SH31B-01 INVITED 

The interaction of turbulence with shock waves

* Zank, G P (zank@ucr.edu), University of California, Riverside, IGPP University of California, Riverside, CA 92521, United States Shaikh, D (dastgeer.shaikh@ucr.edu), University of California, Riverside, IGPP University of California, Riverside, CA 92521, United States Krykov, I (igor.Krykov@ucr.edu), University of California, Riverside, IGPP University of California, Riverside, CA 92521, United States Ao, X (xao001@student.ucr.edu), University of California, Riverside, IGPP University of California, Riverside, CA 92521, United States Pogorelov, N (nikolaip@ucr.edu), University of California, Riverside, IGPP University of California, Riverside, CA 92521, United States Khare, A (ojavinash@yahoo.co.in), University of California, Riverside, IGPP University of California, Riverside, CA 92521, United States Borovikov, S (sergey.borovikov@ucr.edu), University of California, Riverside, IGPP University of California, Riverside, CA 92521, United States

The region downstream of the heliospheric termination shock (HTS) was expected to be turbulent [Zank, 1999], the result of upstream turbulence and disturbances (shock waves, pressure and density enhancements, structures, etc. [Story, 1997]) being transmitted across and interacting with the shock. A turbulent downstream region, the heliosheath, has indeed been observed downstream of the HTS [Burlaga et al., 2006], but the character of the turbulence is significantly different from that of the solar wind. The 48-sec averages of the downstream magnetic field analyzed by Burlaga et al. [2006] reveal that the turbulence is isotropic, and each component has a Gaussian distribution. The distribution of 1-hour averages of the magnetic field was also Gaussian in the heliosheath, unlike the log-normal distributions found in the supersonic solar wind. The Gaussian distribution indicates a scale invariance in the form of the magnetic field distribution. The second intriguing observation was that the turbulence was substantially compressible since the width of the 48-sec magnetic field averages is greater than that for the components. Clearly MHD turbulence in the inner heliosheath is significantly different from that upstream of the HTS in the supersonic solar wind. Since the character of the turbulence is different immediately downstream of the HTS, it suggests that "processing'" by the HTS plays an important role in modifying turbulent upstream fluctuations. Motivated by a need to better understand these observations and related observations at interplanetary shocks and the Earth's bow shock, the interaction of turbulence with shock waves is attracting increasing attention. We will discuss the interaction of upstream disturbances and turbulence with a shock wave both analytically and numerically.

SH31B-02 INVITED 

Interactions of Energetic Particles and Heliospheric Turbulence: Issues and Challenges

* Jokipii, J R (jokipii@lpl.arizona.edu), University of Arizona Planetary Sciences and Astronomy, Kuiper Space Sciences rm 411, Tucson, AZ 85721, United States

The transport and acceleration of energetic charged particles in collisionless astrophysical plasmas is determined significantly by their interactions with turbulence. It has realized for some time that the simple, intuitive "billiard-ball" scattering picture whereby the particles are "scattered" by magnetic fluctuations is inadequate. The spatial transport depends on the nature of the turbulence at all scales, ranging from the small scales of the order of the energetic particle gyro-radius to the outer or coherence scale, which determines the field-line meandering or random walk. Spacecraft observations require that the cross-field transport be much larger than suggested by the simple scattering picture. In addition, observations now show that the turbulence itself is more complex than initially supposed. The anisotropy of the turbulence and intermittency effects impact the particle transport in ways that are yet not fully understood. For example, the "dropouts" observed in impulsive solar-energetic-particle events have been interpreted in two quite-different different ways - either as an initial transient effect or as the result of the behavior turbulent field lines themselves. Some recent work has suggested that subdiffusive or superdiffusive transport may occur and explain some energetic-particle observations. The challenges presented by the observations and possible explanations in terms of new ideas will be critically discussed.

SH31B-03 

Energy Cascades in the Heliospheric Turbulence

* Shaikh, D (dastgeer@ucr.edu), Institute of Geophysics and Planetary Physics, University of California, 900 University Avenue, Pierce Bldg. Room#2112, Riverside, CA 92521, United States

A local turbulence model, to study energy cascades in the interstellar medium (ISM), is developed based on self-consistent two-dimensional fluid simulations. The model describes a partially ionized magnetofluid interstellar medium (ISM) that couples a neutral hydrogen fluid with a plasma primarily through charge exchange interactions, and assumes that the ISM turbulent correlation scales are much bigger than the shock characteristic length-scales. Charge exchange interactions occur ubiquitously in warm ISM plasmas in a subtle nonlinear manner whose strength depends largely on the relative speed between the plasma and the neutral fluid. Unlike small length-scale linear collisional dissipation in the fluid, charge exchange processes can be effective on a variety of ISM length-scales depending upon the neutral and plasma densities, charge exchange cross section and the characteristic length scales. We find, from scaling arguments and nonlinear coupled fluid simulations, that charge exchange interactions modify spectral transfer associated with largescale energy containing eddies. Consequently, the warm ISM turbulent cascade rates lead to a rapid spectral transfer amongst inertial range modes. Turbulent spectra associated with the neutral and plasma ISM fluids are therefore steeper than those predicted by Kolmogorov's phenomenology.

SH31B-04 

The Formation of Magnetic Decreases \(MDs\) in Interplanetary Space: Ulysses

* Tsurutani, B (bruce.tsurutani@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, 91109, United States Guarnieri, F (guarnieri@univap.br), UNIVAP, 2911 Av. Shishima Hifumi, Sao Jose dos Campos, 12244-000, Brazil Echer, E (eecher@inpe.br), INPE, 1758 Av. dos Astronautas, Sao Jose dos Campos, 1227-010, Brazil

Magnetic Decreases are decreases in the magnitude of the ambient magnetic field. The displaced plasma pressure is supplanted by anisotropic \(Tperp/Tpar > 1\), locally heated plasma so that there is pressure balance across the MD structures. Anisotropic spatial distributions of MDs are identified to further understand the conditions under which MDs form/evolve. We will show that Ulysses fast-latitude scans through corotating solar wind streams are good places to search for such spatial anisotropies. The properties of MDs within CIRs and high speed streams will be reported. A general generation mechanism will be described.

SH31B-05 

Rapid directional alignment of velocity and magnetic field in magnetohydrodynamic turbulence

* Matthaeus, W (whm@udel.edu), Bartol Research Institute and Dept. of Physics and Astronomy, University of Delaware, Newark, DE 19711, United States Pouquet, A (pouquet@ucar.edu), NCAR, P.O. Box 3000, Boulder, CO 80307-3000, United States Mininni, P (mininni@df.uba.ar), NCAR, P.O. Box 3000, Boulder, CO 80307-3000, United States Dmitruk, P (pablo@bartol.udel.edu), Departamento de Fi sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires Ciudad Universitaria, Buenos Aires, 1428, Argentina Breech, B (breech@cis.udel.edu), Bartol Research Institute and Dept. of Physics and Astronomy, University of Delaware, Newark, DE 19711, United States

We show that local directional alignment of the velocity and magnetic field fluctuations occurs rapidly in magnetohydrodynamics for a variety of parameters. The phenomenon, which occurs within an eddy turnover time regardless of the initial alignment of the fluctuations, is observed both in direct numerical simulations and in solar wind data. The phenomenon is due to either pressure gradients or shear-associated kinetic energy gradients acting along the magnetic field. A similar alignment, of velocity and vorticity, occurs in the Navier Stokes fluid case. This may be the most rapid and robust relaxation process in turbulent flows, and leads to a local weakening of the nonlinear terms in the small scale vorticity and current structures where alignment takes place.

SH31B-06 

The Evolution of the Spectrum of Velocity Fluctuations in the Solar Wind

* Roberts, D A (aaron.roberts@nasa.gov), NASA GSFC, Code 672, Greenbelt, MD 20771, United States

Previous studies have shown that the power spectra of the magnetic field and velocity in the solar wind do not evolve in the same way with helocentric distance. In particular, the velocity spectrum remains flatter for a substantial distance. However, Voyager observations of the velocity spectrum have demonstrated a likely asymptotic state in which the spectrum steepens to having a spectral index of -5/3, finally mathching the magnetic spectrum and the theoretical expectation of Kolmogoroff turbulence. Here we examine evidence from other spacecraft, in particular studying Ulysses spectra to determine if the Voyager result, based on a very few sufficiently complete intervals, is correct. Preliminary results confirm the -5/3 slope for velocity fluctuations at ~5 AU from the Sun in the ecliptic. We will examine many intervals to develop a more general picture of the spectral evolution in various conditions, and how magnetic and velocity spectra differ in these cases.

SH31B-07 

On the kinetic theory of solar-wind turbulence

* Yoon, P H (phy20723@yahoo.com), MTL, Massachusetts Technological Laboratory, Belmont, MA 02478, United States * Yoon, P H (phy20723@yahoo.com), UMD, University of Maryland, College Park, MD 20723, United States

The present state of knowledge regarding the plasma turbulence is rather incomplete. Standard microscopic (i.e., kinetic) plasma turbulence theory available in the literature was developed largely by the scientists in the former Soviet Union in the 1960s. However, it is applicable only for unmagnetized plasmas. As such, the customary theory is applied largely to Langmuir turbulence problem only. Of course, real plasmas in nature and laboratory are immersed in magnetic fields. At present, the only turbulence theories applicable to magnetized plasmas are macroscopic theories based upon MHD or drift-wave turbulence models. However, macroscopic theories such as MHD models, are not applicable for small spatial and fast time scale structures associated with the turbulence. In short, there is a need for a "kinetic" or microscopic treatment of turbulence theory for magnetized plasmas. The present paper discusses a first step in the formulation of kinetic theory for magnetized plasmas that may be applicable to solar wind and heliospheric turbulence at any dynamical scales. For the sake of simplicity, it is assumed that turbulent fluctuations predominantly propagate along the direction of ambient magnetic field vector, and that the characteristic frequency associated with the fluctuations is much lower than the electron gyrofrequency (but can be as high as or higher than the ion cyclotron frequency), i.e., hydromagnetic turbulence. The basic theoretical formalism outlined in the present paper may have important ramifications for a number of space and astrophysical problems including coronal heating, solar wind acceleration, etc. Research supported by AFOSR

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.