Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “current sheet”

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 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Reconnection of Magnetic Fields

Spacecraft observations of steady and nonsteady reconnection at the magnetopause are reviewed. Computer simulations of three-dimensional reconnection in the geomagnetic tail are discussed. Theoretical aspects of the energization of particles in current sheets and of the microprocesses in the diffusion region are presented. Terrella experiments in which magnetospheric reconnection is simulated at both the magnetopause and in the tail are described. The possible role of reconnection in the evolution of solar magnetic fields and solar flares is discussed. A two-dimensional magnetohydrodynamic computer simulation of turbulent reconnection is examined. Results concerning reconnection in Tokamak devices are also presented.

Source record↗
NASA NTRS1983

An unusual interplanetary event - Encounter with a comet?

The possibility that the slow rise to a sharp maximum and then decay surrounding a strong current sheet observed in the Venus magnetic field 0.72 AU from the sun was caused by passage of Venus through the wake of an active comet is examined. Data were also gathered by the ISEE 3 satellite magnetometer at 0.99 AU 25 h, 20 min later, a delay corresponding to the transit time for the solar wind. No shock structures bounded the phenomenon. The data indicate the presence of a small body in a much larger field of interaction and the magnetometer, solar wind probe, and electron temperature probe support a behavior similar to a planetary magnetosheath. The observed He structure in interplanetary space ruled out a solar wind source, while consideration of the magnetic equator and magnetic pole suggest that an unknown comet passed through the region between the sun and Venus at a distance where the effects would not be detected at earth.

Russell, C. T.↗
NASA NTRS1983

The fine structure of the Saturnian ring system

A dust disk within a planetary magnetosphere constitutes a novel type of dust-ring current. Such an azimuthal current carrying dust disk is subject to the dusty plasma analog of the well known finite-resistivity 'tearing' mode instability in regular plasma current sheets, at long wavelengths. It is proposed that the presently observed fine ringlet of the Saturnian ring system is a relic of this process operating at cosmogonic times and breaking up the initial proto-ring (which may be regarded as an admixture of fine dust and plasma) into an ensemble of thin ringlets. It is shown that this instability develops at a rate that is many orders of magnitude faster than any other known instability, when the disk thickness reaches a value that is comparable to its present observed value.

Houpis, H. L. F.↗
NASA NTRS1983

Delta function excitation of waves in the earth's ionosphere

Excitation of the earth's ionosphere by delta function current sheets is considered, and the temporal and spatial evolution of wave packets is analyzed for a two-component collisional F2 layer. Approximations of an inverse Fourier-Laplace transform via saddle point methods provide plots of typical wave packets. These illustrate cold plasma wave theory and may be used as a diagnostic tool since it is possible to relate specific features, e.g., the frequency of a modulation envelope, to plasma parameters such as the electron cyclotron frequency. It is also possible to deduce the propagation path length and orientation of a remote radio beacon.

Vidmar, R. J.↗
NASA NTRS1983

The reason for magnetospheric substorms and solar flares

It has been proposed that magnetospheric substorms and solar flares are a result of the same mechanism. It is suggested that this mechanism is connected with the escape, or attempted escape, of energized plasma from a region of closed magnetic field lines bounded by a magnetic bottle. In the case of the earth, it must be plasma that is able to maintain a discrete auroral arc, and it is proposed that the cross-tail current connected to the arc is filamentary in nature to provide the field-aligned current sheet above the arc.

Heikkila, W. J.↗
NASA NTRS1984

Magnetopause structure and the question of particle accessibility

A simple plane model of the magnetopause is used to address the question of particle accessibility. Particle motion in the current sheet region is analyzed, showing that the concept of gyromotion is useful even when the electric and magnetic fields vary significantly over the gyroradius, as long as the variation is perpendicular to the drift direction. The first adiabatic invariant for particle motion is defined in such a way that it is preserved in regions of large field gradients, provided that the gradients are in a direction primarily perpendicular to the particle drifts. This generalized invariant provides an adiabatically conserved quantity that can be used to characterize particles as they move from a source region through the drift region and into the magnetopause itself, helping to resolve the accessibility question. Some preliminary results are given for the magnetopause structure based on the location of particle guiding points.

Whipple, E. C.↗
NASA NTRS1984

Research relative to plasma transport across the magnetopause

The concept of how magnetosheath plasma can cross the magnetopause is one of the key elements for a global view of the magnetosphere. According to this view, viscous interaction is the main process to produce convection of plasma in the magnetotail, resulting in auroral phenomena, and other related processes. The mechanism is illustrated herein. The key element is that the electric field is not constant, as assumed in reconnection theories, but reverse within the magnetopause current sheet. Particles, representing magnetosheath ions were traced as they impinge upon the magnetopause.

Heikkila, W. J.↗
NASA NTRS1985

Study of non-thermal photon production under different scenarios in solar flares. 2: The Compton inverse and Bremsstrahlung models and fittings

Energy spectra of photons emitted from Bremsstrahlung (BR) of energetic electrons with matter, is obtained from the deconvolution of the electron energy spectra. It can be inferred that the scenario for the production of X-rays and gamma rays in solar flares may vary from event to event. However, it is possible in many cases to associated low energy events to impulsive acceleration, and the high energy phase of some events to stochastic acceleration. In both cases, flare particles seem to be strongly modulated by local energy losses. Electric field acceleration, associated to neutral current sheets is a suitable candidate for impulsive acceleration. Finally, that the predominant radiation process of this radiation is the inverse Compton effect due to the local flare photon field.

Perez-Peraza, J.↗
NASA NTRS1985

Determination of galactic cosmic ray latitudinal gradient using Earth based detectors

Using cosmic ray intensity data from the Deep River Neutron monitor and the relation between solar wind velocity and heliomagnetic latitude, an attempt is made to evaluate quantitatively the latitudinal gradient of cosmic ray intensity during the periods dominated by a two sector pattern. Assuming a constant orientation of the heliospheric current sheet on a time scale of the order of a year, a relationship is determined between cosmic ray intensity and heliomagnetic latitude.

BADRUDDIN↗
NASA NTRS1985

Cosmic-ray picture of the heliosphere

The existing data base on the characteristics of the heliosphere is discussed. It is known that solar gravity is less than necessary to hold all the solar material, and therefore a supersonic solar wind exists. Skylab soft X-ray photographs revealed the existence of coronal holes, which evolve in an 11 yr cycle. It has been proposed that all but the highest energy cosmic rays detected on earth can be attributed to solar and heliospheric origins, a controversial view which requires further empirical and theoretical work on particle acceleration processes and regions of interaction of the solar wind with interplanetary plasma. It is possible that a warped solar current sheet stretches to interplanetary space and organizes the solar magnetic field and thereby guides cosmic rays. An inverse correlation has been identified between the sunspot cycle and cosmic ray intensity. The features and effects of solar flares, subsequent shock waves and high speed particle streams are also discussed.

Venkatesan, D.↗
NASA NTRS1986

Wave speeds in the corona and the dynamics of mass ejections

A disturbance or coronal mass ejection being advected by the solar wind will expand at the fastest local characteristic speed - typically approximately the fast-mode speed. To estimate this characteristic wave speed and the velocity field in the ambient corona, it is necessary to know the magnetic field, temperature, and density. Only the density is known from coronal observations. The temperature, magnetic field, and velocity are not yet directly measured in the outer corona and must be estimated from a model. In this study, it is estimated that the magnetic field, solar wind velocity, and characteristic speeds use the MHD model of coronal expansion between 1 and 5 solar radii (R solar radii) with a dipole magnetic field at the base. This model, for a field strength of about 2 gauss at the base, gives flow speeds at low latitudes (near the heliospheric current sheet) of 250 km/s at 5 R solar radii and, 50 km/s at 2 solar radii, and fast-mode speeds to 400 to 500 km/s everywhere between 2 and 5 solar radii. This suggests that the outer edge of a velocity of mass ejection reported by MacQueen and Fisher (1983) and implies that the acceleration mechanism for coronal mass ejections is other than simple entrainment in the solar wind.

Suess, S. T.↗
NASA NTRS1986

Equilibrium of magnetic fields with arbitrary interweaving of the lines of force. I - Discontinuities in the torsion

Consideration is given to the static force-free equilibrium of a magnetic field in which all of the lines of force connect without knotting between parallel planes. The field is formed by continuous deformation from an initial uniform field, and is conventiently described in terms of the scalar function psi, which is the stream function for the incompressible wrapping and interweaving of the lines of force. Local compression and expansion of the lines of force is described in terms of the scalar function Phi. Equilibrium in the field requires satisfaction of two independent equations which cannot be accomplished without the full freedom of both psi and Phi. It is shown that discontinuities in the torsional characteristics of the lines occur when psi is predetermined by an arbitrary pattern. Discontinuities in the winding pattern of the lines can lead to discontinuities in the associated current sheets.

Parker, E. N.↗
NASA NTRS1986

Solar radio emission

For this review, a selection has been made of a number of topics which are current active areas of both observational and theoretical research. Observations of Type III bursts are examined, taking into account ground-based observations (above approximately 8 MHz), spacecraft observations (below 1 MHz), and Langmuir waves and electron streams. Microwave bursts are considered along with Type II bursts, moving Type IV bursts, and Type I noise storms. The theory of Type III radio bursts (radio emission from electron streams) is discussed, giving attention to quasi-linear theory, induced scatter of ions, wave-wave effects of the nonlinear refractive and self-focusing variety, the second harmonic emission from Langmuir waves, fundamental emission from Langmuir waves, and density irregularities and ion-acoustic waves. Aspects of radio emission from shock waves and current sheets are also studied.

Goldman, M. V.↗
NASA NTRS1986

The theta aurora

A comprehensive review is presented of the characteristics of theta aurora as revealed from four imaging efforts with the DE 1 and 2 satellites. The theta aurora consists of an auroral oval with a sun-aligned arc extending from the dayside to the nightside sectors of the oval. The DE 1 spacecraft provided high altitude simultaneous measurements of the electric and magnetic fields and plasma and the DE 2 collected equivalent low altitude data on the four events. The plasma was found to convect sunward when the transpolar arc appeared, while the convection was antisunward in other regions over the polar cap. The arc plasmas featured field-aligned electron acceleration into the polar atmosphere and field-aligned current sheets, both of which were sparse over the rest of the polar cap. The ions originated in the ionosphere and the solar wind; ions over the rest of the polar cap mainly arrived from the magnetosphere. Further discussions are provided of the dominant electrons and ions and the associated flow directions into and out of the various regions of the pole, similarities between the transpolar arc and the auroral oval, and interactions between the ionosphere and the auroral phenomena.

Frank, L. A.↗
NASA NTRS1986

Quasistatic evolution of magnetostatic coronal structures

Four separate but related studies of coronal magnetostatic equilibria under a variety of boundary conditions and distributions of coronal current are reviewed. Physically, all four studies assume an axisymmetric corona whose radial magnetic field at the coronal base is dipolar. Electric currents in the model coronas are assumed to flow in the azimuthal direction, giving rise to Lorentz forces that must be balanced by pressure gradients and gravity. Mathematically, such coronas are described by the equation of magnetostatic force balance and Ampere's law. Although highly idealized, the axisymmetric magnetostatic models described provide insights into the behavior of a variety of coronal structures. The models show how more realistic current sheet boundary conditions and coronal density variations may be modeled.

Wolfson, Richard↗
NASA NTRS1986

The Comet Giacobini-Zinner magnetotail: Axial stresses and inferred near-nucleus properties

Utilizing the electron and magnetic field data from the ICE tail traversal of comet Giacobini-Zinner along with the MHD equations, a steady state, stress balance model of the cometary magnetotail was developed, and used to infer important but unmeasured ion properties within the magnetotail at ICE and upstream at the average point along each streamline where cometary ions are picked-up. The derived tailward ion flow speed at ICE is quite constant at approx. -20 to -30 km/sec across the entire tail. The flow velocity, ion temperature, density, and ion source rates upstream from the lobes (current sheet) at the average pick-up locations are approx. -75 km/sec (approx. -12), approx. 4 million K (approx. 100,000), approx. 20 cc (approx. 400), and approx. 15 cu cm/sec. Gradients in the plasma properties between the two regions are quite strong. Implications of inferred plasma properties for the near-nucleus region and for cometary magnetotail formation are examined.

Mccomas, D. J.↗
NASA NTRS1986

The Jovian magnetotail

The current understanding of the plasma, magnetic field, and plasma wave characteristics of the near and distant Jovian magnetotail is discussed. The properties of these two regions are compared, with particular emphasis given to the data on the distant tail obtained by Voyager 2. The Jovian tail has many unusual properties, such as the large scale sausage-string shape of its outer boundary, but some of its properties resemble those of the earth's magnetotail, such as its central current sheet and a surrounding region resembling a plasma sheet consisting of hot ions. Data are presented that support the view that plasma in the Jovian tail plasma sheet comes from Jupiter.

Lepping, R. P.↗
NASA NTRS1987

The topology of force-free magnetic fields and its implications for coronal activity

The topological constraints on coronal magnetic fields are considered. For a field that is initially well-behaved and undergoes deformation by well-behaved ideal MHD motions, it is shown that the topology of the field lines in the corona can be determined at all times solely from the footpoint positions on the photospheric boundary. This result implies that the topology and, consequently, the history of the footpoint motions impose no further constraints on the field beyond those already included in the connectivity boundary conditions, so that there is no reason to expect a lack of equilibrium for fields that are initially well-behaved and evolve by ideal MHD. On the other hand, nonideal processes such as reconnection are bound to occur in the solar corona, and these may lead to magnetic topologies that have no well-bahaved Euler potentials. Hence Parker's hypothesis that footpoint motions lead to the formation of current sheets is still likely to be correct, but only if nonideal processes are included. The effects of reconnection on magnetic topology and the implications for coronal activity are discussed.

Antiochos, Spiro K.↗