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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Intensity variations in plasma flow at the dawn magnetopause

Observations of plasma flows in the region of the dawn magnetopause obtained by the outbound Voyager 1 spacecraft, at a velocity of 11 km/sec, are discussed. Magnetic field and ion data obtained for the period surrounding four magnetopause crossings are presented which reveal energetic anti-sunward flowing ions outside the boundary with a time variability on the order of 400 millisec. These particle intensity variations, observed to vary with frequency, are most likely associated with the particle energization process or with the leakage of magnetospheric protons. The ion flows are considered to have originated sunward of the dawn meridian and were observed to penetrate approximately an ion gyroradius inside the dawn magnetopause.

Lanzerotti, L. J.↗
NASA NTRS1981

Wave-particle interactions at the magnetopause - Contributions to the dayside aurora

The observations on ISEE 1 and ISEE 2 correlate the presence of intense electromagnetic and electrostatic emissions with enhanced fluxes of 1-6 keV electrons at the earth's magnetopause. The measured proton to electron ratio in the 1-10 keV energy range indicates the presence of substantial fluxes of electrons at energies below 1 keV. The 1.3-1.7 keV proton flux was essentially unchanged as the spacecraft moved from the magnetosheath into the wave-particle layer at and inside the magnetopause. The consequences of the magnetopause wave-particle interactions reported are consistent with the known features of the dayside aurora.

Tsurutani, B. T.↗
NASA NTRS1981

The influence of the interplanetary magnetic field and thermal pressure on the position and shape of the magnetopause

An ellipsoidal model, in which the size of an ellipsoid of revolution with a constant eccentricity is inversely proportional to the sixth root of the stream pressure of the solar wind, is used to represent the location of the dayside magnetopause and to study the influences of the interplanetary magnetic field and thermal pressure on its location. The effects of the IMF and thermal pressure on the location of the magnetopause are calculated analytically by using the Chapman-Ferraro theory. The changes in magnetopause size, shape and orientation caused by including the thermal pressure are inversely proportional to the square of the sonic Mach number of the solar wind and are sufficient to explain the observed slight departure of the magnetotail from the expected aberration due to the earth's orbital motion. The results suggest that little angular momentum is being carried away from the sun by the solar wind.

Zhuang, H. C.↗
NASA NTRS1982

Structure of the magnetopause rotational discontinuity

A model is developed for the rotational discontinuities associated with the reconnection configuration on the dayside magnetopause and the energy transfer process in the tail magnetopause. The model, in which the ion dynamics is described by the fluid equations and electrons are assumed to move adiabatically along the magnetic field lines, is valid for a rotational discontinuity with a thickness greater than a few ion gyroradii. It is shown, by the inclusion of self-consistency for trapped electrons, that (1) the trapped electron density profile is uniquely related to the rate of angular rotation of the magnetic field, and (2) the sense of magnetic field rotation is determined by the ratio of the normal and tangential components of the magnetic field. It is found that the electron polarization of magnetopause rotational discontinuities should be in agreement with satellite observations.

Lee, L. C.↗
NASA NTRS1982

The tailward magnetopause field beyond 10 RE

The geomagnetic field in the near-earth magnetotail is made up of three major contributions related to the dipole field, the magnetotail current system, and the magnetopause current system exclusive of the return currents in the magnetotail current system. The present investigation is concerned with the third contribution. The calculation of the magnetospheric field is discussed, taking into account the calculation of the field inside the magnetopause on the basis of the numerical integration of the Biot Savart integral. The representation of the field for distances greater than 10 earth radii on the dark side of the magnetosphere is considered. The field internal to the magnetopause is expressed as the negative gradient of a scalar potential which is expressed as a sum of solutions to Laplace's equation in cylindrical coordinates. The origin of coordinates is the earth.

Beard, D. B.↗
NASA NTRS1982

Observations near the magnetopause at the onset of the July 29, 1977, sudden storm commencement

Characteristics of an earthward movement of the magnetopause caused by the arrival of an interplanetary shockwave are derived from data acquired by the GEOS 1 spacecraft on July 29, 1977. Data were taken on electric field, plasma, magnetic field, energetic particles, and composition before, during, and after leaving the magnetopause. Correlative examinations were made with information gathered by the IMP 7, IMP 8, and ATS 6 satellites. The magnetopause was found to move earthward at 95 km/sec and had a thickness of 500 km. The existence of three different regions is postulated, one with field lines connected directly to the earth, the magnetosphere, a second, the magnetosheath, with no magnetic field line connection to the earth, and a thir; with one end of the field lines connected to the earth and the other to the interplanetary medium, called the transition region.

Knott, K.↗
NASA NTRS1982

Diffusion processes in the magnetopause boundary layer

A quantitative estimate is calculated for the effect of wave-particle scattering on the structure of the magnetopause boundary layer. It is assumed that large cross-B electric fields are absent in the observed penetration of magnetosheath plasma into the magnetopause boundary layer, thus allowing for cross-field transport comparable to 10% of the Bohm diffusion. It is shown that magnetosheath ions, resonant with low frequency electrostatic waves, can account for the typical boundary layer thickness when transported at 10% of the diffusion rate 1000 sq km/sec. The conditions are required to occur at all local times and under all interplanetary conditions. Significant mass and momentum transfer are then possible across the magnetopause when field merging is not occurring.

Tsurutani, B. T.↗
NASA NTRS1984

Patterns of magnetic field merging sites on the magnetopause

Models of the magnetospheric and magnetosheath magnetic fields are used to determine the relative orientations of the two near the dayside magnetopause for the purpose of locating potential merging sites. Areas of the magnetopause with various degrees of antiparallelness for different Interplanetary fields as contour diagrams are studied. For southward and GSE-Y interplanetary field, the patterns obtained are consistent with those envisioned by Crooker in an earlier analysis which used simplified representations for the magnetic field geometry. Here the application of realistic models shows the locations of areas where any antiparallel component occurs. Merging sites for radial interplanetary fields are also illustrated. The results suggest that the geometrical configuration of the fields is suitable for merging over a large fraction of the magnetopause for interplanetary fields that are either primarily southward, GSE-Y, or radial (GSE-X) in direction.

Luhmann, J. G.↗
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

Patterns of potential magnetic field merging sites on the dayside magnetopause

Models of the magnetospheric and magnetosheath magnetic fields are used to determine the relative orientations of these fields at the dayside magnetopause in order to locate potential merging sites. Areas on the magnetopause with different fractional antiparallel components are displayed by contour diagrams for a variety of interplanetary field orientations. For interplanetary fields oriented perpendicular to the solar wind velocity the areas of nearly antiparallel field agree with those obtained by Crooker using simplified representations for the magnetic field geometry. Here, the application of more realistic models gives the locations of areas where any antiparallel component occurs. Potential merging sites for interplanetary fields with radial components are also illustrated. The results suggest that the topology of the magnetosheath and magnetospheric fields provides antiparallel components over a substantial fraction of the magnetopause for most interplanetary field orientations.

Luhmann, J. G.↗
NASA NTRS1984

Mapping the magnetosheath field between the magnetopause and the bow shock - Implications for magnetospheric particle leakage

An approximate picture of the volumes occupied by particles that originate in the vicinity of the magnetopause is obtained by mapping magnetosheath magnetic field lines which drape over the magnetopause through the bow shock. Subsets of these field lines that connect to potential sites of magnetic merging on the magnetopause are also traced in the event that the particle leakage occurs preferentially where normal components of the field are present across that boundary. The results of this modeling exercise suggest that energetic magnetospheric particles which are not scattered by magnetosheath magnetic fluctuations are likely to exit the magnetosheath in the region of the quasi-parallel shock.

Luhmann, J. G.↗
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 NTRS1984

Magnetic field observations at the dayside magnetopause: ISEE-1 and -2 contributions during the IMS

Dayside magnetopause macroscopic characteristics such as velocities and thicknesses of the boundary and internal magnetic field structure of the current layer were studied by ISEE-1 and 2. The ISEE results suggest that the magnetopause is much thicker than previously thought. A current sheet thicker than several proton gyroradii rules out classical kinetic theories of a magnetopause current layer of the order of the ion gyroradius. Such a thick boundary may explain the failure of theories which predict a unique polarization for the field rotation inside the current sheet.

Berchem, J.↗
NASA NTRS1985

A note on the nature of the distant geomagnetic tail magnetopause and boundary layer

In the present comparison of recent plasma and magnetic field measurements of the distant geomagnetic tail magnetopause and boundary layer with numerical simulation results for an 'open' boundary, most aspects revealed are consistent with simulation results for conditions in which the normal magnetic field component at the magnetopause is generally small. On the basis of these results, the simulations of Swift and Lee (1982, 1983) are judged to be able to furnish a theoretical basis for understanding many aspects of the distant geomagnetic tail magnetopause and mantle boundary layer.

Gosling, J. T.↗
NASA NTRS1986

Accelerated plasma flows at the near-tail magnetopause

ISEE-1 and -2 fast plasma data of ion and electron distributions at 16 energies and 16 velocities are used to study accelerated particle flow events at the near-tail dusk magnetopause. The flows separated the plasma sheet from the magnetosheath, a situation which normally arises when the local magnetosheath and plasma sheet magnetic fields are close to being antiparallel. The flows were directed tailward at speeds up to twice those in the adjoining magnetosheath and had densities similar to those in the magnetosheath. The attendant ion and electron temperatures were between those in the magnetosheath and in the plasma sheet. The flow region closest to the magnetosheath had the highest velocity and the lowest density, and a lowered-density field line region appeared on the earthward side of the accelerated flow region. The flows had the velocity changes of a tangential stress balance and are taken as the location of field line merging near (and perhaps tailward) of the dawn-dusk terminator. Significantly fewer accelerated flows are seen in the dawn tail magnetopause, an asymmetry attributed to a seasonally dependent warping of the midtail neutral sheet or a superimposition of interplanetary magnetic field lines over the dusk magnetopause.

Gosling, J. T.↗
NASA NTRS1986

Transport of plasma across the magnetopause

It is here demonstrated numerically, by tracing test particle motions, that solar wind plasma particles can cross the magnetopause and get into the boundary layer if the tangential electric field in the magnetopause rest frame changes in concert with changes in the magnetic field as E = V x B for continuous earthward flow, whether or not the field lines are closed. Such a simultaneous change in the electric and magnetic fields is proposed as a key feature of flux transfer events (FTEs). It is argued that the FTE process could more aptly be described as a transient or impulsive transfer of plasma particles on a single flux tube over a localized region of the magnetopause, generally ending up on different flux tubes after the crossing. Thus, it is proposed that FTE should denote flux 'trade' event, rather than 'transfer'.

Heikkila, Walter J.↗
NASA NTRS1987

The bowshock and magnetopause

Results achieved in magnetopause and the bow-shock research during the years 1983-1986 are discussed. The review on bow-shock phenomenona includes work done on the terrestrial bow shock, interplanetary shocks, and bow shocks of other planets, describing research on macroscopic aspects of shocks as well as on the microphysics of quasi-perpendicular and quasi-parallel shocks. In the area of magnetopause research, global aspects of the boundary are first examined. Next, the issues of reconnection and energy transfer through the magnetopause, including viscous and surface-wave phenomena, are considered. Finally, flux-transfer events are discussed in terms of their characteristic magnetic signatures, energetic-particle anisotropies, and distribution with IMF orientations.

Elphic, R. C.↗
NASA NTRS1987

Cusp displacement at the magnetopause for large IMF Y component

The magnetic field orientation just earthward of the dayside magnetopause from 95 ISEE and 11 HEOS crossings with /By/ greater than /Bz/ in the adjacent magnetosheath indicate a statistically significant-1-earth-radius shift of the cusps toward dawn (dusk) in the Northern Hemisphere and dusk (dawn) in the Southern Hemisphere for positive (negative) By. This small shift near the magnetopause and the contrasting large shifts of some reported ionospheric cusp signatures are explained in terms of a finite thickness magnetopause model through which the cusps shift gradually from the inner to the outer edge.

Crooker, N. U.↗