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NASA NTRS1979

Observations of plasma depletion in the magnetosheath at the dayside magnetopause

In a set of 17 low- to mid-latitude crossings of the dayside and near-dayside magnetopause, Imp 6 plasma measurements show 11 cases of decreases in magnetosheath density just outside the boundary which are consistent with plasma depletion owing to magnetic flux tube compression as the field becomes draped against the magnetopause. Pressure anisotropies in the sense pressure perpendicular to the field direction greater than pressure parallel to it are a predicted result of the plasma depletion and field compression, and such anisotropies are observed. Application of the mirror instability criterion, which predicts growth of slow mode magnetoacoustic waves for values of the ratio between the cited pressures greater than a critical value, suggests that dayside magnetosheath plasma is usually unstable. One of the seventeen cases shows long-period waveforms in the 100-s density data that are 180 deg out of phase with simultaneous waveforms in the magnetic field strength. These data are interpreted as the signature of slow mode magnetoacoustic waves.

Crooker, N. U.↗

NASA NTRS1979

Earth and Jupiter: Comparison of magnetopause structure

Using published data from the Jovian magnetopause crossings of Pioneers 10 and 11, the apparent crossing times as determined from the solar wind plasma detector and from the vector magnetometer are comapred. There were 13 published crossings for which both plasma and magnetic-field signatures were clearly defined. The occurrence distribution of these 13 crossings with respect to the relative timing of plasma and field signatures is shown. Results indicate that Jupiter's magnetopause is more turbulent than the Earth's possibly as the result of the large velocity shear between the magnetosheath and the partially corotating Jovian plasma. However, it is noted that the data set considered is too small to justify a definitive interpretation.

Reiff, P. H.↗

NASA NTRS1979

Measurements of plasma flow at the dawn magnetopause by Voyager 1

Measurements are presented showing strong tailward flow of ions along the dawn magnetopause as the Voyager 1 spacecraft crossed the earth's magnetosphere boundary following launch on September 5, 1977. With one exception all of the observed flows occur outside the magnetopause. The particle flux measurements at energies of at least about 30 keV, together with the observed magnetic-field signatures of the boundary crossing, are consistent with a minimum tailward ion energy flow of about (2-7) x 10 to the 17th erg/s at the time of observation. High-time-resolution particle data indicate that the ion flow can vary on a time scale of about 400 ms. These results, together with recent results from several other spacecraft, show conclusively that a source of energetic particles exists sunward of the dawn-dusk meridian

Lanzerotti, L. J.↗

NASA NTRS1979

Plasma wave turbulence at the magnetopause - Observations from ISEE 1 and 2

An investigation of plasma wave electric and magnetic fields in the vicinity of the magnetopause using measurements from the ISEE 1 and 2 spacecraft is presented. Strong electric and magnetic field turbulence is often observed at the magnetopause; the electric field spectrum of this turbulence extends from less than a few hertz to over 100 kHz, and the magnetic field from a few hertz to about 1 kHz. Similar turbulence spectra are observed in association with flux transfer events and possible 'inclusions' of boundary layer plasma in the magnetosphere. Two possible plasma instabilities, the electrostatic ion-cyclotron and the lower-hybrid-drift instability, should explain the broad-band electric field turbulence; the narrow-band electrostatic emissions near the local electron plasma frequency are believed to be plasma oscillations or electrostatic waves near the upper-hybrid-resonance frequency.

Gurnett, D. A.↗

NASA NTRS1981

Evidence for magnetic field reconnection at the Earth's magnetopause

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause were identified, where the plasma velocity in the magnetopause and boundary layer was substantially larger than in the magnetosheath. The nature of the plasma flow, magnetic field, and energetic particle fluxes in these regions were examined, with a view to determining whether the velocity enhancements can be explained by magnetic field reconnection.

Sonnerup, B. U. O.↗

NASA NTRS1981

Jupiter's magnetopause, bow shock, and 10-hour modulated magnetosheath Voyagers 1 and 2

This summary report discusses Jupiter's magnetopause, bow shock and magnetosheath, based on fine scale magnetic field data from the Voyager 1 and 2 encounters. Explicit models of the dawnside magnetopause and bow shock in Jupiter's orbital plane employ an axisymmetric parabola and hyperbola, respectively, and satisfy average boundary crossing positions, inbound and outbound; these models are determined separately for the two encounters. A new phenomenon has been discovered in Jupiter's magnetosheath. It is manifested as (5 or) 10 hour quasi-periodic modulation of the direction of the magnetic field in the outbound magnetosheath, predominantly in the northward (N) and southward (S) directions. It was seen to occur during both encounters and appears most evident in Voyager 2 outbound observations.

Lepping, R. P.↗

NASA NTRS1981

The magnetopause of the earth and planets

The intrinsic magnetic fields of Mercury, the earth, Jupiter and Saturn, all deflect the solar wind well above the planetary surface. The current layer or magnetopause which flows between the magnetized solar wind and magnetospheric plasmas should play an important role in determining the strength of the interaction but has only been investigated at the earth where the interplanetary magnetic field direction is found to exert a primary control on the structure of the magnetopause and the global dynamics of the magnetosphere. The solar wind interaction with Venus is quite different than that with the earth because Venus has at most a very weak magnetic field. Nevertheless, the current layer on the Venus ionopause has many similarities to that of the earth, in particular the presence of small scale structure.

Russell, C. T.↗

NASA NTRS1981

Surface waves on Saturn's magnetopause

Voyager 1 magnetometer data have shown that small-amplitude surface waves occurred on Saturn's dayside magnetopause, causing multiple inbound crossings of this boundary. These waves were travelling approximately parallel to Saturn's equatorial plane along the magnetopause ('tailward'), suggesting that they were driven by the rotation of Saturn's magnetosphere. Hydromagnetic waves (possibly slow mode) were observed in the adjacent magnetosheath.

Lepping, R. P.↗

NASA NTRS1982

The magnetopause as sensed by energetic particles, magnetic field, and plasma measurements on November 20, 1977

The position of the trapping boundary for ions not less than 24 keV in the vicinity of the magnetopause was determined using energetic particle three-dimensional distributions observed by ISEE 1 and 2. For a short period of time on November 20, 1977, definite periods were observed when the subsolar magnetopause current and magnetosheath plasma penetration layers were not overlapping the trapping region for energetic particles whose outer edge is the trapping boundary. The trapping boundary was found to be in rapid continual motion, representing a sharp, well-defined boundary, and the penetration of the magnetosheath plasma inside this boundary can be limited to a distance comparable to the plasma ion gyroradius of not greater than about 100 km, although plasma penetration was seen up to 200 km earthward of the trapping boundary.

Fritz, T. A.↗

NASA NTRS1982

Energetic ion composition in the subsolar magnetopause and boundary layer

Energetic ion mass spectrometer data obtained on ISEE 1 are analyzed to show that the plasma in the subsolar magnetospheric boundary layer, magnetopause, and adjacent magnetosheath has an ionospheric component, He(+) and O(+), and a solar wind component, H(+) and He(++). Nine intervals when the ISEE 1 and 2 fast plasma and magnetometer data clearly define the magnetopause are examined, and observations indicate that He(+) is more predominant than O(+), in the subsolar boundary layer and the magnetosheath. In five of the boundary layer intervals, keV He(+) ions are observed, and energetic O(+) ions are seen above background in two of the boundary layers where He(+) is observed

Peterson, W. K.↗

NASA NTRS1982

Magnetopause modeling - Flux transfer events and magnetosheath quasi-trapped distributions

Three-dimensional distribution functions for energetic ions are studied numerically in the magnetosphere, through the magnetopause, and in the magnetosheath using a simple one-dimensional quasi-static model and ISEE 1 magnetopause crossing data for November 10, 1977. Quasi-trapped populations in the magnetosheath observed near flux transfer events (FTEs) are investigated, and it is shown that the population in the sheath appears to sandwich the FTE distributions. These quasi-trapped distributions are due to slow, large pitch angle, outward moving particles left behind by the outward rush of the ions more field-aligned at the time the flux was opened. It is found that sheath convective flows can map along the connected flux tube without drastically changing the distribution function, and results suggest that localized tangential fields above the upper limit may exist.

Speiser, T. W.↗

NASA NTRS1982

Unusual structure of the dayside low-latitude magnetopause energetic electron layer

An analysis of high time resolution energetic electron data from an ISEE 1 spacecraft crossing of the magnetopause on Nov. 6, 1977 is presented. Low-energy proton measurements were taken with an ultralow energy and charge analyzer and energetic electron measurements were made with an ultralow energy wide-angle telescope (ULEWAT). Electrons were observed in four rate channels between 75 and 1300 keV and particle angular distributions are provided. The ecliptic was scanned by the particle angular detector as the spacecraft rotated at 19.7 rpm. Trapped energetic electrons were found to drift from dusk to dawn by way of the midnight sector. The electrons which started at midnight met the magnetopause at the dawnside, were entrained on the field lines, and resulted in a filamentary small-scale structure. A diffusion time scale across the magnetosheath was determined to be a few seconds, and electrons diffused tailward or into the interplanetary medium

Scholer, M.↗

NASA NTRS1982

Magnetic field compression at the dayside magnetopause

The degree of compression sustained by the solar wind field as it convects to the magnetopause has been determined empirically with magnetometer data from ISEE 3 in the solar wind and ISEE 1 in the magnetosheath. At the stagnation point, the strength B(SH) of the magnetosheath field may be expressed as B(SH) = square root of 4 X B(SW)' X B(ST), which implies that B(SH) is equal to the geometric mean of the stagnation field strength B(ST) and the shocked solar wind field, approximated by 4 X B(SW)'. For the usual spiral wind pattern of the solar wind field, B(SH) on the duskside of the magnetopause is about 20 percent stronger than it is on the downside. No dependence of B(SH) on the sign of the north-south component of the solar wind field is apparent.

Crooker, N. U.↗

NASA NTRS1983

Rotational discontinuities and the structure of the magnetopause

Symmetric and asymmetric rotational discontinuities are studied by means of a one-dimensional computer simulation and by single-particle trajectory calculations. The numerical simulations show the symmetric rotation to be stable for both ion and electron senses of rotation with a thickness of the order of a few ion gyroradii when the rotation angle of the tangential field is 180 deg or less. Larger rotation angles tend to be unstable. In an expansive discontinuity, when the magnetic field on the downstream side of the discontinuity is larger, an expanding transition layer separating the highfield from a low-field region develops on the downstream side, and a symmetric rotational discontinuity forms at the upstream edge. The implication of these results for magnetopause structure and energy flow through the magnetopause is described.

Swift, D. W.↗

NASA NTRS1983

Transfer of pulsation-related wave activity across the magnetopause - Observations of corresponding spectra by ISEE-1 and ISEE-2

Comparison of power spectra of magnetic field data from ISEE-1 and -2 recorded simultaneously on both sides of the magnetopause showed that power level inside the magnetosphere varied with power level outside in the magnetosheath and suggested that the same frequencies were enhanced on the two sides of the boundary. Power levels were two to three orders of magnitude lower inside than outside the magnetosphere, indicating that wave energy was transmitted inside from the sheath, through a locally stable magnetopause.

Greenstadt, E. W.↗

NASA NTRS1983

Low-energy particles at the bow shock, magnetopause, and outer magnetosphere of Saturn

The characteristics of the dayside bow shock, magnetopause, and outer magnetosphere are emphasized in the present consideration of low energy electrons and protons measured by the Low Energy Charged Particle Experiment during Voyager spacecraft encounters with Saturn. During several of the bow shock crossings, low energy protons were observed streaming from the magnetosphere's dawnside, and in the magnetosheath, the protons were observed to be primarily oriented with pitch angles of about 90 deg. Examination of proton flux distributions suggests that the magnetopause was moving inward with a lower limit speed of 10 km/sec during the Voyager 2 approach to the planet. Intervals of diamagnetic depression observed in the magnetic field energy density during the Voyager 2 encounter nearly coincide with enhancements in the low energy-proton energy density.

Maclennan, C. G.↗

NASA NTRS1984

Nonlinear evolution of magnetopause tearing modes

Since the magnetosheath plasma is highly turbulent, reconnection at the dayside magnetopause is likely to be temporally unsteady. The tearing mode can be viewed as a model for the unsteady development of a reconnecting magnetic topology. Magnetopause tearing occurs in the guide-field limit and has a wave packet spatial structure in the east-west direction. This paper solves for the nonlinear evolution of a single wavelength guide-field tearing mode including the effects of finite transit time on the Landau resonant electrons. Short wavelength modes evolve algebraically in time with perturbation amplitudes proportional to t-squared. Long wavelength modes are fully nonlinear, and the amplitude grows linearly in time.

Coroniti, F. V.↗

NASA NTRS1984

Flux transfer events on the magnetopause - Spatial distribution and controlling factors

The spatial distribution of flux transfer events (FTE) of magnetic flux tubes pulled from the earth's magnetopause is analyzed using ISEE 1 and 2 data from 1977-82. Attention is given to interplanetary conditions influencing different observed FTE polarities. FTEs were observed on nearly 25 percent of the passes near the dayside magnetopause. Direct FTEs were located mainly in the northern dawn sector and reverse FTEs appeared in the southern dusk sector. The distribution indicated an origin in the equatorial sector, and data correlate the appearance of FTEs only when the interplanetary magnetic field (IMF) had southward or nearly horizontal orientation. The presence of a southward component in the IMF was coincident with the appearance of an FTE 45 percent of the events. E-W components in the IMF exhibited no connections with the occurrence of an FTE.

Berchem, J.↗