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NASA NTRS1989

The magnetospheric response to 8-minute period strong-amplitude upstream pressure variations

This paper documents a series of brief, strong (delta p/p = 1), dynamic pressure oscillations that occurred in the region upstream of the earth's bow shock during a period of radial interplanetary magnetic field. The analyzed set of oscillations, which may be either intrinsic solar wind or bow shock-related phenomena, recur approximately every 8-10 min, and their magnetic field signatures occur nearly simultaneously over great distances transverse to the earth-sun line. The pressure oscillations appear to drive tailward-moving magnetopause surface wavelets. In turn, the surface wavelets can be identified as hydromagnetic waves with strong compressional components in the outer magnetosphere and as quasi-periodic variations in electron precipitation and high-latitude ground pulsations. Observations by spacecraft in the outer dayside magnetosphere are used to predict geosynchronous and subsolar magnetic field strengths, the location of the subsolar magnetopause, the solar wind dynamic pressure, and variations in the energetic magnetospheric ion flux.

Sibeck, D. G.↗

NASA NTRS1989

The radiative deceleration of ultrarelativistic jets in active galactic nuclei

A detailed study of the dynamical interaction between a highly relativistic jet and the thermal radiation field from an AGN accretion disk is reported, and the Comptonized spectrum arising from this interaction is self-consistently determined. A simple model that captures the essential radiative and geometrical features of realistic disk configurations is presented, and the disk radiation field is calculated. The results confirm Phinney's (1987) suggestion that the thermal radiation field produced by accretion in an AGN could be very effective in decelerating ultrarelativistic jets that are accreted by electromagnetic or hydromagnetic forces closer to the central black hole. Terminal Lorentz factors are consistent with the values inferred in superluminal radio sources are readily produced in this model for plausible disk and jet parameters without additional acceleration in the interaction zone. A new interpretation of the hard X-ray component detected in BL Lac spectra is proposed.

Melia, Fulvio↗

NASA NTRS1989

The velocity distributions of cometary protons picked up by the solar wind

Velocity space distributions of picked up cometary protons were measured by the ion mass spectrometer on the Giotto spacecraft upstream of the Halley bow shock. Large pitch angle anisotropies were observed at all distances greater than 1.2 x 10 to the 6th km from the comet. As expected, pitch angle diffusion was much more rapid than energy diffusion. When the field was quasi-parallel to the solar wind velocity vector, it was possible to discern the effect of pitch angle scattering by sunward propagating, field-aligned hydromagnetic waves, but there is evidence for other scattering modes as well. For quasi-perpendicular geometries, the pitch angle distribution was very asymmetric with phase space density peaks near pitch angles of 180 deg. It is suggested that the asymmetric pitch angle distribution may be caused by global rather than local wave-particle interactions. Just outside the shock, the pitch angle distribution was nearly isotropic and the radius of the pickup shell increased significantly.

Neugebauer, M.↗

NASA NTRS1989

Distribution function of continuously created newborn and pickup ions in outer cometary exospheres

The time evolution of the distribution function of newborn ions in the solar wind is investigated using a quasi-linear-type diffusion equation. The initial distribution is taken to be a ring beam, which is approximated by delta function in pitch angle and velocity, and it is assumed that the ions are created at a constant rate with a similar distribution. A long-time asymptotic form of ion distribution is obtained, which is a mixture of newborn ions and ions generated throughout the entire process. It is shown that the time asymptotic distribution function exists even in the presence of a continuous ionization process. The stability of the long-time asymptotic distribution was examined for the case of parallel propagation, and the results show that the distribution function can be unstable to low-frequency hydromagnetic waves. The results of the analysis were found to agree with recent satellite observations.

Gaffey, J. D., Jr.↗

NASA NTRS1989

X ray emission from relativistic jets in AGNs and statistical implications

Calculations of the Compton scattering interaction between an ultrarelativistic jet and a thermal radiation field, in an Active Galactic Nuclei (AGN), are presented. This process can be effective in decelerating ultrarelativistic jets that are accelerated by electromagnetic or hydromagnetic forces closer in to the central black hole. A narrow distribution of terminal Lorentz factors gamma sub infinity, consistent with the values inferred in superluminal radio sources, arises naturally in this model. The hard X-ray component detected in the spectra of 3C273 and several BL Lac objects may be due to the inverse Compton radiation produced in the course of the initial deceleration of their relativistic jets. The requirement that the luminosity of the hard X-ray component must exceed the total power in the associated jet is considered.

Melia, Fulvio↗

NASA NTRS1989

The Kelvin-Helmholtz instability at the magnetopause and inner boundary layer surface

Observations by the vector electron spectrometer on the ISEE 1 spacecraft are used to directly check the incompressible hydromagnetic stability condition given by Hasegawa (1975), which indicates how stability is influenced by the velocity shear, density ratio, magnetic field, and the direction of the wave vector of the unstable wave mode. The magnetopause is generally found to be stable, by a large margin. In contrast, many cases of marginal stability or instability are found across density transitions in, and at the inner edge of, the boundary layer for wave vectors in plausible directions. It thus appears that the Kelvin-Helmholtz instability is of importance at the inner edge of the magnetospheric boundary layer, as predicted by MHP theory.

Ogilvie, Keith W.↗

NASA NTRS1989

Evolution of an unstable shell distribution of pickup cometary ions

Consideration is given to the shell distribution formed by pickup ions in the solar wind. Numerical simulations are used to study the linear and nonlinear development of the waves and the evolution of the initial shell distribution. The linear theory is generalized to include additional effects. It is shown that unstable hydromagnetic fluctuations give rise to diffusion in velocity space and thickening of the shell distribution.

Yoon, Peter H.↗

NASA NTRS1990

Line broadening of Mg X 609 and 625 A coronal emission lines observed above the solar limb

A University of Colorado sounding rocket experiment on March 17, 1988, provided high-resolution EUV spectra along a solar diameter and out to 1.2 solar radius with spatial resolution of 20 x 60 arcsec. Each spectrum contains transition region and coronal emission lines in the wavelength range 605-635 A and 1210-1270 A, including the emission lines Mg X 609 and 625 A, Fe XII 1242 A, O V 629 A, N V 1238 and 1242 A, corresponding to a wide range of temperatures of formation. Increased line broadening is observed above the limb for all lines, and this effect is illustrated by presenting observed line widths as a function of height above the limb for the higher temperature lines Mg X 609 and 625 A. On the basis of calculations, the most likely cause of the increased broadening above the limb appears to be the presence of hydromagnetic waves in the corona.

Hassler, Donald M.↗

NASA NTRS1989

Fast Fermi acceleration in the plasma sheet boundary layer

A longstanding question in the field of magnetospheric physics is the source of the energetic particles which are commonly observed along the plasma-sheet boundary layer (PSBL). Several models have been suggested for the acceleration of these particles. Here, a means is suggested by which the fast Fermi acceleration mechanism (Wu, 1984) can accelerate electrons at the plasma sheet and perhaps account for some of the observations. In this scheme, a localized hydromagnetic disturbance propagating through the tail lobe region impinges upon the PSBL, deforming it and displacing it in toward the central plasma sheet. The boundary layer can then act like a moving magnetic mirror. If the disturbance is propagating nearly perpendicular to the layer, then its velocity projected parallel to the layer (and the magnetic field) can be very large, resulting in significant acceleration of reflected particles.

Wu, C. S.↗

NASA NTRS1990

Magnetothermal instability in cooling flows

The effect of magnetic fields on thermal instability in cooling flows is investigated using linear, Eulerian perturbation analysis. As contrasted with the zero magnetic-field case, hydromagnetic stresses support perturbations against acceleration caused by buoyancy - comoving evolution results and global growth rates are straightforward to obtain for a given cooling flow entropy distribution. In addition, background and induced magnetic fields ensure that conductive damping of thermal instability is greatly reduced.

Loewenstein, Michael↗

NASA NTRS1990

Generation of strong MHD Alfvenic turbulence

Strong Alfvenic turbulence containing a number of solitonlike structures propagating at super-Alfvenic speeds is generated self-consistently and studied by means of computer simulation. A one-dimensional hybrid (kinetic ions, fluid electrons) code is used to investigate the nonlinear evolution of an electromagnetic ion-beam instability that generates low-frequency Alfven-like waves. As the instability develops, the field-aligned hydromagnetic waves steepen, forming a soliton that bifurcates several times, leading to a fully turbulent state.

Akimoto, K.↗

NASA NTRS1990

Electromagnetic fire-hose instability in a fully relativistic bi-Maxwellian plasma

Detailed properties of the classical hydromagnetic Alfven wave and the fire-hose instability, driven by an excess in the parallel energy are well understood in the nonrelativistic regime. In this brief communication, the analysis is extended to the fully relativistic regime. The analysis is carried out for fully relativistic bi-Maxwellian distributions of electrons and ions (or positrons). It is shown that the relativistic effect has a nonnegligible effect on the wave and stability properties of the low-frequency modes under certain conditions.

Yoon, Peter H.↗

NASA NTRS1990

Ballooning mode instability at the plasmapause

The ballooning mode instability, which can excite hydromagnetic waves at the plasmapause, is studied in the presence of azimuthal plasma flows induced during geomagnetically disturbed periods. A general sufficient criterion for the ballooning mode stability is derived, for a change in the potential energy greater than or equal to 0, which involves the integration over an entire field line. A local stability analysis at the equatorial plasmapause region shows that the ballooning modes could be spontaneously generated via instability under at least two conditions: one is similar to the usual interchange condition, and the second to the quasi-interchange modes. Both of these local instability conditions can be derived from the general stability criterion. Finally an exact solution for the equilibrium state with flow is derived analytically, and the change in the potential energy is computed numerically. It is found that, in the cases studied, the flow does not spontaneously excite the ballooning modes; it only further stabilizes (or destabilizes) the ballooning spectrum if originally the system is stable (or unstable). The analysis would be useful for the interpretation of some of the low-frequency modes observed at the ground and near the equatorial plasmapause.

Lakhina, G. S.↗

NASA NTRS1989

Magnetic fields in the central engines of active galactic nuclei

Important physical processes which may occur in the central engines of active galactic nuclei and which rely on the presence of a strong magnetic field are discussed. These processes include those involved in the plasma physics of hot tenuous accretion flows, the production of nonthermal continuum radiation, and the radiative manifestation of hydromagnetic jet production. The main arguments which support the hypothesis that supermassive black holes are the prime movers in the central engines are reviewed, and some major deduction regarding the physical state of the accreting gas are pointed out.

Begelman, Mitchell C.↗

NASA NTRS1990

Particle acceleration at shocks with surface ripples

The present treatment of superthermal-ion acceleration on the surface of a fast-mode hydromagnetic shock gives attention to (1) small-amplitude surface ripples characterized by width L and amplitude A that are large relative to the energetic-ion gyroradius, and (2) shocks which are on average quasi-perpendicular. An investigation is made of the effects of the confinement, evolving geometry, and finite shock curvature associated with the ripple, by integrating along the orbits of the proton test particles. As an upstream magnetic field line convects through the surface ripple, it intersects the shock at two points, thereby forming a temporary magnetic trap. Flux-line profiles and angular distributions in a given ripple differ substantially, depending on the path it takes through the ripple and its distance from the shock.

Decker, R. B.↗

NASA NTRS1990

Pitch angle and velocity diffusions of newborn ions by turbulence in the solar wind

The temporal evolution of the distribution function of newborn ions under the influence of intrinsic low-frequency solar wind turbulences is studied. In particular, an initial ring-beam distribution of newborn ions under the influence of hydromagnetic waves is considered. A simplified treatment of the resonance broadening effect is given, and its role in the pickup process is discussed. Two different configurations of wave polarization amd direction of propagation are considered. The conditions that lead either to the formation of anisotropic shells as a long-duration transient state or to rapid isotropization of the ion pitch angle distribution are discussed, as are the conditions which lead to significant acceleration of the ions.

Ziebell, L. F.↗

NASA NTRS1991

Flux-corrected transport techniques for multidimensional compressible magnetohydrodynamics

A prescription is given for conservatively integrating generalized hydromagnetic equations using flux-corrected transport (FCT) techniques. By placing the magnetic-field components at the interface locations of the finite-difference grid, the field is kept divergence-free to within machine roundoff error. The use of FCT techniques allows an integration scheme of high accuracy to be employed, while the numerical ripples associated with large dispersion errors are avoided. The method is particularly well suited for problems involving magnetohydrodynamic shocks and other discontinuities.

Devore, C. Richard↗

NASA NTRS1991

ULF waves in the magnetosphere

Research efforts in the area of magnetospheric ULF waves in the 1987-1990 period are reviewed. Attention is given to externally excited hydromagnetic waves including field line resonance, the global cavity mode, bow-shock-associated upstream waves, and Kelvin-Helmholtz waves. Consideration is given to internally excited Pc 4-5 pulsations and the role of these pulsations in the diffusion of ring-current ions based on the observed properties of the pulsations.

Takahashi, Kazue↗