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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

An atlas of photospheric magnetic field observations and computed coronal magnetic fields: 1976-1985

Daily magnetogram observations of the large-scale photospheric magnetic field have been made at the John M. Wilcox Solar Observatory at Stanford since May of 1976. These measurements provide a homogeneous record of the changing solar field through most of solar cycle 21. Using the photospheric data, the configuration of the coronal and heliospheric fields can be calculated using a Potential Field-Source Surface model. This provides a three-dimensional picture of the heliospheric field evolution during the solar cycle. This paper announces the publication of UAG Report No. 94, an Atlas containing the complete set of synoptic charts of the measured photospheric magnetic field, the computed field at the source surface, and the coefficients of the multipole expansion of the coronal field. The general underlying structures of the solar and heliospheric fields, which determine the environment for solar-terrestrial relations and provide the context within which solar activity related events occur, can be approximated from these data.

Hoeksema, J. T.↗
NASA NTRS1989

Slow twists of solar magnetic flux tubes and the polar magnetic field of the sun

The solar wind model of Weber and Davis (1967) is generalized to compute the heliospheric magnetic field resulting from solar rotation or a steady axisymmetric twist including a geometrical expansion which is more rapid than spherical. The calculated increase in the ratio of the toroidal to poloidal field components with heliocentric radial distance r clarifies an expression derived recently by Jokipii and Kota (1989). Magnetic-field components transverse to r do not in general grow to dominate the radial component at large r. The analysis also yields expressions for the Poynting flux associated with the steady twists.

Hollweg, Joseph V.↗
NASA NTRS1990

Magnetic field structure of interplanetary magnetic clouds at 1 AU

Interplanetary magnetic clouds emerge as a feature of the solar wind at 1 AU, exhibiting enhanced field strength and lower plasma temperature and density than the surrounding plasma. A least-squares program has been developed which fits magnetic field data within a cloud, while estimating such cloud properties as its size, maximum field strength, and axis inclination. The results obtained from a study of 12 clouds observed at 1 AU point to a probable cloud axis direction within 15 deg of the ecliptic plane and about 100 deg from the sun's direction, when projected into the ecliptic plane. A wide variety of orientations is observed; some extend to 80 deg from the ecliptic.

Lepping, R. P.↗
NASA NTRS1991

High-resolution spectra of solar magnetic features. II - Magnetic fields of umbral brightenings

The spectra of Fe I and Fe II Zeeman-sensitive lines enhanced by video processing of CCD images are considered. The magnetic-field variation within umbras is obtained from the nearly complete Zeeman splitting of the Stokes I profile. It is shown that small brightenings within umbrae have magnetic fields nearly equal to or slightly smaller than that of the darker surroundings; these features are also nearly at rest with respect to their surroundings. It is noted that the absence of significant motions in umbral dots implies that radiation transports most of the energy at and immediately below the surface. The small size of the dots implies that, if convective plumes transport energy below the surface of sunspot umbrae, they should have both a lateral extent and a depth of their upper boundary comparable to or smaller than the size of the dot.

Lites, Bruce W.↗
NASA NTRS1991

Probing the magnetic topologies of magnetic clouds by means of solar energetic particles

Solar energetic particles (SEPs) have been used as probes of magnetic cloud topologies. The rapid access of SEPs to the interiors of many clouds indicates that the cloud field lines extend back to the sun and hence are not plasmoids. The small modulation of galactic cosmic rays associated with clouds also suggests that the magnetic fields of clouds are not closed.

Kahler, S. W.↗
NASA NTRS1991

Magnetic field gradients and their uses in the study of the earth's magnetic field

Magnetic field gradients are discussed from the standpoint of their usefulness in modeling crustal magnetizations. The fact that gradients enhance shorter wavelength features helps reduce both the core signal and the signal from external fields in comparison with the crustal signal. If the gradient device can be oriented, then directions of lineation can be determined from single profiles, and anomalies caused by unlineated sources can be identified.

Harrison, C. G. A.↗
NASA NTRS1992

Recent progress towards developing a high field, high-T(sub c) superconducting magnet for magnetic suspension and balance systems

This paper reviews superconducting magnets and high T(sub c) superconducting oxide ceramic materials technology to identify areas of fundamental impasse to the fabrication of components and devices that tap what are believed to be the true potential of these new materials. High T(sub c) ceramics pose problems in fundamentally different areas which need to be solved unlike low T(sub c) materials. The authors map out an experimental plan designed to research process technologies which, if suitably implemented, should allow these deficiencies to be solved. Finally, assessments are made of where and on what regimes magnetic system designers should focus their attention to advance the practical development of systems based on these new materials.

Derochemont, L. Pierre↗
NASA NTRS1991

The effect of low-temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites - A case for acquisition of chemical remanent magnetization

Experiments were performed demonstrating that a chemical remanent magnetization (CRM) can be acquired during the aqueous oxidation of synthetic titanomagnetites (TM40:Fe/2.6/Ti/0.4/O4) by the removal of iron, as it does in the oceanic crust basalts. Results showed that the CRM is acquired along the ambient field direction during oxidation, and that there is no coupling of the CRM and the thermoremanent magnetization. The rate of oxidation was found to be pH-dependent, with the amount of CRM acquired varying inversely with the pH of the oxidizing solution. The reaction produced nascent hydrogen gas. Results support the chemical equation for titanomagnetic oxidation proposed by Worm and Banerjee (1984).

Kelso, Paul R.↗
NASA NTRS1992

The spectrum of random magnetic fields in the mean field dynamo theory of the Galactic magnetic field

The fluctuation spectrum that must arise in a mean field dynamo generation of galactic fields if the initial field is weak is considered. A kinetic equation for its evolution is derived and solved. The spectrum evolves by transfer of energy from one magnetic mode to another by interaction with turbulent velocity modes. This kinetic equation is valid in the limit that the rate of evolution of the magnetic modes is slower than the reciprocal decorrelation time of the turbulent modes. This turns out to be the case by a factor greater than 3. Most of the fluctuation energy concentrates on small scales, shorter than the hydrodynamic turbulent scales. The fluctuation energy builds up to equipartition with the turbulent energy in times that are short compared to the e-folding time of the mean field. The turbulence becomes strongly modified before the dynamo amplification starts. Thus, the kinematic assumption of the mean dynamo theory is invalid. Thus, the galactic field must have a primordial origin, although it may subsequently be modified by dynamo action.

Kulsrud, Russell M.↗
NASA NTRS1994

Coronal Magnetic Structures Observing Campaign. 3: Coronal plasma and magnetic field diagnostics derived from multiwaveband active region observations

Simultaneous soft X-ray, microwave, and photospheric magnetic field observations were taken during the Coronal Magnetic Structures Observing Campaign (CoMStOC '87). The plasma electron temperature and emission measures determined from the X-ray data are used to predict the free-free emission expected at 20 and 6 cm. Comparing these predictions with the microwave observations, it is found that the predicted 20 cm brightness temperatures are higher than the observed, requiring cool absorbing material between the hot X-ray plasma and the observer. The model that is most consistent with all the observations and minimizes the required coronal fields indicates that this 20 cm emission is either free-free or a combination of free-free and fourth harmonic cyclotron emanating from the X-ray plasma with an electron temperature of approximately 3.1 x 10(exp 6) K and an emission measure of approximately 1.3 x 10(exp 29)/cm(exp 5). The observed 20 cm polarization requires a field strength of greater than or equal to 150 G. In addition, the 6 cm emission is free-free, emanating from cooler plasma with an electron temperature of approximately 1.5 x 10(exp 6) K and an emission measure of approximately 3-6 x 10(exp 29)/cm(exp 5). This model is consistent with the rather unusual combination of high 20 cm and low 6 cm polarization as well as the low extrapolated coronal fields.

Schmelz, J. T.↗
NASA NTRS2006

How Does Collisionless Magnetic Reconnection Work in the Presence of a Guide Magnetic Field?

The dissipation mechanism of guide field magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as 2.5 and three-dimensional PIC simulations of guide-field magnetic reconnection. We will show that diffusion region scale sizes in moderate and large guide field cases are determined by electron Larmor radii, and that analytical estimates of diffusion region dimensions need to include description of the heat flux tensor. The dominant electron dissipation process appears to be based on thermal electron inertia, expressed through nongyrotropic electron pressure tensors. We will argue that this process remains viable in three dimensions by means of a detailed comparison of high resolution particle-in-cell simulations.

Hesse, Michael↗
NASA NTRS2021

Magnetic Reconnection in a Sheared Magnetic Flux Tube: Slippage Versus Tearing

The process of magnetic reconnection in a flux tube can occur in a time-stationary fashion as slippage reconnection or in a time-dependent manner based on, for example, the tearing instability. However, it is not well known under which conditions a system can sustain slippage reconnection. Likewise, it is unclear whether systems can exhibit slippage reconnection and time-dependent reconnection simultaneously. In order to investigate these questions, we employ a set of 3D magnetohydrodynamic simulation. Using these simulations, we model a twisted flux tube with a spatially localized resistive region in the center of the simulation box, and an applied driver on one end plane of the simulation box. As a result, the Poynting flux injected at the boundary propagates to the resistive region and dissipates there. If the driver is nearly circular, we find a predominance of slippage reconnection, which decouples the applied driver entirely from one half of the flux tube. Second, a tearing-like instability occurs with slippage reconnection in the resistive region when the resistivity is strong enough and the cross section of the flux tube, as defined by the shape of the applied velocity perturbation, is elliptical enough. In this case we find the surprising feature that magnetic field perturbation generated by the tearing-like instability radiates away from the resistive region to influence the entire flux tube shape.

Hidetaka Kuniyoshi↗
DOE OSTI2024

Polarimetry-Interferometry Diagnostic for Magnetic Field and Magnetic Fluctuation Measurements in High-Performance and Long-Pulse Tokamaks (Final Report)

In this grant, we executed a multi-year plan to develop an advanced Faraday-effect polarimeter-interferometer system to address both fluctuation and equilibrium diagnostic needs by providing a measurement of the plasma internal magnetic field fluctuations (including non-axisymmetric 3-D perturbations) and plasma vertical position. This diagnostic included both fast time response and low phase noise so that both the temporal dynamics of equilibrium quantities as well as high-frequency electromagnetic fluctuations could be measured simultaneously, adding new and unique measurement capabilities for the core of high-temperature plasmas. The combined polarimeter-interferometer diagnostic development was applied primarily to the tokamak magnetic configuration, focusing on high-performance regimes including H-mode, reversed shear advanced tokamak scenarios, and non-inductive current drive.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗