Constraints on dynamo action in plasmas

Upper bounds are derived on the amount of magnetic energy that can be generated by dynamo action in collisional and collisionless plasmas with and without external forcing. A hierarchy of mathematical descriptions is considered for the plasma dynamics: ideal MHD, visco-resistive MHD, the double-adiabatic theory of Chew, Goldberger and Low (CGL), kinetic MHD, and other kinetic models. It is found that dynamo action is greatly constrained in models where the magnetic moment of any particle species is conserved. In the absence of external forcing, the magnetic energy then remains small at all times if it is small in the initial state. In other words, a small "seed" magnetic field cannot be amplified significantly, regardless of the nature of flow, as long as the collision frequency and gyroradius are small enough to be negligible. A similar conclusion also holds if the system is subject to external forcing as long as this forcing conserves the magnetic moment of at least one plasma species and does not greatly increase the total energy of the plasma (i.e., in practice, is subsonic). Dynamo action therefore always requires collisions or some small-scale kinetic mechanism for breaking the adiabatic invariance of the magnetic moment

The flow direction of interstellar neutral H from SOHO/SWAN

Interstellar neutral hydrogen flows into the heliosphere as a mixture of the primary and secondary populations from two somewhat different directions due to splitting occurring in the magnetized outer heliosheath. The direction of inflow of interstellar neutral H observed in the inner heliosphere, confronted with that of the unperturbed flow of interstellar neutral helium, is important for understanding the geometry of the distortion of the heliosphere from axial symmetry. It is also needed for facilitating remote-sensing studies of the solar wind structure based on observations of the helioglow, such as those presently performed by SOHO/SWAN, and in a near future by IMAP/GLOWS. In the past, the only means to measure the flow direction of interstellar hydrogen were spectroscopic observations of the helioglow. Here, we propose a new method to determine this parameter based on a long series of photometric observations of the helioglow. The method is based on purely geometric considerations and does not depend on any model and absolute calibration of the measurements. We apply this method to sky maps of the helioglow available from the SOHO/SWAN experiment and derive the mean flow longitude of interstellar hydrogen. We obtain 253.1\degr \pm 2.8\degr, which is in perfect agreement with the previously obtained results based on spectroscopic observations.Comment: Accepted for Ap

Testing for Markovian Character and Modeling of Intermittency in Solar Wind Turbulence

We present results of statistical analysis of solar wind turbulence using an approach based on the theory of Markov processes. It is shown that the Chapman-Kolmogorov equation is approximately satisfied for the turbulent cascade. We evaluate the first two Kramers-Moyal coefficients from experimental data and show that the solution of the resulting Fokker-Planck equation agrees well with experimental probability distributions. Our results suggest the presence of a local transfer mechanism for magnetic field fluctuations in solar wind turbulence

Multidimensional Hall magnetohydrodynamics with isotropic or anisotropic thermal pressure: numerical scheme and its validation using solitary waves

We present a numerical solver for plasma dynamics simulations in Hall magnetohydrodynamic (HMHD) approximation in one, two and three dimensions. We consider both isotropic and anisotropic thermal pressure cases, where a general gyrotropic approximation is used. Both explicit energy conservation equation and general polytropic state equations are considered. The numerical scheme incorporates second-order Runge-Kutta advancing in time and Kurganov-Tadmor scheme with van Leer flux limiter for the approximation of fluxes. A flux-interpolated constrainedtransport approach is used to preserve solenoidal magnetic field in the simulations. The implemented code is validated using several test problems previously described in the literature. Additionally, we propose a new validation method for HMHD codes based on solitary waves that provides a possibility of quantitative rigorous testing in nonlinear (large amplitude) regime as an extension to standard tests using small-amplitude whistler waves. Quantitative tests of accuracy and performance of the implemented code show the fidelity of the proposed approach

Heliospheric effects caused by Sun-originating versus LISM-advected fluctuations

Context. We investigate the response of the heliosphere to fluctuations in the local interstellar medium (LISM) as compared to the influence of solar-cycle fluctuations. Aims. We discuss the differences between effects coming from the two types of drivers of time-dependent effects in the heliosphere in the context of the shape of the heliosphere, the thickness of the inner heliosheath, and the position of the ribbon of enhanced energetic neutral particle emission, as observed by the IBEX mission. Methods. Our study is based on a comparison of fully time-dependent simulations obtained with a three-dimensional (3D) model of the heliosphere. Results. We show that density fluctuations, taking the form of entropy waves and originating from the LISM, may reduce the thickness of the inner heliosheath to a similar extent as the solar-cycle effects. However, the relative motions of the termination shock and the heliopause in the two types of simulations are different. The amplitude of variation of the heliopause position is greater for the LISM fluctuation. The IBEX ribbon position is shown to be not significantly affected by the two types of drivers, although the effect of LISM fluctuation is stronger than that of the solar cycle. In this context, slight systematic changes of the position of the IBEX ribbon in its different sectors (i.e., changes in the heliospheric nose followed by variations in the heliospheric flanks) may serve as an indicator of the passage of a density fluctuation in the LISM, as suggested by our study. We also discuss the difficulties in fitting the LISM parameters in the presence of time-dependent effects