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At least 1,549 records · Page 86
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NASA NTRS1977

Two types of magnetospheric ELF chorus and their substorm dependences

The distribution of extremely low frequency (10-1500 Hz) magnetospheric chorus to all local times and latitudes is investigated in order to determine dependence on substorms, and to evaluate the conditions under which chorus is generated. The analysis carefully separates space and time effects by an investigation of data obtained by the OGO 5 search coil magnetometer. A study of spatial dependencies shows that chorus occurs in two magnetic regions: equatorial chorus is located near the equator, and high-latitude chorus is located above 15 degrees. An analysis of chorus in each of the regions illustrates that equatorial chorus is definitely related to substorm, whereas high-latitude chorus often occurs within magnetically quiet intervals.

Tsurutani, B. T.↗
NASA NTRS1977

Magnetosphere boundary observations along the Imp 7 orbit. I - Boundary locations and wave level variations

The paper is concerned with magnetosphere boundary phenomena observed by the Imp 7 magnetic field, plasma, and plasma wave instruments in 1972 and 1973. Boundary locations for a 15-month period are surveyed, and the different types of crossings are described. The spacecraft crosses the dawn and dusk boundaries near 25 earth radii downstream, and the physical processes at the Imp 7 magnetopause appear to be intermediate between those observed over the poles and those observed at the lunar orbit. The Imp 7 orbit also traverses a downstream region near where 'fireball' phenomena occur. Electromagnetic wave modes detected in the broad low-frequency channel of the wave instrument are analyzed, and the interpretation of data of this type suggests that the broad low-frequency channel is sensitive to oscillations in the lower hybrid resonance region of the spectrum.

Scarf, F. L.↗
NASA NTRS1977

Characteristics of magnetospheric particle injection deduced from events observed on August 18, 1974

The behavior of particles injected during three well-defined substorm events that occurred on August 18, 1974, has been studied in detail. Plasma characteristics from approximately 200 eV to 80 keV detected at the ATS 6 position have been studied in detail, and we suggest that the results are consistent with the idea that there are two particle sources for electrons during substorms: one at small pitch angles, predominantly in the magnetic field direction, and the other at large pitch angles. The sources of these electrons are probably the ionosphere and the plasma sheet. For protons the results suggest a one-particle source. One possible source for the protons is the plasma sheet. The results have been interpreted in a framework of a model involving magnetospheric convection and parallel electric field.

Parks, G. K.↗
NASA NTRS1978

The statistical magnetic signature of magnetospheric substorms

Daily magnetograms from a midlatitude network of geomagnetic observatories were used in the analysis of approximately 1800 substorm events, and the characteristic magnetic signatures of magnetospheric substorms both on the ground and in space were determined. Auroral electrojet (AE) indices and individual magnetograms at different local times in the auroral zone and at midlatitudes were analyzed with reference to onsets, and superposed epoch averages of individual magnetograms and AE indices confirm the local time magnetic substorm signatures. Superposed epoch averages of the interplanetary magnetic field (IMF) associated with the onsets demonstrates both a distinct southward component prior to the onsets and a dependence of the substorm amplitude on the integrated preceding southward IMF flux. Superposed epoch averages of the tail lobe magnetic field magnitude and vector components are discussed.

Caan, M. N.↗
NASA NTRS1978

A self-consistent model of a corotating Jovian magnetosphere

In the rotation-dominated model of the Jovian magnetosphere, proposed in the present paper, the plasma current distribution is dynamically consistent with the magnetic field. The model is constructed under the assumptions of an ionospheric plasma source without pitch angle scattering; loss-free radiation transport through flux tube interchange diffusion; and a static balance between centrifugal force and magnetic stress. Using these assumptions, a dynamic equation is derived for a self-consistent field configuration having a single adjustable parameter that is related to the plasma source strength. The equation is solved by an iterative technique. The self-consistent field resembles a radially distorted dipole field, rather than the flat magnetodisk configuration inferred from Pioneer data. The implication is that such a magnetodisk field would require an equatorial plasma source rather than a source at the feet of the field lines.

Carbary, J. F.↗
NASA NTRS1978

Whistler mode noise in Jupiter's inner magnetosphere

A study is made of the amplitude and spectral extent of whistler mode noise in the inner magnetosphere of Jupiter. It is found that the 'hat-shaped' pitch angle distributions of energetic electrons (21 and 31 MeV at L=3) are consistent with those predicted in the presence of a band-limited spectrum of whistler mode noise. The equatorial maximum linear growth rate of parallel propagating whistlers are consistent with those necessary to limit the energetic electron intensities by the whistler mode instability. It is noted that the wave phase speeds before wave reflection can occur at high latitudes and that wave growth is limited to a disk-like region centered around the magnetic equator. The frequency extent of the whistler mode noise spectrum may be estimated by the range of frequencies maximally unstable to equatorial linear growth. A value is found for the spectral density of the broadband whistler mode noise necessary to balance radial diffusion of energetic electrons above the critical range, and an expression is derived for the energetic electron system response to fluctuations about the limiting flux value.

Sentman, D. D.↗
NASA NTRS1978

Ionospheric and magnetospheric 'plasmapauses'

The locations of Explorer 45 plasmapause crossings are studied as a likely indicator of ionospheric and magnetospheric trough locations. Attention is given to vertical flows of H(+) ions in the light ion trough, as detected by the magnetic ion mass spectrometer aboard Isis 2 (which was operating in conjunction with Explorer 45 during August 1972). The possibility of an equatorial plasmapause is discussed, whose field lines map into the ionosphere at latitudes poleward of the H(+) density decrease, probably due to the refilling of magnetic flux tubes in the outer plasmasphere.

Grebowsky, J. M.↗
NASA NTRS1978

Magnetic flux transfer associated with expansions and contractions of the dayside magnetosphere

The paper shows that the use of an empirical ellipsoidal model of the sunward magnetopause - which uses Ogo 5 magnetometer data, solar wind data from three satellites, and auroral zone magnetic indices - yields quantitative statistical estimates of the relative values of magnetopause displacements due to solar wind variation, flux transfers, and other causes. While the displacements due to flux transfers are smaller on the average, they are clearly comparable with those due solar wind fluctuations. The fact that expansion and contraction events are observed throughout the region accessible to Ogo 5 satellite on the sunward magnetopause suggests that the entire sunward magnetosphere is affected by flux erosion and return.

Holzer, R. E.↗
NASA NTRS1978

An experiment to study energetic particle fluxes in and beyond the earth's outer magnetosphere

This experiment is designed to take advantage of the ISEE Mother/Daughter dual spacecraft system to study energetic particle phenomena in the earth's outer magnetosphere and beyond. Large geometric factor fixed voltage electrostatic analyzers and passively cooled semiconductor detector telescopes provide high time resolution coverage of the energy range from 1.5 to 300 keV for both ions and electrons. Essentially identical instrumentation is placed on the two spacecraft to separate temporal from spatial effects in the observed particle phenomena.

Anderson, K. A.↗
NASA NTRS1978

A set of satellite experiments for studying the magnetosphere-ionosphere communications

Magnetospheric substorms are one of the basic problems in the physics of near earth space. They appear as magnetic and ionospheric perturbations, disturbances in radio communication, polar auroras and electromagnetic radiation in different frequency ranges. These phenomena were studied from single satellites. However, it is impossible to study these phenomena adequately without simultaneous measurements from at least two different positions with identical instruments. A suitable program can be developed based on the Interkosmos work using new achievements in electronics.

Gringauz, K. I.↗
NASA NTRS1978

Magnetospheric electrostatic emissions and cold plasma densities

A synoptic study of electric wave, magnetometer, and plasma data from IMP-6 was carried out for times when banded electrostatic waves are observed between harmonics of the electron gyrofrequency in the earth's outer magnetosphere. Four separate classes of such waves were previously identified. The spatial and temporal occurrences of waves in each class are summarized here, as are correlations of occurrence with geomagnetic activity. Most importantly, associations between the observations of waves of different classes and the relative portions of cold and hot electrons present at the position of the spacecraft are established. Finally, evidence for the signature of the loss cone is sought in the plasma data.

Hubbard, R. F.↗
NASA NTRS1978

The convection electric field model for the magnetosphere based on Explorer 45 observations

Large-scale electric fields in the magnetosphere, represented by the scalar potentials of corotation and convection fields, are examined in order to interpret both plasmapause positions at different local times and energetic particle penetrations inside the plasmapause observed by the Explorer 45 (S3-A) dc E field measurements and particle detectors. The Volland-Stern type convection electric field is assumed. A uniform dawn-dusk convection electric field which corresponds to an exponent (gamma) of unity is not consistent with the observational data, but the Volland-Stern model with gamma = 2 more likely represents the convection electric field during the hours of electric field increases during magnetic activity. In the time-independent case, the shape of the plasmapause does not depend on the intensity of the convection electric field, so that by using two plasmapause distances at different local times along the S3 outbound and inbound orbits the shape factor gamma of the convection field could be determined. The average value of gamma for 42 cases was found to be 2.4.

Ejiri, M.↗
NASA NTRS1978

Electrostatic emissions between electron gyroharmonics in the outer magnetosphere

Electrostatic emissions are often observed by spacecraft in the outer magnetosphere at frequencies between low electron gyroharmonics. The emissions include the well-known odd half-harmonic emissions as well as weaker emissions which occur near the electron plasma frequency. A scheme is constructed for classifying the emissions, and it is shown that a theoretical model which has been previously used to explain '3/2' emissions can be extended to the other classes of emissions. All of the emissions therefore appear to be generated by the same basic mechanism: an unstable electron plasma distribution consisting of cold electrons (below 100 eV) and hot loss cone electrons (about 1 keV). Each emission class is associated with a particular range of model parameters; the wide-band electric field data can thus be used to infer the density and temperature of the cold plasma component. The model predicts that gyroharmonic emissions near the plasma frequency require large cold plasma densities.

Hubbard, R. F.↗
NASA NTRS1978

Electric fields and conductivity in the nighttime E-region - A new magnetosphere-ionosphere-atmosphere coupling effect

Calculations have been made of the effects of intense poleward-directed electric fields upon the nighttime ionospheric E-region. The results show the Pedersen and Hall conductivities are substantially changed, thereby decreasing the ionospheric electrical load seen by magnetospheric sources. It appears that relatively large electric fields can exist in the absence of accompanying large field-aligned currents, as long as the underlying ionosphere remains in darkness and/or energetic particle precipitation is absent.

Banks, P. M.↗
NASA NTRS1979

On the phase relationship between the energetic particle flux modulation and current disc penetrations in the Jovian magnetosphere - Pioneer 10 inbound

The surface defined by the locus of field minima along field lines for an unterminated Jovian model magnetosphere is found to coincide with the surface of the model current disk which provides a good fit to the data. However, when the model is terminated by means of magnetopause currents, preservation of the fit by modifying the current distribution near the outer edge of the disk causes the minimum absolute magnetic field surface to bifurcate. Energetic electron flux time profiles that are based upon such a minimum absolute magnetic field configuration are found to be qualitatively consistent with the inbound Pioneer 10 data.

Jones, D. E.↗
NASA NTRS1978

The use of ground magnetograms to time the onset of magnetospheric substorms

A critical review of magnetospheric substorm timing techniques is presented. Attention is given to substorm timing with auroral zone magnetograms, substorm onset timing with midlatitude magnetograms, multiple onset substorms, and a phenomenological model for multiple onset substorms. Emphasis is placed on some pitfalls that have led some investigators to err in substorm onset determinations. In particular, multiple onset substorms appear to account for many of the reported discrepancies in substorm timing. It is suggested that present idealized models of substorm morphology must be modified to include the multiple onset substorms.

Mcpherron, R. L.↗
NASA NTRS1978

Three-dimensional plasma measurements within the earth's magnetosphere

Magnetospheric measurements of three-dimensional velocity distributions for positive ions and electrons in the energy range from 1 eV to 45 keV are reported which were obtained with quadrispherical Lepedeas on ISEE 1 and 2. The instrumentation is briefly described, and an extension of energy-time spectrograms to three-dimensional summaries via the use of ancillary energy-spin phase spectrograms for multiple analyzers is outlined. Macroscopic plasma parameters for two crossings of the dayside magnetopause are presented, along with a preliminary evaluation of the kinematical motions of low-energy charged particles within the terrestrial ring current. Nonmonotonic electron velocity distributions presumably related to the generation of electron cyclotron harmonic instabilities are examined. The possible signature of diffusion driven by electrostatic waves is considered, and velocity distributions of positive ions in the dawn magnetotail are investigated.

Frank, L. A.↗
NASA NTRS1978

Abundances of He and CNO ions in the ion pulses observed in the earth's magnetospheric environment

The abundance of He and CNO ions in the ion pulses which are frequently observed in the earth's magnetosheath, magnetotail, and near-by interplanetary space was analyzed and found to be highly variable. Based on the assumption that these ions are solar wind particles accelerated in the magnetospheric environment to the observed energies, it is suggested that this variability reflects the inhomogeneity of the solar atmosphere and/or the physical conditions under which the solar wind particles were energized.

Fan, C. Y.↗