Search NASA⌕ Search

SEARCH · Search NASA

Results for “MAGNETIC RECONNECTION”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

A search for the origins of a possible coronal mass ejection in the low corona

Evidence for coronal and chromospheric precursors of a hypothesized coronal mass ejection is sought in OSO-7 observations of a filament eruption and the subsequent flare. Large-scale changes in the corona above the active region were clearly present for at least several minutes before the flare, culminating in the activation and eruption of two widely separated filaments; the eruption of one of the preexisting filaments initiated magnetic reconnections and energy releases in the low corona, generating the observed chromospheric flare.

Neupert, Werner M.↗
NASA NTRS1989

Theory and modeling group

The primary purpose of the Theory and Modeling Group meeting was to identify scientists engaged or interested in theoretical work pertinent to the Max '91 program, and to encourage theorists to pursue modeling which is directly relevant to data which can be expected to result from the program. A list of participants and their institutions is presented. Two solar flare paradigms were discussed during the meeting -- the importance of magnetic reconnection in flares and the applicability of numerical simulation results to solar flare studies.

Holman, Gordon D.↗
NASA NTRS1989

Pre-pulses: Signature of a trigger process in short (less than 60 secs) solar hard x ray flares

The continuing study of short hard x ray events (less than 60 sec duration) from the Solar Maximum Mission (SMM) Hard X ray Burst Spectrometer (HXRBS) instrument has revealed a unique feature. A well-separated distinctly identifiable, narrow (2 to 6 sec wide) pulse occurs prior to the start of the longer-flare lasting emission activity. Light curves are presented for eight events showing this feature. The pre-pulses show symmetrical rise and fall times. Spectral evolution of the pre-pulses are presented and their evolution compared to that of the main event spectra. It is argued that this feature be the elementary flare burst (de Jager, 1978). These pre-pulses could be a signature of the magnetic reconnection phenomenon discussed by Sturrock et al., (1984).

Deasi, U.↗
NASA NTRS1990

Solar and stellar coronae

A review of the observational facts of the X-ray corona of the sun suggests that the dissipation of waves plays at most a minor role in heating the corona. On the other hand, the random continuous shuffling and mixing of the footpoints of the bipolar magnetic fields, in which the X-ray corona resides, causes the spontaneous appearance of current sheets. Current sheets are highly dissipative, with a tendency to transient bursts of magnetic reconnection. Detailed observations show that the X-ray corona is made up of large numbers of small intense impulsive events of the same magnitude as expected from the bursts of reconnection. It appears, then, that it is the spontaneous current sheets that are the direct cause of the X-ray corona, driven by the continuous motions of the footpoints of the field in the photospheric convection.

Parker, E. N.↗
NASA NTRS1990

Interplanetary Alfven waves and auroral (substorm) activity - IMP 8

Almost 1 year of IMP 8 interplanetary magnetic field and plasma data (days 1-312, 1979) have been examined to determine the interplanetary causes of geomagnetic AE activity. The nature of the interplanetary medium (Alfvenic or non-Alfvenic) and the B(s) correlation with AE were examined over 12-hour increments throughout the study. It is found that Alfvenic wave intervals are present over 60 percent of the time, and the southward component of the Alfven waves is well correlated with AE (average peak correlation coefficient 0.62), with a median lag of 43 min. From this statistical study, no major differences in the magnetospheric response to Alfvenic and non-Alfvenic intervals were obvious. The high-intensity long-duration continuous AE activity (HILDCAA) events discussed previously by Tsurutani and Gonzales (1987) are demonstrated to be caused by the southward components of the Alfven waves, presumably through the process of magnetic reconnection.

Tsurutani, Bruce T.↗
NASA NTRS1990

Extreme energetic particle decreases near geostationary orbit - A manifestation of current diversion within the inner plasma sheet

A qualitative model of magnetic field reconfiguration as might result from neutral line formation in the central plasma sheet late in a substorm growth phase is considered. It is suggested that magnetic reconnection probably begins before the substorm expansion phase and that cross-tail current is enhanced across the plasma sheet both earthward and tailward of a limited region near the neutral line. Such an enhanced cross-tail current earthward of the original X line region may contribute to thinning the plasma sheet substantially, and this would in turn affect the drift currents in that location, thus enhancing the current even closer toward the earth. In this way a redistribution and progressive diversion of normal cross-tail current throughout much of the inner portion of the plasma sheet could occur. The resulting intensified current, localized at the inner edge of the plasma sheet, would lead to a very thin plasma confinement region. This would explain the very taillike field and extreme particle dropouts often seen late in substorm growth phases.

Baker, D. N.↗
NASA NTRS1990

Evaporation in the transition region during the gradual phase of flares

Previous observations have revealed that small, but sustained, H-alpha blueshifts occur in flare ribbons during the gradual, or late, phase of flares. These blueshifts suggest that there is a gentle evaporation of chromospheric material throughout the late phase of flares, but ambiguities in the interpretation of H-alpha leave open the possibility that these blueshifts are caused by downflowing, rather than upflowing, material. Using both C IV and soft (3.5-8 keV) X-ray data from SMM observations, evidence is found which supports the interpretation of the H-alpha blueshifts as upflows in the range from 4-12 km/s. The blueshifts are interpreted in terms of the reconnection model proposed by Carmichael (1964). The model produces a sustained energy release through-out the late phase which accounts for the prolonged soft X-ray emission after a flare. This energy release comes from the reconnecting magnetic field above the flare site, and some of the energy is transported along field lines mapping to the chromosphere where it drives chromospheric evaporation.

Schmieder, B.↗
NASA NTRS1990

The ionospheric signatures of flux transfer events and solar wind dynamic pressure changes

Recent observations of vortical flow patterns in the dayside auroral ionosphere are discussed in terms of two alternative mechanisms: (1) the time-dependent magnetic reconnection in 'flux transfer events' (FTEs); and (2) the action of solar wind dynamic pressure changes at the magnetopause. It is argued that the ionospheric flow signature of an FTE should be a twin vortex, with the mean flow velocity in the central region of the pattern being equal to the velocity of the pattern as a whole. On the other hand, the pulse of enhanced or reduced dynamic pressure is also expected to produce a twin vortex, but with the central plasma flow being generally different in speed from (and almost orthogonal to) the motion of the whole pattern. It is found that, while none of the events discussed here are consistent with the theories of the effects of the dynamic pressure changes, all are well explained in terms of the ionospheric signatures of FTEs.

Lockwood, M.↗
NASA NTRS1991

Shuffling foot points and magnetohydrodynamic discontinuities in the solar wind

ISEE three-field and plasma data are used to investigate the frequency of occurrence of isolated, large-amplitude rotational (RD) and tangential (TD) discontinuities in different types of solar wind flow. It is found that there are relatively more TDs in solar wind that originates in closed field regions and is ejected into interplanetary space by coronal transients than in the solar wind that originates in open field regions. The speed of the wind from open field regions is approximately linearly related to the number of RDs per hour; such a relation does not exist for the wind associated with coronal mass ejections. These results are consistent with the hypothesis that the convection-driven shuffling of magnetic foot points at the solar surface leads to TDs, magnetic reconnection, and heating of the corona on closed field lines, while in the open field regions the disturbances created by the shuffling are carried off by waves which contribute to the acceleration of the solar wind.

Neugebauer, M.↗
NASA NTRS1991

The magnetopause

A review of theoretical and observational research on the magnetopause during 1987-1990. Starting with recent work on magnetic reconnection, the review proceeds to magnetopause surface wave phenomena, including the controversy over the role of solar wind pressure pulses in the coupling process and in the mimicking of flux transfer events signatures, and finishes with the magnetopause structure and associated wave phenomena. Recent advances in computer modeling of the magnetopause and the pertinent processes are also discussed.

Smith, M. F.↗
NASA NTRS1991

Large-scale features of the sun at 20 centimeter wavelength

Results are reported from an experimental study of the characteristics of large-scale coronal structures such as active regions, plages, filaments, and coronal holes using data obtained with the VLA at 1.5 GHz during the period September 11-17, 1988. The radio data were supplemented with He 10830- A, H-alpha, and Calcium-K spectroheliograms. A statistical analysis of some of the characteristics of the active regions is performed. Most of the active region sources were found to be about 100 arcsec in size, with bridges between regions common; lower brightness temperature regions showed a higher degree of polarization in general. The maximum polarization was found at the edge of active regions but well within the associated plages. The degree of polarization from bright active regions was small (not more than 20 percent), in agreement with previous results. Evidence was found for compression of preexisting flux by the emerging flux from a new region, which took place in the apparent absence of magnetic reconnection.

Gopalswamy, N.↗
NASA NTRS1991

Evolution on large-scale plasma structures in comets: Kinematics and physics

The disconnection event (DE) consists of the periodic loss of a comet's entire plasma tail and the growth of a new one. This spectacular phenomenon is not understood. The strategy was to assemble a data base of specific events studied in detail, determine the solar wind conditions responsible for DEs, and develop a consistent physical model. Analysis is complete for the sequence of DEs that took place during 13 to 18 April 1986. The first DE correlates well with a sector boundary crossing for the comet and a group of DEs that occurred approximately a day later could have produced by polarity reversals seen in the IMP-8 data. Thus, these DEs are consistent with the frontside, magnetic reconnection mechanism.

Brandt, John C.↗
NASA NTRS1991

The last disconnection events in Comet Halley in April 1986

Disconnection event (DE) in cometary plasma physics is the regular loss of the entire plasma tail and the growth of the new one. Analysis of a sequence of DEs that occurred 13-18 April, 1986 shows that they correlate well with a magnetic, sector-boundary crossing and a complex magnetic structure in the solar wind with polarity reversals that occurred about one day later. These events are entirely consistent with sunward, magnetic reconnection as the DE mechanism.

Brandt, John C.↗
NASA NTRS1991

Generation of field-aligned current in the auroral zone

Generation of a magnetic field-aligned current in the auroral zone connecting the magnetospheric and ionospheric plasmas has been studied by means of a three dimensional particle simulation model. The model is of a magnetostatic variety appropriate for a low beta plasma in which the high frequency transverse displacement current has been eliminated. The simulation model is highly elongated along the magnetic field lines in order to model a highly elongated flux tube in the auroral zone. An enhanced field-aligned current was generated by injection of a magnetospheric plasma across the auroral zone magnetic field at the center of the model. Such a plasma injection may correspond to a plasmoid injection at the geomagnetic tail associated with magnetic reconnection during a substorm or a transverse plasma flow along the low latitude magnetopause boundary layer. The results of the simulations show that the field-aligned current can be enhanced over the thermal current by a factor of 5 - 10 via such injection. Associated with the enhanced current are the electrostatic ion cyclotron waves and shear Alfven waves excited in the auroral zone.

Okuda, Hideo↗
NASA NTRS1992

The disconnection event of 16.0 March 1986 in comet Halley

From kinematic extrapolation of tail/nucleus distance measurements on photographic images in the International Halley Watch (IHW) archive, we calculated the disconnection time of the 16-19 March 1986 event to be 16.0 (plus or minus 0.1) March. The solar wind conditions around Comet Halley at the time of the DE, inferred by corotation of IMP-8 satellite data to the comet, were such that (1) Comet Halley had just crossed the interplanetary magnetic field (IMF) sector boundary; (2) the solar wind density was approximately 8 cm(exp -3); (3) the solar wind speed was approximately 600 km/sec; (4) the IMF magnitude was approximately 8 nT. Given these conditions, we conclude that the most likely cause of the 16.0 March DE was front-side magnetic reconnection, as described in the model of Niedner and Brandt (1978).

Randall, Cora E.↗
NASA NTRS1992

A global magnetohydrodynamic simulation of the magnetosheath and magnetosphere when the interplanetary magnetic field is northward

We have used a new high-resolution global magnetohydrodynamic simulation model to investigate the configuration of the magnetosphere when the interplanetary magnetic field (IMF) is northward. For northward IMF the magnetospheric configuration is dominated by magnetic reconnection at the tail lobe magnetopause tailward of the polar cusp. This results in a local thickening of the plasma sheet equatorward of the region of reconnection and the establishment of a convection system with two cells in each lobe. In the magnetosheath the plasma density and pressure decrease near the subsolar magnetopause, forming a depletion region. Along the flanks of the magnetosphere the magnetosheath flow is accelerated to values larger than the solar wind velocity. The magnetopause shape from the simulations is consistent with the empirically determined shape.

Ogino, Tatsuki↗
NASA NTRS1993

Mass density and pressure changes across the dayside magnetopause

Plasma composition measurements at about 2-min time resolution from 27 magnetopause crossings are used to determine if mass density rho, modified by the anisotropic pressure term (1-alpha), is constant across the magnetopause. Pressure and mass density in the magnetosheath and low-latitude boundary layer were found to be dominated by H(+) except on one occasion when O(+) was the dominant contributor to the total mass density in the low-latitude boundary layer. Using these measurements, it is found that rho(1-alpha) is not constant for almost all magnetopause crossings even when there is independent evidence for magnetic reconnection and an open magnetopause. These results indicate that the reason why rho(1-alpha) is not constant across the magnetopause is that the H(+) density decrease from the magnetosheath to the low-latitude boundary layer is not compensated for by either a change in the pressure anisotropy or a change in the total mass density and suggest that the magnetopause can never be described as a time-stationary 1D rotational discontinuity.

Fuselier, S. A.↗
NASA NTRS1993

Evidence for ion jets in the high-speed solar wind

Results of an investigation of shapes of proton and alpha particle velocity distribution functions in the high-speed solar wind for the time period from 1973 to 1987 are presented. It is suggested that a modest amount of heating results from the damping of waves, whereas additional heating and the bulk of the acceleration is associated with the direct conversion of magnetic energy flux to plasma convection and enthalpy flux through the process of magnetic reconnection. The present two-component model of heating and acceleration predicts that the ambient flow of plasma from coronal holes is characterized by a low-speed and a low-mass flux. The proton flux is insufficient to drag alpha particles out of the solar gravitational potential well.

Feldman, W. C.↗