A simple model of the effect of the Kerguelen Plateau on the strength of the Antarctic Circumpolar Current

The strong westerly winds of the Southern Ocean drive a northwards Ekman transport which increases in magnitude from 9 Sv, at the southern latitudes of Drake passage, to a maximum of 37 Sv at 45°S. The return flow can occur as a western boundary current along the coasts of South America and the Antarctic Peninsula but at depths above 3000m such a boundary current cannot cross Drake Passage and so another mechanism must be involved. In this paper it is shown that one possible mechanism is for the flow to continue south as a western boundary current attached to the islands and other topographic barriers, which span the latitudes of the passage. A simple model of this process shows that it also generates a strong circumpolar current. The model is then applied to the Southern Ocean using the Kerguelen Plateau as the second topographic barrier. The Kerguelen Plateau occludes the Drake Passage between 1600m and 3000m depths which other model studies have shown to include the Ekman return flow. Using Hellerman and Rosenstein estimates of the wind stress, the model predicts an Antarctic Circumpolar Current with a transport of 151 Sv. This is in reasonable agreement with observations and other model studies. The model does less well south of the Kerguelen Plateau, where it predicts a second strong current. In practice such a current is not observed

Polymer photonic crystal fibre for sensor applications

Polymer photonic crystal fibres combine two relatively recent developments in fibre technology. On the one hand, polymer optical fibre has very different physical and chemical properties to silica. In particular, polymer fibre has a much smaller Young's modulus than silica, can survive higher strains, is amenable to organic chemical processing and, depending on the constituent polymer, may absorb water. All of these features can be utilised to extend the range of applications of optical fibre sensors. On the other hand, the photonic crystal - or microstructured - geometry also offers advantages: flexibility in the fibre design including control of the dispersion properties of core and cladding modes, the possibility of introducing minute quantities of analyte directly into the electric field of the guided light and enhanced pressure sensitivity. When brought together these two technologies provide interesting possibilities for fibre sensors, particularly when combined with fibre Bragg or long period gratings. This paper discusses the features of polymer photonic crystal fibre relevant to sensing and provides examples of the applications demonstrated to date

Polymer optical fiber grating as water activity sensor

Controlling the water content within a product has long been required in the chemical processing, agriculture, food storage, paper manufacturing, semiconductor, pharmaceutical and fuel industries. The limitations of water content measurement as an indicator of safety and quality are attributed to differences in the strength with which water associates with other components in the product. Water activity indicates how tightly water is "bound," structurally or chemically, in products. Water absorption introduces changes in the volume and refractive index of poly(methyl methacrylate) PMMA. Therefore for a grating made in PMMA based optical fiber, its wavelength is an indicator of water absorption and PMMA thus can be used as a water activity sensor. In this work we have investigated the performance of a PMMA based optical fiber grating as a water activity sensor in sugar solution, saline solution and Jet A-1 aviation fuel. Samples of sugar solution with sugar concentration from 0 to 8%, saline solution with concentration from 0 to 22%, and dried (10ppm), ambient (39ppm) and wet (68ppm) aviation fuels were used in experiments. The corresponding water activities are measured as 1.0 to 0.99 for sugar solution, 1.0 to 0.86 for saline solution, and 0.15, 0.57 and 1.0 for the aviation fuel samples. The water content in the measured samples ranges from 100% (pure water) to 10 ppm (dried aviation fuel). The PMMA based optical fiber grating exhibits good sensitivity and consistent response, and Bragg wavelength shifts as large as 3.4 nm when the sensor is transferred from dry fuel to wet fuel

Optimisation of polymer optical fibre based interferometric sensors

A numerical model for studying the performance of polymer optical fibre-based interferometric sensors is presented. The strain sensitivity of Fabry-Perot and two-beam interferometric sensors is investigated by varying the physical and optical properties corresponding to frequently used wavelengths. The developed model was used to identify the regimes in which these devices offer enhanced performance over their silica counterparts when used for stress sensing. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Percent grade scale amplifies racial or ethnic inequities in introductory physics

In previous work we analyzed databases for 95 classes to show that the percent grade scale was correlated with a much higher student fail rate than the 4.0 grade scale. This paper builds on this work and investigates equity gaps occurring under both scales. By employing a “course deficit model” we attribute the responsibility for closing the gaps to those who are responsible for the policies that guide the course. When comparing course grades in classes graded using the percent scale with those in courses graded using the 4.0 scale, we find that students identifying as belonging to racial or ethnic minorities underrepresented in physics suffer a grade penalty under both grade scales but suffer an extra penalty under percent scale graded courses. We then use the fraction of A grades each student earns on individual exam items as a proxy for the instructor’s perception of each student’s understanding of the course material to control for student understanding and find that the extra grade penalty students from groups underrepresented in physics students suffer under percent scale grading is independent of the student’s understanding of physics. When we control for more student level variables to determine the source of the grade scale dependent penalty, we find that it is primarily the low F grades (partial credit scores) on exam problems that are the source of these inequities. We present an argument that switching from percent scale grading to a 4.0 grade scale (or similar grades scale) could reduce equity gaps by 20%–25% without making any other course changes or controlling for any incoming differences between students

Nonlocal ultrafast demagnetization dynamics of Co/Pt multilayers by optical field enhancement

The influence on ultrafast demagnetization dynamics of metallic nano-structured gratings deposited on thin films of magnetic Co/Pt multilayers is investigated by the time-resolved optical Kerr effect. Depending on the polarization of the pump pulse, a pronounced enhancement of the demagnetization amplitude is found. Calculation of the inhomogeneous optical field distribution due to plasmon interaction and time-dependent solutions of the coupled electron, lattice, and spin temperatures in two dimensions show good agreement with the experimental data, as well as giving evidence of non-local demagnetization dynamics due to electron diffusion.BMBF, 05K10KTB, Verbundprojekt: FSP 301 - FLASH: Nanoskopische Systeme. Teilprojekt 1.1: Universelle Experimentierkammer für Streuexperimente mit kohärenten Femtosekunden-Röntgenpulsen Multi Purpose Coherent Scattering Chamber for FLASH and XFEL 'MPscatt

Gender-grade-gap zeroed out under a specific intro-physics assessment regime

Evidence is presented that offering introductory physics courses with an explicit focus on mastery can reduce the gender gap to zero. Taken together with a previous study showing that a concepts-first course may zero out another demographic gap leads one to speculate that demographic grade gaps in introductory physics are just artifacts of the design of the courses and that none of these classes/grades should be assumed to be demographically neutral