Engineering topics

Zweibel, Ellen G.

Publications and source records attributed to Zweibel, Ellen G..

29 records · Page 2

NASA NTRS1990

Virial theorem analysis of the structure and stability of magnetized clouds

The tensor virial theorem is used to analyze the structure and stability of self-gravitating, magnetized spheroids surrounded by a low-density medium with pressure and magnetic field. Analytical expressions are developed for the effect of a weak field and calculate critical states when the effect of the field is arbitrarily strong, comparing the results with full magnetohydrostatic calculations. This analysis suggests that a magnetic field may prevent gravitational collapse but may also be destabilizing, depending on its degree of concentration within the cloud.

Zweibel, Ellen G.↗

NASA NTRS1990

Virial theorem analysis of the structure and stability of magnetized clouds

Zweibel, Ellen G.↗

NASA NTRS1989

Magnetic reconnection in partially ionized gases

Magnetic field lines in a plasma reconnect at a rate scaled by the Alfven speed. In a partially ionized gas there are two natural Alfven speeds: one determined by the ionized mass density alone, which applies when ion-neutral friction is negligible, and one determined by the total mass density, which applies when ion-neutral friction is strong. When the ionization fraction is low, as in a dense molecular cloud, these two speeds differ by several orders of magnitude. Both time-dependent tearing modes and steady-state magnetic reconnection in partially ionized gas are considered, and the regimes in which the charged and neutral components are strongly, intermediately, and weakly coupled are delineated. Molecular clouds are probably in the intermediate regime, while reconnection in solar prominences probably has strong ion-neutral coupling. Reconnection proceeds more rapidly when coupling is not strong.

Zweibel, Ellen G.↗

NASA NTRS1989

Magnetohydrodynamic waves and particle acceleration in a coronal loop

A model is presented for the acceleration of electrons in a flaring solar coronal loop. It is suggested that the primary energy release event in the flare establishes a spectrum of MHD waves within the loop which accelerate electrons by wave-particle resonant interactions. The nature of the MHD oscillations are described and numerical and analytic solution of the dispersion relation are examined. It is found that the behavior of a particle depends on whether or not it is in resonance with the wave spectrum. Resonant particles follow chaotic trajectories and can diffuse in velocity space between wave effective potentials, thereby gaining energy.

De La Beaujardiere, Jean-Francois↗

NASA NTRS1988

Hydromagnetic wave heating of the low-density interstellar medium

A simple model for supernova remnant sources of MHD waves is used to calculate the energy spectrum of waves in the intercloud medium and the heating rate resulting from their dissipation. Models of thermal phases of interstellar gas in ionization and thermal equilibrium are then constructed, and it is demonstrated that wave dissipation can be an important heating mechanism which can account for the observed high H I temperatures in low-density (intercloud) neutral gas.

Ferriere, Katia M.↗

NASA NTRS1988

Ambipolar diffusion drifts and dynamos in turbulent gases

Ambipolar drift in turbulent fluids are considered. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, it is shown that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvenic (helical) turbulence. A simple dynamo theory is made and is compared with standard theories in which dissipation is caused by turbulent diffusion. The redistribution of magnetic flux in interstellar clouds is also discussed.

Zweibel, Ellen G.↗

NASA NTRS1988

The growth of magnetic fields prior to galaxy formation

Hydromagnetic dynamo theory is applied to the problem of magnetic field amplification in the epoch after recombination but prior to galaxy formation. Large-scale density fluctuations acquire angular momentum from tidal torques exerted by neighboring fluctuations, and small-scale subfluctuations possess helicity due to the Coriolis forces associated with the rotation of the large-scale host fluctuations. Cosmic expansion and the interaction of matter with the ambient radiation field are taken into account. Despite many uncertainties, it is concluded that significant dynamo amplification may have occurred.

Zweibel, Ellen G.↗

NASA NTRS1988

Hydromagnetic wave heating of low density interstellar gas

The origin of the observed wave spectrum for hot gas in the ISM is considered theoretically. The governing equations for the generation, propagation, and dissipation of compressive waves are reviewed, and particular attention is given to the heating of warm neutral gas and the implications for radio-wave scattering. It is shown that little power from interactions between SN shocks and hot coronal gas reaches short wavelengths, and that scintillation probably does not originate in a warm weakly ionized gas.

Zweibel, Ellen G.↗

NASA NTRS1987

The scattering of Alfven waves by density fluctuations

The propagation of an Alfven wave packet through a medium containing time-dependent random density fluctuations is considered. The Alfven wave interaction with these density irregularities causes the transfer of wave power to both shear (Alfvenic) and compressive (magnetosonic) disturbances. The latter are dissipated and heat the plasma. The Alfven wave decay rate is computed in the limit of short-wavelength density fluctuations, the results are applied to the solar corona and interstellar medium.

Li, He-Sheng↗

NASA NTRS1987

The formation of current sheets in the solar atmosphere

A simple model of the effect of slow photospheric motions on the coronal magnetic field is examined. In this model, a magnetic field is stretched between two conducting plates and slow tangential displacements of the plates cause the field to evolve quasi-statically. If the field is treated as a continuum, the interior magnetic topology is uniquely specified by the displacement of the boundaries, while the topology of a field made up of discrete flux tubes is not determined by the boundary displacement. Treating the field as a continuum, it is found that small perturbations of a constant vertical or weakly sheared field can come to an equilibrium free of singularities if the separation between the plates is finite.

Zweibel, Ellen G.↗

NASA NTRS1987

The theory of the Galactic magnetic field

The paper discusses the role of the magnetic field in determining the large scale structure and dynamics of the interstellar medium. It then discusses the origin and maintenance of the Galactic field. The two major competing theories are that the field is primordial and connected to an intergalactic field or that the field is removed from and regenerated within the Galaxy. Finally, cosmic ray acceleration and confinement in the interstellar medium are discussed.

Zweibel, Ellen G.↗