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Power spectra of the components of the magnetic field parallel (Pzz) and perpendicular (Pzz+Pyy) to the local mean magnetic field direction were determined by wavelet methods from Ulysses’ MAG instrument data during eighteen 10-day segments of its first North Polar pass at high latitude at solar minimum in 1995. The power depends on frequency f and the angle θ between the solar wind direction and the local mean field, and with distance from the Sun. This data includes the solar wind whose total power (Pxx + Pyy + Pzz) in magnetic fluctuations we previously reported depends on f and the angle θ nearly as predicted by the GS95 critical balance model of strong incompressible MHD turbulence. Results at much wider range of frequencies during six evenly-spaced 10-day periods are presented here to illustrate the variability and evolution with distance from the Sun. Here we investigate the aniso tropic scaling of Pzz(f,θ) in particular because it is a reduced form of the Poloidal (pseudo-Alfvenic) component of th...

The von Karman-Howarth (vKH) hierarchy of equations relate the second-order correlations of the turbulent fluctuations to the third-order ones, the third-order to the fourth-order, and so on. We recently demonstrated [1] that for MHD, self-similar solutions to the vKH equations seem to require at least two independent similarity lengthscales (one for each Elsasser energy), so that compared to hydrodynamics a richer set of behaviors seems likely to ensue. Moreover, despite the well-known anisotropy of MHD turbulence with a mean magnetic field (B0), the equation for the second-order correlation does not contain explicit dependence on B0. We show that there is, however, implicit dependence on B0 via the third-order correlations, which themselves have both explicit B0-dependence and also their own implicit dependence through fourth-order correlations. Some subtleties and consequences of this implicit-explicit balance are summarized here. In addition, we present an analysis of simulation results showing that t...

We examine the effects of including effects of both protons and electrons on the heating of the fast solar wind through two different approaches. In the first approach, we incorporate the electron temperature in an MHD turbulence transport model for the solar wind. In the second approach, we adopt more empirically based methods by analyzing the measured proton and electron temperatures to calculate the heat deposition rates. Overall, we conclude that incorporating separate proton and electron temperatures and heat conduction effects provides an improved and more complete model of the heating of the solar wind.

We present a model for the transport of solar wind fluctuations, based on the assumption that they can be well‐represented using two distinct components: a quasi‐2D turbulence piece and a wave‐like piece. For each component, coupled transport equations for its energy, cross helicity, and characteristic lengthscale(s) are derived, along with an equation for the proton temperature. This energy‐containing “two‐component” model includes the effects of solar wind expansion and advection, driving by stream shear and pickup ions, and nonlinear cascades. Nonlinear effects are modeled using a recently developed one‐point phenomenology for such a two‐component model of homogeneous MHD turbulence [1]. Heating due to these nonlinear effects is included in the temperature equation. Numerical solutions are discussed and compared with observations.

We apply a recently developed 2‐component phenomenology to the turbulent heating of the core solar wind protons as seen at the Voyager 2 spacecraft. We find that this new description improves the model predictions of core temperature and correlation scale of the fluctuations, yielding excellent agreement with the Voyager measurements. However, the model fluctuation intensity substantially exceeds the Voyager measurements in the outer heliosphere, indicating that this picture needs further refinement.

There is a growing body of literature that demonstrates the ability of a turbulent cascade within the solar wind to heat the thermal protons. Several sources of energy are required to accomplish the observed heating. Wind shear and shocks originating with the multiple source of wind plasma heat the wind inside ∼10 AU. However, beyond this distance little is left of these sources and all that remains is the energy injected into the plasma by the pickup of newborn protons originating from interstellar neutrals. Recent advances in the theory of wave excitation by the newborn protons allows us to return to the published heating theory and remove a previously unexplained parameterization of the heating due to pickup protons. Furthermore, recent observational evidence suggests that large‐scale correlations between the wind speed and the proton temperature exist into the distant outer heliosphere that motivate an attempt to connect the two within the structure of the heating theory.

We present a brief review of observations and theory regarding the nature and radial evolution of MHD‐scale solar wind fluctuations. Emphasis is placed on the fact that the fluctuations consist of both waves and turbulence, and on their dual dynamical roles.

In the presence of a weakly inhomogeneous background, magnetohydrodynamic fluctuations are transported, reflected and at small scales, dissipated. In contrast to orderings appropriate to outer solar wind conditions, here we explore transport in a regime relevant for solar coronal heating and solar wind acceleration, in which effects of the order of the Alfven speed are retained while disregarding the solar wind velocity. We consider the general properties of the transport equations as well as some solutions of interest.

Models based upon anisotropic magnetohydrodynamic (MHD) cascade offer promising explanations for observations of both interplanetary and coronal turbulence and heating, which are reviewed here. In the standard picture the cascade proceeds by driving at the energy‐containing scales, transfers through the inertial range, and into small scales where it drives small‐scale random turbulent reconnection events. In order to understand more fully the heating and dissipation processes, one also needs to understand how small‐scale MHD‐driven reconnection — involving current sheets and filaments — induces kinetic plasma processes that thermalize the fluid energy. Here we suggest that in these reconnection sites MHD electric fields drive ion beam instabilities and nonlinear electron dynamics involving electron solitary wave structures, in analogy with the kinetic physics observed near parallel electric field auroral regions by the FAST spacecraft.

A technique for analyzing multiple-time, single-point datasets is presented, which yields information about the (spectral) variation of MHD fields in directions perpendicular to the mean flow. The analysis can be performed for any homogeneous solenoidal fields, including magnetic and incompressible velocity fluctuations. A summary of the associated theory is given, along with initial results obtained using an interval of Voyager data.

We present a candidate mechanism for heating of the solar corona, via the interaction of (chromospherically-generated) Alfven waves, their reflections, and quasi-two dimensional (relative to the mean magnetic field) MHD turbulence. The non-propagating nature of the latter means that the restrictive Alfven timescale constraints associated with high frequency wave heating models are avoided. A phenomenology for this mechanism is described here, and “proof of concept” support from reduced MHD (RHMD) simulations is also discussed. Estimates of the achievable heating efficiency based on turbulence models are favorable, and encourage further investigation of the model’s quantitative feasibility.

The deposition of energy into the solar wind beyond 1 AU is thought to result from the dissipation of low frequency magnetohydrodynamic (MHD) turbulence via kinetic processes at spatial scales comparable to the ion gyroradius. Beyond 1 AU, solar wind turbulence is comprised of both a decaying component generated in the corona and turbulence generated dynamically in situ by processes such as stream shear, interplanetary shocks, and, beyond the ionization cavity, the pickup of interstellar neutral atoms. A turbulence-theoretic model describing the radial evolution of the power in magnetic fluctuations in the solar wind has been developed recently and the predictions were compared successfully with Voyager data. Using the dissipation rate predicted by this model, we evaluate the expected heating of the solar wind by MHD turbulence. The effective adiabatic index of the solar wind is reduced from 5/3 and the theory accounts for the apparent heating of ions in the outer heliosphere.

We investigate the role of three-point decorrelation due to Alfven wave propagation in three-dimensional incompressible homogeneous MHD turbulence. By comparing numerical simulations with theoretical expectations, we have studied how this effect influences the decay of turbulent energy caused by both an external mean magnetic field and the fluctuating turbulent field. Decay is initially suppressed by a mean magnetic field, as expected, but the effect soon saturates. The decay rate does not scale with mean magnetic field for higher values. The disagreement with theoretical predictions can be accounted for by anisotropic spectral transfer. Thus, phenomenological models for energy decay that include decorrelation due to Alfvenic propagation are not substantiated. This work complements our detailed study of various models of energy decay in homogeneous MHD [Hossain et al., 1995].