoff than the grass covered one with sandy silt. Thus, vetiver was more effective in erosion and runoff reduction for soil with a greater percentage of sand, and soil type dominated the erosion process.     

The atmospheric general circulation: Some unresolved issues

Several questions concerning the general circulation for which satisfactory answers are not yet available are discussed. The focus is on the zonal mean heat balance, since problems in our understanding of this balance are a fundamental limitation on our ability to model climate and climate change. The questions are: How strong is the atmosphere's poleward heat transport? What are the relative roles of large-scale eddies and small-scale convection in stabilizing the mid-latitude atmosphere? What are the dynamical mechanisms that maintain the time mean zonal mean state in mid-latitudes? Some suggestions for addressing these questions are given.     

日地关系研究的一种新观点介绍

介绍了Ｇ．Ｐ．Ｇｒｅｇｏｒｉ提出的关于日地关系的新因果链：太阳→太阳风（及月球引潮力）→地幔内地磁感应々→“发电机”感应电流→焦耳热→火山（及地热流）→水或其它微量元素进入大气圈→控制长期气候变化。对这种因果链提出的背景知识、资料证明及应用前景作了详细说明。介绍了国内一些学者关于日地关系的看法，并将二者加以比较。提出了对日地关系研究的看法及急需解决的问题。