SH33B-01
Behavior of solar wind energy flux near density holes upstream of the bow shock
We present new observations of density holes upstream of Earth's bow shock. Density holes are regions of
density depletions below the solar wind level with scale length of an ion gyroradius. They represent the smallest
nonlinear ion structures produced by the solar wind. The edges of density holes have strong currents and as the
holes propagate earthward the magnetic field evolves nonlinearly into a shock-like structure.We have examined
how the solar wind energy flux changes as it approaches the density holes. The ith component of the energy flux
2Qi = nm
SH33B-02
Measurements of the wavevector anisotropy of solar wind turbulence
We present a new, wavelet-based analysis of solar wind magnetic field data which makes it possible to determine the wavevector anisotropy of the fluctuations with respect to the local mean field direction. We demonstrate, consistent with published work, that the fluctuations within the inertial range are dominated by power in wavevectors at large angles to the field (so-called "2D"). In an important new result, we also show how the spectral index, γ, varies with the angle between the radial (measurement) and mean field directions. At most angles γ ≈ 5/3 , however this smoothly transitions to 2 for small angles, i.e., those associated with fluctuations with wavevectors nearly parallel to the field ("slab"). We also demonstrate significant deviations from a simple slab and 2D model, perhaps due to power at intermediate angles or finite wave propagation speeds. Finally, we discuss the anisotropy of the field magnitude spectrum, a proxy for density variations, and how this relates to the anisotropy in the field components.
SH33B-03
Strong Imbalanced Turbulence
We consider imbalanced, or cross-helical MHD Alfvenic turbulence where the waves traveling in one direction have higher amplitudes than the opposite waves. This paper is dedicated to so-called strong turbulence, which cannot be treated perturbatively. Our main result is that the anisotropy of the weak waves is stronger than the anisotropy of a strong waves. This seemingly contradicts the conventional interpretation of so-called critical balance (Goldreich & Sridhar 1995). We propose that critical balance, that was originally conceived as a causality argument, has to be amended by what we call a propagation argument. This revised formulation is consistent with the old one in the balanced case, and is able to include the imbalanced case. We also provide phenomenological model of energy cascading and discuss possibility of self-similar solutions in a realistic setup of driven turbulence.
SH33B-04
Cosmic-ray Electrons as Probes of the Dissipation Range of Solar Wind Turbulence
In numerical modulation models low-energy cosmic-ray nuclei experience the adiabatic limit, where their intensity becomes proportional to their kinetic energy per nucleon, independent of the properties of the diffusion tensor and interstellar spectrum. Electrons, on the other hand, remain sensitive to changes in the diffusion tensor and associated turbulence quantities down to very low energies. In principle, electron modulation can therefore be used as an indirect probe of the properties of turbulence at small scales, notably the wavenumber kD where the dissipation range occurs, and the latter's spectral index. We consider two relationships for kD suggested by Leamon et al. (2000 ApJ, 537), based on observational results. In one case, the proton gyrofrequency is used as predictor for the steepening wavenumber kD where the dissipation range occurs, and in the second case, the ion inertial scale is used. We use a steady-state three-dimensional modulation model for this study, with the parallel mean free path (MFP) given by an analytical expression based on results of Teufel and Schlickeiser (2002, A&A, 393), and the perpendicular MFP an approximation derived by Shalchi et al. (2004, ApJ, 604) for the nonlinear guiding center model (NLGC) of Matthaeus et al. (2003, ApJ, 590). The drift coefficient is from Burger et al. (2000, JGR, 105). We show that different combinations of models for kD, the spectral index in the dissipation range, and the strength of dynamical turbulence lead to orders of magnitude differences in the predicted intensity of electrons at Earth. Our preliminary results suggest that a model for kD based on the ion inertial scale is possibly the more appropriate choice.
SH33B-05
The Turbulent Cascade at 1 AU: Energy Transfer and the Third-Order Scaling for MHD
We perform a test of MHD turbulent cascade theory in the solar wind and directly evaluate the contribution of local turbulence to heating the solar wind at 1 AU. We look at turbulent fluctuations in the solar wind velocity V, and magnetic field B, using the vector Elsasser variables Z± \equiv V ± B / \sqrt{4 π ρ} measured at the ACE spacecraft stationed at Earth L1. We combine the fluctuations δ Z± over time lags in the inertial range, from 64 seconds to several hours, to form components of the mixed vector third moments that Politano and Pouquet (1998a,b) show obey an exact law, similar to the Kolmogorov 4/5 law, but valid in anisotropic MHD turbulence. This effort is vital to studies of dissipation processes because it provides both the rate that energy is delivered to the dissipation process, but also the form in that the cascade in directions parallel and perpendicular to the mean magnetic field can be measured separately. We demonstrate that the scaling is reasonably linear as expected for the inertial range. The total turbulent energy injection/dissipation rate we derive this way agrees with the in situ heating of the solar wind inferred from the temperature gradient, while methods using the power spectra only seldom agree with heating rates derived from gradients of the thermal proton distribution. We derive expressions of the third-order moments that are applicable to the spectral cascade parallel and perpendicular to the mean magnetic field. We apply these expressions to fast- and slow-wind subsets of the data with additional subsetting for mean field direction. We find that both the fast wind and the slow wind exhibit an active energy cascade over inertial range scales. Furthermore, we find that the energy flux in the parallel cascade is consistently smaller than in the perpendicular cascade. This is especially true of high-speed wind conditions where we see that the turbulence is moving away from the pre-existing field-aligned geometry of Dasso et al.\ [2005]. This work does not assume a particular MHD theory for the power spectrum such as Iroshnikov [1964], Kraichnan [1965], Goldreich and Sridhar [1995], or Boldyrev [2005, 2006]. Although this is a study of fluctuations within the inertial range of interplanetary turbulence, it has direct bearing on the rate and manner that energy is injected into the dissipation range.
SH33B-06
Flux tubes in the solar wind from Cluster measurements
Recent studies of solar wind turbulence suggest that tangential discontinuities in the interplanetary medium can introduce significant intermittency and these discontinuities may be a natural manifestation of flux-tube-like structures in the solar wind. Because the existence of these flux tubes can affect our understanding of the solar wind MHD turbulence, it is necessary that we verify their existence and are able to identify them individually. Using the magnetic field data from FGM instrument onboard Cluster spacecraft, we examine the existence of flux tubes in the solar wind. Cluster/FGM has a high time resolution of magnetic field data and the orbits of Cluster also traverse through various dynamic regions, including the solar wind, Earth's magnetosheath and magnetotail, making Cluster's dataset ideal for studying the differences between, for example, solar wind turbulence and those inside the magnetosphere (e.g. turbulence in plasma sheet). Using a recent data analysis by {\sl Li}, [2007a,b], we show that flux tubes exist in the solar wind, but not inside Earth's magnetotial. The existence of flux-tube-like structures in the solar wind implies that current studies of solar wind MHD turbulence must be carefully re-examined because these flux tubes will inneviatablly cause intermittency and affect turbulence power spectrum.
SH33B-07
Turbulence and Plasma Heating in the Solar Wind
Measurements of the solar wind plasma in the inner heliosphere show non-adiabatic temperature profiles for the plasma species, suggesting in situ heating of the solar wind. Heating of the plasma via the dissipation of a turbulent cascade is considered a promising explanation; phase space instabilities may also play an important role in constraining the temperature anisotropies of each species as the solar wind expands radially. I will present the results of nonlinear, kinetic simulations of the plasma turbulence aimed at determining directly the heating of each species. In addition, an analytical model of the plasma heating in the inner heliosphere is used to identify the influence of temperature anisotropy instabilities and to constrain the required heating by the plasma turbulence. This work is supported by the DOE Center for Multi-scale Plasma Dynamics, Fusion Science Center Cooperative Agreement ER54785.
SH33B-08
Spectra of moderately strong anisotropic MHD turbulence in high beta plasma
Spectra of MHD turbulence are studied at a moderate level of beta. For small amplitude waves the interactions can be neglected. The increase of wave amplitudes results in the regime of weak turbulence. For larger amplitudes the nonlinear effects become comparable or larger than the wave dispersion, the weak turbulence approach is not applicable anymore. In this case the main nonlinear effect for acoustic waves is the wave breaking that results in the formation of shocks. This process is purely coherent and the expanding caustics should be treated as coherent elementary entities of wave turbulence. The jumps of density as well as the density singularities are known to result in a power-law tails in the short-wavelength turbulence spectrum. Kadomtsev and Petviashvili suggested that the ion acoustic turbulence in a plasma can be considered as a randomly distributed set of shocks and found the spectrum of the isotropic energy distribution E(k)~k-2. Our goal is to determine how the KP spectrum is modified in the presence of the strong enough anisotropy in a plasma.