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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

An active role for magnetic fields in solar flares

Observations of photospheric magnetic fields are reviewed to determine whether changes in such fields can be related to flare activity, assuming that magnetic fields play an active role in providing flare energy. An intimate relation between emerging fields and bright flare knots is noted, and it is shown that the activation and eruption of an H-alpha filament is indicative of a major disruption of a magnetic field just prior to a flare. Observations of twisting motions in a filament just before a flare are discussed, erupting untwisting filaments are taken as unambiguous evidence for restructuring of the magnetic fields associated with flares, and it is argued that magnetic-field changes in the midst of most flares are obvious. It is concluded that successive brightenings in a family of loops may be evidence for the spread of a magnetic-field reconnection point from one field concentration to another and that flares may well take place in regions of field-line reconnection. This latter conclusion is illustrated using an empirical flare model that involves field-line reconnection, filament activation, and emerging magnetic flux.

Rust, D. M.↗
NASA NTRS1976

Evolution of fibrils with special reference to flare activity

A series of H alpha filtergrams are presented and used to examine the preflare evolution of H alpha structures along with the development of H alpha chromospheric flares. Three stages are found in the development of preflare neutral line filaments: (1) formation of a filament as a result of reconnection, (2) increase of the shear of the filament, and (3) reconnection of the fine components of the filament to form an elongated component immediately prior to the flare. Two particular examples of solar flares are examined: (1) the flare of July 5, 1974 which began with the activation of the elongated component of the filament after reconnection of the fine components, and (2) the flare of September 10, 1974 which occurred at the region where fibrils connect the sunspots in distorted form.

Tanaka, K.↗
NASA NTRS1976

On flares, substorms, and the theory of impulsive flux transfer events

Solar flares and magnetospheric substorms are discussed in the context of a general theory of impulsive flux transfer events (IFTE). IFTE theory, derived from laboratory observations in the Double Inverse Pinch Device (DIPD), provides a quantitative extension of 'neutral sheet' theories to include nonsteady field line reconnection. Current flow along the reconnection line increases with magnetic flux storage. When flux build-up exceeds the level corresponding to a critical limit on the current, instabilities induce a sudden transition in the mode of conduction. The resulting IFTE, indifferent to the specific modes and instabilities involved, is the more energetic, the lower the initial resistivity. It is the more violent, the greater the resulting resistivity increase and the faster its growth. Violent events can develop very large voltage transients along the reconnection line. Persistent build-up promoting conditions produce relaxation oscillations in the quantity of flux and energy stored (build-up-IFTE cycles). It is difficult to avoid the conclusion: flares and substorms are examples of IFTE.

Bratenahl, A.↗
NASA NTRS1979

On the origin of chondrules

A new mechanism for primordial melting of chondrules based upon heating by relativistic electrons accelerated by reconnecting magnetic field lines is discussed. This mechanism is free of the creation-annihilation problem of collisions used for heating. The basic requirements for reconnection are an interplanetary magnetic field with regions of reversals. An early magnetic field is inferred from the paleomagnetism of meteorites and the requirements of solar spin-down, while the field reversals upon which the reconnection is based are a common property of the present epoch solar wind.

Sonett, C. P.↗
NASA NTRS1982

Plasma and magnetic field characteristics of magnetic flux transfer events

Plasma and magnetic field data from ISEE 1 and 2 are examined for 5 passes of the magnetopause region at 20 and 40 deg northern latitudes, and are presented in terms of moments of the distribution function, calculated from two-dimensional or three-dimensional data. Flux transfer events are characterized by a mixture of magnetosheath and magnetospheric particles, which supports the hypothesis that flux transfer events represent encounters of reconnected flux tubes. An excess pressure appears to be balanced by the tension of the ambient magnetic field lines as they are draped around the reconnected flux tube, and the different observed magnetic field signatures are consistent with expectations for encounters of the flux tubes at different relative locations. It is suggested that increased flow speeds are caused by continued reconnection at the low-latitude boundaries of the flux tubes.

Paschmann, G.↗
NASA NTRS1983

Magnetic structure of the boundary layer

While magnetospheric convection appears to be driven by both magnetic reconnection and the 'viscous' dragging of closed flux tubes acting in varying proportions but with reconnection predominating as a cause, the action of the closed flux tubes of the low latitude boundary layer seems to predominate in driving system 1 field aligned currents and discrete auroras. ISEE boundary layer data for the magnetic shear effects associated with field aligned currents has occasionally shown a reverse draping of boundary layer field lines whose cause may be plasma entry around the cusps, a north-south asymmetry in current flow to the polar regions, or magnetic reconnection at high latitudes. Boundary layer encounter character and distribution asymmetries suggest that the boundary layer and/or plasma mantle may differ substantially at the dawn and dusk sides of the magnetosphere, and that their properties may have seasonal dependencies.

Hones, E. W., Jr.↗
NASA NTRS1983

Hall current effect on tearing mode instability

From a linear 2-D eigenmode analysis, it is found that the Hall current effect on collisional tearing mode instability becomes important for the thin magnetic reversal layer whose width is comparable to the ion inertia length; Hall currents produce a three-dimensional field structure and increase the reconnection (growth) rate. Since the magnetaic reversal layer widths both in the magnetopause and in the magnetotail are reported to become as thin as the ion inertial length (several hundred km) when the reconnection process is supposed to occur, the Hall current effect may explain the appearance of the dawn-dusk component of the magnetic field in the magnetotail reconnection region.

Terasawa, T.↗
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 NTRS1984

Boundary layers of the earth's outer magnetosphere

The magnetospheric boundary layer and the plasma-sheet boundary layer are the primary boundary layers of the earth's outer magnetosphere. Recent satellite observations indicate that they provide for more than 50 percent of the plasma and energy transport in the outer magnetosphere although they constitute less than 5 percent by volume. Relative to the energy density in the source regions, plasma in the magnetospheric boundary layer is predominantly deenergized whereas plasma in the plasma-sheet boundary layer has been accelerated. The reconnection hypothesis continues to provide a useful framework for comparing data sampled in the highly dynamic magnetospheric environment. Observations of 'flux transfer events' and other detailed features near the boundaries have been recently interpreted in terms of nonsteady-state reconnection. Alternative hypotheses are also being investigated. More work needs to be done, both in theory and observation, to determine whether reconnection actually occurs in the magnetosphere and, if so, whether it is important for overall magnetospheric dynamics.

Eastman, T. E.↗
NASA NTRS1985

Current sheets in solar flares

Numerical simulations of current sheets in solar flares are described, including new features such as the presence of a shock in Petschek's mechanism and impulsive burst-like reconnection due to secondary tearing and coalescence. The general properties of magnetic reconnection are discussed in connection with the basic requirements of numerical current sheet models. Emphasis is given to the need for realistic criteria for energy balance, the Lundquist number, and line tying in calculations of tearing and reconnection modes. The need for analytical models of current sheet processes to compare with the numerical simulations is also stressed.

Priest, E. R.↗
NASA NTRS1986

Quasi-stagnant plasmoid in the middle tail - A new preexpansion phase phenomenon

From the analysis of ISEE 3 data it is found that a plasmoid is sometimes formed in the middle tail outside the intervals of the substorm expansion phase. This plasmoid is produced by reconnection at the X-type neutral line, which is located earthward of the distant neutral line but beyond the substorm-associated near-tail neutral line, and it is almost stagnant in that the associated flow speed is less than 300 km/s. The blocking effect of the distant neutral line is the most probable reason for the slow movement. The quasi-stagnant plasmoid is observed at x = -60 to - 100 earth radii for a duration of a few tens of minutes, and in about one half of the cases it is followed by the fast tailward streaming. The onset of this streaming tends to coincide with the onset of the substorm expansion phase, and this probably occurs when the reconnection at the middle-tail neutral line comes close to processing the last closed field line. Intensification of the dawn-to-dusk electric field that causes the mantle plasma to reach the plasma sheet boundary closer to the earth is suggested as the reason for the formation of the middle-tail neutral line earthward of the distant neutral line. The effects on the energetic particle flux and relation to the near-tail reconnection are also discussed.

Nishida, A.↗
NASA NTRS1986

Magnetic energy dissipation in force-free jets

It is shown that a magnetic pressure-dominated, supersonic jet which expands or contracts in response to variations in the confining external pressure can dissipate magnetic energy through field-line reconnection as it relaxes to a minimum-energy configuration. In order for a continuous dissipation to occur, the effective reconnection time must be a fraction of the expansion time. The dissipation rate for the axisymmetric minimum-energy field configuration is analytically derived. The results indicate that the field relaxation process could be a viable mechanism for powering the synchrotron emission in extragalactic jets if the reconnection time is substantially shorter than the nominal resistive tearing time in the jet.

Choudhuri, Arnab Rai↗
NASA NTRS1987

AMPTE IRM observations of waves associated with flux transfer events in the magnetosphere

The AMPTE IRM wave instrument has been applied to the study of flux transfer events (FTE's). This initial investigation concentrates on FTE's observed in the magnetosphere during the fall of 1984. The wave morphology consists of four significant features: at frequencies below the ion gyrofrequency, magnetic fluctuations occur with amplitudes of the order of 1 nT; at frequencies from a few hertz to a few hundred hertz, electric field fluctuations are observed which have a broadband amplitude of a few millivolts per meter, which are perpendicularly polarized at the lowest frequencies, and which are partly electrostatic and partly electromagnetic; in the frequency range from 100 Hz to 10 kHz, broadband spikelike waves occur with a time-averaged amplitude of about 0.1 mV/m; and near the electron plasma frequency, bursts of waves occur at the boundaries of FTE's. In none of the 25 events surveyed does the total broadband amplitude of all the waves exceed a few millivolts per meter, an amplitude far too small to provide the dissipation required by 'traditional' reconnection models in which the dissipation region has a thickness the order of an ion gyroradius. Thus, either all of the FTE's are observed at some distance from the diffusion region, or the observable waves play no significant role in the diffusion process, or 'traditional' reconnection models do not apply to reconnection in FTE's.

Labelle, J.↗
NASA NTRS1987

The cause of two plasma-tail disconnection events in comet P/Haley during the ICE-Halley radial period

The causes of two plasma-tail disconnection events (DEs), which occurred in Halley's comet on March 20-22 and April 11-12, 1986, during the ICE-Halley radial period, are analyzed using the ICE magnetometer and electron plasma data. It is concluded that the DE of March 20-22 was most likely caused by an IMF polarity reversal. The DE of April 11-12 on the other hand, is attributed to either a compression region in the solar wind, an IMF polarity reversal, or a combination of the two. Assuming that the two DEs are due to frontside reconnection after an IMF reversal, it was estimated that the time period between the onset of reconnection and the final disconnection of the tail is between 0.1 and 0.6 day, suggesting that the average speed at which reconnection proceeds through the cometary magnetic field pile-up region is between 1 and 6 km/sec, or several tenths of the local Alfven speed.

Brosius, J. W.↗
NASA NTRS1987

MHD aspects of magnetotail dynamics

The equations of ideal MHD represent the conservation of mass, momentum, and energy; thus, MHD must, in some broad sense, provide a correct description of large-scale structure of the earth's magnetosphere. However, steady-state, ideal MHD (the simplest description) allows only a static interaction between the solar wind and the magnetosphere. Hence an MHD theory of magnetospheric dynamics must involve unsteady flow turbulence, such as might be generated by Kelvin-Helmholtz turbulence at the magnetopause, and dissipative transport, such as reconnection, which violates ideal MHD. This brief review summarizes recent theoretical efforts to explore and extend the early MHD magnetospheric models of Axford and Petschek. Analytic calculations show that steady convection in the magnetotail is nearly impossible. Global magnetospheric simulations exhibit substorm reconnection phenomena for southward IMF and predict a novel convection pattern for northward IMF. Reconnection simulations show plasmoid formation and field-aligned current flows in the tail. The Kelvin-Helmholtz instability can provide a strong anomalous viscosity which can drive the viscous convection system.

Coroniti, F. V.↗
NASA NTRS1988

Observations of photospheric magnetic fields and shear flows in flaring active regions

Horizontal flows in the photosphere and subsurface convection zone move the footpoints of coronal magnetic field lines. Magnetic energy to power flares can be stored in the corona if the flows drive the fields far from the potential configuration. Videodisk movies were shown with 0.5 to 1 arcsecond resolution of the following simultaneous observations: green continuum, longitudinal magnetogram, Fe I 5576 A line center (mid-photosphere), H alpha wings, and H alpha line center. The movies show a 90 x 90 arcsecond field of view of an active region at S29, W11. When viewed at speeds of a few thousand times real-time, the photospheric movies clearly show the active region fields being distorted by a remarkable combination of systematic flows and small eruptions of new flux. Magnetic bipoles are emerging over a large area, and the polarities are systematically flowing apart. The horizontal flows were mapped in detail from the continuum movies, and these may be used to predict the future evolution of the region. The horizontal flows are not discernable in H alpha. The H alpha movies strongly suggest reconnection processes in the fibrils joining opposite polarities. When viewed in combination with the magnetic movies, the cause for this evolution is apparent: opposite polarity fields collide and partially cancel, and the fibrils reconnect above the surface. This type of reconnection, driven by subphotospheric flows, complicates the chromospheric and coronal fields, causing visible braiding and twisting of the fibrils. Some of the transient emission events in the fibrils and adjacent plage may also be related.

Tarbell, T.↗
NASA NTRS1988

Plasma motions in an emerging flux region

Ultraviolet spectra of C IV show large nonthermal broadening in an area of emerging flux. These nonthermal motions are observed at a very early stage of reconnecting field lines. The spectra can be traced to small, rapidly changing surge or filament-like features which are seen in the center, blue, and red wing of H-alpha. They seem to have the characteristics of macrospicules or explosive events. They are precursors of the reconnection process. Plasma turbulence and/or plasma waves in the reconnecting plasma may cause the broad C IV line profiles. The activity in the transition zone precedes the formation of a bright chromospheric loop system.

Brueckner, G. E.↗
NASA NTRS1989

Polar cap deflation during magnetospheric substorms

The expanding/contracting polar cap model has been used to simulate DE-2 ion drift data during substorms as determined using the AL index. Of the 39 cases modeled, 57 percent required the opening of a nightside gap which maps to where reconnection occurs in the tail; 75 percent of the 16 recovery phase cases required a nightside gap, while only 29 percent of the 17 expansion phase cases required a nightside gap. On the basis of this result, it is concluded that if a nightside gap implies tail reconnection, then reconnection probably occurs after expansion phase onset and continues throughout most of the recovery phase of a substorm.

Moses, J. J.↗