SM54A-01 INVITED
Three-dimensional current systems and turbulence generation by colliding plasmas in a background magnetoplasma.
Results are presented from an experiment in which two plasmas, initially far denser than a background magnetoplasma (n~ 2× 1012 cm -3 dia ~ 60 cm, L ~ 18m ), collide as they move across the background magnetic field (B = 600 G). The dense plasmas are formed when two laser beams, nearly orthogonal to the background magnetic field strike two carbon targets. The experiment is repeated at 1 Hz and data is acquited at 10 ns intervals and over ten thousand spatial locations. The merging plasmas are observed to carry large diamagnetic currents, which initially shield out most of the background field. A magnetic reconnection event is triggered by the collision and the induced electric field generates a field aligned current, which is the first step in the development of a fully three-dimensional current system. After several ion gyro- periods the current systems become those of shear Alfvèn waves. As local currents move, small reconnection" flares" occur at many locations throughout the plasma volume but they do not seem to affect the overall system dynamics. The data clearly show that the induced electric field is transported though the system by shear Alfvèn waves. The magnetic helicity ∫A· B dV is derived from the data and observed to grow with the Alfvèn wave currents and decay on the timescale of the wave magnetic energy. Osscillations in the helicity at the Alfvèn frequency are observed using a wavelet transfomation. Movies and data illustrating the three dimension-spacetime nature of the interaction will be presented.
SM54A-02
Phase- and group-velocity in the aurora and its relationship to laboratory plasma wave dispersion
The dispersion of inertial-scale Alfven waves in the near-Earth magnetosphere produces field-aligned electron populations with sufficient energy to excite the optical aurora. In these cases, luminous structure should reflect structure in the overlying wave field, thus providing a framework for comparison of auroral imagery with laboratory measurements of Alfven wave dispersion. The challenge lies in collecting images at sufficient cadence and resolution. An analysis of narrow-field video recorded at 50 images/s has revealed two distinct motions in dynamic aurora: a group motion associated with a translating wave packet, and a phase motion associated with the periodic substructure. The latter appears to be consistent with a standing wave pattern. Our working hypothesis is that these ephemeral field patterns result from a rapid drift between the wave source and the background plasma, which would produce Airy-like patterns in the auroral fine structure.
SM54A-03
Alfven Wave Cones in theory, Lab Experiments and Space
It is now well established that shear Alfven waves (SAWs) dump a large amount of electrognatic power in the topside auroral ionosphere. How is this energy dissipated? Obervations also show that the SAWs have narrow transverse structures. How are such narrow (filamentary) structures created from SAWs having long transverse wavelengths generated in the distant parts of the magnetosphere? We demonstrate by kinetic simulations that when long wavelength SAWs encounter localized density cavities, they induce electric dipoles in the cavities due to the divergence in the ion polarization current. The dipoles radiate narrow structures of Alfven waves like the inertial Alfven waves detected from Freja, FAST and Polar. The radiated wave patterns are the Alfven wave resonance cones. The existence of Alfven wave resonance cones has already been known theoretically as well as from laboratory experiments. Refined space observations have recently demonstrated that Alfven wave resonance cones do indeed occur and they introduce fine structures in the auroral electron accelerations and hence in the auroral arc structures. Comparing the structures in the Alfven wave resonance cone radiated from the localized cavities with satellite data we find that the transversely narrow structures of the inertial Alfven waves detected from Freja, FAST and Polar are indeed the Alfven wave resonance cones. The localized density cavities themselves are generated by double layers, which are an integral part of the auroaral acceleration processes. Thus we discuss the entire chain of processes involved in the dissipation of SAWs; the chain involves SAWs as the source of energy, the localized plasma density cavities, generation of nature-made electric dipole antennas in the cavities, radiation of short scale inertial Alfven waves, which facilitate the transfer of electromagnetic energy into the kinetic energy of the plasma ions. Thus the Alfven wave cones represent an important plasma phenomenon, which is predicted from theory, has been quantitatively measured in laboratory and now detected in space as well. It is also a strategically important phenomenon because it facilitates in the transfer of Alfven wave power into the ions' kinetic energy and thereby in their outflow.
SM54A-04 INVITED
Fast Reconnection and its Nonlinear Stabilization in Laboratory and Magnetospheric Plasmas
The onset of fast reconnection is widely studied in toroidal fusion plasmas, dedicated laboratory experiments, and by in situ satellite measurements in the Earth's magnetosphere. These observations place strong constraints on theory, which must explain not only a fast reconnection rate but also a sudden increase in the time- derivative of the reconnection rate, seen, for instance, in sawtooth oscillations in tokamaks and magnetotail substorms. We will show that such dynamics can be accounted for by Hall MHD reconnection models in one unifying framework. The criteria predicted by theory and simulation for the onset of fast reconnection has been tested recently by targeted laboratory experiments such as MRX (at Princeton) and VTF (at MIT). The theory also elucidates the role of diamagnetic drifts that can quench nonlinearly the onset of fast reconnection. Thus, the theory explains not only when reconnection is near-explosive, but also when it is not. We will compare the predictions of theory with data from tokamaks, laboratory experiments, and satellite observations.
SM54A-05
Occurrence and signatures of the Hall-mediated regimes of magnetic reconnection
A complete two-fluid description of the resistive tearing instability in a sheared force-free magnetic field is presented. A role of the Hall effect, which accelerates the pace of magnetic reconnection above that achieved in the single-fluid MHD case, is essentially determined by two parameters: the plasma beta, and the normalised ion inertial skin-depth d-i. Altogether, there are four different regimes of reconnection, each of which corresponds to the particular parameters domain in the (beta, d-i) plane. An important signature of the Hall-mediated magnetic reconnection is a quadrupole pattern of the out-of the reconnection plane component of the magnetic field, which has been already observed both in laboratory and space plasmas. Therefore, we also discuss the structure of this field component for all possible regimes of the two-fluid tearing instability.
SM54A-06 INVITED
High Beta Plasma Disruptions in Space and Laboratory Plasmas
Explosive magnetohydrodynamic (MHD) instabilities in laboratory plasmas, the solar corona and the Earth's magnetosphere play a major role in disrupting configurations associated with magnetic confinement and plasma energy storage. Central to our approach to this problem is the fact that the presence of resonances in Hamiltonian systems can have a destabilizing effect on the system as a whole. In Tokamaks, toroidally localized, high-n (toroidal) ballooning modes are driven to instability due to toroidally localized changes in the pressure gradient caused by low frequency, low-n modes. As an example of generic high beta disruptions we shall examine the nonlinear stability of the magnetic field topology and possible nonlinear plasma instabilities that might occur in the near Earth magnetotail (8-10 RE) during the substorm growth phase. These nonlinear instabilities lead to the initiation of the substorm intensification at the Earthward edge of the plasma sheet. Central to our model are ultralow frequency (1-4 mHz), normal modes (shear Alfven waves). The work we present is based, in part, on a Lagrangian-Hamiltonian approach, with possible further refinements on measures of nonlinear instability in MHD systems.
SM54A-07 INVITED
Laboratory Modeling of Space experiments on Expulsion of CO2 ions. Application to Global Warming.
An approach to expel minority species which can contribute to global warming from the upper atmosphere in the
Arctic region by the use of HF electromagnetic waves has been proposed [1]. Laboratory plasma experiments
have been designed to model various aspects of this concept - from the acquisiton of negative charges by green
house gases such as CO2 to their ascent to the upper atmosphere and their acceleration and expulsion along
the open magnetic field lines. Laboratory results are presented which confirmed the efficient gyro-resonance
acceleration of minority ion species made possible through the space charge cancellation by majority species.
The outflow of CO2 ions from the divergent magnetic field of a laboratory plasma device is measured at various
background neutral pressures and for different amount of currents along the axial magnetic field.
The central idea is to impart perpendicular energy to a selective ion species gyrating around the geomagnetic
field at its cyclotron resonance. The wave field is produced by either modulating the auroral electrojet or from the
nonlinear interaction between two electron plasma resonances. In the presence of the divergent polar
geomagnetic field the accelerated perpendicular ion velocity is converted into an upward motion along open
magnetic field lines. The ions thus removed will unlikely find their way back to the lower atmosphere.
Negatively charged particles move upward by the fair-weather electric field and by atmospheric convection. When
these ions reach above 120 km altitude where the ion gyro frequency is comparable to or greater than the ion-
neutral collision frequency, they can be accelerated by EM fields through the gyro resonance interaction. The
propagation of these low frequency waves to the upper atmosphere along the earth's magnetic field is permitted
by the plasma dispersion relation.
Laboratory experiments play an important role in confirming the theoretical prediction that ion cyclotron waves can
grow in the presence of an axial electron current. This allows the utilization of free energy sources in the auroral
ionosphere to expel the selected species. The feasibility of this concept depends on how efficiently the free
energy can be directed towards this remediation.
Experimental excitation of these low frequency waves using the HIPAS facility will be presented. By exciting ELF
waves over a range of ion gyro frequencies of dominant ion species, dips were observed in the low frequency
magnetometer at these frequencies suggesting that the ELF wave energy was absorbed by ion species at
specific frequencies. Ion acceleration and expelling phenomenon over the polar regions have been observed by
high latitude satellites as a natural process. A method of using the ground-based HIPAS LIDAR to directly
observe this selective ion acceleration will be presented along with laboratory laser- induced- fluorescent
experiments on doppler shifts.
1. Wong, A.Y. et al. AIP CIP 96-27719, Chap 3, pp 41-75, 1997
http:www.hipas.alaska.edu