Understanding Helical Magnetic Dynamo Spectra with a Nonlinear Four-Scale Theory

Recent MHD dynamo simulations for magnetic Prandtl number $>1$ demonstrate that when MHD turbulence is forced with sufficient kinetic helicity, the saturated magnetic energy spectrum evolves from having a single peak below the forcing scale to become doubly peaked with one peak at the system (=largest) scale and one at the forcing scale. The system scale field growth is well modeled by a recent nonlinear two-scale nonlinear helical dynamo theory in which the system and forcing scales carry magnetic helicity of opposite sign. But a two-scale theory cannot model the shift of the small-scale peak toward the forcing scale. Here I develop a four-scale helical dynamo theory which shows that the small-scale helical magnetic energy first saturates at very small scales, but then successively saturates at larger values at larger scales, eventually becoming dominated by the forcing scale. The transfer of the small scale peak to the forcing scale is completed by the end of the kinematic growth regime of the large scale field, and does not depend on magnetic Reynolds number $R_M$ for large $R_M$. The four-scale and two-scale theories subsequently evolve almost identically, and both show significant field growth on the system and forcing scales that is independent of $R_M$. In the present approach, the helical and nonhelical parts of the spectrum are largely decoupled. Implications for fractionally helical turbulence are discussed.Comment: 19 Pages, LaTex, (includes 4 figs at the end), in press, MNRA

Current status of turbulent dynamo theory: From large-scale to small-scale dynamos

Several recent advances in turbulent dynamo theory are reviewed. High resolution simulations of small-scale and large-scale dynamo action in periodic domains are compared with each other and contrasted with similar results at low magnetic Prandtl numbers. It is argued that all the different cases show similarities at intermediate length scales. On the other hand, in the presence of helicity of the turbulence, power develops on large scales, which is not present in non-helical small-scale turbulent dynamos. At small length scales, differences occur in connection with the dissipation cutoff scales associated with the respective value of the magnetic Prandtl number. These differences are found to be independent of whether or not there is large-scale dynamo action. However, large-scale dynamos in homogeneous systems are shown to suffer from resistive slow-down even at intermediate length scales. The results from simulations are connected to mean field theory and its applications. Recent work on helicity fluxes to alleviate large-scale dynamo quenching, shear dynamos, nonlocal effects and magnetic structures from strong density stratification are highlighted. Several insights which arise from analytic considerations of small-scale dynamos are discussed.Comment: 36 pages, 11 figures, Spa. Sci. Rev., submitted to the special issue "Magnetism in the Universe" (ed. A. Balogh

Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

Challenges in Developing Recommendations Based on Low-Quality Evidence in Thyroid Guidelines.

Not applicable (editorial)

Expression of biomarkers (p53, transforming growth factor alpha, epidermal growth factor receptor, c-erbB-2/neu and the proliferative cell nuclear antigen) in oropharyngeal squamous cell carcinomas

Using immunohistochemistry, expression of p53, transforming growth factor-alpha (TGF-α), epidermal growth factor receptor (EGFR), c-erbB-2/neu and proliferating cell nuclear antigen (PCNA) was examined in 26 fresh frozen tissue specimens of oropharyngeal squamous cell carcinomas (SCCs). p53 gene mutations were examined by polymerase chain reaction (PCR)/DNA sequencing methods in 22 carcinomas. The findings were examined for correlations with patients’ clinicopathological parameters. Expressions of p53 and PCNA were also examined in 21 formalin-fixed corresponding tissues. Of the fresh frozen tissue specimens, 77% (20/26) showed expression and 68% (15/22) showed mutations (substitutions) of the p53, with significant clustering of the mutations in exons 5 (8/22; 36%), 7 (4/22; 18%) and 8 (5/22; 23%). No mutations were found in exon 6. There was a discordance between expression of p53 protein and mutations of the gene. Parallel to expression and mutations of the p53 found in most of the specimens, expression of TGF-α, EGFR, c-erbB-2/neu and PCNA was found in 88% (22/25), 92% (23/25), 58% (14/24) and 91% (21/23) of the specimens, respectively. For the formalin-fixed tissue specimens, 62% (13/21) and 90% (19/21) expressed p53 and PCNA, respectively. Examining for correlations with patients’ clinicopathological parameters, expression of p53, TGF-α, EGFR and c-erbB-2/neu seemed to negatively correlate with the increase of the tumour grade. The present work suggests that: (1) lack of negative growth regulation due to inactivation of the p53 gene together with activation of other proto-oncogenes are necessary genetic events in the carcinogenesis of oropharyngeal SCCs; (2) in oropharyngeal SCCs, p53 gene mutations were clustered in exons 5 (codons 130–186), 7 (codons 230–248) and 8 (codons 271–282) which perhaps suggests that tobacco carcinogens probably affect the mutational hot spots of the p53 gene at codons 157, 175, 186, 248, 273 and 282; and (3) fresh frozen and formalin-fixed tissue specimens give similar results when an immunohistochemical method is applied. The importance of p53, TGF-α, EGFR, c-erbB-2/neu and PCNA as biomarkers in oropharyngeal SCCs deserves particular attention because it might offer further understanding of the development of these carcinomas