1,327 research outputs found

    Update of Arkansas Odonata (Anisoptera)

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    Seventy-five species of dragonflies have been reported in Arkansas. The present study provides 43 species; records of the other 32 are drawn from the literature and personal communication with individuals. A new species, Gomphus ozarkensis, described from western Arkansas in 1975 on the basis of minimal data emphasizes the need for extensive work in this area. Opportunities to collect and identify additional species are discussed

    Abundance, Diversity and Distribution of Benthic Macro-Invertebrates in the Flat Bayou Drainage Area, Jefferson County, Arkansas

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    The main ditch of Flat Bayou Drainage in north central Jefferson County carries water southward into Plum Bayou which then shortly empties into the Arkansas River. Flat Bayou proper flows northward into Wabbaseka Bayou which in turn flows into the Arkansas River in northeastern Lincoln County. Two sites on the main ditch were sampled for physico-chemical parameters and benthic macroin vertebrates on 9 September, 7 October and 11 November 1978. No visible detrimental effects were attributed to physico-chemical characteristics. Thirty-one below-family taxa and 19 families were found in abundance from zero at one site and date to 3580 per m² at one site and date. Corbicula was by far the most numerous taxon whereas several taxa (e.g. Placobdella, Piscicolarla, Lampsilis, Uniomerus, Campeloma, Berosus and Palpomyia) were represented by a per m² density of 8.6 on one date at one site. Dominance indices were generally greater, and distribution values indicated stronger clumping at Site 1 whereas diversity values were generally greater at Site 2. These indicated the substrate at Site 2 was more suitable for community development

    Solar cycle evolution of the solar wind in three dimensions

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    Measurements of the solar wind speed both in and out of the ecliptic are presented for 1971-82. The speed estimates, which were made with the interplanetary scintillation system at UC San Diego, have been compared to in situ for large, slowly evolving structures, and thus such structures can be studied up to 60 degrees north and south heliographic latitude. Annual average wind speeds are presented versus latitude for an entire solar cycle. Fast wind streams from the poles persisted through declining and low solar activity, but were closed off during four years of high activity. This evolution follows that of the polar coronal holes, as displayed by comparing averaged speed and coronal density over latitude and longitude. The most recent data (1982) show the reestablishment of large tilted polar holes and associated fast streams. Coronal magnetic field data show that the neutral sheet is confined to low latitudes at solar minimum and extends to high latitudes at solar maximum; thus the slow solar wind comes from the same latitude range as that of the neutral sheet

    Fluctuations and Relationships of Selected Physiochemical Parameters in Dardanelle Reservoir, Arkansas, 1975-1982

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    Annual and seasonal fluctuations and relationships are described for discharge, turbidity, chloride, total hardness, conductivity and suspended solids over an eight-year period in Dardanelle Reservoir. The parameters fluctuated rather widely primarily in response to seasonal patterns of rainfall. Chloride and conductivity were related and generally fluctuated together as did turbidity and suspended solids. Hardness appeared to vary independently of the others prior to 1979 then varied more closely with chloride after March 1979. Inherent differences between the Illinois Bayou arm and the main Arkansas River sections complicated the precise identification of any overall impact of power plant operation. No significant long term changes were seen, but chloride declined gradually whereas hardness and conductivity increased slightly. Suspended solids exhibited a significant rise in 1982

    Dragonflies of (Anisoptera) Arkansas

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    Previous publications have recorded 69 species of dragonflies for Arkansas. Three of these are deleted, but state records for 21 new species are reported herein, bringing the list to 87 species. Based on lists from adjacent states, an additional nine species are listed as probably occurring in Arkansas. County records are given for both naiads and adults of each species, as well as first and last capture dates for adults. Specific location and capture date are given for new state records when such data are available. The most species (39) have been reported from Washington County. Twenty-nine counties list from 1-5 species, and six counties list no records

    Phytoplankton Community Structure in Dardanelle Reservoir, Arkansas, 1975-1982

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    Phytoplankton data were collected with standard equipment and procedures over an eight-year period (1975-1982) in Dardanelle Reservoir, Arkansas. Community abundance and diversity at the genus level are described. Sixty-five genera representing 35 families and five divisions were identified. Total phytoplankton abundance and diversity were quite uniform among the stations but fluctuated considerably with time. These fluctuations did not correspond clearly with season. Dominant taxa were seasonal, though, with diatoms being usually dominant in January, April and October, and blue-greens dominant in July. The phytoplankton community structure has not been significantly altered by the operation of ANO Unit I

    Radio Scintillation due to Discontinuities in the Interstellar Plasma Density

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    We develop the theory of interstellar scintillation as caused by an irregular plasma having a power-law spatial density spectrum with a spectral exponent of 4 corresponding to a medium with abrupt changes in its density. An ``outer scale'' is included in the model representing the typical scale over which the density of the medium remains uniform. Such a spectrum could be used to model plasma shock fronts in supernova remnants or other plasma discontinuities. We investigate and develop equations for the decorrelation bandwidth of diffractive scintillations and the refractive scintillation index and compare our results with pulsar measurements. We consider both a medium concentrated in a thin layer and an extended irregular medium. We conclude that the discontinuity model gives satisfactory agreement for many diffractive measurements, in particular the VLBI meaurements of the structure function exponent between 5/3 and 2. However, it gives less satisfactory agreement for the refractive scintillation index than does the Kolmogorov turbulence spectrum. The comparison suggests that the medium consists of a pervasive background distribution of turbulence embedded with randomly placed discrete plasma structures such as shocks or HII regions. This can be modeled by a composite spectrum following the Kolmogorov form at high wavenumbers and steepening at lower wavenumbers corresponding to the typical (inverse) size of the discrete structures. Such a model can also explain the extreme scattering events. However, lines of sight through the enhanced scattering prevalent at low galactic latitudes are accurately described by the Kolmogorov spectrum in an extended medium and do not appear to have a similar low-wavenumber steepening.Comment: Accpeted for ApJ vol 531, March 200
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