SM43B-01 13:40h
Universal Aspects of the Local Plasma Environment
There are two long-range forces that determine the structure and behavior of matter in the universe: gravitational and magnetic. Traditional astrophysics emphasizes the first and plasma astrophysics the second. Regarding the second, fifty years of investigating the local plasma environment-that accessible to in situ measurements by spacecraft but including the sun-has revealed structures and processes that appear to be universal and, thus, to characterize modes in which plasma physics operates at least on the scale of the local cosmos. These universal aspects of cosmic plasma behavior can be divided into five categories: creation and annihilation of magnetic fields, explosive energy conversion, generation of penetrating radiation, spontaneous generation of structures and transients, and magnetic coupling. This talk will illustrate the five categories with solar system examples, describe their treatment by scaling and dimensional analysis, and as an applied example, show that Earth's magnetosphere manifests two such modes of universal behavior.
SM43B-02 INVITED 13:55h
Physical Processes in the Solar Interior: the Ultimate Sun-Weather Engine
The ultimate source of the magnetic fields that drive the Sun-Weather is located in the solar interior, where the dynamo mechanism (or mechanisms) operate. To model these processes, it is necessary to combine the tools of plasma physics (MHD) together with those of stellar structure and evolution, and large eddy simulations (LES) of the convective envelope. Because of the differences in methodologies and space and time scales, it has not been possible, until now, to produce models that incorporate all aspects of the problem. However, the modeling of each separate component has been progressing with ever increasing sophistication, and the first attempts are being made to combine several approaches at once. This presentation will describe where these efforts are at the present time, what the next steps are likely to be, and what should ultimately be done in order to produce a dynamo model capable of explaining (and forecasting) the most significant aspects of Sun-Weather.
SM43B-03 14:15h
Relation Between Magnetic Fields and Electric Currents in Cosmic Plasmas
Maxwell's equations allow the magnetic field ${\bf B}$ to be calculated if the electric current density ${\bf J}$ is assumed to be completely known as a function of space and time. The charged particles that constitute the current, however, are subject to Newton's laws as well, and ${\bf J}$ can be changed by forces acting on charged particles. Particularly in plasmas, where the concentration of charged particles is high, the effect of the electromagnetic field calculated from a given ${\bf J}$ on ${\bf J}$ itself cannot be ignored. Whereas in ordinary laboratory physics one is accustomed to take ${\bf J}$ as primary and ${\bf B}$ as derived from ${\bf J}$, in plasmas ${\bf B}$ may be viewed as primary and ${\bf J}$ as derived from ${\bf B}$ simply as $(c/4\pi )\nabla \times {\bf B}$ -- a view proposed by Cowling (1957) and Dungey (1958) and in recent years strongly argued by Parker. Here I investigate the relation between $\nabla \times {\bf B}$ and ${\bf J}$ in the same terms and by the same method as previously applied to the MHD relation between electric field and plasma bulk flow [Vasyli\=unas, {\it Geophys. Res. Lett.}, {\it 28}, 2177--2180, 2001]: assume that one but not the other is present initially, and calculate what happens. The result is that, for all configurations with spatial scales much larger than the electron inertial length, (1) a given $\nabla \times {\bf B}$ produces the corresponding ${\bf J}$, (2) a given ${\bf J}$ does not produce any $\nabla \times {\bf B}$ but disappears instead. The result can be understood by noting that $\nabla \times {\bf B} \neq (4\pi/c){\bf J}$ implies a time-varying electric field (displacement current) which acts to change both terms (in order to bring them toward equality); the changes of the two terms, however, proceed on different time scales, light travel time for ${\bf B}$ and electron plasma period for ${\bf J}$, and clearly the term changing much more slowly is the one that survives. (The electron inertial length, by definition, is where the two time scales are equal.) Some illustrative simple examples will be discussed, together with implications for determining magnetic fields in space plasmas.
SM43B-04 14:30h
Intermittency, Self- Similarity and a stochastic dynamical model for Solar Wind Turbulence
The solar wind provides a natural laboratory for observations of MHD turbulence over extended temporal scales. We identify approximate self-similarity in the Probability Density Functions (PDF) of fluctuations in certain solar wind in- situ bulk plasma parameters. Whereas the fluctuations of speed for example are multi-fractal, we find that under certain conditions the fluctuations in the ion density, field and particle energy densities and MHD-approximated Poynting flux are approximately self-similar on timescales up to tens of hours. The intermittency of thesystem is then expressed in these parameters through the non-Gaussian nature of the single curve that describes the fluctuations PDF up to this timescale. Self- similarity implies that a simple Fokker-Planck model exists for the timeseries and we derive this here along with the associated Langevin equation- a stochastic dynamical equation for the fluctuations in the timeseries. This is an example of a generic approach to turbulence, with connections with our understanding of the statistical mechanics of correlated systems generally.
SM43B-05 14:45h
The Role of Interchange Reconnection at the Sun, in the Heliosphere, and in Earth's Magnetosphere
"Interchange reconnection" is the name given to reconnection between open and closed magnetic field lines. It interchanges closed loops, and it transports open field lines over distances comparable to the spans of the reconnecting loops. First recognized as the process for maintaining coronal hole boundaries on the Sun in the face of differential rotation, it has more recently become a touchstone for understanding a wide range of solar and heliospheric phenomena like the opening of magnetic fields in interplanetary coronal mass ejections, the release of plasma parcels at the tops of helmet streamers, and the global circulation of solar magnetic footpoints and its wide-ranging implications. Most recently, interchange reconnection has been recognized as a magnetospheric process. As in solar models of global circulation, interchange reconnection prevents the build up of magnetic flux in magnetospheric convection flows by allowing open field lines to leap over volumes of closed field lines. With the aid of numerical models, we report on progress in applying this concept to understand convection patterns during periods of northward interplanetary magnetic field.
SM43B-06 15:00h
The Effects of Topology on Magnetic Reconnection
Magnetic reconnection is widely believed to be the dominant process by which plasma and magnetic field exchange energy in the cosmos. Although certain aspects of reconnection are universal, the nature of the process depends strongly on the particular topology of the reconnecting system. In the Earth's magnetosphere, the topology is fixed -- a four flux system with a pair of nulls and separators. In the Sun's corona, on the other hand, the topology can vary greatly depending on the complexity of the active region. We argue that the usual coronal topology is a two-flux system with an isolated 3D null, but four flux systems that are topologically equivalent to the magnetosphere are possible. We contrast and compare the dynamics of reconnection for these two topologies. We present both theoretical models and fully 3D simulations using ARMS, the NRL adaptively-refined MHD solver. The implications of the results for observations will be discussed. This work was supported in part by NASA and ONR.
http://solartheory.nrl.navy.mil
SM43B-07 INVITED 15:15h
Experiments on skin depth thick current sheets with Alfv\'{e}nic Scintillations
Exploring the formation and dynamics of sheet-like current structures in plasmas is key to understanding phenomena ranging from auroral arcs, to reconnection in the earth's magnetosphere, to disruptions of tokamak plasmas. We present results from a laboratory experiment designed to produce a thin electron sheet of current within a uniform, magnetized ambient plasma. The experiment is conducted in the Large Plasma Device (LaPD) at UCLA. The linear device is 20 meters in length and one meter in diameter. The electron current sheet is produced using a thin, biased copper plate inserted into the plasma. The initial dimensions of the sheet are 0.4 by 45 ion Larmour radii, $\rho_{i}$, perpendicular to $B_{0}$. Two main observations are made. First, in addition to the electron flow in the sheet, a thin ($2\rho{i}$) sheet of parallel ion flow is measured using a mach probe. This flow has an 'S'-shape in the cross-field plane. Second, a deep (20%) and broad ($8\rho_{i}$) density cavity forms which gives rise to the spontaneous growth of drift-Alfven waves along the steepest density gradient contours. These waves, which appear as spontaneous scintillations are measured by correlating the fluctuating density and magnetic fields on planes parallel to the background field. The waves are also correlated with cross-field particle flux and lead to a relaxation of the density gradient. The correlations are made apparent in computer generated movies. The current sheet is also host to high frequency waves ($\omega \ge \omega_L_H$) as measured with microprobes. An attempt will be made to relate the experimental results to naturally occuring current systems in space.