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P C H Martens – One of the best experts on this subject based on the ideXlab platform.

  • magnetic quenching of turbulent Diffusivity reconciling mixing length theory estimates with kinematic dynamo models of the solar cycle
    The Astrophysical Journal, 2011
    Co-Authors: Andres Munozjaramillo, P C H Martens, Dibyendu Nandy
    Abstract:

    The turbulent magnetic Diffusivity in the solar convection zone is one of the most poorly constrained ingredients of mean-field dynamo models. This lack of constraint has previously led to controversy regarding the most appropriate set of parameters, as different assumptions on the value of turbulent Diffusivity lead to radically different solar cycle predictions. Typically, the dynamo community uses double-step Diffusivity profiles characterized by low values of Diffusivity in the bulk of the convection zone. However, these low Diffusivity values are not consistent with theoretical estimates based on mixing-length theory, which suggest much higher values for turbulent Diffusivity. To make matters worse, kinematic dynamo simulations cannot yield sustainable magnetic cycles using these theoretical estimates. In this work, we show that magnetic cycles become viable if we combine the theoretically estimated Diffusivity profile with magnetic quenching of the Diffusivity. Furthermore, we find that the main features of this solution can be reproduced by a dynamo simulation using a prescribed (kinematic) Diffusivity profile that is based on the spatiotemporal geometric average of the dynamically quenched Diffusivity. This bridges the gap between dynamically quenched and kinematic dynamo models, supporting their usage as viable tools for understanding the solar magnetic cycle.

  • magnetic quenching of turbulent Diffusivity reconciling mixing length theory estimates with kinematic dynamo models of the solar cycle
    arXiv: Solar and Stellar Astrophysics, 2010
    Co-Authors: Andres Munozjaramillo, P C H Martens, Dibyendu Nandy
    Abstract:

    The turbulent magnetic Diffusivity in the solar convection zone is one of the most poorly constrained ingredients of mean-field dynamo models. This lack of constraint has previously led to controversy regarding the most appropriate set of parameters, as different assumptions on the value of turbulent Diffusivity lead to radically different solar cycle predictions. Typically, the dynamo community uses double step Diffusivity profiles characterized by low values of Diffusivity in the bulk of the convection zone. However, these low Diffusivity values are not consistent with theoretical estimates based on mixing-length theory — which suggest much higher values for turbulent Diffusivity. To make matters worse, kinematic dynamo simulations cannot yield sustainable magnetic cycles using these theoretical estimates. In this work we show that magnetic cycles become viable if we combine the theoretically estimated Diffusivity profile with magnetic quenching of the Diffusivity. Furthermore, we find that the main features of this solution can be reproduced by a dynamo simulation using a prescribed (kinematic) Diffusivity profile that is based on the spatiotemporal geometric-average of the dynamically quenched Diffusivity. Here, we provide an analytic fit to the dynamically quenched Diffusivity profile, which can be used in kinematic dynamo simulations. Having successfully reconciled the mixing-length theory estimated Diffusivity profile with kinematic dynamo models, we argue that they remain a viable tool for understanding the solar magnetic cycle.

Andres Munozjaramillo – One of the best experts on this subject based on the ideXlab platform.

  • magnetic quenching of turbulent Diffusivity reconciling mixing length theory estimates with kinematic dynamo models of the solar cycle
    The Astrophysical Journal, 2011
    Co-Authors: Andres Munozjaramillo, P C H Martens, Dibyendu Nandy
    Abstract:

    The turbulent magnetic Diffusivity in the solar convection zone is one of the most poorly constrained ingredients of mean-field dynamo models. This lack of constraint has previously led to controversy regarding the most appropriate set of parameters, as different assumptions on the value of turbulent Diffusivity lead to radically different solar cycle predictions. Typically, the dynamo community uses double-step Diffusivity profiles characterized by low values of Diffusivity in the bulk of the convection zone. However, these low Diffusivity values are not consistent with theoretical estimates based on mixing-length theory, which suggest much higher values for turbulent Diffusivity. To make matters worse, kinematic dynamo simulations cannot yield sustainable magnetic cycles using these theoretical estimates. In this work, we show that magnetic cycles become viable if we combine the theoretically estimated Diffusivity profile with magnetic quenching of the Diffusivity. Furthermore, we find that the main features of this solution can be reproduced by a dynamo simulation using a prescribed (kinematic) Diffusivity profile that is based on the spatiotemporal geometric average of the dynamically quenched Diffusivity. This bridges the gap between dynamically quenched and kinematic dynamo models, supporting their usage as viable tools for understanding the solar magnetic cycle.

  • magnetic quenching of turbulent Diffusivity reconciling mixing length theory estimates with kinematic dynamo models of the solar cycle
    arXiv: Solar and Stellar Astrophysics, 2010
    Co-Authors: Andres Munozjaramillo, P C H Martens, Dibyendu Nandy
    Abstract:

    The turbulent magnetic Diffusivity in the solar convection zone is one of the most poorly constrained ingredients of mean-field dynamo models. This lack of constraint has previously led to controversy regarding the most appropriate set of parameters, as different assumptions on the value of turbulent Diffusivity lead to radically different solar cycle predictions. Typically, the dynamo community uses double step Diffusivity profiles characterized by low values of Diffusivity in the bulk of the convection zone. However, these low Diffusivity values are not consistent with theoretical estimates based on mixing-length theory — which suggest much higher values for turbulent Diffusivity. To make matters worse, kinematic dynamo simulations cannot yield sustainable magnetic cycles using these theoretical estimates. In this work we show that magnetic cycles become viable if we combine the theoretically estimated Diffusivity profile with magnetic quenching of the Diffusivity. Furthermore, we find that the main features of this solution can be reproduced by a dynamo simulation using a prescribed (kinematic) Diffusivity profile that is based on the spatiotemporal geometric-average of the dynamically quenched Diffusivity. Here, we provide an analytic fit to the dynamically quenched Diffusivity profile, which can be used in kinematic dynamo simulations. Having successfully reconciled the mixing-length theory estimated Diffusivity profile with kinematic dynamo models, we argue that they remain a viable tool for understanding the solar magnetic cycle.

Chunsheng Zhou – One of the best experts on this subject based on the ideXlab platform.

  • Unified determination of relative molecular Diffusivity and fluid permeability for partially saturated cement-based materials
    Cement and Concrete Research, 2015
    Co-Authors: Chunsheng Zhou, Wei Chen, Wei Wang, Frédéric Skoczylas
    Abstract:

    From conductivity theory, general models for relative molecular Diffusivity and fluid permeability are first derived with unknown tortuosity function and modification coefficient, which can be respectively deduced from hydraulic Diffusivity and water retention curve (WRC). Based on empirical laws for hydraulic Diffusivity and WRC of cement-based material, unified models for relative molecular Diffusivity and fluid permeability are further formulated with only two measurable parameters. Because of practical difficulty for measuring water permeability and pure gaseous molecular Diffusivity, only relative gas permeability and relative chloride Diffusivity models are verified by the reported data. It is found that the predicted relative gas permeability agrees with measured values and exponential law is a little more preferable than power law for quantifying hydraulic Diffusivity. Moreover, relative chloride Diffusivity from the unified model also agrees well with experimental data derived via Nernst–Einstein Equation. However, the unified model doesn’t capture the possibly overestimated relative chloride Diffusivity from Fick’s law.

  • General solution of hydraulic Diffusivity from sorptivity test
    Cement and Concrete Research, 2014
    Co-Authors: Chunsheng Zhou
    Abstract:

    A general approach to solving hydraulic Diffusivity from sorptivity test is established and verified in this paper. The diffusion equation governing capillary water absorption is first converted into normalized ordinary differential and integral forms via Boltzmann transformation, which are then directly solved by the method of weighted residuals. By this method, the approximate solution of Boltzmann variable is determined for any distribution law of Diffusivity. The relationship between sorptivity and Diffusivity is further analytically established. It’s found that initial Diffusivity is proportional to square of the ratio of sorptivity to the water content difference between saturated and initial states. Ignoring the water vapor diffusion leads to the underestimation of derived water content profile and Diffusivity. The Boltzmann variable and Diffusivity calculated by the proposed method are verified by experimental data. Finally, the relationship between coefficients of general solution for diffusion equation and shape parameter for exponential Diffusivity is also derived.

William M. Mcmahon – One of the best experts on this subject based on the ideXlab platform.

  • Diffusion tensor imaging of white matter in the superior temporal gyrus and temporal stem in autism.
    Neuroscience Letters, 2007
    Co-Authors: Jee Eun Lee, Erin D. Bigler, Andrew L. Alexander, Mariana Lazar, Molly B. Dubray, Moo K. Chung, Michael D. Johnson, Jubel Morgan, Judith Miller, William M. Mcmahon
    Abstract:

    Recent MRI studies have indicated that regions of the temporal lobe including the superior temporal gyrus (STG) and the temporal stem (TS) appear to be abnormal in autism. In this study, diffusion tensor imaging (DTI) measurements of white matter in the STG and the TS were compared in 43 autism and 34 control subjects. DTI measures of mean Diffusivity, fractional anisotropy, axial Diffusivity, and radial Diffusivity were compared between groups. In all regions, fractional anisotropy was significantly decreased and both mean Diffusivity and radial Diffusivity were significantly increased in the autism group. These results suggest that white matter microstructure in autism is abnormal in these temporal lobe regions, which is consistent with theories of aberrant brain connectivity in autism.

Dw Moon – One of the best experts on this subject based on the ideXlab platform.