High-Resolution Simulations of Nonhelical MHD Turbulence

According to the kinematic theory of nonhelical dynamo action, the magnetic energy spectrum increases with wavenumber and peaks at the resistive cutoff wavenumber. It has previously been argued that even in the dynamical case, the magnetic energy peaks at the resistive scale. Using high resolution simulations (up to 1024³ meshpoints) with no large-scale imposed field, we show that the magnetic energy peaks at a wavenumber that is independent of the magnetic Reynolds number and about five times larger than the forcing wavenumber. Throughout the inertial range, the spectral magnetic energy exceeds the kinetic energy by a factor of two to three. Both spectra are approximately parallel. The total energy spectrum seems to be close to k ^?3/2, but there is a strong bottleneck effect and we suggest that the asymptotic spectrum is instead k ^?5/3. This is supported by the value of the second-order structure function exponent that is found to be ζ₂ = 0.70, suggesting a k ^?1.70 spectrum. The third-order structure function scaling exponent is very close to unity,—in agreement with Goldreich–Sridhar theory. Adding an imposed field tends to suppress the small-scale magnetic field. We find that at large scales the magnetic energy spectrum then follows a k ^?1 slope. When the strength of the imposed field is of the same order as the dynamo generated field, we find almost equipartition between the magnetic and kinetic energy spectra.     

The helicity constraint in turbulent dynamos with shear

The evolution of magnetic fields is studied using simulations of forced helical turbulence with strong imposed shear. After some initial exponential growth, the magnetic field develops a large-scale travelling wave pattern. The resulting field structure possesses magnetic helicity, which is conserved in a periodic box by the ideal magnetohydrodynamics equations and can hence only change on a resistive time-scale. This strongly constrains the growth time of the large-scale magnetic field, but less strongly constrains the length of the cycle period. Comparing this with the case without shear, the time-scale for large-scale field amplification is shortened by a factor Q , which depends on the relative importance of shear and helical turbulence, and which also controls the ratio of toroidal to poloidal field. The results of the simulations can be reproduced qualitatively and quantitatively with a mean-field α Ω-dynamo model with alpha-effect and turbulent magnetic diffusivity coefficients that are less strongly quenched than in the corresponding α ²-dynamo.     

Greenhouse gas emission mitigation plan for the State of Israel: strategies,incentives and reporting

In the context of the negotiations under the United Nations Framework Convention on Climate Change and its accompanying Kyoto Protocol, participating nations have recognized the need for formulating Nationally Appropriate Mitigation Actions (NAMAs). These NAMAs allow countries to take into account their national circumstances and to construct measures to mitigate GHG emissions across economic sectors. Israel has declared to the UN that it would strive to reduce its GHG emissions by 20% in the year 2020 relative to a ‘business as usual' scenario. With its growing population and an expanding economy, the national GHG mitigation plan was developed to draw a course for steering the Israeli economy into a low-carbon future while accommodating continued economic growth. The article describes relevant policy measures, designed to aid in the implementation of the plan and compares them with measures being undertaken by different countries. Emphasis is placed on analysing the progress to date, opportunities and barriers to attaining the ultimate GHG emissions reduction goals. The objective of this article is to contribute to the knowledge base of effective approaches for GHG emissions reduction. We emphasize the integrated approach of planning and implementation that could be especially useful for developing countries or countries with economies in transition, as well as for developed countries. Yet, in the article we argue that NAMAs’ success hinges on structured tracking of progress according to emerging global consensus standards such as the GHG Protocol Mitigation Goals Standard.

Policy relevance:

The study is consistent with the NAMA concept, enabling a country to adopt a ‘climate action plan’ that contributes to its sustainable development, while enabled by technology and being fiscally sound.

The analysis shows that although NAMAs have been framed in terms of projects, policies, and goals, current methodologies allow only the calculation of emission reductions that can be attributed to distinct projects. Currently, no international guidance exists for quantifying emissions reduction from policy-based NAMAs, making it difficult to track and validate progress. This gap could be addressed by an assessment framework that we have tested, as part of a World Resources Institute pilot study for an emerging voluntary global standard.     

Standardization of baseline and additionality determination under the CDM

Clean Development Mechanism (CDM) project developers have long complained about the complexities of project-specific baseline setting and the vagaries of additionality determination. In response to this, the CDM Executive Board took bold steps towards the standardization of CDM methodologies, culminating in the approval of guidelines for the establishment of performance standards in November 2011. The guidelines specify a performance standard stringency level for both baseline and additionality of 80% for several priority sectors and 90% for all other sectors. However, an analysis of 14 large-scale CDM methodologies that use performance standard approaches challenges this top-down approach to the performance standard design. An appropriate performance standard stringency level strongly depends on sector and technology characteristics. A single stringency level for baseline and additionality determination is appropriate only for greenfield projects, but not for retrofit ones. Overly simple, highly aggregated performance standards are unlikely to ensure high environmental integrity, and difficult questions regarding stringency and updating frequency will eventually have to be addressed on a rather disaggregated level. A careful balance between data requirements and the practicability of performance standards is essential because the heavy data requirements of the existing performance standard methodologies have been the key barrier to their actual implementation.

Policy relevance

CDM regulators have been pushed by many stakeholders to standardize baseline setting and eliminate project-specific additionality determination. At first glance, performance standards seem to provide the perfect solution for both tasks. However, a one-size-fits-all political decision – e.g. the average of the top 20% performers as enshrined in the Marrakech Accords – is inappropriate. Substantial disaggregation of performance standards is required both technologically and geographically in order to limit over- and under-crediting and close loopholes for non-additional projects. As a lack of reliable and complete data has been and will be a key bottleneck for the development of performance standards, international support for data collection will be indispensable, but costly, and time-consuming. Empirically driven, techno-economic assessments of performance standard stringency levels must be the central task of the future work on standardized methodologies, and should not be sidelined by perceived needs of policy makers to take bold decisions under time pressures.     

CCD-DRIFT SCAN MEASUREMENTS OF THE SOLAR DIAMETER: METHOD AND FIRST RESULTS

A new method of measuring the solar diameter, combining the advantages of both photoelectric and visual drift-timing techniques, has been put into operation at Observatorio del Teide/Tenerife: a CCD camera is placed in the focus of the Gregory–Coudé Telescope, and a number of two-dimensional pictures are quickly read out during the passage of each of the two opposite limbs of the Sun across the CCD. Accurate time marks are stored in each frame, and image motion due to the Earth's atmosphere is compensated by fitting the limb positions in each series of frames with respect to time and by reducing them to a common reference position. The first results are described and compared with those of visual measurements.