Session 8S - Space.
POSTER session, Thursday afternoon, November 14
Exhibit Hall - Concourse Level, Adam's Mark

Tenuous, weakly ionized but highly conducting gases display ambipolar drift of magnetic fieldlines: the lines are tied to the plasma component, but drift with respect to the neutral (or center of mass) component. The drift velocity v_D is approximately J\timesB/\rho\nu c, where \rho is the gas density and \nu is the neutral - ion collision frequency. Many of the effects of ambipolar drift are scaled by an ambipolar diffusion coefficient \lambda_AD\equiv V_A^2/\nu, from which can be formed the ambipolar Reynolds number R_AD\equiv VL/\lambda_AD. Ambipolar drift is believed to operate in the cool to cold portions of the interstellar gas in galaxies.

Ambipolar drift is known to have three properties: it sets a minimum scale L_min below which the field is poorly coupled to the neutral fluid (defined approximately by R_AD(L_min)\approx 1, (2) it acts as a nonlinear diffusion, and mediates the formation of current sheets and sharp moving fronts, and (3) it drives the fluid to a relaxed state in which J\times B\approx 0 to the extent permitted by topological constraints. We obtain the RAMHD equations, 3D reduced MHD with ambipolar drift, and use them to explore the effects of ambipolar drift on magnetic reconnection and on mean field dynamo theory as they apply to galaxies.

[8S.02] The Pregalactic Origin for Galactic Magnetic Fields

It has been generally accepted that there is no natural mechanism to create a strong primordial magnetic field. For this reason all the attention has been concentrated on the generation of the magnetic field by hydrodynamic turbulence in the galactic disk. But this approach suffers from the problem of the rapid amplification of small scale magnetic fields(R. Kulsrud and S. Anderson ApJ 306, 606, 1992). However, as the result of numerical simulations, it is now clear that there is a lot of turbulence present in the pregalactic state, when the galaxy is arising out of gravitational instabilities. The simulations further show that the thermolelectric term in Ohm's law produces a weak magnetic field, even from zero initial conditions. Further, the smallest eddy of the turbulence turns over several hundred times before the galaxy collapses to a virial state. This many turnovers amplifies the weak magnetic field by a large enough factor for it to reach saturation with the hydrodynamic turbulence at a considerable field strength. Lastly, it appears from a physical argument, and also by a DIA calculation that when the field becomes strong enough it straightens itself out and becomes coherent on a galactic scale. this coherence arises even in the absence of an `` \alpha '' effect! It is proposed that this pregalactic process is the true origin of the galactic magnetic field. .

[8S.03] Nonlinear Dynamos

Cosmic and galactic magnetic fields are believed to result from amplification by turbulence. To understand this process the buildup of magnetic energy in a zero-resistivity turbulent MHD plasma is considered in numerical simulations using both the Direct Interaction Approximation and the Realizable Markovian Closure of Bowman and Krommes. In the earliest stages of growth when the magnetic field is dynamically unimportant, magnetic energy grows rapidly on scales much smaller than the scale of the smallest turbulent eddies, if the resistivity is sufficiently small.(Kulsrud, R., and Anderson, S. 1992, ApJ, 396, 606) However, it is shown that at some point before the magnetic field reaches equipartition with even the smallest turbulent eddies, dynamic effects prevent further growth of the smallest scale magnetic fields. When the magnetic energy reaches equipartition with the smallest turbulent eddies, the magnetic energy is concentrated near the smallest turbulent scale. The magnetic energy continues to grow for a few large-eddy turnover times until it reaches rough equipartition with the velocity turbulence at all scales, and a k^-3/2 spectrum is set up. Implications for both the galactic and protogalactic dynamos will be discussed.

[8S.04] Observational Evidence for Highly Compressible Turbulence in the Interstellar Medium

The presence of plasma turbulence in the interstellar medium is revealed by propagation effects on radio waves. These measurements, referred to as interstellar scintillations, are sensitive only to plasma density fluctuations in the interstellar medium. An important question in determining the nature of this turbulence is the relation between fluctuations in density and magnetic field, plasma velocity, etc. We have recently published measurements of the Faraday rotation along many closely-spaced lines of sight through the interstellar medium (Ap.J. 458, 194, 1996). When compared with other measurements which are sensitive to only the density fluctuations, these Faraday rotation observations permit determination of the amplitude and spectral characteristics of the small scale magnetic field fluctuations in the interstellar medium. Our observations are consistent with \delta n/n_0 \simeq \delta b/ B_0 on all spatial scales smaller than about 3 parsecs. In this expression, \delta n and n_0 are the fluctuating and static components of the plasma density, and similarly with the magnetic field. These are very large density fluctuations; it seems that interstellar turbulence is not describable by some of the more obvious theoretical models, such as ponderomotive density fluctuations or slightly obliquely propagating magnetohydrodynamic waves.

[8S.05] Turbulence Engendered Meridional Circulation in Astrophysical Bodies; Magnetic Reynolds Stress Tensor Expressible Wholly in Terms of Reynolds Stress Tensor.*

We evaluate generally small magnetic Reynolds stress tensor. We previously remarked^1 that it is easier to solve the eq. of secular fluid motion (esfm) by artificially inserting magnetic B-field for Sun, Earth rather than solving esfm simultaneously with amplification eq., \partial B / \partial t = \nu_m \nabla^2 B_m + \nabla X (u x B_m ) + \nabla X \langle v_\epsilon X b_\epsilon \rangle, with \langle ... \rangle \equiv ensemble mean. In Sun this may require dividing 11-yr cycle into, say, 10 phases and describing B-field for each phase in terms of Laplace surface harmonics, upper magnetic toroid reaching 3600 gauss thru central regions and being 4000 km thick (est.) while its upper surface lies 2450 km below top of photosphere, determined from 5-minute sound wave oscillations.^2 Lower toroid is 10^5 km below upper toroid.^3 We established that remaining difficult term in esfm, magnetic Reynolds stress tensor, to be proportional to Reynolds stress tensor, by assuming that turbulent mag. energy/cm^3, b_\epsilon^2 / 8 \pi \mu, is proportional to kinetic energy/cm^3. To derive space dependency we recognize that b_\epsilon arises solely from v_\epsilon, and \langle v_\epsilon r^2 \rangle \gg \langle v_\epsilon \theta^2 \rangle, \langle v_\epsilon \theta^2 \rangle since convection has a stronger radial component. The simplest geometrical configuration is Sun's or Earth's toroidal field, B_T, in which convection interacts normally with B-field lines, which are everywhere horizontal, thus amplifying the field by stretching lines of force. This leads to our basic assumption, justified in Part II: ( 4 \pi \mu )^-3/2 b_\epsilon i = ....

Work not supported to defray costs, qualify jour. publ. ^1,2 K.L. McDonald, Bul. APS, 40(12) 2035; 27, 1179 (1982). ^3 ____, Solar-Terrestrial Electromagnetic Phenomena: ... Utah Engin. Expt. Sta. Bull. No. 145, Univ. Utah (1984), 135 pp.

[8S.06] Angular Momentum Transfer in Accreting Disks.

The observed radiation emission around compact, heavy centers ( X--ray stars, black holes) is known to require rates of mass accretion and angular momentum transfer that can not be attributed to classical collisional ion viscosity^1,2.\ The angular momentum flow in particular is ``anomalous'' in at least two respects: it is much higher than predicted classically, and its flow is outward, directed along its gradient. A collective mode instability is clearly needed to drive this transport^2. We consider a magnetized disk, in the ``thin disk'' approximation, to investigate the accretion dynamics under the assumption that the macroscopic mode responsible for the angular momentum transfer is driven by the differential rotation of the disk as suggested in reference , and first considered by Velikhov^4 and Chanrasekhar^5.

(1) Pringle, J.E. 1981\ Ann. Rev. Astron. Astrophys.\ 19:137-162 (2) Coppi, B.\ 1994\ Plasma Phys. Control. Fusion\ 36, B107-B121 (3) Balbus, S.A., Hawley, J.F., 1991\ Ap. J.\ 376, 214-222 (4) Velikhov, E.P.\ 1959\ Soviet JETP.\ 36, 995 (5) Chandrasekhar, S.\ 1960\ Proc. Nat. Acad. Sci.\ 46, 253

[8S.07] Accretion Processes, Momentum Transport and Gravitation

Accretion processes are considered to account for the radiation emission by a large variety of objects and the plasma parameters of the accretion disks associated with them are quite different. A common feature is the need to have collective modes that can transport angular momentum at the high rate required to explain the accretion rate (J.E. Pringle, Ann. Rev. Astron. Astrophys. 19, 137 (1981).) inferred from the object luminosity. Referring to a magnetized plasma disk, a simple model is considered where an inhomogeneous flow velocity in the direction of the field exists. The gravitational force is parallel to the velocity gradient, transverse to the field, and in the same direction as that of the density gradient. The combination of these two factors (gravity and velocity gradient) can drive a macroscopic mode unstable provided the frozen-in-law is violated for instance by the presence of a finite electrical resistivity. Contrary to the case of the dissipative electrostatic, velocity gradient driven modes(B. Coppi, Plasma Phys. Cont. Fusion 36, B107 (1994).) where instead of gravity the mediating factor is the finite electron temperature, the longitudinal ion viscosity does not play an essential role.

\$^*Supported by U.S. Department of Energy.

[8S.08] Magnetohydrodynamical Energy Relaxation in Solar Corona

The nonlinear dynamics of solar coronal magnetic arcades, those are subject to a shearing motion at the feet, is studied by numerical simulations. In particular, the effects of the plasma pressure in the corona on the energy relaxation and on magnetic reconnection are intensively considered. It is found that, when the pressure distributes uniformly in the coronal region, the plasma pressure has a stabilizing effect on the symmetric mode instability [Kusano and Nishikawa, ApJ 461, 415 (1996)] in arcade groups, even if the plasma \beta is as small as 10^-3. It implies that a larger amount of the magnetic helicity is required to be injected into the corona for the instability to drive the energy relaxation with reconnections, compared to the case in which the pressure effects are neglected. Consequently, we suggest that the energy relaxation in the solar corona must be more explosive than the predictions by zero \beta plasma models. Furthermore, the structure of the diffusion region as well as the slow shock in the reconnection process are examined in detail.

[8S.09] Three Dimensional Magnetic Reconnection and the Topology of Interacting Twisted Flux Tubes

3D nonlinear resistive MHD simulations of twisted tubes of magnetic flux will be presented. The initial states consist of two parallel tubes with initial field either parallel or antiparallel, twisted such that the current density in the two tubes is either parallel or antiparallel, giving four cases. For the case in which the initial fields are antiparallel and the twist is in the opposite direction (parallel currents) the tubes attract, spontaneously producing a pair of magnetic nulls. Subsequently, a bifurcation occurs in which the nulls are replaced by a single closed X-line. At this time the reconnection rate, as measured by the rate of increase of flux connecting the two tubes, increases rapidly. The X-line eventually undergoes a sequence of period doubling bifurcations in reverse, producing an O-type line and a region of flux surfaces. The current density on these surfaces is nearly zero because they are not connected to the twisting surfaces. The rotational transform is due to stellarator transform. Possible connections with coronal mass ejections and spheromak formation will be discussed.

[8S.10] On Non-Equilibrium and Current Sheet Formation in Line-Tied Magnetic Fields

Parker's model of coronal heating is considered using the ideal reduced magnetohydrodynamic equations. It is shown that there can be at most one smooth magnetostatic equilibrium for each smooth footpoint mapping between two plates with line-tied boundary conditions. It follows that if such a static equilibrium is driven unstable by footpoint motions, then there is no other equilibrium for the plasma to relax to, leading to magnetic non-equilibrium and the formation of current sheets. It is shown that this process can occur as the system relaxes asymptotically to a state of minimum energy, possibly in infinite time. Numerical simulations for line-tied island coalescence and equilibria containing current layers are presented which suggest that as current layers become intense and cross a threshold for instability, the magnetic relaxation observed is consistent with the formation of non-equilibrium states with current sheets. A criterion is given to determine geometrically the sites of current sheet formation and magnetic reconnection in models without nulls or closed field lines. This criterion requires the rate of velocity amplification, which is analogous to the Lyapunov exponent in nonlinear dynamics, to become arbitrarily large.

\rule2in1pt ^\astWork supported by NSF and AFOSR.

[8S.11] Current Sheet Formation in the Solar Corona---Topological Considerations

An outstanding problem in solar physics has been to understand how the solar corona, with its temperature of millions of degrees, can be so much hotter than the underlying photosphere, which is typically of only a few thousand degrees. A simple model has been proposed for the heating of low-\beta, low resistivity plasmas, such as the solar corona, which possess highly tangled magnetic fields, and which evolve very slowly from external stresses compared to a typical Alfven crossing time. These plasmas are considered to evolve ``quasi-statically,'' i.e. arbitrarily close to equilibrium at all stages in their evolution. In the model, the heating has been assumed to result from the formation of regions of intense electrical current, or current sheets. A key ingredient in this model has been the topological restrictions imposed on current sheets, owing to the fact that the low-\beta property constrains current to flow only along the magnetic field lines. Assuming that the most probable sites for the formation of current sheets are closed loops of magnetic field, an estimate has been made of the statistical properties of current sheets. The special role of magnetic neutral points has been considered, and the implications for coronal heating have been discussed.

[8S.12] DYNAMIC OF ELECTRON BEAMS RESPONSIBLE FOR TYPE III SOLAR BURSTS FINE STRUCTURE

Recent observations and theoretical reasons point in favor of the fact that the source of type III solar bursts emission consists of some electron beams. Propagating along opened magnetic field lines these electrons are supposed to generate radiation observed. Conditions in solar corona gives us that the quasilinear time is much smaller than the time of particle flying-off. Using this fact the main equation describing propagation of N monoenergetic beams are obtained and their solution is found. Electrons generating and absorbing Langmuir waves move as a row of beam-plasma structures. As a result of "interaction" between beam-plasma structures a part of slow structure electrons absorbing plasma waves accelerates into fast ones that leads to have their shapes changed and fast structures accelerated.

[8S.13] Electron Fluxes After Explosive Injection in a Turbulent Solar Wind.*

The dispersion problem for energetic-charged-particle-cloud in a turbulent space plasma in a weak magnetic field is considered. Solution for electron distribution function is found for the case when a level of the plasma turbulence depends on cloud particles. It is shown that characteristic velocity of the cloud expansion is determined mainly by the low energetic part of the cloud. The features of expansion dynamics of the diffusion electron flux from solar bursts are analyzed. It is shown that there are turbulent boundaries in front of the cloud and a slowly cooling plateau in its center. A small part of particles whose velocities are directed almost along the dispersion axes can ``evaporate'' from a relatively slowly expanding electron cloud and could be a cause of the third type solar radiation bursts. Supported in part by Tesla Labs, Inc., La Jolla, CA 92038-2946. ^1Also with Tesla Labs, Inc., La Jolla, CA 92038-2946.

[8S.14] Auroral Na Lidar in Alaska

A lidar system consisting of a 2.7 m diameter rotating mercury collector and a 7 mJ/pulse 10 Hz excimer pumped dye laser tuned to the Na-D_2 (589 nm), resonance, has been installed at the HIPAS Observatory, Alaska (65^\circN-147^\circW). The lidar passes under the electrojet and the aurora. It is inside an arctic building with a glass skylight for winter operation (\geq-40^\circ outside). During an auroral storm (3/18/96), we observed the formation of sporadic Na layers 30 km above the ``normal Na'' layer at 90 km altitude. It is proposed that, in this case, Na was released from compound reservoirs by earth captured solar protons, penetrating no further than 120 km altitudes,(F.Sigernes, et all, "Calculation and ground-based observations of pulsed proton events in the dayside aurora, J.Atmos. Terrs. Phys., 58, 1281-1291, 1996. ) and that a Na lidar is a new diagnostic for such events. Our lidar will include a ``Doubled flashlamp pumped Ti-sapphire laser'' (125 mJ/pulse at 10 Hz at 391.4 nm, the wavlength of the N2^+ bandhead). Work Supported by ONR N00014-91-C-0191.

[8S.15] BUMP-ON-TAIL INSTABILITY IN THE EARTH'S ELECTRON FORESHOCK

This theoretical investigation is motivated by measurements of a beam-like feature in the Earth's electron foreshock. The power-law electron tail tends to inhibit backscatter of the unstable waves and preventing the development of strong turbulence [1]. The evolution equations for turbulence spectrum and electron distribution function are derived. It is shown that dispersion law of turbulence differs strongly from Langmuir's law. The spectrum of quasistationary high-frequency turbulence and the electron distribution function are constructed. 1. M.V.Goldman, D.L.Newman, J.G.Wang, L.Muschietti. Bull. APS. 37th Meeting DPP. Louisville. USA. 1995. v.40. N.11. p.1816.

[8S.16] Transport at the Dayside Magnetopause Resulting from Mode Conversion of Compressional Magnetosheath Waves to Kinetic Alfvén Waves \thanksThis work supported by DoE contract No. DE--AC02--76--CHO--3073.

Low frequency MHD waves driven by pressure anisotropy or intrinsic solar wind fluctuations are nearly always observed in the high \beta (\beta \ge 1) magnetosheath and magnetopause. Substantial background gradients (on the order of several ion gyroradii) in pressure, density and magnetic field couple compressional waves and kinetic Alfvén waves at the magnetopause near the Alfvén resonance location (ømega = k_\parallel V_A). Mode converted kinetic Alfvén waves found at the magnetopause can provide significant particle transport for both northward and southward IMF. Moreover, for southward IMF additional transport can arise because the kinetic Alfvén wave can propagate to the location where k \cdot B = 0 leading to the formation of islands in phase space which induces massive particle transport. We present a solution of the kinetic-MHD wave equations for this process using a 1-D model equilibrium profile which includes a sheared magnetic field; magnetic curvature; and gradients in the background density, pressure and magnetic field. We incorporate wave-particle resonance interactions for electrons and ions to determine wave structure. Implications for plasma transport will be discussed and compared with other magnetic reconnection mechanisms.

[8S.17] Three Whistler Excitation Bands in Jupiter's Radiation Belts.*

The instability of Jupiter's radiation belts is studied from the perspective of whistler wave excitation at cyclotron resonance. In accordance with direct measurement a dumbbell-shaped distribution function is used for relativistic electrons with both transverse and longitudinal anisotropy, i.e., the maximum of the angular distribution in the equatorial plane is not perpendicular to the magnetic field. It is shown that instability occurs in three bands: one band is situated below the relativistic gyrofrequency while the other two are centered at half the nonrelativistic gyrofrequency. These results are important because the experiments on board the ``Voyager-1'' indicated that the whistler emission in the magnetosphere of Jupiter is typically registered in three spectral bands. Supported in part by Tesla Labs, Inc., La Jolla, CA 92038-2946, within the project ``Plasma Astrophysics.''. ^1Permanent address: Institute for Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia.

[8S.18] Phase Synchronization for Clock Event in the Jovian Electron Radiation Belts.*

This work deals with nonlinear time-dependent processes in the outer Jupiter's electron radiation belts. The cyclotron instability dynamics of a plasma magnetospheric maser are described by a relativistic system of quasilinear equations. This system takes into account the diffusion of particles in the adiabatic invariant space, the synchrotron losses, and the electromagnetic radiation evolution. Analysis shows the importance of the effect of global resonance, i.e., the oscillation eigen-frequencies of the radiation belt parameters are virtually independent on the magnetic shell and coincide with the planet's angular rotation velocity. Supported in part by Tesla Labs, Inc., La Jolla, CA 92038-2946. ^1Also with Tesla Labs, Inc., La Jolla CA 92038-2946.

[8S.19] Precipitation of Magnetosphere Electrons Caused by Relativistic Effect Enhanced Chaotic Motion in the Whistler Wave Fields

In the magnetosphere, energetic electrons in the radiation belts are trapped by the Earth's dipole magnetic field and undergo bounce motion about the geomagnetic equator. It is shown that the trajectories of some of the trapped particles can in the presence of a whistler wave become chaotic and wander into the loss cone. The threshold field for the commence- ment of chaos in the trajectories of electrons with energies of a few hundred keV is found to be lowered by the inclusion of relativistic effects by about an order of magnitude. Waves with these smaller magnetic field amplitudes (about 1ing between hemispheres. Since this chaotic scattering process does not have a directional preference, it offers a plausible explanation for the simultaneous observation of electron precipitation into the upper atmosphere at geomagnetically conjugate regionsdue to a single lightning flash.

[8S.20] Kinetic Model of Plasmaspheric Morphology

We have constructed a kinetic, multi-species model of plasmaspheric morphology, which predicts the spatial dependence of the distribution function. The plasma is collisionless at high altitudes, with the the low-altitude boundary conditions determined by the field line interhemispheric plasma (FLIP) model. In the collisionless region, ballistic, escaping and trapped particles are included, and the ambipolar electric field is determined self-consistently. Several important effects are considered, including the convective and co-rotational electric fields, diurnal motion, and possible instabilities due to the nonequilibrium nature of the distribution function. Our results are compared to satellite observations, e.g., EUV imaging data from the STP 72-1 satellite [C.S. Weller and R.R. Meier, J. Geophys.\ Res.\ 79, 1572 (1974)], and possible predictions of future observations (e.g., POLAR and IMAGE missions) are outlined.

[8S.21] Stimulated Electromagnetic Emissions near the Second Electron Cyclotron Harmonic

First results of broadband stimulated electromagnetic emissions (SEE) near the second electron cyclotron harmonic (2Ømega_e) are presented. The results were obtained at a recent HF heating campaign at the HIPAS Observatory with the heater frequency ømega_o near 2Ømega_e at 2.85 MHz. Experiments were performed for both O and X-mode polarizations, and under both continuous (CW) and low duty-cycle short pulse heating conditions. Typical SEE spectral features, such as the Downshifted Maximum (DM), the Broad Upshifted Maximum (BUM), and the Broad Symmetric Sidebands (BSS) were observed. While such spectral features were observed routinely at heater frequencies near the third electron cyclotron harmonic and higher at other heating facilities, this is the first observation that demonstrates that such features can also be excited near 2Ømega_e. Comparison will be made between our results and past observations at higher frequencies. Physics issues involving the generation of these features such as the formation of field aligned striations and the conversion of HF pump wave to upper hybrid wave will also be discussed.

[8S.22] Signatures of Two-Plasmon Decay at Arecibo

We study the Two-Plasmon Decay (TPD) instability and the turbulence it engenders at one quarter of reflection density in the hf-driven ionosphere over Arecibo using a reduced (generalized Zakharov) model that couples (magnetized) Langmuir wave and ion acoustic wave fluctuations to the TPD driving current.(D.F.DuBois, D.Russell,and H.A.Rose, PRL 74, 3983(1995).) Recent advances in Incoherent-Scatter Radar (ISR) diagnostics and the upgrade of the heater transmission lines promise to make Arecibo a good place to study TPD, however, the 430 MHz radar cannot directly detect the linearly most unstable TPD Langmuir waves. But secondary Langmuir waves produced by such turbulence effects as cascade and caviton collapse may be detected by the ISR, and we characterize these enhanced fluctuations by their energy- and power-spectra. Earlier work established the precedent of observing nonlinearly produced fluctuations instead of those predicted by linear theory. (M.P.Sulzer and J.A.Fejer, JGR 99, 15035, (1994).) (D.F.DuBois, D.Russell and H.A.Rose, PRL 60, 581(1988).)

[8S.23] Three dimensional structuring of plasma patches in the high = latitude ionosphere

We have developed a 3D nonlinear fluid code for investigating the structuring of plasma patches in the high latitude ionosphere, caused by the gradient drift instability. The code uses a trapezoidal leapfrog algorithm for the time stepping and higher order upwind finite differencing for the convection of the density. The potential is obtained by solving the vorticity equation by a relaxation method. The structuring, in the direction perpendicular to the equilibrium flow and the magnetic field, is found to be very sensitive to the scalelength along the direction of the magnetic field. We have derived a scaling for the numbers of finger-like structures as a function of a dimensionless parameter associated with the parallel scalelength, which is borne out by the 3D simulations. The present work will be discussed in context with observations of the structuring of plasma patches and auroral blobs.

[8S.24] The self-focusing instability in the presence of quasi-random = density fluctuations

We have studied the thermal self-focusing instability (SFI) in the presence of quasi-random density irregularities in under-dense plasma . The role of the amplitude of the irregularity and its correlation length on SFI has been examined. We find that a modest level of density fluctuations leads to the excitation of a broad band spectrum of SFI. Typically modes with wave-numbers five to ten times larger than the marginally stable mode for the homogeneous plasma, can be excited because of the background fluctuations. The spectrum of the side band falls off as k^-2 . The present model predicts that the long wavelength and the short wavelength modes, coupled through the pre-existing density irregularity, grow at the same rate. The study of SFI is being extended to the critical surface in an inhomogeneous plasma.

[8S.25] Simulations of Strong Langmuir Turbulence and Hot Electron Production During Ionospheric Modification.

We study electron acceleration, heating, and wavebreaking due to strong Langmuir turbulence (SLT) using a hybrid simulation code. Heated electron distributions are characterized and modeled in various regimes. Comparisons are made with analytic models derived from PIC simulations. Suprathermal electron-induced turbulence above the reflection region is studied. Comparisons are made to data from recent ionospheric heating experiments.

[8S.26] Breakdown of quasilinear electron diffusion in the auroral ionosphere at high Langmuir wave intensities.

We assess the validity of quasilinear diffusion for the self-consistent interaction of magnetized Langmuir waves with an electron beam in the auroral ionosphere. Large amplitude Langmuir waves have been measured by UC Berkeley sounding rockets and by the Freja satellite. Limited theoretical analysis(Newman, D.L., M.V. Goldman, R.E. Ergun, and M.H. Boehm, J. Geophyis. Res.) 99, 6377, 1994. suggests that Langmuir waves of high intensity in the auroral ionosphere have random phases due to dissipation by nonthermal electron tail (halo) distributions. For incoherent wave spectra, a self-consistent study of the plasma is often made by coupling the weak turbulence equations for wave-wave interactions to the quasilinear diffusion equation for wave-particle interactions. We show that this may not be valid at high altitudes (Freja) for intense wave fields. We employ a test particle simulation to estimate the maximum wave amplitude for which the quasilinear diffusion approximation is valid at these altitudes.\hfill

Work supported by NASA ITM SRamp;T NAGW 4453 and NSF ATM-9417116.

[8S.27] Evaluation of Suprathermal Electron Distributions from Incoherent Scatter Measurements During HF Heating of the Ionosphere

Bernhardt, et al (Berhardt, PA., C.A. Tepley and L.M. Duncan, JGR 94, 9071, (1989).) have theoretically inferred suprathermal (power-law) electron distributions capable of producing the oxygen airglow observed during ionospheric modification by an HF EM-wave heater. These electrons, which may come from Langmuir turbulence in the heated region, stream out along geomagnetic field lines. They are also expected to enhance Langmuir waves outside the heated region, just as daytime photolelectrons ( Fremouw, E.J., Petriceks, J., and Perkins, F.W., PF 12), 869, (1969). enhance Langmuir waves in the ambient ionosphere. Photoelectron-induced enhancements are due to a combination of increased Cerenkov emission and decreased Landau damping. They have been observed in daytime Thompson-scattering measurements of the plasma line. Similar experiments are planned during nighttime modification of the ionosphere. We assume various power-law electron distribution models and theoretically calculate the Thompson-scatter cross-section around the plasma line in order to be able to infer from scattering experiments the shape of the suprathermal electron distribution. Work supported by NSF.

[8S.28] Meridian Scan of the Electrojet using ELF/VLF Modulation

Systematic correlation of ELF and VLF signal strenghts with spatial distributions of electrojet and auroral arcs has been investigated in a series of experiments carried out using the HIPAS heater array (f_0 = 2.85 MHz, P_rad \simeq 800 kW). The heater beam was scanned along the magnetic N-S direction and ELF/VLF signals were aquired with the magnetometer setup at Gilmore Creek, AK. Electrojet parameters were obtained from the UAF magnetometer chain and from the HLMS Auroral Radar facility at Elmendorf AFB, and auroral arc positions from the all-sky camera at the Poker Flat Research Range. VLF was generated by AM modulation of the heater beam at 1.11 and 2.5 kHz, while ELF was produced by runnning the heater array in double-frequency (DF) mode at 24 or 29 Hz frequency separation. In some of the experiments simultaneous ELF and VLF modulation was successfully achieved. Preliminary results show strong time-varying dependency of received signals (particulary ELF) with beam zenith angle.

[8S.29] The Ionospheric Forerunners of Earthquakes.*

A comprehensive analysis of various seismoionospheric precursors was carried out. This made it possible to select three main precursor types in ionosphere characterized by location and time of appearance. It is shown that common property of all seismoionospheric precursors is the fact that horizontal dimensions of precursor observations exist within the radius of earthquake epicenter originally defined by Dobrovolsky theory for ground precursor measurement. Our argument is in favor of atmospheric electricity as a possible cause for appearance of seismoionospheric precursors. Supported in part by Tesla Labs, Inc., La Jolla, CA 92038-2946. ^1Permanent address: IZMIRAN, Troitsk, Moscow Region, Russia.

[8S.30] The Earthquake as a Possible Cause of Appleton Equatorial Anomaly Generation.*

We assume that the earthquake build up manifests itself at ionospheric altitudes in a different way at high, middle and low latitudes due to the various kinds of prevailing physical processes depending on earthquake location. To check this idea we have analyzed ALOUETTE ionospheric data obtained several days before and during equatorial earthquake with magnitude M-6,8 that happened on April 13, 1963 with epicenter 600 km north-west from Huancayo, Peru. We have obtained some confirmation of the idea of the local display or image of ionospheric earthquake precursor. Nevertheless it is important to study this phenomenon in different geophysical conditions in order to make the final conclusion about the concrete image of seismoionospheric precursor in equatorial ionosphere.

Supported in part by Tesla Labs, Inc., La Jolla, CA 92038-2946 within the project ``Seismo-Ionospheric Physics''. ^1Also with Tesla Labs, Inc., La Jolla, CA 92038-2946.

[8S.31] Compatibility of Payload Equipment for Monitoring of the Seismoionospheric Precursors.*

The earthquake prediction including precursors appearance and their detection on the Earth's surface and at ionospheric altitudes is a matter of high interest.^2 Present warning systems are based on ground-based monitoring data. Satellite monitoring represents, due to some well known advantages (global scale, continuous records, etc.) a significant progress in the area. It is shown that by means of one payload it is possible to monitor all main parameters of seismoionospheric precursors. The active methods and airglow imager are used for detection of anomalous plasma structure during a few days before first shock. Supported in part by Tesla Labs, Inc., La Jolla, CA 92038-2946. ^1Also with Tesla Labs, Inc., La Jolla, CA 92038-2946. ^2V. Stefan (Editor-in-Chief). Environmental Physics. Frontiers in Interdisciplinary Physics series of the La Jolla International School of Physics, The Institute for Advanced Physics Studies, La Jolla, CA (IAPS Press, La Jolla, 1996).

[8S.32] Particle dynamics in a strongly-coupled dusty plasma

We have used video imaging to study the dynamics of 9 \mu m plastic spheres suspended in low-power Krypton discharges. The spheres, which are highly charged and levitated by the electrode sheath, form a strongly-coupled system. Using a digitized series of images, we tracked individual particles and measured collective and random particle motions.\footnote J. B. Pieper and J. Goree, submitted to PRL Dust acoustic waves were excited at \leq 10 Hz and their dispersion relation verified. Fitting the measured and theoretical dispersion relations also give a measurement of the particle charge and the "linearized" Debye length. The temperature of random particle motion in the horizontal plane (parallel to the electrode) was measured to be 2-10 times room temperature and about 2 times the temperature in the vertical plane. It is proposed that the particles are heated by low-frequency (kHz) electrostatic plasma fluctuations. Work supported by NSF and NASA

[8S.33] Debye Shielding and Particle Correlations in Strongly Coupled Dusty Plasmas

A one-dimensional strongly-coupled dusty plasmas is simulated by the particle-in-cell method. For coupling parameters of order unity, the dust particles exhibit Debye shielding on the spatial scale of the dust Debye length. The average potential as well as the two-particle correlation function of dust particles do not exhibit short-range spatial order if the system is not annealed, due to the substantial amount of the system energy residing in electrostatic wave fluctuations. The plasma exhibits liquid-like and crystal-like correlations when annealed. \rule2in1pt ^\astWork supported by NSF and AFOSR.

[8S.34] Coulomb Crystallization of Dust Grains in Magnetized Dusty Plasmas

It is well known that a magnetized dusty plasma can support a great variety of plasma waves. Our objective is to investigate the potential of a dust test charge in the presence of unmagnetized dust grains and magnetized electrons and ions. Collective interactions introduce an oscillator wake potential which may provide an attractive force [1] between charged dust grains having the same polarity. We have found that the presence of an external magnetic field significantly enhances the wake potential and the attraction length. The present result should help to understand the properties of Coulomb lattices in low temperature laboratory dusty plasmas.

[1] M. Nambu, S.V. Valaslimirov and P.K. Shukla, Phys. Lett. A \bf203, 40 (1995)

Work supported by AFOSR contract number F49620-96-1-0113

[8S.35] New Approach to Finding Convective and Absolute Parametric Instabilities in a 1-D Density Gradient with a Background Magnetic Field

The three-wave parametric decay of a spatially homogeneous (k=0) pump electric field into daughter Langmuir and ion-acoustic waves is studied numerically for the case where the embedding plasma has a one-dimensional density gradient and can contain a uniform background magnetic field of arbitrary orientation. The distribution of complex eigenfrequencies of the pump-plasma system are determined from the linear response function: If there are normal modes with positive growth rates, the corresponding absolutely unstable Langmuir wavefunctions are computed. Alternatively, if all of the normal modes are stable, the steady-state response to a boundary Langmuir source is found, yielding convective amplification profiles. The resulting instabilities are interpreted in terms of magnetized or unmagnetized plasma wave optics, and are compared to previous analytical models of convective and absolute growth. Applications to both ionospheric modification experiments and laboratory laser-plasma interactions will be given.

Work supported under by NSF Atmospheric Physics Grant ATM-9314409 and by DoE contract No. W-7405-ENG-48.

Part 8 of program listing