SM54A-01
Multiscale Complexity Reflects Substorm Dynamics: Toward Designing a New Auroral Index
Earth's magnetosphere is known to generate complex spatiotemporal patterns of activity reflecting its strongly nonlinear response to the solar wind driver as well as the intrinsic stochasticity of plasma phenomena in the near-Earth environment. On average, and over large time scales involving many typical susbstorm loading- unloading times, this activity tends to self-organize into a nearly perfect scale-invariant dynamical state reminiscent of that in critical "avalanching" sandpile models. In this talk, we present evidence that on shorter time scales the magnetosphere exhibits distinct deviations from the scale-free statistics that correlate with the development of the substorm. Based on an analysis of auroral images from POLAR and IMAGE satellites, we propose a set of scaling parameters of auroral activity that can provide some essential new information on substorm phases and location and have the potential of being a new dynamical measure of the geoeffectiveness of the solar wind input under a variety of IMF conditions. Our results confirm that the magnetosphere operates as a critical avalanching system on a global scale, but they also show that the individual energy release events underlying this critical state seem to be shaped by a variety of inhomogeneous and transient processes violating most of the symmetries assumed in simplistic sandpile models. http://phas.ucalgary.ca/~vuritsky/SM07
SM54A-02 INVITED
The observed scaling properties of fluctuations in the solar wind and in geomagnetic indices: intermittent turbulence and coronal driver.
The solar wind provides a natural laboratory for observations of MHD turbulence over extended temporal scales. We quantify the ‘macroscopic' scaling seen in extended intervals of solar wind by testing for scaling in the Probability Density Functions (PDF) of fluctuations in the timeseries both directly and via structure function analysis. In practice there are statistical limitations presented by a finite length time series which we will first discuss. The anisotropic nature of solar wind fluctuations can be accessed by decomposing the vector velocity linearly into two coexistent components perpendicular and parallel to the local average magnetic field. These show distinct scaling. That of the perpendicular fluctuations is consistent with recent predictions for anisotropic MHD. That of the parallel fluctuations is close to the scaling which we find in the number and magnetic energy density, and Poynting flux. One interpretation of the co- existence of these scalings in the solar wind is that they reflect both local and nonlocal phenomenologies, with implications for our understanding of the evolving solar wind. Intriguingly, a more detailed analysis of magnetic energy density reveals a solar cycle dependence, and at solar maximum, self affine rather than multifractal scaling, suggesting the scaling is of solar origin. To see how these fluctuations impact on magnetospheric activity, we consider the same analysis performed on fluctuations of the AU and AL geomagnetic indices that provide a measure of magnetospheric activity, and of the epsilon parameter which is a measure of the solar wind driver.
SM54A-03
Do Sandpile Models Explain Observations of Complex Dynamics in Solar Flares and Polar Aurora?
Many complex systems in nature are at present not accessible for description based on first physical principles, partly due to the complexity of the dynamics and partly due to lack of necessary observational data. For some phenomena, like solar flare activity or optical aurora, detailed spatiotemporal information is only available as fluctuations of a scalar 2D radiation field emitted from the system, while the internal 3D dynamics is not accessible to direct observation. Analysis of such fields presented to this date strongly indicates that some astro and geospace systems may exhibit simultaneous statistical signatures that are traditionally attributed to either intermittent turbulence or self-organized avalanche dynamics. A paradigm for the latter is the Bak-Tang-Wiesenfeld (BTW) sandpile model or related models, but it is not obvious, however, if and how it is possible to derive from this model a scalar field with the properties observed in these geospace phenomena. The problem is that the sandpile toppling activity field has only two states at every site (toppling or non-toppling), while the occupation number field looks basically like random noise in space, and hence neither have the properties of the observation data that are used to define patches that are interpreted as avalanches, and which allows computation of structure functions. In this work we consider an approach by which the binary toppling field can be transformed into a field with a continuous range. This feature is necessary for large patches to be defined by a threshold condition, and for generation of structure functions which are not flat and trivial. The method involves hypothesizing that a toppling site emits radiation which does not disappear at the next time step, but decays with a given time constant. This creates a field with continuous range and, if the time constant is long, a much smoother spatial structure. We analyze this field generated from numerical simulations of the BTW sandpile by the same methods which have been used for observational data, and compare the results with those from observations and with avalanche statistics obtained from the traditional treatment of the BTW sandpile.
SM54A-04 INVITED
Crossover Phenomenon and Dynamical Complexity of Intermittent Turbulence in Space Plasmas
The ability of plasmas to form multitudes of coherent structures with varying sizes and the resulting interactions of such entities have been suggested as the prime reason for the abundance of observations of intermittent turbulence in the space environment. Such phenomenon bears the hallmark of dynamical complexity. One procedure to gauge the intermittency of plasma turbulence is via the method of multifractals using the structure and/or partition functions. We demonstrate that the crossover phenomenon of the exponents for the structure and partition functions with moment order may be understood in terms of the generalized concept of scale invariants. Relations between the ideas of self-organized criticality and intermittent turbulence will be discussed. Theoretical concepts and results of analyses of dynamical models, direct numerical simulations, and observational data will be presented to elucidate such ideas and concepts.
SM54A-05 INVITED
Multiscale magnetospheric physics from a simple model of self-organized dynamics
Distributions of geomagnetic indices and aurora have been used to show the nonlinear characteristics of magnetospheric dynamics. However, the physics underlying these distributions is often not clear. Self organization of micro-scale perturbations has been suggested as a possibility whereby robust power-law distributions of magnetospheric dynamics indices can be produced. Earlier we proposed that some aspects of observed substorm distributions can be explained qualitatively by a model featuring interactive discrete flux tubes which are used to simulate the central plasma sheet dynamics driven by a constant energy input. A one- dimensional simulation of the model yielded scale-free distributions of auroral activity and quasiperiodic injection. This result raised the question what dynamical category the substorm belongs to, scale-free or with definite scales. In this talk, we present the latest two-dimensional simulation and auroral observational results, in an attempt to elucidate the dynamical nature of the substorm.
SM54A-06
Modeling the Turbulent Reconnection Dynamics of Earth's Magnetotail Plasma Sheet
Substorms, regardless of how they are triggered, lead to global relaxations of the magnetotail as excess magnetic flux and energy are released both earthward and tailward through reconnection in the plasma sheet. Throughout all substorm phases, in particular during the expansion phase, reconnection remains intermittent, impulsive, spatially localized, and distributed over a significant portion of the plasma sheet. The plasma sheet is turbulent. The turbulence is strong; it may be best characterized as eddy turbulence that is driven by localized fast flows acting as jets in the plasma sheet. The turbulence has been studied extensively; its existence, many of its scaling properties, and its direct relationship to the localized fast flows has been well established. In the magnetotail, substorms are organized global events. Nevertheless, reconnection in the magnetotail, a critical element in the substorm evolution, remains sporadic and embedded in a turbulent plasma sheet whose turbulence is largely generated by the reconnection and, likely, whose turbulence plays a role in triggering the reconnection. What is the relationship between these interior stochastic multiscale phenomena and the organized global substorm? We are investigating this question using a 3-D driven reconnection model. The model is based on the full MHD system with hysteretic-thresholded current-driven resistivity added to include some effects of kinetic phenomena that are beyond the MHD approximation. We will show that under continuous driving (1) the model evolves into a loading-unloading cycle reminiscent of the substorm loading-unloading cycle, (2) that unloading is supported by multiple intermittent localized reconnection sites in the simulation volume, and (3) that reconnection in the model drives intermittent turbulence with characteristics similar to those observed in the plasma sheet.
SM54A-07 INVITED
Reconnection Outflow Associated Multi-Scale Statistics in the Earth's Plasma Sheet
The objective of the presentation is to asses the specific spectral scaling properties of magnetic reconnection associated fluctuations/turbulence at the Earthward and tailward outflow regions observed simultaneously by the Cluster and Double Star spacecraft. The comparisons provide further evidence for asymmetry of physical processes in Earthward/tailward reconnection outflow regions. The spectral parameters and spectral anisotropy angles estimated from the multi-scale magnetic fluctuations in the tailward outflow region show features which are characteristic for magnetohydrodynamic cascading turbulence in the presence of a local mean magnetic field. The multi-scale magnetic fluctuations in the Earthward outflow region are exhibiting more power and larger anisotropies, but the scale-dependent anisotropy signatures of a turbulent cascade are missing. In this region the magnetic field is more dipolar, the main processes driving fluctuations are flow breaking, dipolarization or tail current disruption for which the scale dependent spectral characteristics were not studied in detail yet.
SM54A-08
Study of turbulence in the central plasma sheet during quiet and substorm time intervals using the Interball-Tail satellite data
Recent studies are shown that the turbulent processes in the space plasmas are very important. It includes the behavior of the plasma sheet plasma during geomagnetic substorms. Study of the plasma turbulence in the central plasma sheet were made using the Corall instrument onboard Interball-Tail satellite. Fluctuations of the plasma bulk velocity across the plasma sheet, and corresponding eddy-diffusion coefficients, were calculated for different locations inside the plasma sheet. It was found that all components of eddy diffusion coefficients have a very wide range of values, generally between 104 and 106 km2/s. However, their average values increase with the distance from the Earth in the tailward direction for both quiet and substrom time intervals. Nevertheless, it also was the values of eddy-diffusion coefficients increase a few times during substorms in comparison with quiet time intervals, especially at the distances of 15-20 Earth's radii. We also found that the eddy-diffusion coefficients increase with the ion temperature and decrease with the ion number density. These studies made it possible to create a three-dimensional distribution of the eddy-diffusion coefficients for quiet and substorm times and compare with those predicted by the Antonova and Ovchinnikov (1998) model for the turbulent plasma sheet.