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94 records · Page 6

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NASA NTRS1993

Three-dimensional magnetotail equilibria by numerical relaxation techniques

Consideration is given to a numerical method to iteratively approach 3D magnetostatic force equilibria, with modeling emphasis on the development of a suitable model of the earth's magnetotail, including a portion of the inner magnetosphere, i.e., on models which violate the 'tail approximation' commonly employed in analytical models. The numerical approach is compared to methods developed for laboratory plasma physics. The method is applied to the magnetotail outside of 10 R(E), using Tsyganenko's (1987) model for the quiet magnetosphere as an initial condition. The changes of the magnetic field necessary to yield an equilibrium configuration and the resulting distribution of the self-consistently derived pressure are discussed. It is shown that a self-consistent magnetotail equilibrium based on a close approximation to Tsyganenko's requires a region 1 type current system, which is not present in the initial configuration.

Hesse, Michael↗

NASA NTRS1985

Magnetic canopies and models of the solar chromosphere

Magnetic canopies are the topologies formed by magnetic field lines as they spread from compact, nearly vertical concentrations of flux in the low photosphere into the large-scale organization of the corona and heliosphere. Analytical techniques for inferring the base-heights of canopies from magnetographic data are reviewed together with observational evidence that much of the sun is covered by canopies which lie two or more pressure scale heights below the level which is traditionally inferred from thin flux tube models. Implications of these results for modeling the structure and energetics of the chromosphere are discussed, and it is argued that future models should be based upon both spectroscopic and polarimetric data. Recent improvements in magnetostatic model atmospheres are reviewed, and new observational data, including C IV Doppler-grams from the Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission, are considered. Directions for future research in MHD modeling of canopies, in simulating spectrographic and polarimetric data from such models, and in observational programs are discussed.

Jones, H. P.↗

NASA NTRS1983

Closed and open magnetic fields in stellar winds

A numerical study of the interaction between a thermal wind and a global dipole field in the sun and in a giant star is reported. In order for closed field lines to persist near the equator (where a helmet-streamer-like configuration appears), the coronal temperature must be less than a critical value Tc, which scales as M/R. This condition is found to be equivalent to the following: for a static helmet streamer to persist, the sonic point above the helmet must not approach closer to the star than 2.2-2.6 stellar radii. Implications for rapid mass loss and X-ray emission from cool giants are pointed out. The results strengthen the case for identifying empirical dividing lines in the H-R diagram with a magnetic topology transition locus (MTTL). Support for the MTTL concept is also provided by considerations of the breakdown of magnetostatic equilibrium.

Mullan, D. J.↗

NASA NTRS1976

Test particle propagation in magnetostatic turbulence. 1. Failure of the diffusion approximation

The equation which governs the quasi-linear approximation to the ensemble and gyro-phase averaged one-body probability distribution function is constructed from first principles. This derived equation is subjected to a thorough investigation in order to calculate the possible limitations of the quasi-linear approximation. It is shown that the reduction of this equation to a standard diffusion equation in the Markovian limit can be accomplished through the application of the adiabatic approximation. A numerical solution of the standard diffusion equation in the Markovian limit is obtained for the narrow parallel beam injection. Comparison of the diabatic and adiabatic results explicitly demonstrates the failure of the Markovian description of the probability distribution function. Through the use of a linear time-scale extension the failure of the adiabatic approximation, which leads to the Markovian limit, is shown to be due to mixing of the relaxation and interaction time scales in the presence of the strong mean field.

Klimas, A. J.↗