Collisional and Radiative Processes in Optically Thin Plasmas

Most of our knowledge of the physical processes in distant plasmas is obtained through measurement of the radiation they produce. Here we provide an overview of the main collisional and radiative processes and examples of diagnostics relevant to the microphysical processes in the plasma. Many analyses assume a time-steady plasma with ion populations in equilibrium with the local temperature and Maxwellian distributions of particle velocities, but these assumptions are easily violated in many cases. We consider these departures from equilibrium and possible diagnostics in detail

The Origin, Early Evolution and Predictability of Solar Eruptions

Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt

57Fe MÖSSBAUER EFFECT STUDY OF HYPERFINE INTERACTIONS IN HfFe2-xSix

Les champs hyperfins et le gradient de champ électrique ont été obtenus expérimentalement dans l'intervalle de température de 4.2 K à 700 K, pour le composé intermétallique de Laves HfFe2 et pour les composés pseudo-binaires HfFe2-xSix, avec x = 0,05 ; 0,l et 0,3 en utilisant la spectroscopie Mössbauer du 57Fe. On a rapporté récemment /1/ que la présence du silicium stabilise la phase cubique Cl5 de HfFe2 pour des concentrations 0,05 < x < 0,24. Etant donné que cette stabilisation pourrait être le résultat des perturbations de densité électronique dans la matrice de HfFe2 nous avons fait une étude des données Mössbauer pour déterminer ces quantités sur le site du fer. Les champs hyperfins obtenus à 300 K pour le composé HfFe2 ont été 179 ± 2 kOe et 173 ± 3 kOe avec un rapport de population de 3 : l pour les deux sites non équivalents du point de vue magnétique dans la structure C15. Pour le composé à x = 0,1, trois champs hyperfins ont été mesurés à 4,2 K : 218 ± 3 kOe, 205 ± 3 kOe et 175 ± 5 kOe. Le champ le plus petit est associé aux atomes de fer qui ont un Si comme premier voisin. Les champs hyperfins moyens saturés pour x = 0,05 et 0,l0 sont plus grands que ceux pour HfFe2. A part cela, nos mesures indiquent que la température de transition magnétique est aussi plus haute pour ces alliages que pour HfHe2. Pour x = 0,3 la structure est hexagonale Cl4 et le champ hyperfin ainsi que la température de transition sont plus bas. De plus, les spectres indiquent l'existence d'une distribution de champs hyperfins. Le dédoublement quadrupolaire mesuré au-dessus de la température de Curie est de 0,32 mm/s pour HfFe2 et 0,29 mm/s pour le composé avec x = 0,l. Pour x = 0,3 nous avons observé une diminution du dédoublement. Le déplacement isomérique augmente avec la concentration en Si jusqu'à la concentration maximum étudiée. Nous donnons une interprétation de ces résultats, dans laquelle le Si provoque une réduction de la densité électronique moyenne à travers des effets d'écrantage. A l'appui de cela, on pourrait citer l'augmentation du caractère d localisé pour les fers qui ne sont pas proches voisins de Si et la réduction de la densité moyenne d'électrons s indiquée par les valeurs du déplacement isomérique. Cet écrantage peut inclure des effets sur les électrons de bande et aussi bien que sur ceux qui sont localisés dans les proches voisins des atomes de silicium.Hyperfine fields and the electric field gradient (EFG) were experimentally obtained, in temperature interval of 4.2 K to 700 K for the Laves phase intermetallic compound HfFe2 and for the pseudo binary compounds HfFe2-xSi with x = 0.05, 0.1 and 0.3 using 57Fe Mössbauer effect spectroscopy. As was recently reported /1/, the addition of Si results in the stabilization of the cubic Cl5 phase of HfFe2 for concentrations 0.05 < x < 0.24. Since this stabilization could be the result of the charge and spin density perturbations by Si in the HfFe2 matrix, an analysis of the 57Fe Mössbauer data was made to determine these quantities at the Fe site. The magnetic hyperfine fields obtained at 300 K for the compound HfFe2 were 179 ± 2 kOe and 173 ± 3 kOe with a population ratio of 3 : l for the two magnetically non-equivalent sites in the Cl5 structure. For the compound x = 0.1, three hyperfine fields were measured at 4.2 K : 218 ± 3 kOe, 205 ± 3 kOe and 175 ± 5 kOe. The smallest field is associated with iron atoms that have Si as a first nearest neighbor. The mean saturated hyperfine fields for both x : 0.05 and 0.10 are larger than for HfFe2. Furthermore our measurements indicate that the magnetic transition temperature is also larger for these alloys than for HfFe2. For x = 0.3, the structure is hexagonal Cl4 and both the hyperfine field and transition temperature decrease. In addition, the spectra show evidences of a distribution of hyperfine fields. The quadrupole splitting measured above the Curie temperature was 0.32 mm/s for HfFe2 increasing to 0.38 mm/s for the compound with x = 0.1. For x = 0.3, a decrease in the splitting was observed. No direct evidence for more than one quadrupole interaction was seen for any of the compounds studied. In addition the isomer shift increases with increasing Si content up to the maximum concentration studied. An interpretation of these results is given in which the Si provokes an overall reduction in the mean electronic density through shielding effects. This would be supported by the increase in localized d character on those Fe which are not nearest neighbors of Si and the reduction of the average s-electron density indicated by the isomer shift results. The exact nature of the shielding may involve band electrons and localized ones on Fe which are nearest neighbors of Si

Feeding associations between capybaras Hydrochoerus hydrochaeris (Linnaeus) (Mammalia, Hydrochaeridae) and birds in the Lami Biological Reserve, Porto Alegre, Rio Grande do Sul, Brazil

Feeding associations between capybaras Hydrochoerus hydrochaeris (Linnaeus, 1766) and some bird species were registered in the Lami Biological Reserve, southern Brazil, through observations in a set of transects established in the five major vegetation types of the study area: shrubby and herbaceous swamps, wet grasslands, sandy grasslands and forests. Data included: date and time, vegetation type, bird species, number of individuals (birds and capybaras), type of prey consumed, foraging strategy of the birds and the behavior of the capybaras in relation to the presence of birds. Five species of birds were registered: Caracara plancus (Miller, 1777), Furnarius rufus (Gmelin, 1788), Machetornis rixosus (Vieillot, 1819), Milvago chimachima (Vieillot, 1816) and Molothrus bonariensis (Gmelin, 1789). The interactions were observed in the shrubby swamp (M. bonariensis), forest (C. plancus) and wet grassland (F. rufus, M. rixosus, M. chimachima). The foraging strategies were: (1) use of the capybara as a perch, hunting from its back (M. rixosus, M. bonariensis); (2) use of the capybara as a beater, hunting in the ground (F. rufus, M. rixosus, M. bonariensis); (3) foraging in the skin of the capybara, by picking the ectoparasites (C. plancus, F. rufus, M. chimachima). Strategies (1) and (2) were employed to catch arthropods flushed from the vegetation. Sometimes, capybaras lay down and exposed the abdomen and lateral areas of their bodies to facilitate cleaning by M. chimachima, but the presence of other bird species seemed to be neutral to capybaras.<br>Foram registradas associações alimentares entre capivaras Hydrochoerus hydrochaeris (Linnaeus, 1766) e aves na Reserva Biológica do Lami, sul do Brasil, por meio de observações em um conjunto de transecções estabelecidas nos cinco principais tipos de vegetação existentes na área: banhado arbustivo, banhado herbáceo, campo úmido, campo arenoso e mata. As informações coletadas foram: data, horário, tipo de vegetação, espécie de ave, número de indivíduos (para aves e capivaras), tipo de presa consumida e estratégia de forrageamento das aves e comportamento das capivaras em relação à presença das aves. Cinco espécies de aves foram registradas: Caracara plancus (Miller, 1777), Furnarius rufus (Gmelin, 1788), Machetornis rixosus (Vieillot, 1819), Milvago chimachima (Vieillot, 1816) and Molothrus bonariensis (Gmelin, 1789). As interações foram observadas no banhado arbustivo (M. bonariensis), na mata (C. plancus) e no campo úmido (F. rufus, M. rixosus, M. chimachima). As estratégias de forrageamento registradas foram: (1) uso da capivara como "poleiro"; (2) uso da capivara como "batedor"; (3) forrageio no pêlo da capivara, para captura de ectoparasitas. As estratégias (1) e (2) foram empregadas pelas aves para capturar artrópodos que saltavam da vegetação pela movimentação das capivaras. Foi observado que as capivaras, em algumas ocasiões, apresentavam comportamento de exposição do abdômen e partes laterais do corpo para facilitar a retirada de ectoparasitas por M. chimachima. A presença das demais espécies de aves não interferiu no comportamento das capivaras