Session 7E - Space and Basic.
ORAL session, Thursday morning, November 14
Majestic Ballroom, Adam's Mark

Observations of X-ray emission from comet Hyakutake (Science 272,194, 1996) appear to have been a surprise to most comet researchers. This was the first time that an x-ray image of a comet has been obtained. Existing plasma data suggests that this observation should not have been a surprise. Data obtained from the encounter with comet Halley by spacecraft (Nature 321, 1986; GRL 13, 1986) showed strong plasma wave turbulence and energetic electrons in the region of comet Halley which corresponds closely to the region where the X-rays were observed from comet Hyakutake. Plasma wave turbulence observed was close to the lower-hybrid frequency range where waves are very effective in energizing electrons to the required energies. We show that the interaction of the commentary plasma and the solar wind produce waves in the lower-hybrid frequency range which produce super thermal electrons with energies in the range of 100eV to 1000eV. These produce bremsstrahlung and excite k shell electrons of oxygen, carbon and heavy elements to produce the observed X-rays. The X-ray production is of the order of 10^7 watts, which is of order of that observed. X-ray brightness changed with time; probably reflecting variations in solar wind speed and density. Observations of this type show changing solar wind conditions o the comet.

[7E.02] Generation of X-Rays From Comet Hyakutake

An explanation of recent observations of a strong X-ray emission from comet Hyakutake (Science, 272, 194, 1996) is proposed. It is based on an idea that interaction between solar wind and photoionized cometary plasma produces strong plasma turbulence in the lower hybrid frequency range which is responsible for the acceleration of suprathermal electrons with the energies up to several keV. Estimations for the lower hybrid electric field, typical energies and fluxes of suprathermal electrons are obtained in the framework of quasilinear theory of the corresponding instability. Proposed scenario is compared with in-situ observations of wave and electron energy spectra during the encounter in 1986 of armada of spacecrafts with HalleyUs comet. Two possible mechanisms of producing X-Rays by energetic keV electrons are analyzed-bremsstrahlung and cometary gas (oxygen) K-shell radiation of X-Rays. It is shown that for conditions under investigation line K-shell radiation is dominant resulting in total comet luminosity in X-ray equal to 10^14 erg/s in agreement with Rossat observations.

[7E.03] Accretion Flow Penetration of Compact Object Magnetospheres

We discuss the problem of plasma penetration of magnetospheres, an important issue in a wide variety of astrophysical contexts, ranging from accretion in cataclysmic variables to flows in protostellar systems. It has been argued that cross-field accretion on strongly magnetized stars (e.g., white dwarfs and neutron stars) is impeded when the kinetic energy density of the free-falling plasma becomes comparable to or smaller than the magnetic field energy density; consequently, the accretion should occur only in the close vicinity of magnetic poles. Despite that, accretion is frequently observed in the vicinity of the magnetic equator (assuming the stellar magnetic field is dipolar).

It is well known that high-permittivity, bounded plasma can E\timesB drift across the magnetic field due to the polarization charge build-up on the plasma stream boundary. We discuss this mechanism in application to the problem of accretion on AM Her magnetic cataclysmic variables. For typical parameters, we find that this mechanism can lead to deep penetration of the accreting matter, if depolarizing effects are weak. We consider various depolarization mechanisms and their effects on plasma propagation.

[7E.04] Vortices generated by a localized shear flow in magnetospheric plasmas

Using theory and numerical simulations, we investigate the nonlinear evolution of vortices generated by the Kelvin-Helmholtz (KH) instability of a sheared ion current in the Earth's magnetosphere. The extent of broadening of the shear flow, and the energy and enstrophe exchange between the shear flow and KH vortices, are characterized in the two-dimensional plane across the magnetic field lines. A new stationary vortex street solution is found, and two distinct phases of the nonlinear dynamics are identified. The first involves a transient phase in which burst like pulsations of the flow lead to a rapid dissipation of enstrophe. After the transient phase, an asymptotic smooth state is reached which corresponds to a periodic chain of dipolar vortices. Some effects of the KH wave form evolution arising due to the radiating field aligned current in three-dimensional geometry will be also described. The consequences of the model for the dynamics of field line resonances in the Earth's magnetosphere will be discussed.

[7E.05] Ponderomotive saturation of magnetospheric field line resonances. Effects of the shear Alfvén wave spatial dispersion.

Compressional Alfvén waves in the terrestrial magnetospheric cavity constitute a discrete spectrum of global modes which can resonate with specific components of the continuum spectrum of standing shear Alfvén wave field line resonances (FLRs). The compressional modes are excited by solar wind impulsive perturbations and the experimental evidence suggests that their resonance absorption results in FLR intensifications and is responsible for the optical emissions from discrete auroral arc structures. We investigate the effect of the ponderomotive force of standing shear Alfvén waves on the nonlinear saturation of FLRs. In the low \beta magnetospheric plasmas the FLR saturation occurs due to a nonlinear phase slip between the compressional mode driver and the shear wave filed. Ponderomotive force also leads to density depletions at the ionospheric ends of the resonator, nonlinear narrowing of FLR up to meridional scales comparable to the electron inertia scale length or to the scale of ion cyclotron radius. At these scales the parametric decay instability of the shear Alfvén wave takes place which is responsible for the FLR structuring comparable to scales comparable to those embedded within temporally modulated discrete auroral arcs. Observational features related to the effects of shear Alfvén wave dispersion will be discussed.

[7E.06] High-Voltage Tethers For Enhanced Particle Scattering In Van Allen Belts

New applications of space tethers (HVTSS) are discussed in relation with the idea\footnote Yu.V.Vasilyev, V.V.Danilov, Physics-Doklady, (1995) 342, 5. of an active experiment at the Earth's radiation belts. Two conducting strings are supposed to be tethered between the main satellite and two small subsatellites flying through the ERB. A large potential difference \sim1MV is applied between the tethers by means of a generator carried on the main satellite. The tethers effectively scatter the high energy particles into loss cone, providing a control of particle life time in ERB. The rigorous theory of the sheath layer formed by relatively cold plasma is developed for both DC and AC regimes yielding an electric field profile, which is then used for the treatment of the scattering problem. With the help of the Fokker-Planck equation, the average rate of particle losses, normalized per 1 km of the theter's length is found to be: (2.5 \div 14)\times 10^16 sec^-1km^-1 for electron belts and 1.8\times 10^14\div 2.5\times 10^20 sec^-1km^-1 for proton belts. New active experiments in ERB become possible under the joint realization of HVTSS and HAARP\footnote D.Papadopoulos, P.Bernhardt et al. A Joint Program of Phillips Lab and the Office of Naval Research, June, 1995 projects.

[7E.07] Nonlinear wave-driven currents in the E-region ionosphere

Ionospheric two-stream waves and gradient-drift waves nonlinearly drive D.C. currents in the E-region ionosphere. These currents flow parallel to, and with a comparable magnitude as, the fundamental Pederson current, acting to discharge the electrojet. The fundamental mechanism of this instability is quite simple. E-region plasma waves generate oscillating electric fields which cause electrons to \vec E \! \times \! \vec B drift perpendicular to the wave propagation direction on the density maxima and minima of the waves. On the maxima these electrons drift with the same velocity but in the opposite direction as they do on the minima. Since more electrons exist at the maxima than the minima, a net D.C. current results. This wave-driven current can be of the same order of magnitude as the fundamental ion-Pederson current, modifying the large scale dynamics of the E-region during highly active periods. It may also be responsible for a number of observed features of E-region waves, including the oft-repeated radar observation that equatorial Farley-Buneman waves travel at the acoustic velocity as well as the squaring-off of gradient-drift waves observed by rockets.

[7E.08] Two Step Temperature Decay in Self Accelerated Plasma for Laser Induced Lightning

In the last meeting in Louisville, the plasma decay of mm scale tiny plasma ball with spheromak type helicity ( 10 microsecond, 100 kA) was discussed with experimental results. This plasma is closely related to the laser produced plasma train to attract and to induce lightning discharge from thunder clouds and the temperature decay time constant is found to be divided into two categories. The fast process is explained reasonably well but the latter, few hundred micro second time scale, is at least one order of magnitude larger than the conventional chemical reaction models. In this paper we show the time history of travelling arcs with a velocity around the sound speed. The arc behaviour as a fluid is reasonably well understood in Newtonian Fluid Model under a reasonable assumptions. However the decay of the temperature in the slower time scale again is somewhat different from the conventional chemical models. Bull.American Phys.Soc., 40,11(1995) OCT, 1775

[7E.09] Mesoscale modeling of laser-plasma instabilities in macroscopic sized systems

A modeling procedure for macroscopic systems is proposed which is based on the large separation of space and time scales between the macro scales and the microscopic correlation lengths of the induced Langmuir tubulence excited by instabilities such as SRS, TPD and IADI. As a first example we have modeled in 1D the propagation of the pump wave and the modification of the intially linear density profile by the turbulence induced near critical density by the IADI ( ion acoustic decay instabilty) and the related modulational instabilty. The effect of the induced turbulence on the equation for the pump propagation is reduced to an effective (or anomalous) absorption coefficient and a frequency shift parameter, obtained from homogeneous simulations. The density profile is perturbed primarily by the ponderomotive pressure of the induced turbulence. Solution of the resulting mesoscale equations shows a dramatic, time dependent, alteration of the usual linear Airy profile. An application to SRS is proposed in which the standard envelope equation for the daughter Langmuir wave is modified by turbulence-induced anomalous aborption and frequency shift parameters which can be obtained from homogeneous simulations. * Research supported by the USDOE and by NSF grant #ATM-9503147

[7E.10] New Results On Ballooning Modes In Open Traps

The stability of ballooning modes is studied in an application for the experiments developed at Budker Institute of Nuclear Physics, Novosibirsk, Russia. Specifically, two concepts are considered: a mirror-based neutron generator (NG)\footnote I.A.Kotel'nikov, V.V.Mirnov, V.P.Nagorniy, D.D.Ryutov, in Proc. of the 10th IAEA Conf., London, v.2, Vienna, (1985) and a multiple mirror trap (MMT)\footnote A.Lichtenberg, V.V.Mirnov, Reviews of Plasma Physics, v.19, Plenum Corp., New York, 1996. To enhance the neutron flux in NG, oblique injection of fast tritium atoms is used yielding P_\parallel \gg P_\perp, that drives "fire hose" instability if \beta = 8\pi P_\parallel/B^2 > 2. Besides the "fire hose" instability ballooning modes also exist. Their stability and connection with the "fire hose" modes are treated on the basis of the paraxial MHD energy principle. The ballooning limits on \beta are found to be not so severe such that "fire hose" criterion \beta \leq 2 can be used for a rough estimate of marginal stability. Axisymmetric MMT where the margin of stability is shown to be \beta \leq \pi ^2 /N^2I , I =(3l/r_max^4) \int_-l/2^l/2 dz r^2(z) \left (dr/dz \right )^2 is strongly different from the previous case. For an optimal "square root" profile of the cells r(z) and number of cells, N=10, a low ballooning margin, \beta \simeq 5%, is predicted in line-tying stabilized MMT.

[7E.11] One-Dimensional Intense Laser Pulse Solitons in a Plasma

The nonlinear dispersion relation between the group velocity and laser amplitude of a class of one-dimensional isolated envelope solitons for modulated light pulse coupled to electron plasma waves has been found analytically for \epsilon^2=ømega_p^2/ømega^2 \ll 1, where ømega_p is the plasma frequency and ømega is the laser frequency. The solitons are classified by the integer N which is the number of nodes of the laser vector potential envelope. The shift in the nonlinear group velocity from the linear group velocity is given by \Delta/2, where the eigenvalue \Delta<1. The conventional N=0 soliton is the continuum limit \Delta \ll \epsilon^2, \Delta \propto \phi_0\propto A_0^2, where \phi_0 and A_0 are the maximum electrostatic and vector potential respectively. As the laser vector potential amplitude is increased, the continuum of eigenvalues \Delta ceases to exist and the first discrete axisymmetric soliton with N = 1 occurs at \Delta\simeq 0.5\,\epsilon^2. In the large N limit, \Delta \sim \epsilon^8/5 N^4/5 and \phi_0 \sim \epsilon^2 N^2. Unlike the case of N=0, the width w of the large N soliton increases with amplitude, w\propto\sqrt\phi_0. The amplitude and group velocity of the solitons with N \geq 1 are quantised. This agrees with the numerical results [1,2]. [1]. V. A. Kozlov et al., Sov. Phis. JETP 49, 75 (1979). [2]. P. K. Kaw, Phys. Rev. Lett. 68, 3172 (1992).

[7E.12] Two-Dimensional Child-Langmuir Law

By considering uniform emission of electrons over a finite strip of width w in a planar gap of gap separation D, we extend the classical 1-dimensional Child-Langmuir law to 2 dimensions. The analysis is done with the use of the simulation codes, MAGIC and OOPIC. The classical one-dimensional Child-Langmuir law is recovered when w >> D. Both cases of zero external magnetic field and large external magnetic field (immersed flows) are studied. Our preliminary results indicate that, for the case of zero magnetic field, R is a monotonically increasing function of W/D, approaching unity when W/D >> 1. Here R is the maximum current density on the emitting strip that would permit laminar flow behavior, expressed in units of the classical Child-Langmuir value.

Part 7 of program listing