Plasma Seminar Abstracts: Winter 2002 |

Last Updated: Tue Feb 12 11:49:19 EST 2002

January 08, 15, 22, 29 February 05, 12, 19, 26 March 05

Coronal Mass Ejections: The Calm Before the Storm

by

Dr. Richard Wolfson

Department of Physics

Middlebury College

Abstract

Coronal mass ejections (CMEs) have been called "the most energetic events in the solar system." But where does all that energy come from? Theory and observation alike suggest that the energy is stored gradually in the corona, over a timescale of days or more. The sudden release of that stored energy powers mass ejections. But what form does the stored energy take? How does it get into the corona? What triggers its release? This talk explores these questions about what goes on in the relatively quiet time leading to the eruption of a CME.

3:30 p.m.

Tuesday, January 08, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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Two-Dimensional Channel Flow Simulations

by

Brian Kress

Department of Physics and Astronomy

Dartmouth College

Abstract

In addition to providing interesting non-linear dynamics, investigation of the two-dimensional Navier-Stokes equation gives us insight into the dynamics of oceanographic and atmospheric flows. We solve the Navier-Stokes equation in 2D "channel" geometry. A spectral method is used to advance the flow. It is a Galerkin approximation using a set of expansion functions which individually obey the boundary conditions at the walls. Attention is paid to the effects of rigid wall boundary conditions on decaying turbulence in two dimensions. There is an emphasis on finding and characterizing large-scale coherent structures which persist at late times in the decay.

3:30 p.m.

Tuesday, January 15, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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Modes of the Magnetosphere: Toroidal, Poloidal, and Compressional

by

Dr. Richard Denton

Department of Physics and Astronomy

Dartmouth College

Abstract

The three basic MHD waves are the fast/magnetosonic wave, the Alfven wave, and the slow/ion acoustic wave. In the magnetosphere, the Alfven wave has two limits depending on the azimuthal mode number; these are the toroidal mode (with azimuthal magnetic fluctuations) and the poloidal mode (with radial magnetic fluctuations). We review some basic properties of these waves, and then present new results. Our recent studies have examined the spatial dependence of the toroidal Alfven frequency, used toroidal Alfven frequencies to infer mass density along field lines, examined the radial structure of the poloidal mode, and calculated fast/magnetosonic (cavity mode) resonance frequencies in the plasmasphere.

3:00 p.m.

Tuesday, January 22, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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Limits on Transpolar Versus Reconnection Potentials: The Unknown Magnetosphere

by

Dr. George Siscoe

Center for Space Physics

Boston University

Abstract

MHD simulations give about the same dependence of transpolar potential on solar wind electric field (IEF) as the Hill model of transpolar potential saturation. MHD simulations suggest further that transpolar potential saturation most likely results from solar wind ram pressure limiting the amount to which region 1 currents can grow rather than from a limit on total reconnection potential. Total reconnection potential appears to be unrestricted by the ram-pressure limitation and continues to grow approximately linearly with IEF. The resulting difference between transpolar potential and reconnection potential that arises when the magnetosphere enters the saturation domain implies parallel potential drops, which, during magnetic storms, can be tens to hundreds of kilovolts.

3:00 p.m.

Tuesday, January 29, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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Open Spot

by

3:00 p.m.

Tuesday, February 5, 2002

Room 202 Cummings Hall

Thayer School of Engineering

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The Maryland Centrifugal Experiment (MCX) : Centrifugal Confinement for Magnetic Fusion Energy

by

Dr. Richard Ellis

Institute for Electronics and Applied Physics

University of Maryland

Abstract

The concept of centrifugal confinement is to employ centrifugal forces from supersonic rotation perpendicular to B to enhance magnetic confinement along B, allowing an open magnetic configuration. Nonuniform plasma rotation produces large velocity shear to stabilize MHD instabilities and possibly quell microturbulence, as well as provide viscous heating. MCX is a modest sized experiment designed to test this concept. The design principles and status of MCX will be summarized.

http://www.ireap.umd.edu/mcx/

3:00 p.m.

Tuesday, February 12, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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A Simple Parameterization of the High Latitude Ionospheric Convection Pattern for use in Space Weather Metrics

by

Dr. Kile Baker

Magnetospheric Physics Program Director

National Science Foundation

Abstract

The National Space Weather Program (NSWP) requires the development and use of appropriate metrics for validating space weather models. One of the obvious metrics for magnetospheric modeling is a comparison of the high-latitude plasma convection pattern produced by the model with observational data. However, comparing the results of a model with physical measurements is not always a simple task, particularly when dealing with 2-dimensional data such as the convection pattern. The initial metric established for magnetospheric modeling was to compare the model electric fields with the electric fields measured by a DMSP satellite pass. Small errors in the positioning of the model convection pattern, or a systematic error in the magnitude of the electric field can result in a large error in model vs observation metric. We therefore seek a simple parameterization of the convection pattern that will give us independent pieces of information about the magnitude of the polar cap potential drop and the morphology of the convection pattern.

Data from the SuperDARN radars will be used to demonstrate one possible simple parameterization of the convection pattern. With this parameterization we can also use linear regression analysis to examine how the convection pattern is affected by the interplanetary magnetic field and the solar wind velocity and density.

3:00 p.m.

Tuesday, February 19, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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What We Would Like to Know About Magnetosphere-Ionosphere Coupling

by

Dr. William Lotko

Thayer School of Engineering

Dartmouth College

Abstract

Why is aurora more intense at night, in winter and during solar minimum? What is the vertical structure of an auroral acceleration region, what determines the altitude of formation, and how does it vary with time? How are scale-interactive processes activated, and how do they regulate energy deposition in the ionosphere and lower magnetosphere? Are cusp ion injections caused by patchy or pulsating magnetic reconnection? Are their austral and boreal signatures synchronous and conjugate? What is the genesis of ionospheric outflow, and what are its magnetospheric impacts? What is the role of auroral substorms in the development of geomagnetic storms? This seminar will describe how in situ and optical measurements from variable-separation, dual spacecraft in low-earth polar orbit can be combined with regional and global dynamic models to resolve longstanding problems in magnetosphere-ionosphere coupling.

3:00 p.m.

Tuesday, February 26, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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Where Do Large-Scale, Spontaneously-Appearing Magnetic Fields Come From?

by

Dr. David Montgomery

Department of Physics and Astronomy

Dartmouth College

Abstract

One of the most common features seen in astrophysical and geophysical settings is the spontaneous generation of large-scale magnetic fields in electrically conducting media: the "dynamo effect." Turbulent motions in electrically conducting fluids naturally generate enhanced magnetic fields through stretching of flux tubes, any time the electrical conductivity is high enough. This mechanism would appear to generate disordered magnetic fields at scale sizes as small as, or smaller than, the scales of the mechanical motions, and it is something of a mystery as to how the magnetic excitations get transferred to the largest spatial scales available. A mechanism proposed in 1975, and verified by numerical computation in 1981, is the inverse cascade of magnetic helicity. This is a non-intuitive statistical mechanism discovered by analogy with the inverse cascade phenomenon in two-dimensional Navier-Stokes fluids. Here, we show numerically that the presence of a uniform dc magnetic field in effect suppresses the inverse magnetic cascade. The framework used is one that has become standard in turbulence theory and computation: triply-periodic rectangular boundary conditions in space. We have given arguments [1] that, for reasons that do not apply to hydrodynamics, such boundary conditions are inconsistent with 3D magnetohydrodynamics in situations with a dc magnetic field present. Thus we believe the observed suppression to be an artifact of the numerics. We suggest [2] other boundary conditions and geometries in which we believe the inverse cascade of magnetic helicity will not be suppressed by the presence of a dc magnetic field, and where dynamo action will again be operative.

[1] D.C. Montgomery and J.W. Bates, Phys. Plasmas 6, 2727 (1999).

[2] http://arXiv.org/abs/physics/0202027

3:00 p.m.

Tuesday, March 5, 2002

Room 200 Cummings Hall

Thayer School of Engineering

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