Double layers on auroral field lines

Time-stationary solutions to the Vlasov-Poisson equation for ion holes and double layers were examined along with particle simulations which pertain to recent observations of small amplitude (e phi)/t sub e approx. 1 electric field structures on auroral field lines. Both the time-stationary analysis and the simulations suggest that double layers evolve from holes in ion phase space when their amplitude reaches (e phi)/t sub e approx. 1. Multiple small amplitude double layers which are seen in long simulation systems and are seen to propagate past spacecraft may account for the acceleration of plasma sheet electrons to produce the discrete aurora

ALS Resistant Smooth Pigweed in Western Kentucky

Pigweeds The pigweed, or Amaranthus, family contains some of the most commonly occurring weeds of midwest agriculture. Species from this family that occur in Kentucky include smooth pigweed (Amaranthus hybridus, most common), tumble pigweed, prostrate pigweed, spiny amaranth,Palmer amaranth,common waterhemp, and tall waterhemp. Research has shown that some pigweed species respond differently to various herbicides, therefore, proper identification is necessary to achieve acceptable control. Pigweed identification in early stages of seedling growth can be difficult because the distinguishing physical characteristics do not appear until plants are mature or have produced seed. Also, some pigweed species may cross-pollinate to produce hybrid plants that exhibit characteristics of both parents

Kinetics and Mechanisms of Atrazine Adsorption and Desorption in Soils Under No-Till and Conventional Management

Both soils (Maury silt loam and Sadler) exhibited three apparent mechanisms of atrazine adsorption. The first two mechanisms were very rapid (10 minutes) and were assigned to soil-clay surface adsorption reactions via hydrogen bonding. The quantity of atrazine involved in these two reactions for the 0.5 mg/1 solution atrazine varied, depending on the soil, from 67 μg/100 g clay to 219 μg/100 g clay. The reason there were two possible atrazine sinks in this range of atrazine adsorption was believed to be the presence of two types of reactive surfaces, the clay inorganic phase and the organic carbon phase. The latter phase exhibited more influence on the Maury silt loam soil than on the Sadler soil, where the Maury silt loam soil contained more organic carbon than the Sadler soil. The third mechanism involved an atrazine condensation mechanism. It was a relatively slow reaction and it appeared to persist for at least 2 hours. This mechanism accounted for about three fourths of the total atrazine adsorbed. After 75 minutes of solution flow the total atrazine adsorbed by the soil clay samples varied from 333 μg/100 g to 710 μg/100 g. Reversibility of the adsorption process was shown to be limited. Approximately one-third of the adsorbed atrazine was desorbed after a 2 hour leaching with l mmol L-1 CaCl2 solution. The desorption process was shown to be controlled by two types of reactions. A short rapid one and a long extremely slow one (diffusion controlled). The above findings suggest that the amount of atrazine leaching into surface water or groundwater would depend on the amount of time atrazine had to react with the soil. If it rained immediately following atrazine application then most of the atrazine would be carried in the runoff, making water the main mechanism of atrazine movement. If, on the other hand, a significant amount of time passed after atrazine was applied then a much smaller proportion of the applied atrazine would be leached, making soil erosion the main mechanism of atrazine movement. Equations for all these processes have been developed to aid in modeling the movement of atrazine during rain fall events

Quaestor: Query web caching for database-as-a-service providers

Today, web performance is primarily governed by round-trip latencies between end devices and cloud services. To improve performance, services need to minimize the delay of accessing data. In this paper, we propose a novel approach to low latency that relies on existing content delivery and web caching infrastructure. The main idea is to enable application-independent caching of query results and records with tunable consistency guarantees, in particular bounded staleness. Q uaestor (Query Store) employs two key concepts to incorporate both expiration-based and invalidation-based web caches: (1) an Expiring Bloom Filter data structure to indicate potentially stale data, and (2) statistically derived cache expiration times to maximize cache hit rates. Through a distributed query invalidation pipeline, changes to cached query results are detected in real-time. The proposed caching algorithms offer a new means for data-centric cloud services to trade latency against staleness bounds, e.g. in a database-as-a-service. Q uaestor is the core technology of the backend-as-a-service platform Baqend, a cloud service for low-latency websites. We provide empirical evidence for Q uaestor 's scalability and performance through both simulation and experiments. The results indicate that for read-heavy workloads, up to tenfold speed-ups can be achieved through Q uaestor 's caching. </jats:p

Exotic Spaces in Quantum Gravity I: Euclidean Quantum Gravity in Seven Dimensions

It is well known that in four or more dimensions, there exist exotic manifolds; manifolds that are homeomorphic but not diffeomorphic to each other. More precisely, exotic manifolds are the same topological manifold but have inequivalent differentiable structures. This situation is in contrast to the uniqueness of the differentiable structure on topological manifolds in one, two and three dimensions. As exotic manifolds are not diffeomorphic, one can argue that quantum amplitudes for gravity formulated as functional integrals should include a sum over not only physically distinct geometries and topologies but also inequivalent differentiable structures. But can the inclusion of exotic manifolds in such sums make a significant contribution to these quantum amplitudes? This paper will demonstrate that it will. Simply connected exotic Einstein manifolds with positive curvature exist in seven dimensions. Their metrics are found numerically; they are shown to have volumes of the same order of magnitude. Their contribution to the semiclassical evaluation of the partition function for Euclidean quantum gravity in seven dimensions is evaluated and found to be nontrivial. Consequently, inequivalent differentiable structures should be included in the formulation of sums over histories for quantum gravity.Comment: AmsTex, 23 pages 5 eps figures; replaced figures with ones which are hopefully viewable in pdf forma

X-ray Halos and Large Grains in the Diffuse Interstellar Medium

Recent observations with dust detectors on board the interplanetary spacecraft Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 mu entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by a a^-3.5 power law in grain radii a, and extends to a maximum grain radius of 0.25 mu. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size, and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to investigate whether this population of larger dust particles prevails in the general interstellar medium, or if it is merely a local phenomenon. The presence of large interstellar grains can be mainly inferred from their effect on the intensity and radial profiles of scattering halos around X-ray sources. In this paper we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study confirm the need to extend the interstellar grain size distribution in the direction of this source to and possibly beyond 2.0 mu. The model that gives the best fit to the halo data is characterized by: (1) a grain size distribution that follows an a^-3.5 power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0 mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundances constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.Comment: 17 pages, incl. 1 figure, accepted for publ. by ApJ Letter

Dust in the Local Interstellar Wind

The gas-to-dust mass ratios found for interstellar dust within the Solar System, versus values determined astronomically for the cloud around the Solar System, suggest that large and small interstellar grains have separate histories, and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, $R_{g/d} = 94^{+46}_{-38}$. Hubble observations of the cloud surrounding the solar system yield a gas-to-dust mass ratio of Rg/d=551+61-251 when B-star reference abundances are assumed. The exclusion of small dust grains from the heliosheath and heliosphere regions are modeled, increasing the discrepancy between interstellar and in situ observations. The shock destruction of interstellar grains is considered, and comparisons are made with interplanetary and presolar dust grains.Comment: 87 pages, 9 figures, 6 tables, accepted for publication in Astrophysical Journal. Uses AASTe

Thermodynamics and Stability of Flat Anti-de Sitter Black Strings

We examine the thermodynamics and stability of 5-dimensional flat anti-de Sitter (AdS) black strings, locally asymptotically anti-de Sitter spacetimes whose spatial sections are AdS black holes with Ricci flat horizons. We find that there is a phase transition for the flat AdS black string when the AdS soliton string is chosen as the thermal background. We find that this bulk phase transition corresponds to a 4-dimensional flat AdS black hole to AdS soliton phase transition on the boundary Karch-Randall branes. We compute the possibility of a phase transition from a flat AdS black string to a 5-dimensional AdS soliton and show that, though possible for certain thin black strings, the transition to the AdS soliton string is preferred. In contrast to the case of the Schwarzschild-AdS black string, we find that the specific heat of the flat AdS black string is always positive; hence it is thermodynamically stable. We show numerically that both the flat AdS black string and AdS soliton string are free of a Gregory-Laflamme instability for all values of the mass parameter. Therefore the Gubser-Mitra conjecture holds for these spacetimes.Comment: 14 pages, 1 figur

The Ammount of Interstellar Carbon Locked in Solid Hydrogenated Amorphous Carbon

We review the literature and present new experimental data to determine the amount of carbon likely to be locked in form of solid hydrogenated amorphous carbon (HAC) grains. We conclude on the basis of a thorough analysis of the intrinsic strength of the C-H stretching band at 3.4 micron that between 10 and 80 ppM H of carbon is in the form of HAC grains. We show that it is necessary to know the level of hydrogenation (H/C) of the interstellar HAC to determine more precisely the amount of carbon it ties up. We present optical constants, photoluminescence spectroscopy, and IR absorption spectroscopy for a particular HAC sample that is shown to have a 3.4 micron absorption feature that is quantatively consistent with that observed in the diffuse interstellar medium.Comment: This paper is 14 pages long with 5 figures and will appear in the 1 December 1999 issue of Ap