Analysis of a white-light flare spectrum

The continuum emission of an X₁ flare on 26 March, 1970 observed close to the solar limb (N 05 E 64) was analyzed by a photometric determination of the contrast I()/I ₀() in the wavelength range 3558–5920 Å. Two possible mechanisms for the emission were investigated, namely hydrogen Paschen and H^– continua. We show the unlikeness of the Paschen possibility and derive strong constraints on the temperature structure and energy deposition mechanism imposed by the H^– continuum process.We conclude that the emission must have originated in deep atmospheric layers. The relevance of this result with respect to flare energy release and transport processes is also discussed.     

Flare loops heated by thermal conduction

A flare observed with the Hard X-Ray Imaging Spectrometer (HXIS) was studied during its rise to maximum temperature and X-ray emission rate. Two proximate flare loops, of lengths 2.8 × 10⁹ cm and 1.1 × 10¹⁰ cm, rose to temperatures of 21.5 × 10⁶ K and 30 × 10⁶ K, respectively, in 30 s. Assuming equal heat flux F into each loop from a thermal source at the point where they met, we derive a simple relationship between temperature T and loop length , which gives a loop temperture ratio of 0.68, in close agreement with the observed ratio of 0.72. The observations imply that heating in each loop was maintained by a thermal flux of 5 × 10⁹ ergs cm^-2 s^-1. It is suggested that conductive heating adequately describes the rise and maximum phase emissions in the loops and that long flare loops reach higher temperatures than short loops during the impulsive phase because of an equipartition of energy between them at their point of interaction.     

Microfabrics of calcite ultramylonites as records of coaxial and non-coaxial deformation kinematics: Examples from the Rocher de l'Yret shear zone (Western Alps)

Microfabrics were analysed in calcite mylonites from the rim of the Pelvoux massif (Western Alps, France). WNW-directed emplacement of the internal Penninic units onto the Dauphinois domain produced intense deformation of an Eocene-age nummulitic limestone under lower anchizone metamorphic conditions (slightly below 300 °C). Two types of microfabrics developed primarily by dislocation creep accompanied by dynamic recrystallisation in the absence of twinning. Coaxial kinematics are inferred for samples exhibiting grain shape fabrics and textures with orthorhombic symmetry. Their texture (crystallographic preferred orientation, CPO) is characterised by two c-axis maxima, symmetrically oriented at 15° from the normal to the macroscopic foliation. Non-coaxial deformation is evident in samples with monoclinic shape fabrics and textures characterised by a single oblique c-axis maximum tilted with the sense of shear by about 15°. From the analysis of suitably oriented slip systems for the main texture components under given kinematics it is inferred that the orthorhombic textures, which developed in coaxial kinematics, favour activity of <10–11> and <02–21> slip along the f and r planes, respectively, with minor contributions of basal-<a> slip. In contrast, the monoclinic textures, which developed during simple shear, are most suited for duplex <a> slip along the basal plane. The transition between the dominating slip systems for the orthorhombic and monoclinic microfabrics is interpreted to be due to the effects of dynamic recrystallisation upon texture development. Since oblique c-axis maxima documented in the literature are most often rotated not with but against the shear sense, calcite textures alone should not be used as unequivocal shear sense indicators, but need to be complemented by microstructural criteria such as shape preferred orientations, grain size estimates and amount of twinning.     

Significance of post‐peak metamorphic reaction microstructures in the ultrahigh temperature Eastern Ghats Province,India

Ultrahigh temperature (UHT) granulites in the Eastern Ghats Province (EGP) have a complex P–T–t history. We review the P–T histories of UHT metamorphism in the EGP and use that as a framework for investigating the P–T–t history of Mg–Al‐rich granulites from Anakapalle, with the express purpose of trying to reconcile the down‐pressure‐dominated P–T path with other UHT localities in the EGP. Mafic granulite that is host to Mg–Al‐rich metasedimentary granulites at Anakapalle has a protolith age of c. 1,580 Ma. Mg–Al‐rich metasedimentary granulites within the mafic granulite at Anakapalle were metamorphosed at UHT conditions during tectonism at 960–875 Ma, meaning that the UHT metamorphism was not the result of contact metamorphism from emplacement of the host mafic rock. Reworking occurred during the Pan‐African (c. 600–500 Ma) event, and is interpreted to have produced hydrous assemblages that overprint the post‐peak high‐T retrograde assemblages. In contrast to rocks elsewhere in the EGP that developed post‐peak cordierite, the metasedimentary granulites at Anakapalle developed post‐peak, generation ‘2’ reaction products that are cordierite‐absent and nominally anhydrous. Therefore, rocks at Anakapalle offer the unique opportunity to quantify the pressure drop that occurred during so‐called M2 that affected the EGP. We argue that M2 is either a continuation of M1 and that the overall P–T path shape is a complex counter‐clockwise loop, or that M1 is an up‐temperature counter‐clockwise deviation superimposed on the M2 path. Therefore, rather than the rocks at Anakapalle having a metamorphic history that is apparently anomalous from the rest of the EGP, we interpret that other previously studied localities in the EGP record a different part of the same P–T path history as Anakapalle, but do not preserve a significant record of pressure decrease. This is due either to the inability of refractory rocks to extensively react to produce a rich mineralogical record of pressure decrease, or because the earlier high‐P part of the rocks history was erased by the M1 loop. Irrespective of the specific scenario, models for the tectonic evolution of the EGP must take the substantial pressure decrease during M2 into account, as it is probable the P–T record at Anakapalle is a reflection of tectonics affecting the entire province.     

Fiscal geographies: “Placing” taxation in urban geography

As the state’s primary means of both redistributing wealth and incentivizing private investment, tax plays an outsized role in a range of critical urban processes, including (re)development, gentrification, financialization, and local and regional governance. We argue, through reference to existing literature in urban and economic geography, as well as our own research on taxation and the state, that urban scholarship could benefit by close and careful engagement with taxation and the tax system. We term this new vein of research “fiscal geographies” and see it as offering potential for more nuanced study of urban political economy, politics, and processes.     

Contemporary and palaeo channel patterns and the late quaternary stratigraphy of Cooper Creek,Southwest Queensland,Australia

Braided and anastomosing channels make up two major coexistent networks in the mud-dominated fluvial system of Cooper Creek, Southwest Queensland. The floodplain is characterized by a system of mud braids operative when floods inundate the whole alluvial surface. Anastomosing channels are inset deeper into the floodplain, operate at modern flows, and transport a traction load of sand. Shallow stratigraphic data show that an underlying sand sheet is unrelated to surface channel patterns and was formed by a system of meandering streams. According to preliminary dates based on thermoluminescence, the change from a sand- to mud-dominated fluvial regime took place between 50 000 and 200 000 years B.P., and probably reflects increasing aridity.     

Inverted-polarity electrical structures in thunderstorms in the Severe Thunderstorm Electrification and Precipitation Study (STEPS)

Balloon-borne electric field soundings and lightning mapping data have been analyzed for three of the storms that occurred in the Severe Thunderstorm Electrification and Precipitation Study field program in 2000 to determine if the storms had inverted-polarity electrical structures. The polarities of all or some of the vertically stacked charge regions in such storms are opposite to the polarities observed at comparable heights in normal storms. Analyses compared the charge structures inferred from electric field soundings in the storms with charges inferred from three-dimensional lightning mapping data. Charge structures were inferred from electric field profiles by combining the one-dimensional approximation of Gauss's law with additional information from three-dimensional patterns in the electric field vectors. The three different ways of inferring the charge structure in the storms were found to complement each other and to be consistent overall. Charge deposition by lightning possibly occurred and increased the charge complexity of one of the storms.Many of the cloud flashes in each case were inverted-polarity flashes. Two storms produced ground flash activity comprised predominantly of positive ground flashes. One storm, which was an isolated thunderstorm, produced inverted-polarity cloud flashes, but no flashes to ground. The positive and negative thunderstorm charge regions were found at altitudes where, respectively, negative and positive charge would be found in normal-polarity storms. Thus, we conclude that these storms had anomalous and inverted-polarity electrical structures. Collectively, these three cases (along with the limited cases in the refereed literature) provide additional evidence that thunderstorms can have inverted-polarity electrical structures.     

Use of GIS mapping and modeling approaches to examine the spatial distribution of seagrasses in Barnegat Bay, New Jersey

Due to the ecological importance of seagrasses and recent indications of disease and dieback, we have synthesized existing mapped survey information concerning the spatial and temporal distribution of seagrass beds (primarily eelgrass,Zostera marina) in Barnegat Bay, New Jersey. Mapped surveys from the 1960s, 1970s, 1980s, and 1990s were digitized and compiled in a geographic information system to facilitate analysis. Comparison of the earlier maps with the 1990s survey shows an overall decrease of approximately 2,000 to 3,000 ha in the area of seagrass beds. While there are indications of seagrass decline, due to the great difference in mapping methods used for each of the surveys, we are cautious in directly attributing the decrease in mapped eelgrass acreage to a large-scale dieback. We examined the extent to which light could be used to predict the distribution of seagrass in Barnegat Bay. Data on Secchi depth throughout the bay were combined with a modification of an existing model (Duarte 1991) of the relationship betweenZ. marina compensation depths and light attenuation coefficients to predict the distribution of seagrasses in Barnegat Bay. When compared with mapped seagrass distribution in the bay, the model correctly predicts seagrass presence-absence over two-thirds of the time. The majority of the model error is due to errors of commission, i.e., the model predicts seagrass occurrence where it was not observed to occur. Most of this commission error is located in specific geographic areas (i.e., southern third of Little Egg Harbor and the western shoreline of the bay).     

Magnetic fields,loop prominences and the great flares of August, 1972

Lines of magnetic force, computed under the assumption that the solar corona is free of electric currents, have been compared with loop prominence systems associated with three flares in August, 1972. The computed fields closely match the observations of loops at a height of 40000 km at times 3–4 h after onset of the associated flares. Inferred magnetic field intensities in the loops range from 1300 G where the loops converge into a sunspot to 50–80 G at 40 000 km above the photosphere. The first-seen and lowest-lying loops are sheared with respect to the calculated fields. Higher loops conform more closely to the current-free fieldlines. A model of Barnes and Sturrock is used to relate the degree of shear to the excess magnetic energy available during the flare of August 7. On various lines of evidence, it is suggested that magnetic energy was available to accelerate particles not only during the impulsive phase of the flare, but also during the following 2–3 h. The particle acceleration region seems to be in the magnetic fields just above the visible loops. The bright outer edges of the flare ribbons are identified as particle impact regions. The dense knots of loop prominence material fall to the ribbons' inner edges.On leave from Tel Aviv University, Tel Aviv, Israel.