Perry C. Gray

Phenomenology for the decay of energy-containing eddies in homogeneous MHD turbulence

We evaluate a number of simple, one‐point phenomenological models for the decay of energy‐containing eddies in magnetohydrodynamic (MHD) and hydrodynamic turbulence. The MHD models include effects of cross helicity and Alfvenic couplings associated with a constant mean magnetic field, based on physical effects well‐described in the literature. The analytic structure of three separate MHD models is discussed. The single hydrodynamic model and several MHD models are compared against results from spectral‐method simulations. The hydrodynamic model phenomenology has been previously verified against experiments in wind tunnels, and certain experimentally determined parameters in the model are satisfactorily reproduced by the present simulation. This agreement supports the suitability of our numerical calculations for examining MHD turbulence, where practical difficulties make it more difficult to study physical examples. When the triple‐decorrelation time and effects of spectral anisotropy are properly taken i...

A novel method for preclinical detection of PrPSc in blood.

In this study, we demonstrate that a moderate amount of protein misfolding cyclic amplification (PMCA) coupled to a novel surround optical fibre immunoassay (SOFIA) detection scheme can be used to detect the disease-associated form of the prion protein (PrP(Sc)) in protease-untreated plasma from preclinical and clinical scrapie sheep, and white-tailed deer with chronic wasting disease, following natural and experimental infection. PrP(Sc), resulting from a conformational change of the normal (cellular) form of prion protein (PrP(C)), is considered central to neuropathogenesis and serves as the only reliable molecular marker for prion disease diagnosis. While the highest levels of PrP(Sc) are present in the central nervous system, the development of a reasonable diagnostic assay requires the use of body fluids that characteristically contain exceedingly low levels of PrP(Sc). PrP(Sc) has been detected in the blood of sick animals by means of PMCA technology. However, repeated cycling over several days, which is necessary for PMCA of blood material, has been reported to result in decreased specificity (false positives). To generate an assay for PrP(Sc) in blood that is both highly sensitive and specific, we have utilized limited serial PMCA (sPMCA) with SOFIA. We did not find any enhancement of sPMCA with the addition of polyadenylic acid nor was it necessary to match the genotypes of the PrP(C) and PrP(Sc) sources for efficient amplification.

Scaling of field‐line random walk in model solar wind fluctuations

The diffusion coefficient associated with random walk of magnetic field lines is computed by direct calculation of a large number of field lines from a specified statistical ensemble. Two-component magnetic field models are examined, consisting of a mixture of slab and two-dimensional fluctuations. The scaling of the diffusion coefficient with magnetic field strength is established numerically for a wide range of relative fluctuation amplitudes ( δb/B0). The results confirm the recent nonperturbative theory of field line random walk (Phys. Rev. Lett., 75, 2136, 1995), but depart considerably from the quasilinear B0−2scaling, for models thought to be appropriate to the solar wind.

Simple and specific detection of abnormal prion protein by a magnetic bead-based immunoassay coupled with laser-induced fluorescence spectrofluorometry.

Transmissible spongiform encephalopathies (TSEs), also termed prion diseases, are fatal neurodegenerative conditions that affect both humans and animals. The transmissibility and fatal nature of TSEs necessitate their rapid and accurate diagnosis. Laser-induced fluorescence (LIF) spectrofluorometry is useful for obtaining measurements on fluorescence-labeled targets with a high degree of sensitivity. In the present study, we applied this technology to the immunological detection of abnormal prion protein, PrPSc, which is a universal diagnostic marker for TSEs. The assay format consists of a magnetic bead-based sandwich immunoassay utilizing a biotin-conjugated capture antibody and a fluorophore-labeled detector antibody. By using one pair of anti-PrP monoclonal antibodies (MAbs), PrPSc in brain homogenates from various experimental and natural TSEs can be easily detected with high specificity. Furthermore, the assay proved to be applicable for the detection of PrPSc in the lymph nodes from deer with TSE. The sensitivity of the assay was shown to be comparable to standard immunoblotting, but has several advantages over conventional tests, in terms of flexibility, simplicity, specificity, and run time. These results provide an important basis for the development of an early diagnostic test with potential for multi-sample analysis.

Prion Disease Detection, PMCA Kinetics, and IgG in Urine from Sheep Naturally/Experimentally Infected with Scrapie and Deer with Preclinical/Clinical Chronic Wasting Disease

ABSTRACT Prion diseases, also known as transmissible spongiform encephalopathies, are fatal neurodegenerative disorders. Low levels of infectious agent and limited, infrequent success of disease transmissibility and PrPSc detection have been reported with urine from experimentally infected clinical cervids and rodents. We report the detection of prion disease-associated seeding activity (PASA) in urine from naturally and orally infected sheep with clinical scrapie agent and orally infected preclinical and infected white-tailed deer with clinical chronic wasting disease (CWD). This is the first report on PASA detection of PrPSc from the urine of naturally or preclinical prion-diseased ovine or cervids. Detection was achieved by using the surround optical fiber immunoassay (SOFIA) to measure the products of limited serial protein misfolding cyclic amplification (sPMCA). Conversion of PrPC to PrPSc was not influenced by the presence of poly(A) during sPMCA or by the homogeneity of the PrP genotypes between the PrPC source and urine donor animals. Analysis of the sPMCA-SOFIA data resembled a linear, rather than an exponential, course. Compared to uninfected animals, there was a 2- to 4-log increase of proteinase K-sensitive, light chain immunoglobulin G (IgG) fragments in scrapie-infected sheep but not in infected CWD-infected deer. The higher-than-normal range of IgG levels found in the naturally and experimentally infected clinical scrapie-infected sheep were independent of their genotypes. Although analysis of urine samples throughout the course of infection would be necessary to determine the usefulness of altered IgG levels as a disease biomarker, detection of PrPSc from PASA in urine points to its potential value for antemortem diagnosis of prion diseases.

Surround optical fiber immunoassay (SOFIA): an ultra-sensitive assay for prion protein detection.

We describe the development of a new technology (SOFIA) and demonstrate its utility by establishing a sensitive and specific assay for PrP(Sc). SOFIA is a surround optical fiber immunoassay which is comprised of a set of specific monoclonal antibodies and comprehensive capture of high energy fluorescence emission. In its current format, this system is capable of detecting less than 10 attogram (ag) of hamster, sheep and deer recombinant PrP. Approximately 10 ag of PrP(Sc) from 263 K-infected hamster brains can be detected with similar lower limits of PrP(Sc) detection from the brains of scrapie-infected sheep and deer infected with chronic wasting disease. These detection limits allow protease treated and untreated material to be diluted beyond the point where PrP(C), non-specific proteins or other extraneous material may interfere with PrP(Sc) signal detection and/or specificity. This not only eliminates the issue of specificity of PrP(Sc) detection but also increases sensitivity since the possibility of partial PrP(Sc) proteolysis is no longer a concern. SOFIA will likely lead to early antemortem detection of transmissible encephalopathies and is also amenable for use with additional target amplification protocols. SOFIA represents a sensitive means for detecting specific proteins involved in disease pathogenesis and/or diagnosis that extends beyond the scope of the transmissible spongiform encephalopathies.

Heating of the solar wind by pickup ion driven Alfvén ion cyclotron instability

Pickup ions in a ring velocity distribution are unstable to several kinetic plasma instabilities. At large heliocentric distances where the overall plasma β (ratio of kinetic to magnetic energy) is dominated by the energy density of interstellar pickup ions and pickup is perpendicular to the interplanetary magnetic field, the dominant of these is the Alfven ion cyclotron instability (AIC). We demonstrate by hybrid particle simulation that, for conditions where the solar wind β is low, AIC driven by the pickup ions couples to the solar wind. The result is perpendicular heating, leading to an anisotropic solar wind distribution. This process may contribute to enhanced solar wind temperatures at large heliocentric distances and may allow for indirect measurement of interstellar pickup ions.

Thermal anisotropies in the solar wind: Evidence of heating by interstellar pickup ions?

A recent paper by Gray et al. [1996] shows that the Alfven ion cyclotron instability is generated by newly created pickup ions and heats the thermal solar wind protons in the direction perpendicular to the magnetic field. This instability operates most effectively in regions where the plasma β is low, so this mechanism predicts that the ratio of the temperatures perpendicular and parallel to the magnetic field should be larger in low-β regions of the solar wind. We look for this effect in ISEE-3 and Voyager 2 data. The near-Earth ISEE-3 data show no evidence for greater thermal anisotropies at low β. Voyager 2 data obtained between 1 and 8 AU show that the ambient proton thermal anisotropy is a function of β, with parallel temperatures generally greater than perpendicular temperatures except when β is small. For Voyager 2 data, the average ratio of perpendicular to parallel temperature is about 0.9, but this ratio is ∼2 when β is less than 0.1 and ∼4 when β is less than 0.01.

Observation of Bernstein Waves Excited by Newborn Interstellar Pickup Ions in the Solar Wind

A recent examination of 1.9 s magnetic field data recorded by the Voyager 2 spacecraft in transit to Jupiter revealed several instances of strongly aliased spectra suggestive of unresolved high-frequency magnetic fluctuations at 4.4 AU. A closer examination of these intervals using the highest resolution data available revealed one clear instance of wave activity at spacecraft frame frequencies from 0.2 to 1 Hz. Using various analysis techniques, we have characterized these fluctuations as Bernstein mode waves excited by newborn interstellar pickup ions. We can find no other interpretation or source consistent with the observations, but this interpretation is not without questions. In this paper, we report a detailed analysis of the waves, including their frequency and polarization, that supports our interpretation.

Use of Fluorometry to Differentiate Among Clipped Species in the Genera Astragalus, Oxytropis, and Pleuraphis

Abstract A rapid and reproducible method to determine botanical composition of forage is an ecological and economic goal for range animal ecologists. Multidimensional fluorometry previously demonstrated the possibility of a unique optical approach for accurately determining species composition of clipped and digested plant materials. Fluorometry may be used to detect toxic plants in standing crop as well as diets by using electronic transitions in chemical structures at wavelengths between 370 and 580 nm. Grass hay (genus Pleuraphis) and 6 clipped forbs (4 species of Astragalus and 2 species of Oxytropis) were examined. The resulting spectral signatures were evaluated for differences in the blue and green regions of the visible spectrum using Principal Component Analysis (PCA). This represents the first published data using chemometrics to differentiate among fluorophores from these plant extracts. It was possible to distinguish between the grass and forbs and among forbs. Further research will be required to evaluate these same plant species in mixed diets and fecal samples.