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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Laminar and turbulent plasmoid ejection in a laboratory Parker Spiral current sheet

Quasi-periodic plasmoid formation at the tip of magnetic streamer structures is observed to occur in experiments on the Big Red Ball as well as in simulations of these experiments performed with the extended magnetohydrodynamics code, NIMROD. This plasmoid formation is found to occur on a characteristic time scale dependent on pressure gradients and magnetic curvature in both experiment and simulation. Single mode, or laminar, plasmoids exist when the pressure gradient is modest, but give way to turbulent plasmoid ejection when the system drive is higher, which produces plasmoids of many sizes. However, a critical pressure gradient is also observed, below which plasmoids are never formed. A simple heuristic model of this plasmoid formation process is presented and suggested to be a consequence of a dynamic loss of equilibrium in the high- $\beta$ region of the helmet streamer. This model is capable of explaining the periodicity of plasmoids observed in the experiment and simulations, and produces plasmoid periods of 90 minutes when applied to two-dimensional models of solar streamers with a height of $3R_\odot$ . This is consistent with the location and frequency at which periodic plasma blobs have been observed to form by Large Angle and Spectrometric Coronograph and Sun Earth Connection Coronal and Heliospheric Investigation instruments.

Physics↗
NASA NTRS1981

The coronal and interplanetary current sheet in early 1976

A comparison of Helios 1 and 2 observations of the interplanetary sector pattern in early 1976 with the maximum brightness curves in the K coronameter data at 1.5 solar radii shows that the latter may be identified with the footprints of the sector boundary surface to an accuracy of about 10 deg. The sector boundary surface probably extended to about 15 deg in the Northern Hemisphere and to about 30 deg in the Southern Hemisphere, with little change between 1.5 solar radii and 1 AU. The surface was warped appreciably from a single titled plane (a dipole configuration) suggesting a significant magnetic quadrupole contribution.

Burlaga, L. F.↗
NASA NTRS1983

The structure of the heliospheric current sheet, 1978 - 1982

The structure of the heliospheric magnetic field changes substantially during the 11 year sunspot cycle. Its configuration for the period 1976 through 1982 using a potential field model was calculated. The structure during the rising phase, maximum, and early decline of sunspot cycle 21, from 1978 to 1982 is considered.

Hoeksema, J. T.↗
NASA NTRS1985

Electric and magnetic drift of non-adiabatic ions in the earth's geomagnetic tail current sheet

It has been shown recently that nonadiabatic particles in the earth's magnetotail drift across the tail roughly as predicted for adiabatic particles with 90 deg pitch angles. In this paper it is shown that this result implies the existence of an approximate invariant of the motion. Adding the effect of convection associated electric fields, the approximate bounce averaged motion of nonadiabatic particles in the magnetotail can be obtained. Thus the particle motion and energization due to combined magnetic and electric drifts in the magnetotail are easily predicted.

Beard, D. B.↗
NASA NTRS1984

Magnetotail energy storage and the variability of the magnetotail current sheet

The antiparallel field configuration in the earth magnetotail is a prime location to search for the important astrophysical process known as magnetic reconnection. The magnetic evidence that reconnection occurs in the earth's magnetotail is that the energy of the tail invariably decreases at the time of global substorm onsets, and that an increased number of closed field lines are seen in the near-earth magnetotail after substorms. This increased northward flux is often convecting earthward from the expected site of the X line after the substorm onset, while southward flux is often convecting tailward. Prior to a substorm, energy accumulates in the tail as the field assumes a more taillike configuration with a smaller field component across the equatorial plane. The high-beta plasma sheet becomes thin and has been observed to be at least as thin as a few thousand km near substorm onset times. The small Bn component and thin plasma sheet are the conditions which favor the onset of the tearing-mode instability, which is thought to lead to neutral-line formation and the reconnection associated with substorm onset. Intervals of weak and highly variable fields in the plasma sheet provide evidence for the tearing-mode instability.

Fairfield, D. H.↗
NASA NTRS1986

Magnetic nonequilibrium and current sheet formation

This paper investigates the equilibrium of a cluster of long, straight, twisted magnetic flux tubes extending in the z-direction in a highly conducting fluid, subject to the boundary condition that the cluster is surrounded by a uniform pressure P. It is shown that there is equilibrium only for axial symmetry, i.e., only for a single twisted flux tube. Any more complicated cluster of twisted tubes is subject to nonequilibrium reconnection of the transverse component of the field. The example provides another perspective on the general absence of equilibrium, and consequent rapid dissipation, of any magnetic field subject to random small-scale internal strains, such as the fields extending outward from the convective zones of stars.

Vainshtein, S. I.↗
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.↗