NG43B-01 INVITED
ROMA (Rank-Ordered Multifractal Analyses) Spectra for Intermittent Fluctuations in Space Plasmas
The hallmark of nonlinear complexity phenomena in MHD and plasma turbulence is the appearance of intermittent fluctuating events. Such intermittent fluctuations are the results of the chaotic interactions of multiple coherent structures of varied sizes of the dynamical media. We discuss here a new method, the Rank-Ordered Multifractal Analysis (ROMA) that is both physically explicable and quantitatively accurate in deciphering the multifractal characteristics of intermittency. The generic character of the procedure provides a natural connection between the ROMA spectrum based on parametric rank ordering and the one- parameter scaling idea of monofractals. The properties of such multifractal spectra and their generalizations may be understood theoretically from the point of view of global scale invariants of coarse-graining transformations. We demonstrate the utility of the method using results obtained from large-scale 2D MHD simulations as well as in-situ observations of magnetic field fluctuations from the interplanetary and cusp/magnetosheath regions, and broadband electric field fluctuations from the auroral zone.
NG43B-02 INVITED
A New Technique Using Electron Velocity Data From the Four Cluster Spacecraft to Explore Magnetofluid Turbulence in the Solar Wind
It is now possible in certain circumstances to use velocity moments computed from the Plasma Electron and Current Experiment (PEACE) on the four Cluster spacecraft to determine a number of turbulence properties of the solar wind, including direct measurements of the vorticity and compressibility. Assuming that the four spacecraft are not co-planar and that there is only a linear variation of the plasma variables across the volume defined by the four satellites, one can estimate the curl of the fluid velocity, i.e., the vorticity. From the vorticity it is possible to explore directly intermittent regions in the solar wind where dissipation is likely to be enhanced. In addition, one can estimate directly the Taylor microscale.
NG43B-03 INVITED
Turbulent Cascade Dynamics in the Interplanetary Plasma
For years it was common to assume that the spectrum of interplanetary fluctuations was a remnant signature of solar photospheric sources imparting a spectrum on propagating Alfven waves as they passed through the acceleration region and into the solar wind. Such a spectrum was merely a remnant of the acceleration region dynamics and assumed to possess no intrinsic evolution apart from wave propagation and weakly nonlinear WKB dynamics. We now know that while there may be elements of truth to the view that the early solar wind possesses a remnant signature of the acceleration region, the eventual evolution of the interplanetary spectrum is the result of interplanetary MHD dynamics that result in a self-generated spectrum of magnetic and velocity field fluctuations. We know this because we can measure directly the rate at which energy cascades through the inertial range spectrum and we can compare this rate to the rate at which the background plasma is heated. We find these rates to be in good agreement. We can also show that the MHD dynamics are acting to move energy toward perpendicular wave vectors as shown in numerous simulations in a manner that is likely to result in a nearly 2D geometry which is also seen in numerous simulations of MHD turbulence.
NG43B-04 INVITED
Nonlinear Processes at the Heliospheric Boundaries
With both Voyager 1 and 2 having crossed the heliospheric termination shock recently and now traversing the inner heliosheath, it is appropriate to review the nonlinear processes and structures in the heliospheric boundary region. Time permitting, four topics will be discussed. 1) Our current understanding, based on Voyager 2 magnetic field and plasma measurements, of the structure of the heliospheric termination shock will be reviewed. A basic theory describing the observations will be presented. 2) Turbulence in the heliosheath appears to be quite different from that observed in the supersonic solar wind. We will discuss briefly the processing of supersonic solar wind fluctuations and turbulence by the termination shock, showing that the downstream turbulence can become quite compressive. 3) Large-amplitude nonlinear structures have been observed in the heliosheath, some possessing a distinctive single hole or hump structure. A model describing the Voyager 2 observations will be presented, and the close correspondence between model results and observations will be shown. Finally, 4) in the outer heliosheath, beyond the heliopause, the evolution of turbulence will occur in a partially ionized environment (a mix of neutral H interstellar gas and weakly shocked interstellar plasma). Recent simulation results describing turbulence in a partially ionized plasma will be presented, together with some theoretical extensions to Kolmogorov theory.