| Plasma Seminar Abstracts: Winter 2004 |
Some new results in the statistical mechanics of vortices
Dr. David Montgomery, Dartmouth College
Charge densities in a strong dc magnetic field are a source of vorticity. Vortex motion is the principal kind of incompressible motion available to continua inside a closed container. Strong dc magnetic fields are known to suppress spatial variations in plasma quantities in the direction parallel to themselves, leading to an essentially two-dimensional motion. If it is demanded that the ExB drift motions of a plasma be electrostatic and self-consistent (i.e., that the drifts are produced by the electric fields which the charges themselves generate), the dynamics become identical with those of a two-dimensional neutral fluid. This is perhaps the simplest strongly nonlinear evolution that can be considered in all of continuum mechanics in more than one dimension. The turbulent evolution of two-dimensional Navier-Stokes fluids (or guiding center plasmas) may be studied numerically, and the results can be predicted to a considerable extent with an unorthodox statistical mechanics involving negative temperatures. Doing any kind of combinatorics requires an idealized discretization of the fields and here, we consider the differences between two such idealizations: those starting with delta-function line (or "point") two-dimensional vortices, and those that start with mutually-exclusive, finite-area "patches" of vorticity. This is the difference between what are commonly called "Boltzmann statistics" and "Lynden-Bell statistics." It is interesting that the predictions of turbulent evolution in some cases depend sensitively on the choice of model for the vorticity field.
Reference: Z. Yin et al, Phys. Fluids 15, 1937 (2003).- top - back -
Interstellar Matter to Touch - Exploration Beyond the Solar System
Dr. Eberhard Möbius, Space Science Center and Department of Physics, University of New Hampshire
Abstract.
The solar wind carves out a large void into the surrounding interstellar plasma, the heliosphere. Because the sun moves relative to the local interstellar cloud at about 26 km/s an interstellar wind of neutral gas blows through our solar system, and in spite of the size of the heliosphere we have already access to in-situ sampling of interstellar matter. This surrounding interstellar cloud contains an accessible sample of cosmic material that does not belong to our solar system and therefore presents a window into the evolution of galactic matter. Great progress in determining parameters and composition of this local interstellar material and its interaction with the heliosphere has been made by utilizing pickup ions and direct neutral gas imaging. Recent progress with low energy neutral atom instrumentation and neutral imaging of the heliospheric boundary will allow further breakthroughs in the understanding of the interaction of the heliosphere with its neighborhood.- top - back -
X-ray emission due to solar wind charge exchange with interstellar and geocoronal neutrals
Dr. Ina Robertson, University of Kansas
X-rays are generated throughout the heliosphere and the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and interstellar and geocoronal neutrals. In this seminar I will first discuss solar wind charge transfer with geocoronal hydrogen. The highest production rate of this type of radiation is in the cone region of the magnetosheath, where the solar wind density is high. This might be observable from an observation point outside the geocorona. I will next discuss solar wind charge exchange with interstellar neutrals. We estimate that heliospheric X-ray emission accounts for roughly half of the observed soft X-ray background intensity. I will discuss the impact this estimate might have on our view of the local bubble.- top - back -
Cluster Observations of Channels of Enhanced Convection Velocity in the Ring Current Region
Dr. Pamela Puhl-Quinn, Space Science Center and Department of Physics, University of New Hampshire
For decades, spacecraft-borne instruments have detected enhanced convection features in the duskside and premidnight subauroral region, variously known as polarization jet [Galperin et al., 1973] or subauroral ion drifts (SAID) [Spiro et al., 1979]. We report Cluster observations of this phenomenon, detected with the electron drift instrument (EDI) which uses a novel particle-detection method to measure electric fields in the plane perpendicular to the magnetic field. We have identified enhanced convection velocity striations within the dusk-side ring current region. Such striations with durations 1-5 minutes were observed in AMPTE/IRM data [LaBelle et al., 1988] and also may be the source of fine features observed recently in ground-based radar observations of SAID [Erickson et al., 2002]. Cluster, due to its more polar orbit, finds them on time scales of tens of minutes implying that the channels are extended along magnetic flux tubes. A survey of Cluster/EDI data from February, 2001 until July, 2003 was performed, using data from 14 to 24 hours local time, from 60-69 degrees invariant latitude, and for Kp > 4. Enhanced electric fields were defined as those in the anti-corotation direction (westward) with amplitude exceeding twice the corotation speed. The data show a tendency for the enhanced electric fields to occur at lower latitudes in premidnight/midnight local time than on the duskside, consistent with previous observations. Many previous observations report widths as narrow as 0.1 degrees for SAID events, and for example the 1-5 minute crossing times at IRM imply striation widths on the order of 1000-2000 km. The variable separations of the Cluster spacecraft over the multi-year data set provides a unique opportunity to improve the determination of this width. In this talk, we elaborate about statistical properties, theoretical implications, and association with the SAPS (sub-auroral polarization streams) and SAID phenomena.
Erickson, P.J., et al., Inferred electric field variability in the polarization jet from Millstone Hill E-region coherent scatter observations, Radio Sci., 37, 10.1029/2000RS002531, 2002.
Galperin, Yu., et al., Direct measurements of ion drift velocity in the upper ionosphere during a magnetic storm, Cosmicheskie Issled., 11, 273, 1973.
LaBelle, J., et al., The duskside plasmapause/ring current interface: Convection and plasma wave observations, J. Geophys. Res., 93, 2573, 1988.
Spiro, R.W., et al., Rapid subauroral ion drifts observed by Atmosphere Explorer C, Geophys. Res. Lett., 6, 660, 1979.- top - back -
Anisotropic hydromagnetic turbulence in space and solar physics
Dr. Leo Milano, Bartol Research Institute, University of Delaware
A broad range of astrophysical phenomena occur on plasmas embedded in large-scale magnetic fields. A similar scenario is also ubiquitous in plasma laboratories. In particular, these plasmas may develop low frequency, fluid-like turbulence. But unlike regular fluid turbulence, magnetofluid turbulence is inherently anisotropic when a large-scale magnetic field is present. In this seminar I will present a brief discussion of this topic, and an overview of some of our work in this topic over the last years:
(i) local anisotropy;
(ii) anisotropy in solar coronal heating models;
(iii) solar wind turbulent correlation measurements.
The talk is intended for a fairly general physics and astronomy audience.- top - back -
Relativistic Quasilinear Diffusion
Dr. Alain Brizard, St. Michael's College, Colchester VT
A relativistic bounce-averaged quasilinear diffusion equation is derived to describe stochastic particle transport associated with arbitrary-frequency electromagnetic fluctuations in a nonuniform magnetized plasma.
Expressions for the elements of the full relativistic quasilinear diffusion tensor are calculated explicitly from first principles for magnetically-trapped particle distributions in axisymmetric magnetic geometry.- top - back -
Ionospheric feedback instability at high and low latitudes
Dr. Anatoly Streltsov, Thayer School of Engineering, Dartmouth College
This talk is about how the small-scale, intense electric fields and currents can be generated in the low-altitude magnetosphere by the interaction between large-scale, slowly evolving, field-aligned current system and the ionosphere. The study is based on numerical simulations of the reduced, two-fluid MHD model describing interaction between shear Alfven waves and the ionosphere. It is shown that the small-scale electromagnetic structures can be generated by the ionospheric feedback mechanism in the downward current channels. The application of these results to the observations performed by satellites above the auroral and sub-auroral ionosphere is discussed.- top - back -
The source and acceleration of energetic He+
Dr. Harald Kucharek, Space Science Center and Department of Physics, University of New Hampshire
Over the past 10 years substantial progress has been made in the understanding of shock acceleration since the advent of powerful enough simulations of the related processes. However, there is still a debate about the injection of particles from the original thermal or suprathermal distributions into the acceleration process. What percentage of particles is actually accelerated under given circumstances? What causes the widely observed selective acceleration? This has been known as the “injection problem”. Recent observations at interplanetary disturbances have shown that the He+ that originates from interstellar pickup ions represents the third most abundant ion species in the energetic particle population in the heliosphere, because it is apparently injected and accelerated with very high efficiency compared with coronal and solar wind He2+. Therefore, the ion pair He+ and He2+ represents an ideal probe for the injection efficiency, because it comes from two well-defined sources with different velocity distributions. In addition, pickup ions (of which He+ is a key example) are known to be the source of anomalous cosmic rays. In this talk I present recent results from a long-term observations of energetic He+/He2+ ratio by ACE/SEPICA and SOHO/CELIAS/STOF to determine the source of energetic He+ and their contribution to solve the injection and acceleration problem at interplanetary discontinuities.- top - back -
The SERSIO mission: rocket, EISCAT, and ground camera observations of reconnection and ion outflow in dayside aurora
Dr. Kristina Lynch, Dartmouth College
Abstract not available.- top - back -
Magnetohydrodynamic turbulence in coronal heating and particle acceleration
Dr. Pablo Dmitruk, Bartol Research Institute, University of Delaware
Development of magnetohydrodynamic (MHD) turbulence in space and astrophysical plasmas, involving complex motions across a wide range of scales, provides a mechanism by which the energy in low-frequency and large scale motions is transferred to small scales, where it can be more efficiently dissipated. I will talk about a model for the heating of the lower solar corona based in that idea. Sustainment of turbulence becomes an issue and I will discuss the conditions which favor that situation, based on numerical simulations of the MHD equations. The results have also implications for models of the origin of the solar wind. Another situation where MHD turbulence may play a role is acceleration of charged particles in short time scales. I will talk about the relation with magnetic reconnection models and show test particle simulation results where substantial acceleration occurs.- top - back -
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