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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Magnetic observations during the recent declining phase of solar activity

Changes in the heliospheric magnetic field during the recent declining phase in solar activity are reviewed and compared with observations during past sunspot cycles. The study is based principally on data obtained by IMP-8 and Ulysses. The field magnitude is found to have increased during the declining phase until it reached a maximum value of 11.5nT in approximately 1991.5, approximately two years after sunspot maximum. The field of the sun's south pole became negative after a reversal in early 1990. The sector structure disappeared at Ulysses in April 1993 when the latitude of the spacecraft was -30 deg revealing a low inclination of the heliospheric current sheet. A large outburst of solar activity in March 1991 caused four Coronal Mass Ejections (CMEs) and numerious shocks at the location of Ulysses. Following a delay of more than a year, a series of recurrent high speed streams and Corotating Interaction Regions commenced in July 1992 which were observed by IMP-8, Ulysses and Voyager 2. In all these respects, the behavior of the magnetic field mimics that seen in the two earlier sunspot cycles. The comprehensive data set suggests a correlation between the absolute value of B and sunspot number. The major solar cycle variations in the radial component (and magnitude) of the field have been successfully reproduced by a recent model consisting of a tilted solar dipole, whose strength and tilt undergo characteristic changes over the sunspot cycle, and the heliospheric current sheet. The large outbursts of activity in mid-1972, mid-1982 and the first quarter of 1991 may represent a characteristic last 'gasp' of solar activity before the sun evolves to a different state. The recurrent high speed streams in 1973, 1984 and 1992 accompany the developemnt of large asymetrical polar coronal holes and the growth in intensity of the polar cap fields. After they endure for about one year, the polar coronal holes recede and the high speed streams are replaced by weaker streams more characteristic of solar minimum.

Smith, E. J.↗
NASA NTRS2010

Searching for Turbulent Spectra in Mutliple Spacecraft Observations

The subject of magnetohydrodynamic simulations in the 1980's, magnetic turbulence has reemerged as a favored explanation for the production of fast magnetic reconnection. Recent three-dimensional simulations reveal the formation of secondary islands that serve to shorten the current sheet and increase the accelerating electric field, while both simulations and observations witness electron holes whose collapse energizes electrons. However, little work has been done to investigate the presence of turbulent magnetic and electric field spectra in association with in situ reconnection observations in space. In this study we address this deficiency by analyzing the wavelength spectra of two reconnection events established by multiple spacecraft observations in previous publications. We apply a k-filtering analysis to determine the wave vectors. To optimize the quality of the k-filtering analysis and minimize aliasing, we select multi-spacecraft observations having optimal spacecraft tetrahedral geometry and dimensions that resolve wavelengths within the ion diffusion region. We have chosen one dayside and one nightside reconnection event each observed by the Cluster spacecraft. Spatial correlation between the reconnecting current sheet and a turbulent magnetic spectrum supports an association between turbulence and reconnection in space. The k-filtering method, a wavelet time-frequency spectrum, and a polarization analysis serve to identify wave modes associated with the turbulence. A calculation of the reconnection electric field based on the normal component of the reconnecting magnetic field provides an estimate of the reconnection rate.

Wendel, Deirdre E.↗
NASA NTRS1994

Effect of B(sub y) on neutral line ridges and dynamical source ordering

In this paper we study the effect of a uniform magnetic field B(sub y) in the cross-tail y direction on the dynamics and distribution of energetic ions in a current sheet model including a neutral line. Martin and Speiser (1988) have previously shown that a 'ridge' in the velocity space distribution function is a remote neutral line signature in a two-dimensional field without B(sub y). Our results show that as B(sub y) is increased, using nominal tail parameters, there is very little change in the ridge signature for small B(sub y) (up to about 4 times B(sub z)). For intermediate values (up to about 10 times times B(sub Y)) the ridge becomes observable further from the neutral line, while close to the X line the ridge is strongly modified. For large B(sub y) (of the order of B(sub x)) the ridge evolves into the new neutral line signatures which depend strongly on whether the observations point is above or below the current sheet. These results are used to estimate B(sub y) less than 2 nT in the Active Magentospheric Particle Traces Explorers/Ion Release Module (AMPTE/IRM) event modeled by Speiser and Martin (1994), which is consistent with onboard magnetometer measurements. We further show that the cause of the new structures appearing at large B(sub Y) is the same source seperation effect that produced the original ridge and that all these signatures are relatively insensitive to a small parallel electric field. Finally, we point our a dynamical ordering which occurs at large B(sub y): Using high-resolution simulations, we find a common boundary separating regions of positive and negative initial x, y, and z position, as well as initial pitch angle. This boundary is purely dynamical in nature and is independent of modeled source distributions.

Martin, R. F., Jr.↗
NASA NTRS1995

The heliospheric sector boundary as a distented magnetic cloud

A magnetic cloud was detected both near Earth and by Pioneer 11 located 43 deg east of Earth at 4.8 AU. The magnetic field within the cloud rotated smoothly from toward to away polarity, marking sector boundary passage. Interpreted as a flux rope, the cloud had a vertical axis, implying that its cylindrical cross-section in the ecliptic plane was distended along the sector boundary by at least 43, forming an extensive occlusion in the heliospheric current sheet. At 1 AU the cloud had plasma signatures typical of a fast coronal mass ejection with low temperature and a leading shock. In contrast, at 4.8 AU, only the cloud signature remained. Its radial dimension was the same at both locations, consistent with little expansion beyond 1 AU. Energetic particle data at 4.8 AU show high fluxes preceding the cloud but not extending forward to the corotating shock that marked entry into the interaction region containing the cloud. The streaming direction was antisunward, consistent with possible acceleration in a low-beta region of field line draping around the cloud's western (upstream) end. The fluxes dropped upon entry into the cloud and became essentially isotropic one third of the way through it. On the basis of sector boundary characteristics published in the past, we suggest that distended clouds may be common heliospheric current sheet occlusions.

Crooker, N. U.↗
DOE OSTI2020

Fast Magnetic Reconnection with Turbulence in High Lundquist Number Limit

Here, we use extensive 3D resistive MHD simulations to study how large-scale current sheets will undergo fast reconnection in the high Lundquist number S limit (above ~104), when the system is subject to different externally driven turbulence levels and the self-generated turbulence produced by 3D reconnection dynamics. We find that the normalized global reconnection rate ~0.01–0.13 is weakly dependent on S. Global reconnection with the classic inflow/outflow configurations is observed, and 3D flux ropes are hierarchically formed and ejected from reconnection regions. A statistical separation of the reconnected magnetic field lines follows a superdiffusive behavior, from which the rate is measured to be very similar to that obtained from the mixing of tracer populations. We find that the reconnection rate scales roughly linearly with the turbulence level during the peak of reconnection. This scaling is consistent with the turbulence properties produced by both the externally driven and self-generation processes. These results imply that large-scale thin current sheets tend to undergo rigorous reconnection.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

A Mechanism for Bulk Energization in the Impulsive Phase of Solar Flares: MHD Turbulent Cascade

We propose that the large production rate (approximately 10(exp 36)/s) of energetic electrons (greater than or approximately equal to 25 keV) required to account for the impulsive-phase hard X-ray burst in large flares is achieved through MHD turbulent cascade of the bulk kinetic energy of the outflows from many separate reconnection events. Focusing on large two- ribbon eruptive flares as representative of most large flares, we envision the reconnection events to be the driven reconnection of oppositely directed elementary flux tubes pressing into the flare-length current-sheet interface that forms in the wake of the eruption of the sheared core of the preflare bipolar field configuration. We point out that, because the outflows from these driven reconnection events have speeds of order the Alfven speed and because the magnetic field reduces the shear viscosity of the plasma, it is reasonable that the outflows are unstable and turbulent, so that the kinetic energy of an outflow is rapidly dissipated through turbulent cascade. If the largest eddies in the turbulence have diameters of order the expected widths of the outflows (10(exp 7)-10(exp 8)cm), then the cascade dissipation of each of these eddies could produce approximately 10(exp 26) erg burst of energized electrons (approximately 3 x (10(exp 33) 25 keV electrons) in approximately 0.3 s, which agrees well with hard X-ray and radio sub-bursts commonly observed during the impulsive phase. Of order 10(exp 2) simultaneous reconnection events with turbulent outflow would produce the observed rate of impulsive-phase plasma energization in the most powerful flares (approximately 10(exp 36) 25 keV electrons/ s); this number of reconnection sites can easily fit within the estimated 3 x 10(exp 9) cm span of the overall current-sheet dissipation region formed in these large flares. We therefore conclude that MHD turbulent cascade is a promising mechanism for the plasma energization observed in the impulsive phase of solar flares.

LaRosa, T. N.↗
NASA NTRS1988

Alfven waves from an electrodynamic tethered satellite system

The ionospheric plasma currents associated with the operation of an orbiting, constant current electrodynamic tethered satellite system are calculated using linear cold plasma theory and a tethered system current distribution model that takes into account the peculiar dumbbell shape of the system. The field line current calculations show plasma sheet currents at each end of the tethered system whose structure is roughly that of the Alfven wings. The disturbance caused by the moving tethered system extends behind its dimensions along the line of sight. The form of the wings alters as they travel away from the system. The front boundary becomes sharper while the ripples in J(z) in the wake extend to greater distances with larger amplitude. The wave impedance is calculated for a wide range of system dimension values. The expression is in sharp disagreement with a recently published result by Dobrowolny and Veltri (1986).

Estes, Robert D.↗
NASA NTRS1988

Does the resistive tearing instability nonlinearly evolve to a fast reconnection mode

A fundamental problem in applying linear tearing instability theory to the rapid processes (particle acceleration, heating) in flares was the characteristically slow rate of reconnection. This problem can be at least partially overcome if the tearin mode nonlinearly evolves to a regime in which the reconnection rate is substantially enhanced, such as that for the Petschek configuration. This possibility was often suggested, and some numerical simulations appear to provide support for such a view. Numerical simulation are used to study the nonlinear evolution of the tearing stability and show that a fast Petschek-like regime may not be achieved. This conclusion follows when there are sufficient grid points within the diffusion region to completely resolve the nonlinear dynamic interactions in the diffusion layer. When the numerical resolution is not adequate, the solution does appear to approach a Petschek configuration. The resolved solution contains reverse flow vortices and current sheets, terminated with a current reversal, similar to those obtained by Syrovatsky (JEPT, 33, 933, 1971).

Steinolfson, R. S.↗
NASA NTRS1972

Magnetic field signatures of substorms on high latitudes field lines in the nighttime magnetosphere

Two types of magnetic field changes are repeatedly observed in the high latitude nightside magnetosphere in association with magnetic substorms. One type is characterized by sudden decrease in the field strength, accompanied by an abrupt perturbation in the field declination angle. These changes are attributed to field aligned-sheet currents flowing on the high latitude boundary of an expanding plasma sheet following substorms. Single sheets of field-aligned currents on this boundary tend to flow toward the earth in the morning quadrant and away in the evening. Multiple sheets of current may also occur, with the direction of the high latitude sheet generally being the same as for a single sheet. A second type of field change is a decrease in field inclination during substorms. This is regarded as a manifestation of the changing field configuration during substorms and can be described in terms of azimuthal currents.

Fairfield, D. H.↗
NASA NTRS1984

Driven magnetic reconnection in three dimensions - Energy conversion and field-aligned current generation

The energy conversion processes occurring in three-dimensional driven reconnection is analyzed. In particular, the energy conversion processes during localized reconnection in a taillike magnetic configuration are studied. It is found that three-dimensional driven reconnection is a powerful energy converter which transforms magnetic energy into plasma bulk flow and thermal energy. Three-dimensional driven reconnection is an even more powerful energy converter than two-dimensional reconnection, because in the three-dimensional case, plasmas were drawn into the reconnection region from the sides as well as from the top and bottom. Field-aligned currents are generated by three-dimensional driven reconnection. The physical mechanism responsible for these currents which flow from the tail toward the ionosphere on the dawnside of the reconnection region and from the ionosphere toward the tail on the duskside is identified. The field-aligned currents form as the neutral sheet current is diverted through the slow shocks which form on the outer edge of the reconnected field lines (outer edge of the plasma sheet).

Sato, T.↗
NASA NTRS1976

Measurements of field-aligned currents in a multiple auroral arc system

A rocket-borne experiment to study the currents associated with a system of multiple auroral arcs was conducted at Poker Flat, Alaska, at 1122 UT on Feb. 2, 1972. The magnetic field in the vicinity of the auroral system was measured with a cesium vector magnetometer. Possible configurations were inferred by constructing model current systems that reproduced the magnetic field variations measured along the flight path. The data are interpreted in terms of a model current system consisting of two eastward electrojets and one westward electrojet and three pairs of oppositely directed Birkeland sheet currents, all lying in a plane approximately parallel to the auroral arcs. Sheet thicknesses ranged from 20 to 60 km and current densities from 10 to 45 microampere/sq m; the electrojet currents ranged from 1000 to 2000 A. A possible alternate model consisted of four pairs of sheets whose thicknesses range from 10 to 40 km with current densities from 10 to 90 microampere/sq m. There was quite good agreement between the locations of the visual arcs and the upward current sheets. The overall current configuration is discussed in view of the theoretical models constructed by Atkinson and Sato and Holzer and of other observations.

Sesiano, J.↗
NASA NTRS1994

Comprehensive Plasma Instrumentation (CPU) for the Geotail spacecraft

Geotail was launched on 24 July 1992 as the first new spacecraft of the International Solar Terrestrial Physics Program (ISTP). A primary objective of the Geotail mission is the acquisition of particles and fields measurements in the tail region of the magnetosphere. Some of the specific scientific objectives of the Geotail mission are; (1) assessment of the relative importance of solar-wind and ionospheric sources to the plasmas of the magnetotail, (2) a search for plasmoids and flux ropes, (3) direct observation of the effects of plasma acceleration and heating in the current sheet, and (4) measurements of currents and convective flows carried by magnetotail plasmas. The Comprehensive Plasma Instrumentation (CPI) aboard the Geotail spacecraft provides observations of the charged particles that comprise the plasma populations of the magnetosphere and the solar wind. In many cases the physical processes that contribute to the flow of energy and mass through the Solar-Terrestrial plasma system can be understood only through a detailed examination and analysis of the distribution of the particle velocities in the ambient plasmas. The CPI provides detailed measurements of these distributions. It employs three plasma analyzers: (1) the Hot Plasma analyzer (CPI-HP) for hot electrons and ions found in the plasma sheet and the inner magnetosphere; (2) a Solar Wind analyzer (CPI-SW) for cool plasmas with high bulk speeds such as those found in the solar wind and magnetosheath; and (3) an Ion Composition analyzer (CPI-IC) for identification of ion species such as H(+), He(+), He(++), and O(+). Along with measurements from other fields and particles instrumentation on Geotail, the CPI measurements provide a fundamental experimental base for investigations of solar-terrestrial plasmas and fields.

Frank, Louis A.↗
NASA NTRS1974

Identifications of the polar cap boundary and the auroral belt in the high altitude magnetosphere: A model for field aligned currents

Using the OGO-5 fluxgate magnetometer data, the polar cap boundary is identified in the high altitude magnetosphere by a sudden transition from a dipolar field to a more tail like configuration. The basic pattern of the magnetic field variations observed during the satellite's traversal of the auroral belt is presented. This pattern shows the existence of a field aligned current layer on the equator side of the polar cap boundary. Currents flow in the opposite directions in the two field aligned current layers. The current directions in these layers as observed by OGO-5 in the high-altitude magnetosphere are the same as those observed at low altitudes by the polar orbiting TRIAD satellite. The magnetic field in the region where the lower latitude field aligned current layer is situated is essentially meridional. Thus the equatorial current closure of this current system must be via the equatorial current sheet. The two field aligned current systems, one at the polar cap boundary and the other on the low latitude side of the auroral belt, are coupled through the Pedersen current in the ionosphere.

Sugiura, M.↗
NASA NTRS1978

Chatanika radar observations of ionospheric and field-aligned currents

A method for estimating field-aligned current densities from ground-based measurements of the horizontal ionospheric currents at two or more latitudes using the Chatanika incoherent scatter radar is presented. It is assumed that the one-dimensional divergence of the horizontal currents approximates the current density of a field-aligned current sheet. The field-aligned current densities found via radar data agree with those given by simultaneous magnetic observations by the Triad satellite. It is noted that downward or upward field-aligned currents having densities of 10 to the -6th to 10 to the -5th A/sq m are achieved in the evening or morning sector. The densities are further increased during periods of increased geomagnetic activity and when current directions are reversed. This reversal generally occurs during auroral oval expansion.

Kamide, Y.↗
NASA NTRS2007

Slow Mode Waves in the Heliospheric Plasma Sheet

We report the results of a search for waves/turbulence in the Heliospheric Plasma Sheet (HPS) surrounding the Heliospheric Current Sheet (HCS). The HPS is treated as a distinctive heliospheric structure distinguished by relatively high Beta, slow speed plasma. The data used in the investigation are from a previously published study of the thicknesses of the HPS and HCS that were obtained in January to May 2004 when Ulysses was near aphelion at 5 AU. The advantage of using these data is that the HPS is thicker at large radial distances and the spacecraft spends longer intervals inside the plasma sheet. From the study of the magnetic field and solar wind velocity components, we conclude that, if Alfven waves are present, they are weak and are dominated by variations in the field magnitude, B, and solar wind density, NP, that are anti-correlated.

heliospheric magnetic field↗
NASA NTRS1990

A multisatellite case study of the expansion of a substorm current wedge in the near-earth magnetotail

Results are presented on observations from four satellites (GOES 5, GOES 6, AMPTE CCE, and AMPTE IRM) and two ground stations (San Juan and Tucson) on a substorm that occurred on April 19, 1985 at about 0830 UT. The four spacecraft were arrayed in a configuration that made it possible to separate the effects arising from the longitudinal versus the radial expansion of the substorm current wedge and thus to examine its spatial evolution. The sequence of events that was observed suggests that, during this substorm, the disruption of the cross-tail current sheet, the formation of the substorm current wedge, and the expansion of the plasma sheet began in the near-earth region, and subsequently spread tailward as well as longitudinally.

Lopez, R. E.↗
NASA NTRS1997

Force Balance and Substorm Effects in the Magnetotail

A model of the quiet time middle magnetotail is developed using a consistent orbit tracing technique. The momentum equation is used to calculate geocentric solar magnetospheric components of the particle and electromagnetic forces throughout the current sheet. Ions generate the dominant x and z force components. Electron and ion forces almost cancel in the y direction because the two species drift earthward at comparable speeds. The force viewpoint is applied to a study of some substorm processes. Generation of the rapid flows seen during substorm injection and bursty bulk flow events implies substantial force imbalances. The formation of a substorm diversion loop is one cause of changes in the magnetic field and therefore in the electromagnetic force. It is found that larger forces are produced when the cross-tail current is diverted to the ionosphere than would be produced if the entire tail current system simply decreased. Plasma is accelerated while the forces are unbalanced resulting in field lines within a diversion loop becoming more dipolar. Field lines become more stretched and the plasma sheet becomes thinner outside a diversion loop. Mechanisms that require thin current sheets to produce current disruption then can create additional diversion loops in the newly thinned regions. This process may be important during multiple expansion substorms and in differentiating pseudoexpansions from full substorms. It is found that the tail field model used here can be generated by a variety of particle distribution functions. However, for a given energy distribution the mixture of particle mirror or reflection points is constrained by the consistency requirement. The study of uniqueness also leads to the development of a technique to select guiding center electrons that will produce charge neutrality all along a flux tube containing nonguiding center ions without the imposition of a parallel electric field.

Kaufmann, Richard L.↗