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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The heliospheric current sheet - 3-dimensional structure and solar cycle changes

The reversal in polarity of the interplanetary current sheet/sector structure is investigated during the recent solar maximum. Multipoint observations by ISEE-3 and Pioneer 11 show that a simple two sector or occasional four sector structure persisted throughout the maximum and out to distances of 10 AU. The polarity reversal occurred between March 1979 and October 1980, without any indication of an abrupt transition that might permit a more precise timing. The reversal coincided approximately with the reversal in the sun's polar cap fields. The current sheet appeared to be highly inclined during the ascending and descending phases of the solar cycle but was apparently too complex to describe as a simple inclined current sheet during solar maximum.

Smith, E. J.↗
NASA NTRS1986

The recovery phase of galactic cosmic ray modulation in the outer heliosphere

Energetic hydrogen and helium nuclei from the galaxy (cosmic rays) enter the solar system with varying degrees of difficulty during the 11-year solar magnetic cycle, resulting in spatial and temporal variations of particle intensities in the heliosophere. Observations of these variations from the ISEE-3, Voyager 1 and 2, and Pioneer 10 spacecraft during the period from early 1981 to early 1986 and for heliocentric distances ranging from 1 to 37 AU are reported. During this time the high energy intensities in the outer heliosphere have been increasing exponentially at a rate of approximately 20 percent/year with occasional interruptions due to enhanced solar activity. The radial intensity gradients measured in the ecliptic plane are observed to remain constant or to decrease at the time of the reversal of the solar magnetic poles in 1980.

Mcdonald, F. B.↗
NASA NTRS1986

Radio evidence for a delayed acceleration process in solar flares

Observations of six delayed solar radio bursts at dm and mm frequencies are analyzed. The events included five Type II bursts. The data comprise 17 GHz interferometer data and ISEE-3 and SMM hard X-ray spectrometry data which peaked 0.5-1.0 hr after the main radio bursts. The data indicate the electrons with energies in the MeV range continue to be excited for tens of minutes after the impulsive phase acceleration. The continuing acceleration occurs in a large magnetic structure extending to at least 200,000 km altitude. The radio signals arise from a columnar source, the microwave signals being emitted near a leg or legs and meterwave emissions originating from the top of the magnetic structure.

Kai, K.↗
NASA NTRS1986

The resolved layer of a collisionless, high beta, supercritical, quasi-perpendicular shock wave. I - Rankine-Hugoniot geometry, currents, and stationarity

Data collected by the ISEE dual-spacecraft mission (on November 7, 1977) on a slowly moving, supercritical, high-beta, quasi-perpendicular bow shock are presented, and the local geometry, spatial scales, and stationarity of this shock wave are assessed in a self-consistent Rankine-Hugoniot-constrained frame of reference. Included are spatial profiles of the ac and dc magnetic and electric fields, electron and proton fluid velocities, current densities, electron and proton number densities, temperatures, pressures, and partial densities of the reflected protons. The observed layer profile is shown to be nearly phase standing and one-dimensional in a Rankine-Hugoniot frame, empirically determined by the magnetofluid parameters outside the layer proper.

Scudder, J. D.↗
NASA NTRS1986

The resolved layer of a collisionless, high beta, supercritical, quasi-perpendicular shock wave. II - Dissipative fluid electrodynamics

Using the results of Scudder et al. (1986) on the bow shock wave observed by ISEE satellites, a quantitative description is presented of the electrodynamics of ion and electron fluids, and phase-standing wave interaction which manifests itself as a supercritical MHD shock. The cross-shock electrical profile was determined in both the normal incidence frame and in the deHoffman-Teller frame by two different methods, and the results were compared with dc electric field measurements.

Scudder, J. D.↗
NASA NTRS1986

The resolved layer of a collisionless, high beta, supercritical, quasi-perpendicular shock wave. III - Vlasov electrodynamics

The analysis of the fluid continuum level within the shock layer observed on November 7, 1977 by the ISEE satellites (Scudder et al., 1986) is extended to examine, within the framework of Vlasov electrodynamics, direct observational evidence for 'collisionless' resistivity. The analysis is based on the measured wave turbulence and the deviation of the observed electron velocity distribution function from the form predicted using the reversible individual electron trajectories in the presence of dc forces.

Scudder, J. D.↗
NASA NTRS1986

Simultaneous polar cap and magnetotail observations of intense polar rain

The significance of acceleration processes in accounting for the energies and temperatures of the electrons seen during periods of intense polar rain is investigated along with the role of the IMF and solar wind in controlling the polar rain. Evidence for an acceleration region is sought by comparing DMSP observations of the precipitating electrons over the polar cap with simultaneous ISEE 1 electron measurements in the conjugate tail lobe at distances between 10 and 22.6 earth radii. The second question is explored by examining the solar wind and IMF conditions during which the intense polar rain events occur. It is found that intense polar rain can pass through the tail lobes without undergoing acceleration between 22.6 earth radii and 800 km altitude.

Greenspan, M. E.↗
NASA NTRS1986

Bidirectional solar wind electron heat flux and hemispherically symmetric polar rain

THe paper examines ISEE 3 solar wind electron data obtained concurrent with reported symmetric polar rain events and it is found that a bidirectional solar wind electron heat flux is present whenever such polar cap events occur. In contrast to the normal situation when only one of the earth's polar caps is magnetically connected to the sun, during hemispherically symmetric polar rain events either both of the earth's polar caps are magnetically connected to the sun, or else both are connected to a magnetic loop which is entirely disconnected from the sun. The relative timing between bidirectional solar wind heat flux and symmetrical polar rain events can be utilized to determine certain magnetospheric quantities such as the cross-tail convection speed.

Gosling, J. T.↗
NASA NTRS1986

A test of Lee's quasi-linear theory of ion acceleration by interplanetary traveling shocks

Lee's (1983) quasi-linear theory of ion acceleration is tested using ISEE-3 measurements of the November 12, 1978 quasi-parallel interplanetary shock. His theory accounts with varying degrees of precision for the energetic proton spatial profiles; the dependence of the spectral index of the power law proton velocity distribution upon the shock compression ratio; the power law dependence of the upstream proton scalelength upon energy; the absolute magnitude of the upstream proton scale length; the behavior of the energetic proton anisotropy upstream and downstream of the shock; the behavior of the alpha-particle proton ratio upstream; the equality of the spatial scale lengths at the shock of the upstream waves and of the protons that resonate with them; and the dependence of the integrated wave energy density upon the proton energy density at the shock. However, the trace magnetic field frequency spectra disagree with his theory in two ways. The part of the spectrum that can resonate with the observed protons via first-order cyclotron resonance is flat, whereas Lee's theory predicts an f exp - 7/4 frequency dependence for the November 12 shock. Higher frequency waves, which could not resonate with the observed upstream protons, increased in amplitude as the shock approached, suggesting that they too were generated by the shock.

Kennel, C. F.↗
NASA NTRS1986

Subcritical and supercritical interplanetary shocks - Magnetic field and energetic particle observations

A study of 34 forward interplanetary shocks observed by ISEE 3 during 1978 and 1979 has been conducted. Magnetic field and high-energy particle data have been used, and for each shock the first critical Mach number has been determined. The first surprising result is that the majority of the observed shocks appear to be supercritical, and consistent with their supercritical character, many shocks have a foot and/or an overshoot in the magnetic field structure. Large-amplitude low-frequency waves (period of about 20 s in the spacecraft frame) are commonly observed upstream of all supercritical shocks (except for a few quasi-perpendicular shocks) and also upstream of the few subcritical shocks. Intense particle events are frequently observed at many shocks: spikes at quasi-perpendicular shocks and energetic storm particle events associated with quasi-parallel shocks can be comparably intense. The correlation of the high-energy particle peak flux with various shock parameters is in agreement with the acceleration mechanisms proposed by previous studies.

Bavassano-Cattaneo, M. B.↗
NASA NTRS1985

Subcritical collisionless shock waves

The development history of theoretical accounts of low Mach number collisionless shock waves is related to recent observational advancements, with attention to weaker shocks in which shock steepening is limited by dispersion and/or anomalous resistivity and whose character is primarily determined by the dispersive properties of the ambient plasma. Attention has focused on nearly perpendicular shocks where dispersive scale lengths become small and the associated cross-field currents become strong enough to generate significant plasma wave turbulence. A number of oblique, low Mach number bow shocks have been studied on the basis of data from the ISEE dual spacecraft pair, allowing an accurate determination of shock scale lengths.

Mellott, M. M.↗
NASA NTRS1985

Numerical simulations of quasi-perpendicular collisionless shocks

Numerical simulations of collisionless quasi-perpendicular shock waves are reviewed. The strengths and limitations of these simulations are discussed and their experimental (laboratory and spacecraft) context is given. Recent simulation results are emphasized that, with ISEE bow shock observations, are responsible for recent progress in understanding quasi-steady shock structure.

Goodrich, C. C.↗
NASA NTRS1985

The electron foreshock

ISEE particle and wave data are noted to furnish substantial support for the basic features of the velocity dispersed model at the foreshock boundary that was proposed by Filbert and Kellogg (1979). Among many remaining discrepancies between this model and observation, it is noted that unstable reduced velocity distributions have been discovered behind the thin boundary proposed by the model, and that these are at suprathermal energies lying far below those explainable in terms of an oscillating, two-stream instability. Although the long-theorized unstable beam of electrons has been found in the foreshock, there is still no ready explanation of the means by which it could have gotten there.

Klimas, Alexander J.↗
NASA NTRS1986

He-3 in galactic cosmic rays

Cosmic-ray He-3/He-4 observations, including a new measurement around 65 MeV per nucleon from ISEE-3, are compared with interstellar propagation and solar modulation calculations in an effort to understand the origin of cosmic-ray He nuclei. A survey of spacecraft and balloon observations of the He-3/He-4 ratio shows improved consistency among measurements in the 50-300 MeV per nucleon energy range when a previously neglected contribution from atmospheric secondary He-3 is taken into account. These low-energy observations imply a mean escape length of 6-8 g/sq cm in the standard 'leaky box' model for cosmic-ray propagation in the Galaxy, a value consistent with that derived from studies of heavier nuclei. No evidence is found for an excess of low-energy He-3 such as that reported at high energies.

Mewaldt, R. A.↗
NASA NTRS1987

Properties of a large-scale interplanetary loop structure as deduced from low-energy proton anisotropy and magnetic field measurements

Correlated particle and magnetic field measurements by the ISEE 3 spacecraft are presented for the loop structure behind the interplanetary traveling shock event of Nov. 12, 1978. Following the passage of the turbulent shock region, strong bidirectional streaming of low-energy protons is observed for approximately 6 hours, corresponding to a loop thickness of about 0.07 AU. This region is also characterized by a low relative variance of the magnetic field, a depressed proton intensity, and a reduction in the magnetic power spectral density. Using quasi-linear theory applied to a slab model, a value of 3 AU is derived for the mean free path during the passage of the closed loop. It is inferred from this observation that the proton regime associated with the loop structure is experiencing scatter-free transport and that either the length of the loop is approximately 3 AU between the sun and the earth or else the protons are being reflected at both ends of a smaller loop.

Tranquille, C.↗
NASA NTRS1987

Structure of a slow mode shock observed in the plasma sheet boundary layer

A detailed study of the plasma parameters and magnetic field measured using instrumentation aboard ISEE 2 during the recovery phase of a substorm on Apr. 24, 1979, is presented. The purpose of this study was to determine the nature and structure of the lobe-plasma sheet boundary layer. The analysis shows that the lobeward edge of the boundary is consistent with a freestanding slow mode shock. This shock is followed by a slow compression wave containing a trailing ion cyclotron wave which also stands in the flow. Measurement of the cross-tail current in the shock and the current system within the wave gives a shock thickness and wavelength comparable to but larger than both the upstream ion inertial length and the downstream proton gyroradius. Equality of the shock thickness with the standing ion cyclotron wavelength suggests that the steepness of the front is limited by dispersion rather than current-driven anomalous resistivity.

Feldman, W. C.↗
NASA NTRS1987

Jovian electrons at 1 AU - 1978-1984

Data on Jovian electrons sensed with a spectrometer on board the ISEE spacecraft at 1 AU are studied in terms of intensity variations over time and shape and variations of the electron spectra with time. Design and performance features of the electron spectrometer are described. The analyses cover solar cycle modulation of intensity, the synodic intensity modulation, micromodulation over a period of days, and adiabatic deceleration of electrons propagating in the expanding solar wind. Emphasis is placed on the effects of maximum solar activity on Jovian electron emission and propagation, noting a confirmed 13 mos intensity variation in the Jovian electrons.

Moses, Dan↗
NASA NTRS1987

Ion distributions in a two-dimensional reconnection field geometry

ISEE observations have shown trapped ion distributions in the magnetosphere along with streaming ion distributions in the magnetosheath. The more energetic ion beams are further away from the magnetopause than lower-energy ion beams. Predictions made with a simple two-dimensional reconnection model which contains a neutral line and an azimuthal electric field were compared with the experimental data of Sept., 1978. The model explains trapped particles in the magnetosphere due to nonadiabatic mirroring in the magnetosheath and streaming ions in the magnetosheath due to energization at the magnetopause. The model also shows the higher-energy ions extending further into the magnetosheath and farther away from the magnetopause than the lower-energy ions. This suggests the ion data of Sept., 1978 are consistent with a reconnection geometry.

Curran, D. B.↗