MHD wave propagation from the sub-photosphere to the corona in an arcade-shaped magnetic field with a null point

The aim of this work is to study the energy transport by means of MHD waves propagating in quiet Sun magnetic topology from layers below the surface to the corona. Upward propagating waves find obstacles, such as the equipartition layer with plasma b=1 and the transition region, and get converted, reflected and refracted. Understanding the mechanisms by which MHD waves can reach the corona can give us information about the solar atmosphere and the magnetic structures. We carry out two-dimensional numerical simulations of wave propagation in a magnetic field structure that consists of two vertical flux tubes separated by an arcade shaped magnetic field. This configuration contains a null point in the corona, that significantly modifies the behaviour of the waves. We describe in detail the wave propagation through the atmosphere under different driving conditions. We also present the spatial distribution of the mean acoustic and magnetic energy fluxes and the spatial distribution of the dominant frequencies in the whole domain. We conclude that the energy reaches the corona preferably along vertical magnetic fields, inside the flux tubes, and it has an acoustic nature. Most of the magnetic energy keeps concentrated below the transition region due to the refraction of the magnetic waves and the continuous conversion of acoustic-like waves into fast magnetic waves in the equipartition layer located in the photosphere. However, part of the magnetic energy reaches the low corona when propagating in the region where the arcades are located, but waves are sent back downwards to the lower atmosphere at the null point surroundings. This phenomenon, together with the reflection and refraction of waves in the TR and the lower turning point, act as a re-feeding of the atmosphere. In the frequency distribution, we find that high frequency waves can reach the corona outside the vertical flux tubes.Comment: 13 pages, 13 figure

Numerical simulations of conversion to Alfven waves in sunspots

We study the conversion of fast magneto-acoustic waves to Alfven waves by means of 2.5D numerical simulations in a sunspot-like magnetic configuration. A fast, essentially acoustic, wave of a given frequency and wave number is generated below the surface and propagates upward though the Alfven/acoustic equipartition layer where it splits into upgoing slow (acoustic) and fast (magnetic) waves. The fast wave quickly reflects off the steep Alfven speed gradient, but around and above this reflection height it partially converts to Alfven waves, depending on the local relative inclinations of the background magnetic field and the wavevector. To measure the efficiency of this conversion to Alfven waves we calculate acoustic and magnetic energy fluxes. The particular amplitude and phase relations between the magnetic field and velocity oscillations help us to demonstrate that the waves produced are indeed Alfven waves. We find that the conversion to Alfven waves is particularly important for strongly inclined fields like those existing in sunspot penumbrae. Equally important is the magnetic field orientation with respect to the vertical plane of wave propagation, which we refer to as "field azimuth". For field azimuth less than 90 degrees the generated Alfven waves continue upwards, but above 90 degrees downgoing Alfven waves are preferentially produced. This yields negative Alfven energy flux for azimuths between 90 and 180 degrees. Alfven energy fluxes may be comparable to or exceed acoustic fluxes, depending upon geometry, though computational exigencies limit their magnitude in our simulations.Comment: Accepted for publication in Ap

Keys to unblocking multilateral nuclear arms control

Includes bibliographical references (p. 9.) "July 2002"This article outlines the activities that must be undertaken and the issues that must be addressed in order for multilateral arms control to succeed in the 21st century, given the current US opposition to most international agreements.unpublishednot peer reviewe

Half-Remembrance of Things Past: Critics and Cuts of Old

[No abstract][No keywords]

Israel/Palestine/Lebanon: Climbing Out of the Abyss

The Middle East is immersed in its worst crisis in years following the capture of three Israeli soldiers by the Palestinian Islamic Resistance Movement (Hamas) and Lebanese Party of God (Hizbollah) in late June 2006 and early July, Israel's comprehensive offensive throughout the Gaza Strip and Lebanon, and the daily firing of rockets deep into Israel. And horrific as it is, the current toll of death and destruction could reach entirely different proportions should a new threshold be crossed -- a Hizbollah rocket that strikes a chemical plant or a heavily populated area in Tel Aviv or Haifa, an Israeli bombing raid resulting in massive casualties, a major ground offensive, or the expansion of the war to Syria or Iran. A political solution to the twin crises of Lebanon and Palestine must be the international community's urgent priority. Waiting and hoping for military action to achieve its purported goals will have not only devastating humanitarian consequences: it will make it much harder to pick up the political pieces when the guns fall silent. This report is also available in Arabic at http://www.crisisgroup.org/home/index.cfm?id=4282&l=6

No Peace in the House: Witchcraft Accusations as an Old Woman\u27s Problem in Ghana

In Ghana, older women may be marginalized, abused, and even killed as witches. Media accounts imply this is common practice, mainly through stories of “witches camps” to which the accused may flee. Anthropological literature on aging and on witchcraft, however, suggests that this focus exaggerates and misinterprets the problem. This article presents a literature review and exploratory data on elder advocacy and rights intervention on behalf of accused witches in Ghana to help answer the question of how witchcraft accusations become an older woman’s problem in the context of aging and elder advocacy work. The ineffectiveness of rights based and formal intervention through sponsored education programs and development projects is contrasted with the benefit of informal conflict resolution by family and staff of advocacy organizations. Data are based on ethnographic research in Ghana on a rights based program addressing witchcraft accusations by a national elder advocacy organization and on rights based intervention in three witches camps

The N-K Problem in Power Grids: New Models, Formulations and Numerical Experiments (extended version)

Given a power grid modeled by a network together with equations describing the power flows, power generation and consumption, and the laws of physics, the so-called N-k problem asks whether there exists a set of k or fewer arcs whose removal will cause the system to fail. The case where k is small is of practical interest. We present theoretical and computational results involving a mixed-integer model and a continuous nonlinear model related to this question.Comment: 40 pages 3 figure