Developing generative themes for community action

This chapter explores a range of participative methods for working with groups to achieve social change in community settings. It draws on the work of Paulo Freire and examines how workers can facilitate processes of dialogue and critical reflection which enable local people to develop solutions to the issues they are dealing with

Parity-violating electron scattering and nucleon structure

The measurement of parity violation in the helicity dependence of electron-nucleon scattering provides unique information about the basic quark structure of the nucleons. This review presents the general formalism of parity-violating electron scattering, with emphasis on elastic electron-nucleon scattering. The physics issues addressed by such experiments are discussed, and the major goals of the presently envisioned experimental program are identified. Results from a recent series of experiments are summarized and the future prospects of this program are discussed

Magnetic field reversals and galactic dynamos

We argue that global magnetic field reversals similar to those observed in the Milky Way occur quite frequently in mean-field galactic dynamo models that have relatively strong, random, seed magnetic fields that are localized in discrete regions. The number of reversals decreases to zero with reduction of the seed strength, efficiency of the galactic dynamo and size of the spots of the seed field. A systematic observational search for magnetic field reversals in a representative sample of spiral galaxies promises to give valuable information concerning seed magnetic fields and, in this way, to clarify the initial stages of galactic magnetic field evolution

The relation between magnetic and material arms in models for spiral galaxies

Context. Observations of polarized radio emission show that large-scale (regular) magnetic fields in spiral galaxies are not axisymmetric, but generally stronger in interarm regions. In some nearby galaxies such as NGC 6946 they are organized in narrow magnetic arms situated between the material spiral arms. Aims. The phenomenon of magnetic arms and their relation to the optical spiral arms (the material arms) call for an explanation in the framework of galactic dynamo theory. Several possibilities have been suggested but are not completely satisfactory; here we attempt a consistent investigation. Methods. We use a 2D mean-field dynamo model in the no-z approximation and add injections of small-scale magnetic field, taken to result from supernova explosions, to represent the effects of dynamo action on smaller scales. This injection of small scale field is situated along the spiral arms, where star-formation mostly occurs. Results. A straightforward explanation of magnetic arms as a result of modulation of the dynamo mechanism by material arms struggles to produce pronounced magnetic arms, at least with realistic parameters, without introducing new effects such as a time lag between Coriolis force and {\alpha}-effect. In contrast, by taking into account explicitly the small-scale magnetic field that is injected into the arms by the action of the star forming regions that are concentrated there, we can obtain dynamo models with magnetic structures of various forms that can be compared with magnetic arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered as coherent magnetic structures generated by large-scale dynamo action, and associated with spatially modulated small-scale magnetic fluctuations, caused by enhanced star formation rates within the material arms.Comment: 13 pages, 8 figures, accepted for publication to A&

Spectroscopy at the solar limb: II. Are spicules heated to coronal temperatures ?

Spicules of the so-called type II were suggested to be relevant for coronal heating because of their ubiquity on the solar surface and their eventual extension into the corona. We investigate whether solar spicules are heated to transition-region or coronal temperatures and reach coronal heights (>6 Mm) using multi-wavelength observations of limb spicules in different chromospheric spectral lines (Ca II H, Hepsilon, Halpha, Ca II IR at 854.2 nm, He I at 1083 nm). We determine the line width of individual spicules and throughout the field of view and estimate the maximal height that different types of off-limb features reach. We derive estimates of the kinetic temperature and the non-thermal velocity from the line width of spectral lines from different chemical elements. We find that most regular spicules reach a maximal height of about 6 Mm above the solar limb. The majority of features found at larger heights are irregularly shaped with a significantly larger lateral extension than spicules. Both individual and average line profiles in all spectral lines show a decrease in their line width with height above the limb with very few exceptions. Both the kinetic temperature and the non-thermal velocity decrease with height above the limb. We find no indications that the spicules in our data reach coronal heights or transition-region or coronal temperatures.Comment: Accepted for publication in Solar Physics, 52 pages, 32 figure

The polarization signature of photospheric magnetic fields in 3D MHD simulations and observations at disk center

Before using 3D MHD simulations of the solar photosphere in the determination of elemental abundances, one has to ensure that the correct amount of magnetic flux is present in the simulations. The presence of magnetic flux modifies the thermal structure of the solar photosphere, which affects abundance determinations and the solar spectral irradiance. We compare the polarization signals in disk-center observations of the solar photosphere in quiet-Sun regions with those in Stokes spectra computed on the basis of 3D MHD simulations having average magnetic flux densities of about 20, 56, 112 and 224 G. This approach allows us to find the simulation run that best matches the observations. The observations were taken with the Hinode SP, TIP, POLIS and the GFPI, respectively. We determine characteristic quantities of full Stokes profiles in a few photospheric spectral lines in the visible (630 nm) and near-infrared (1083 and 1565 nm). We find that the appearance of abnormal granulation in intensity maps of degraded simulations can be traced back to an initially regular granulation pattern with numerous bright points in the intergranular lanes before the spatial degradation. The linear polarization signals in the simulations are almost exclusively related to canopies of strong magnetic flux concentrations and not to transient events of magnetic flux emergence. We find that the average vertical magnetic flux density in the simulation should be less than 50 G to reproduce the observed polarization signals in the quiet Sun internetwork. A value of about 35 G gives the best match across the SP, TIP, POLIS and GFPI observations.Comment: 12 pages, 11 figures; accepted for publication in Ap

Filamentary Diffusion of Cosmic Rays on Small Scales

We investigate the diffusion of cosmic rays (CR) close to their sources. Propagating individual CRs in purely isotropic turbulent magnetic fields with maximal scale of spatial variations Lmax, we find that CRs diffuse anisotropically at distances r <~ Lmax from their sources. As a result, the CR densities around the sources are strongly irregular and show filamentary structures. We determine the transition time t* to standard diffusion as t* ~ 10^4 yr (Lmax/150 pc)^b (E/PeV)^(-g) (Brms/4 muG)^g, with b ~ 2 and g = 0.25-0.5 for a turbulent field with Kolmogorov power spectrum. We calculate the photon emission due to CR interactions with gas and the resulting irregular source images.Comment: 5 pages (2 columns), 4 figures. Published in Physical Review Letter

Aphidophagous Coccinellids in Alfalfa, Small Grains, and Maize in Eastern South Dakota

In a 13-year study of aphidophagous coccinellids associated with alfalfa (Medicago sativa), maize (Zea mays), and small grain crops in eastern South Dakota, the following species were consistently associated with the crops: Hippodamia convergens, H. tredecimpunctata tibialis, H. parenthesis, Coleomegilla maculata lengi, Coccinella transversoguttata richardsoni, Cycloneda munda, and Adalia bipunctata. All species except A. bipunctata were associated with each of the three crops, while A. bipunctata occurred only in maize. Relative abundances of each species varied among crops and among years. Although only seven species were associated with the crops, additional species were captured on sticky traps stationed adjacent to sampled fields. The species diversity of immature coccinellids did not differ among crops but did differ among years. The diversity of adults differed among crops and years. The site from which samples were taken had no influence on the diversity of immatures or adults. Species relative abundances in alfalfa and small grains were more similar to each other than they were to relative abundances in maize