Chirality, extended MHD statistics and solar wind turbulence

We unite the one-flow-dominated-state (OFDS) argument of \citet{MeyrandGaltierPRL12} with the one-chiral-sector-dominated-state (OCSCS) one \citep[][]{hydrochirality} to form a nonlinear extended-magnetohydrodynamics (XMHD) theory for the solar wind turbulence (SWT), \textbf{ranging from the MHD- to subproton-, and even to subelectron-scale regimes} \citep[modifying the theory of][]{AbdelhamidLingamMahajanAPJ16}. Degenerate chiral states in \citet{MiloshevichLingamMorrisonNJP17}'s XMHD absolute equilibria are exposed with helical representation, to offer the basis of replacing the linear wave (of infinitesimal or arbitrarily finite amplitudes) arguments of previous theories with OCSDS. Possible connection of the OFDS-plus-OCSDS theory with the local minimal-energy/stability principle is also discussed.Comment: uncorrected versio

Statistical mechanics of dd-dimensional flows and cylindrically reduced passive scalars

Statistical properties of $d$-dimensional incompressible flows with and without cylindrical reduction are studied, leading to several explanations and conjectures about turbulent flows and passive scalars, such as the de-correlation between the flow and scalar, reduction of passive scalar intermittency in the bottleneck regime, et al. The absolute-equilibrium analyses assure the correctness of a recent numerical result. It is implied that passive scalar(s) in two-dimensional (2D) space can be fundamentally different to those in $d>2$, concerning the correlations with the flow, which is not considered in the celebrated Kraichnan model. The possibility of genuine inverse transfer to large scales of 2D passive scalar energy, together with the advection energy, is indicated. The compressible situation is also briefly remarked in the end, in particular the absence of density in a nontrivial Casimir which, without boundary contribution, also vanishes for $d=4$.Comment: three figures added for better illustratio

Fast rotating flows in high spatial dimensions

The central result about fast rotating-flow structures is the Taylor-Proudman theorem (TPT) which connects various aspects of the dynamics. Taylor's geometrical proof of TPT is reproduced and extended substantially, with Lie's theory for general frozen-in laws and the consequent generalized invariant circulation theorems, to compressible flows and to $d$-dimensional Euclidean space ($\mathbb{E}^{d}$) with $d\ge 3$. The TPT relatives, the reduced models (with particular interests on passive-scalar problems), the inertial (resonant) waves and the higher-order corrections, are discussed coherently for a comprehensive bird view of rotating flows in high spatial dimensions.Comment: plasmas are neutralized, for the time being, and the bibliography is greatly enriched with in particular many more relevant references of mathematical analyse

Isotropic polarization of compressible flows

The helical absolute equilibrium of a compressible adiabatic flow presents not only the polarization between the two purely helical modes of opposite chiralities but also that between the vortical and acoustic modes, deviating from the equipartition predicted by {\sc Kraichnan, R. H.} [1955 The Journal of the Acoustical Society of America {\bf 27}, 438--441.]. Due to the existence of the acoustic mode, even if all Fourier modes of one chiral sector in the sharpened Helmholtz decomposition [{\sc Moses, H. E.} 1971 SIAM ~(Soc. Ind. Appl. Math.) J. Appl. Math. {\bf 21}, 114--130] are thoroughly truncated, leaving the system with positive definite helicity and energy, negative temperature and the corresponding large-scale concentration of vortical modes are not allowed, unlike the incompressible case.Comment: an Erratum or Clarification is added to a remark on Page

Note on specific chiral ensembles of statistical hydrodynamics: "order function" for transition of turbulence transfer scenarios

Hydrodynamic helicity signatures the parity symmetry breaking, chirality, of the flow. Statistical hydrodynamics thus respect chirality, as symmetry breaking and restoration are key to their fundamentals, such as the spectral transfer direction and its mechanism. Homochiral sub-system of three-dimensional (3D) Navier-Stokes isotropic turbulence has been numerically realized with helical representation technique to present inverse energy cascade [Biferale et al., Phys. Rev. Lett., {\bf 108}, 164501 (2012)]. The situation is analogous to 2D turbulence where inverse energy cascade, or more generally energy-enstrophy dual cascade scenario, was argued with the help of a negative temperature state of the absolute equilibrium by Kraichnan. Indeed, if the helicity in such a system is taken to be positive without loss of generality, a corresponding negative temperature state can be identified [Zhu et al., J. Fluid Mech., {\bf 739}, 479 (2014)]. Here, for some specific chiral ensembles of turbulence, we show with the corresponding absolute equilibria that even if the helicity distribution over wavenumbers is sign definite, different \textit{ansatzes} of the shape function, defined by the ratio between the specific helicity and energy spectra $s(k)=H(k)/E(k)$, imply distinct transfer directions, and we could have inverse-helicity and forward-energy dual transfers (with, say, $s(k)\propto k^{-2}$ resulting in absolute equilibrium modal spectral density of energy $U(k)=\frac{1}{\alpha +\beta k^{-2}}$, exactly the enstrophy one of two-dimensional Euler by Kraichan), simultaneous forward transfers (with $s(k)=constant$), or even no simply-directed transfer (with, say, non-monotonic $s(k) \propto \sin^2k$), besides the inverse-energy and forward-helicity dual transfers (with, say, $s(k)=k$ as in the homochiral case)

Equilibrium and non-equilibrium time-reversible dynamical ensembles relevant to chiral turbulence

Ideas and theories of turbulence based on modifying the Navier-Stokes equation, to obtain equilibrium and non-equilibrium time-reversible dynamical ensembles relevant to helical turbulence, are presented. Discussions of controlling helicity to control the aerodynamic force, heat and noise are presented, together with the compressible turbulence relevant statistical mechanics analysis. A helical time-reversible system for nonequilibrium dynamical ensemble is constructed. Applications are also remarked.Comment: main text in Chinese with English tittle and abstrac

Comment on "Energy Transfer and Dual Cascade in Kinetic Magnetized Plasma Turbulence"

We argue that the constraints on transfers, given in the Letter [G. Plunk and T. Tatsuno, Phys. Rev. Lett. {\bf 106}, 165003 (2011)], but not correctly, do not give the transfer and/or cascade directions which however can be assisted by the absolute equilibria calculated in this Comment, following Kraichnan [R. H. Kraichnan, Phys. Fluids {\bf 102}, 1417 (1967)]. One of the important statements about the transfers with only one or no diagonal component can be shown to be inappropriate according to the fundamental dynamics. Some mathematical mistakes are pointed out.Comment: the typos in some formulae corrected; some rewording (say, acknowledging the Letter's contributions in the beginning of the Comment) mad

On the exact solutions of (magneto)hydrodynamic systems and the superposition principles of nonlinear helical waves

The principles of restricted superposition of circularly polarized arbitrary-amplitude waves for several hydrodynamic type models are illustrated systematically with helical representation in a unified sense. It is shown that the only general modes satisfying arbitrary-amplitude superposition to kill the generic nonlinearity are the mono-wavelength homochiral Beltrami mode and the one-dimensional-two-component stratified vorticity mode, which we call the XYz flow/wave; while, there are other special superposition principles for some specific cases. We try to remark on the possible connections with the geo- and/or astro-physical fluid and magnetohydrodynamic turbulence issues, such as the rotating turbulence, dynamo and solar atmosphere turbulence, especially with the introduction of disorder locally frozen in some (randomly distributed) space-time regions. Recent disagreements about exact solutions of Hall and fully two-fluid magnetohydrodynamics are also settled down by such a treatment. This work complements, by studying the modes which completely kill the triadic interactions or the nonlinearities, previous studies on the thermalization purely from the triadic interactions, and in turn offers alternative perspectives of the nonlinearities

Taming heavy-tailed features by shrinkage

In this work, we focus on a variant of the generalized linear model (GLM) called corrupted GLM (CGLM) with heavy-tailed features and responses. To robustify the statistical inference on this model, we propose to apply $\ell_4$-norm shrinkage to the feature vectors in the low-dimensional regime and apply elementwise shrinkage to them in the high-dimensional regime. Under bounded fourth moment assumptions, we show that the maximum likelihood estimator (MLE) based on the shrunk data enjoys nearly the minimax optimal rate with an exponential deviation bound. Our simulations demonstrate that the proposed feature shrinkage significantly enhances the statistical performance in linear regression and logistic regression on heavy-tailed data. Finally, we apply our shrinkage principle to guard against mislabeling and image noise in the human-written digit recognition problem. We add an $\ell_4$-norm shrinkage layer to the original neural net and reduce the testing misclassification rate by more than $30\%$ relatively in the presence of mislabeling and image noise.Comment: 24 page

Group Variable Selection via a Hierarchical Lasso and Its Oracle Property

In many engineering and scientific applications, prediction variables are grouped, for example, in biological applications where assayed genes or proteins can be grouped by biological roles or biological pathways. Common statistical analysis methods such as ANOVA, factor analysis, and functional modeling with basis sets also exhibit natural variable groupings. Existing successful group variable selection methods such as Antoniadis and Fan (2001), Yuan and Lin (2006) and Zhao, Rocha and Yu (2009) have the limitation of selecting variables in an "all-in-all-out" fashion, i.e., when one variable in a group is selected, all other variables in the same group are also selected. In many real problems, however, we may want to keep the flexibility of selecting variables within a group, such as in gene-set selection. In this paper, we develop a new group variable selection method that not only removes unimportant groups effectively, but also keeps the flexibility of selecting variables within a group. We also show that the new method offers the potential for achieving the theoretical "oracle" property as in Fan and Li (2001) and Fan and Peng (2004).Comment: 43 pages, 2 figure