Energy cascade rate in isothermal compressible magnetohydrodynamic turbulence [CL]

Three-dimensional direct numerical simulations are used to study the energy cascade rate in isothermal compressible magnetohydrodynamic turbulence. Our analysis is guided by a two-point exact law derived recently for this problem in which flux, source, hybrid, and mixed terms are present. The relative importance of each term is studied for different initial subsonic Mach numbers $M_S$ and different magnetic guide fields ${\bf B}_0$. The dominant contribution to the energy cascade rate comes from the compressible flux, which depends weakly on the magnetic guide field ${\bf B}_0$, unlike the other terms whose modulus increase significantly with $M_S$ and ${\bf B}_0$. In particular, for strong ${\bf B}_0$ the source and hybrid terms are dominant at small scales with almost the same amplitude but with a different sign. A statistical analysis made with an isotropic decomposition based on the SO(3) rotation group is shown to generate spurious results in presence of ${\bf B}_0$, when compared with an axisymmetric decomposition better suited to the geometry of the problem. Our numerical results are eventually compared with previous analyses made with in-situ measurements in the solar wind and the terrestrial magnetosheath.

N. Andres, F. Sahraoui, S. Galtier, et. al.
Fri, 16 Feb 18
32/42

To the question about perturbations of solar-terrestrial characteristics [CL]

Data obtained over the last three solar cycles have been analysed to reveal the relationships between the intensity of the photospheric field measured along the line of sight by the WSO group at heliolatitudes from -75 to 75 degrees and the intensity of the interplanetary magnetic field and absolute values of the perturbations of the different characteristics of the solar wind at the Earth orbit, and geomagnetic parameters. provided by the OMNI team.
The heliospheric and geomagnetic data are found to be divided into two groups characterized by their response to variability of the solar magnetic field latitudinal structures on short and on long time scales.

E. Gavryuseva
Wed, 14 Feb 18
7/68

Comments: 14 pages, 6 Postscript figures. arXiv admin note: substantial text overlap with arXiv:1802.03135

Generation of large-scale magnetic fields due to fluctuating $α$ in shearing systems [GA]

We explore the growth of large-scale magnetic fields in a shear flow, due to helicity fluctuations with a finite correlation time, through a study of the Kraichnan-Moffatt model of zero-mean stochastic fluctuations of the $\alpha$ parameter of dynamo theory. We derive a linear integro-differential equation for the evolution of large-scale magnetic field, using the first-order smoothing approximation and the Galilean invariance of the $\alpha$-statistics. This enables construction of a model that is non-perturbative in the shearing rate $S$ and the $\alpha$-correlation time $\tau_\alpha$. After a brief review of the salient features of the exactly solvable white–noise limit, we consider the case of small but non–zero $\tau_\alpha$. When the large–scale magnetic field varies slowly, the evolution is governed by a partial differential equation. We present modal solutions and conditions for the exponential growth rate of the large-scale magnetic field, whose drivers are the Kraichnan diffusivity, Moffatt drift, Shear and a non-zero correlation time. Of particular interest is dynamo action when the $\alpha$-fluctuations are weak; i.e. when the Kraichnan diffusivity is positive. We show that in the absence of Moffatt drift shear does not give rise to growing solutions. But shear and Moffatt drift acting together can drive large scale dynamo action with growth rate $\gamma \propto |S|$.

N. Jingade, N. Singh and S. Sridhar
Wed, 14 Feb 18
14/68

Comments: 18 pages, 4 figures, Submitted to Journal of Plasma Physics

Laboratory Space Physics: Investigating the Physics of Space Plasmas in the Laboratory [CL]

Laboratory experiments provide a valuable complement to explore the fundamental physics of space plasmas without the limitations inherent to spacecraft measurements. Specifically, experiments overcome the restriction that spacecraft measurements are made at only one (or a few) points in space, enable greater control of the plasma conditions and applied perturbations, can be reproducible, and are orders of magnitude less expensive than launching spacecraft. Here I highlight key open questions about the physics of space plasmas and identify the aspects of these problems that can potentially be tackled in laboratory experiments. Several past successes in laboratory space physics provide concrete examples of how complementary experiments can contribute to our understanding of physical processes at play in the solar corona, solar wind, planetary magnetospheres, and outer boundary of the heliosphere. I present developments on the horizon of laboratory space physics, identifying velocity space as a key new frontier, highlighting new and enhanced experimental facilities, and showcasing anticipated developments to produce improved diagnostics and innovative analysis methods. A strategy for future laboratory space physics investigations will be outlined, with explicit connections to specific fundamental plasma phenomena of interest.

G. Howes
Tue, 13 Feb 18
11/76

Comments: Invited paper from APS DPP 2017 Plenary Review Talk, 30 pages, 489 references

Relations between variability of the photospheric and interplanetary magnetic fields, solar wind and geomagnetic characteristics [SSA]

Large scale solar magnetic field topology has a great influence on the structure of the corona, heliosphera and geomagnetic perturbations.
Data obtained over the last three solar cycles have been analysed to reveal the relationships between the photospheric field measured along the line of sight by the WSO group at 30 levels of heliolatitudes from -75 to 75 degrees and the interplanetary magnetic field The main aim of this first paper is to make a direct comparison between the basic structure and dynamics of the photospheric magnetic field and components and intensity of the interplanetary magnetic field % solar wind and geomagnetic parameters without using theoretical assumptions, models, physical expectations, etc.
The second paper by Gavryuseva, 2018d presents the raports between different characteristics of the solar wind at the Earth orbit, and geomagnetic parameters provided by the OMNI team. % Data obtained over the last three solar cycles have been analysed % to reveal the relationships % between the photospheric field measured along the line of sight % by the WSO group % at heliolatitudes from -75 to 75 degrees averaged over one year % and the interplanetary magnetic field, different characteristics % of the solar wind at the Earth orbit, and geomagnetic parameters. % provided by the OMNI team.
The heliospheric and geomagnetic data are found to be divided into two groups characterized by their response to variability of the solar magnetic field latitudinal structures on short and on long time scales.

E. Gavryuseva
Mon, 12 Feb 18
13/53

Comments: 28 pages, 15 Postscript figures

Longitudinal structure of the photospheric magnetic field in the Carrington system [SSA]

The observations of the Sun have been performed over the years, even centuries- Whether are there active longitudes? If yes how stable are they? One of the first The Wilcox Solar Observatory data taken over three cycles N 21, N 22, N 23 have been used to reveal the longitudinal structure of the photospheric magnetic field. Mean over three cycles magnetic field distribution has been calculated in the North and in the South hemispheres as well as at 30 levels of latitude from -75 to 75 degrees. This study was performed using observations of the magnetic field taking into account its polarity or only intensity. The longitudinal structure of the magnetic field was calculated in the coordinate system rotating with Carrington rate. These structures were compared with a model of random longitudinal distribution of the magnetic field. Random character of the longitudinal structure was refused. The results agree with the presence of two active meridians seen in different phenomena of solar activity at longitudes separated by 150-170 degrees in the Carrington coordinate system.

E. Gavryuseva
Fri, 9 Feb 18
50/57

Comments: 9 pages, 4 Postscript figures

Magnetohydrodynamic Turbulence in the Plasmoid-Mediated Regime [CL]

Magnetohydrodynamic turbulence and magnetic reconnection are ubiquitous in astrophysical environments. In most situations, these processes do not occur in isolation, but interact with each other. This renders a comprehensive theory of these processes highly challenging. Here, we propose a theory of magnetohydrodynamic turbulence driven at large scale that self-consistently accounts for the mutual interplay with magnetic reconnection occurring at smaller scales. Magnetic reconnection produces plasmoids that grow from turbulence-generated noise and eventually disrupt the sheet-like structures in which they are born. The disruption of these structures leads to a modification of the turbulent energy cascade, which, in turn, exerts a feedback effect on the plasmoid formation via the turbulence-generated noise. The energy spectrum in this plasmoid-mediated range steepens relative to the standard inertial range and does not follow a simple power law. As a result of the complex interplay between turbulence and reconnection, we also find that the length scale which marks the beginning of the plasmoid-mediated range and the dissipation length scale do not obey true power laws. The transitional magnetic Reynolds number above which the plasmoid formation becomes statistically significant enough to affect the turbulent cascade is fairly modest, implying that plasmoids are expected to modify the turbulent path to dissipation in many astrophysical systems.

L. Comisso, Y. Huang, M. Lingam, et. al.
Thu, 8 Feb 18
2/43

Comments: Accepted for publication in The Astrophysical Journal