The decay of beam-generated Langmuir wave into another Langmuir wave and an ion-acoustic wave is a well-known problem with wide-ranging applications. However, most discussions in the literature are based upon simple one-dimensional approximation. Recently, the present authors carried out a fully self-consistent two-dimensional analysis of the beam-driven Langmuir wave decay problem. The main focus of the present authors' work to date had been on the nonlinear evolution of Langmuir turbulence and its influence on the electrons. Relatively little attention had been paid to the ion-acoustic wave generation. In the present discussion, the focus is placed on the dynamics of ion-acoustic turbulence that results from the decay of beam-generated Langmuir wave. The present analysis considers three electron components, the dense core, a primary beam and a counter-streaming beam. We find that the ion-sound turbulence level sensitively depends on the properties of the counter-streaming beam.

BACKGROUND: The aim of the present study was to evaluate the association among menopause, hormone replacement therapy (HRT), and periodontal attachment loss (PAL). METHODS: Three hundred twenty-eight women (40 to 69 years old) who participated in a previous oral survey in South Brazil were included. Women who reported not having their monthly menstrual cycles for >or=12 months were classified as postmenopausal. Postmenopausal women were categorized according to HRT. Women with >or=30% teeth with PAL >or=5 mm were classified as having periodontitis. The data were analyzed using a multivariable logistic regression model and adjusted for age, smoking, socioeconomic status, and dental care. RESULTS: The prevalence of periodontitis was significantly greater among postmenopausal women not using HRT (HRT-) than among premenopausal women (64.4% versus 46.3%; P = 0.005). Similarly, the multivariable analysis demonstrated a significantly higher chance of having periodontitis in postmenopausal HRT- women than in premenopausal women (odds ratio [OR], 2.1; 95% confidence interval [CI]: 1.1 to 4.0). In contrast, no significant differences were observed in the prevalence of periodontitis between postmenopausal women using HRT (HRT+) and premenopausal women (48.8% versus 46.3%; P = 0.77). After adjusting for cofactors, postmenopausal HRT+ women did not have a greater likelihood of having periodontitis than premenopausal women (OR, 1.2; 95% CI: 0.6 to 2.5). No significant differences in tooth loss were observed among the study groups after adjusting for confounders. CONCLUSIONS: Postmenopausal HRT- women had a greater chance of having periodontitis than premenopausal women. In contrast, postmenopausal HRT+ women and premenopausal women had similar periodontal status. HRT may have a beneficial effect on periodontal health.

We utilize a kinetic approach to the problem of wave propagation in dusty plasmas, taking into account the variation of the charge of the dust particles due to inelastic collisions with electrons and ions. The components of the dielectric tensor are written in terms of a finite and an infinite series, containing all effects of harmonics and Larmor radius. The formulation is quite general and valid for the whole range of frequencies above the plasma frequency of the dust particles, which are assumed motionless. The formulation is employed to the study of electrostatic waves propagating along the direction of the ambient magnetic field, in the case for which ions and electrons are described by bi-Maxwellian distributions. The results obtained in a numerical analysis corroborate previous analysis, about the important role played by the dust charge variation, particularly on the imaginary part of the dispersion relation, and about the very minor role played in the case of electrostatic waves by some additional terms appearing in the components of the dielectric tensor, which are entirely due to the occurrence of the dust charge variation.

The electron distributions detected in the solar wind feature varying degrees of anisotropic high-energy tail. In a recent work the present authors numerically solved the one-dimensional electrostatic weak turbulence equations by assuming that the solar wind electrons are initially composed of thermal core plus field-aligned counterstreaming beams, and demonstrated that a wide variety of asymmetric energetic tail distribution may result. In the present paper, the essential findings in this work are tested by means of full particle-in-cell simulation technique. It is found that the previous results are largely confirmed, thus providing evidence that the paradigm of local electron acceleration to high-energy tail by self-consistently excited Langmuir turbulence may be relevant to the solar wind environment under certain circumstances. However, some discrepancies are found such that the nearly one-sided energetic tail reported in the numerical solution of the weak turbulence kinetic equation is not shown.

A kinetic formulation developed to analyze wave propagation in dusty plasmas, which takes into account the charge variation of the dust particles, is utilized to study the propagation and damping of Alfven waves propagating in oblique directions relative to the ambient magnetic field. A dusty plasma containing spherical and immobile dust grains in a homogeneous ambient magnetic field is considered. The charging process of the dust grains is assumed to be associated with the capture of electrons and ions by the dust particles during inelastic collisions between them and plasma particles. The dispersion relation and the damping rates of obliquely propagating Alfven waves are obtained assuming Maxwellian distributions for electrons and ions in equilibrium. For the numerical analysis of the dispersion relation we use the average values of the inelastic collision frequency as an approximation, instead of the momentum dependent expressions originally derived in the kinetic formulation, and study the modifications which the presence of the dust particles causes in both the propagation and the damping of the Alfven waves. In particular is discussed the competition between the different damping mechanisms, namely, the Landau damping and the damping associated with the dust charge variation, and it is shown that the inelastic collision frequency plays a pivotal role in the magnitude of the damping rates.

Solar wind electrons near 1 AU feature wide-ranging asymmetries in the superthermal tail distribution. Gaelzer et al. (2008) recently demonstrated that a wide variety of asymmetric distributions results if one considers a pair of counterstreaming electron beams interacting with the core solar wind electrons. However, the nonlinear dynamics was investigated under the simplifying assumption of one dimensionality. In the present paper, this problem is revisited by extending the analysis to two dimensions. The classic bump-on-tail instability involves a single electron beam interacting with the background population. The bidirectional or counterstreaming beams excite Langmuir turbulence initially propagating in opposite directions. It is found that the nonlinear mode coupling leads to the redistribution of wave moments along concentric arcs in wave number space, somewhat similar to the earlier findings by Ziebell et al. (2008) in the case of one beam-plasma instability. However, the present result also shows distinctive features. The similarities and differences in the nonlinear wave dynamics are discussed. It is also found that the initial bidirectional beams undergo plateau formation and broadening in perpendicular velocity space. However, the anisotropy persists in the nonlinear stage, implying that an additional pitch angle scattering by transverse electromagnetic fluctuations is necessary in order to bring the system to a truly isotropic state.

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Este artigo pretende chamar a atenção para uma exigência a respeito do bom aluno de política, conforme apresentado por Aristóteles no livro I da Ética Nicomaquéia, a saber, ser educado em bons hábitos. O vicioso não é um bom aluno de política porque não é capaz nem mesmo de reconhecer a validade do discurso proposto por ela e não, simplesmente, porque não é capaz de agir em conformidade com esse discurso, como pretende T. Irwin. Reconhecer um argumento prático como bom é, necessariamente, ser motivado a agir conforme à sua conclusão. Aristóteles exige bons hábitos dos seus alunos porque eles são necessários para compreender praticamente o desenvolvimento dos seus argumentos. É essa a compreensão que Aristóteles pretende que seus alunos tenham do discurso político; é essa a compreensão que o vicioso não é mais capaz de ter.PALAVRAS-CHAVE: Conhecimento. Bons hábitos. Compreensão prática.

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