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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

The relationship of energy flow at the magnetopause to geomagnetic activity

Properties of the energetic electron (E greater than or approximately equal to 200 keV) magnetopause layer along the distant magnetotail have been studied with Caltech instrumentation aboard IMP-8 for approximately 60 spacecraft orbits. The cross-sectional area of the layer appears to increase by a factor of approximately 5 with increasing geomagnetic activity, and the average unidirectional electron flux within the layer increases by a factor of approximately 4. The energy carried by electrons greater than or approximately equal to 200 keV ranges from approximately 10 to the 14th ergs per second to approximately 10 to the 15th ergs per second. Extrapolation to include all electrons greater than 1 keV suggests total energy flow ranging from approximately 3 x 10 to the 15th ergs per second at quiet times to approximately 5 x 10 to the 18th ergs per second at magnetically disturbed times.

Baker, D. N.↗
NASA NTRS1977

Observations of large transient magnetospheric electric fields

Transient electric field events were studied by means of the long double-probe instrumentation carried by the Imp 6 satellite, and nine clearly defined exceptionally large amplitude events are described. The events were observed in the midnight sector at geocentric distances of 3.5 to 5.5 earth radii at middle latitudes within a magnetic L shell range of 4.8 to 7.5; the duration is from one to several minutes with peak power spectra amplitudes occurring at a frequency of about 0.3 Hz. The events occur under magnetically disturbed conditions and are often associated with negative dH/dt excursions. The magnetospheric motions calculated for these electric fields indicate a quasi-stochastical diffusive process rather than the general inward magnetospheric collapsing motion expected during the expansive phases of auroral substorm activity. It is likely that the transient electric fields are responsible for the impulsive acceleration and injection of plasma to populate the outer radiation belt.

Aggson, T. L.↗
NASA NTRS1977

Relationship between auroral substorms and the occurrence of terrestrial kilometric radiation

The paper examines the correlation between magnetospheric substorms as inferred from the AE(11) index and the occurrence of terrestrial kilometric radiation (TKR) as observed by the Goddard radio astronomy experiment on board the Imp 6 spacecraft. It is suggested that many TKR events begin at low altitudes and high frequencies (approximately 400 - 500 kHz) and spread to higher altitudes and lower frequencies as the substorm expands. AE and TKR are well correlated for observations in the 1500-300 MLT zone and poorly correlated for the complementary zone. High-resolution dynamic spectra obtained during periods of isolated substorms are described; the substorm expansion phase corresponds to a rapid intensification and bandwidth increase of TKR.

Kaiser, M. L.↗
NASA NTRS1977

On the alignment of plasma anisotropies and the magnetic field direction in the solar wind

One year's Imp 6 solar wind plasma and magnetic field data are examined to determine whether anisotropies in particle velocity distributions are aligned with the measured interplanetary magnetic field vector. Alignment of components in the analysis plane was generally found to be excellent whenever plasma parameter magnitudes were larger than determination uncertainties, although some spread exists (typical rms approximately equal to 10 deg). By assuming cylindrical symmetry about the simultaneously measured magnetic field vector during the 1-year interval under study, three-dimensional values of selected solar wind plasma thermal parameters were constructed from the two-dimensional plasma measurements, and the statistical properties of their distributions have been tabulated.

Asbridge, J. R.↗
NASA NTRS1977

High altitude observations of Birkeland currents

Several models of field-aligned currents (Birkeland currents) in the magnetosphere are discussed, and high altitude observations of these currents, carried out with the aid of highly eccentric earth-orbiting spacecraft of the OGO and IMP series, are reviewed. The essential roles of Birkeland currents are identified: they relieve charge imbalances, transmit stresses, and lead to particle acceleration anomalous resistivity.

Russell, C. T.↗
NASA NTRS1978

Stereoscopic direction finding analysis of a type III solar radio burst - Evidence for emission at 2f/p-/

Stereoscopic direction finding measurements from the Imp 8, Hawkeye 1, and Helios 2 spacecraft over base line distances of a substantial fraction of an astronomical unit are used to directly determine the three-dimensional trajectory of a type III solar radio burst. By comparing the observed source positions with the direct in situ solar wind plasma density measurements obtained by Helios 1 and 2 near the sun, the relationship of the emission frequency to the local plasma frequency can be determined directly without any modeling assumptions. These comparisons show that the type III radio emission occurs near the second harmonic of the local electron plasma frequency. Other characteristics of the type III radio emission, such as the source size, which can be obtained from this type of analysis, are also discussed.

Gurnett, D. A.↗
NASA NTRS1978

Wavy nature of the magnetotail neutral sheet

Crossings of the magnetotail neutral sheet are examined with 20-sec averaged magnetic field data from Imp 5 satellite. It is shown that for some multiple neutral sheet crossings, the y-component of the magnetic field reverses its sign between adjacent crossings. This feature can be explained in terms of a wavy profile of the neutral sheet along the dawn-dusk direction, but not in terms of flapping motion or wavy profile of the neutral sheet along the tailward direction. It is suggested that the magnetotail neutral sheet often shows departure from a plane geometry.

Lui, A. T. Y.↗
NASA NTRS1978

Prompt solar proton events and coronal mass ejections

Data from the HAO white-light coronagraph and the X-ray telescope on Skylab have been used to investigate the coronal manifestations of 18 prompt solar proton events observed on the IMP 7 spacecraft during the Skylab period. Evidence is found that a mass-ejection event is a necessary condition for the occurrence of a prompt proton event. Mass-ejection events can be observed directly in the white-light coronagraph when they occur near the limb and inferred from the presence of a long-decay X-ray event when they occur on the disk. It is suggested that: (1) the occurrence of mass-ejection events facilitates the escape of protons - whether accelerated at low or high altitudes - to the interplanetary medium; and (2) there may exist a proton acceleration region above or around the outward moving ejecta far above the flare site.

Kahler, S. W.↗
NASA NTRS1978

Preferential enhancements of the solar flare-accelerated nuclei carbon to zinc from about 20-300 MeV/nucleon

IMP 8 satellite measurements with a charged-particle telescope are examined which show that three large solar flares in late 1977 displayed preferential enhancements of abundances from C through Fe at energies of about 20 to 400 MeV/nucleon. It is found that the differential energy spectra of C, O, Mg, Si, and Fe, as well as protons and helium, are best fitted by power laws in energy per nucleon or rigidity with an index of 2.3 + or - 0.2, that the spectra of most of the even-Z nuclei have essentially the same slope, and that all the enhancements were independent of energy in the range from 20 to 300 MeV/nucleon. A plot of enhancement factor vs. Z is presented which clearly demonstrates the preferential enhancement of abundances with increasing Z in one of the solar flares, where the nuclei reached such high energies that they must have escaped the corona fully stripped of electrons. Implications for theories of solar-flare acceleration of the nuclei are considered.

Dietrich, W. F.↗
NASA NTRS1978

The heliocentric radial variation of plasma oscillations associated with type III radio bursts

A survey is presented of all of the electron plasma oscillation events found to date in association with low-frequency type III solar radio bursts using approximately 9 years of observations from the Imp 6 and 8, Helios 1 and 2, and Voyager 1 and 2 spacecraft. Plasma oscillation events associated with type III radio bursts show a pronounced increase in both the intensity and the frequency of occurrence with decreasing heliocentric radial distance. This radial dependence explains why intense electron plasma oscillations are seldon observed in association with type III radio bursts at the orbit of the earth. Possible interpretations of the observed radial variation in the plasma oscillation intensity are considered.

Gurnett, D. A.↗
NASA NTRS1978

Sources of magnetic fields in recurrent interplanetary streams

The paper examines sources of magnetic fields in recurrent streams observed by the Imp 8 and Heos spacecraft at 1 AU and by Mariner 10 en route to Mercury between October 31, 1973 and February 9, 1974, during Carrington rotations 1607-1610. Although most fields and plasmas at 1 AU were related to coronal holes and the magnetic field lines were open in those holes, some of the magnetic fields and plasmas at 1 AU were related to open field line regions on the sun which were not associated with known coronal holes, indicating that open field lines may be more basic than coronal holes as sources of the solar wind. Magnetic field intensities in five equatorial coronal holes, certain photospheric magnetic fields, and the coronal footprints of the sector boundaries on the source surface are characterized.

Burlaga, L. F.↗
NASA NTRS1978

Simultaneous multispacecraft observations of energetic proton bursts inside and outside the magnetosphere

Energetic proton bursts were observed simultaneously on Imp 6, 7, and 8 both inside and outside the earth's magnetosphere during the period October 1972 through August 1974. It is found that proton bursts are present nearly simultaneously in the vicinity of the outer belt trapping boundary, in the low-latitude magnetotail, in the magnetosheath, and upstream from the bow shock. Large intensity differences, time delays, and differences in energy spectra recorded by the three spacecraft are described. It is suggested that energetic protons and electrons are probably accelerated inside the plasma sheet and propagate to various regions both inside and outside the magnetosphere.

Sarris, E. T.↗
NASA NTRS1978

The magnetopause energetic electron layer. I - Observations along the distant magnetotail

Instrumentation aboard Imp 8 was used to study the magnetopause energetic electron layer along the distant magnetotail, and the observations are discussed. The inner edge of the layer corresponds to the magnetopause boundary, and energetic electrons within the layer persistently stream tailward along local magnetosheath field lines. Pitch angles of electrons along the dusk magnetopause are often restricted to values less than approximately 40 deg, while along the dawnside of the tail somewhat larger pitch angles are normally seen. The differential energy spectra of electrons in the layer, the intensity enhancements, and the average directional intensities are characterized.

Baker, D. N.↗
NASA NTRS1978

The power spectrum of the solar wind speed for periods greater than 10 days

The use of the more than 11 years of solar wind speed data obtained by Vela 2-6 and Imp 6-8 to study the power spectrum of speed variations in the range near the solar rotational frequency is discussed. The broad bands of power near periods of 27 days (corresponding to the rotational period of the sun), 13.5 days, and higher harmonics are characterized, and it is suggested that the described individual peaks in both the solar wind and the geomagnetic spectra are probably not due to differential rotation. The alternate explanation is that the multipeak nature of the power spectra are explained by a wave packet concept in which recurring highspeed streams are described as a series of pulses (separated by a constant period) that last for a varying number of solar rotations.

Fenimore, E. E.↗
NASA NTRS1977

Multiple-satellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral-zone activity

Particle observations from pairs of satellites (Ogo 5, Vela 4A and 5B, Imp 3) during the recovery of plasma sheet thickness late in substorms were examined. Six of the nine events occurred within about 5 min in locations near the estimated position of the neutral sheet, but over wide ranges of east-west and radial separations. The time of occurrence and spatial extent of the recovery were related to the onset (defined by ground Pi 2 pulsations) and approximate location (estimated from ground mid-latitude magnetic signatures) of substorm expansions. It was found that the plasma sheet recovery occurred 10 - 30 min after the last in a series of Pi bursts, which were interpreted to indicate that the recovery was not due directly to a late, high latitude substorm expansion. The recovery was also observed to occur after the substorm current wedge had moved into the evening sector and to extend far to the east of the center of the last preceding substorm expansion.

Pytte, T.↗
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

Non-thermal radio emission from Saturn

Direct, strong evidence for non-thermal radio emission from Saturn exists in the hectometric data observed by Imp 6. The planet has been tentatively identified as a decametric source, but the most sensitive and most recent data fail to confirm this. At metric or decimetric wavelengths Saturn has no non-thermal emission like Jupiter's synchrotron sources. Finally, a comparative study of Earth and Jupiter radio emissions suggests lightning discharges.

Warwick, J. W.↗
NASA NTRS1979

Observed distribution functions of H, He, C, O, and Fe in corotating energetic particle streams: Implications for interplanetary acceleration and propagation

Distribution functions for H, He, C, O, and Fe derived from our IMP 8 measurements of approximately 0.15 to approximately 8 MeV/nucleon particles in three corotating streams observed near earth are shown to have a simple exponential dependence on the particle speed. The e-folding speed, v sub o, is typically 0.01c, is found to be the same for the distribution functions of all elements examined, and varies little from one corotating event to the next. The relative abundances of energetic particles in these events resemble most closely the solar coronal composition and, thus, presumably that of the solar wind. These results may imply that the acceleration of these particles, which occurs in corotating interaction regions at several AU from the sun, is by a statistical process.

Gloeckler, G.↗