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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Heliocentric distance and temporal dependence of the interplanetary density-magnetic field magnitude correlation

The Helios, IMP 8, ISEE 3, ad Voyager 2 spacecraft are used to examine the solar cycle and heliocentric distance dependence of the correlation between density n and magnetic field magnitude B in the solar wind. Previous work had suggested that this correlation becomes progressively more negative with heliocentric distance out to 9.5 AU. Here it is shown that this evolution is not a solar cycle effect, and that the correlations become even more strongly negative at heliocentric distance larger than 9.5 AU. There is considerable variability in the distributions of the correlations at a given heliocentric distance, but this is not simply related to the solar cycle. Examination of the evolution of correlations between density and speed suggest that most of the structures responsible for evolution in the anticorrelation between n and B are not slow-mode waves, but rather pressure balance structures. The latter consist of both coherent structures such as tangential discontinuities and the more generally pervasive 'pseudosound' which may include the coherent structures as a subset.

Roberts, D. A.↗
NASA NTRS1990

The onset of the new solar modulation cycle in 1987-1988 as a function of heliocentric radius and latitude

The onset of a new solar modulation cycle were studied in 1987 and 1988, on the basis of data on counting rates of particles with E greater than 60 MeV, observed at the IMP, Voyager (V), and Pioneer (P) satellites. It was found that the decrease at earth was rapid after the intensity maximum in early 1987 and was closely correlated with the increase in the average tilt of the heliospheric current sheet. The initial rapid intensity decrease was found to be related to three small Forbush decreases which are superimposed on a more gradual decrease. By the end of 1988, the intensity decreased by about 40 percent at earth, 30 percent at V2, and 18 percent at P10. This overall decrease was accompanied by an increase in the average integral radial gradient as well as a decrease in the radial dependence of the gradient. The onset of the new solar modulation cycle seems to be related to the complete altering of the solar magnetic structure as observed on the surface of the sun.

Webber, W. R.↗
NASA NTRS1990

Differences in the solar modulation of E greater than 60 MeV cosmic rays at earth and 18 AU during a complete 11-year cycle from 1977 to 1988

The cosmic ray intensities for E greater than 60 MeV particles at a constant radius of 18 AU using data from the IMP, Voyager and Pioneer spacecraft. These intensities are compared with the intensities at earth for a complete 11-year cycle from 1977 to 1988. It is found that the variations at 1 AU and 18 AU track very closely during the decreasing portion of the cycle from 1977 to 1982 but during the recovery the intensity at 18 AU increases much less than at earth with the result that in 1987 the intensity is only 82 percent of that at the previous minimum in 1977. These results imply that the integral radial gradient between 1 and 18 AU is constant in time from 1977 to 1982 but decreases from 1982 to 1988. Thus, the amplitude of the 11-year variation observed at a fixed energy is quite strongly radially dependent, with the amplitude becoming less at larger radii.

Lockwood, J. A.↗
NASA NTRS1990

Magnetic field line draping in the plasma depletion layer

Simultaneous IMP 8 solar wind and ISEE 1/2 observations for a northern dawn ISEE 1/2 magnetopause crossing on November 6, 1977. During this crossing, ISEE 1/2 observed quasi-periodic pulses of magnetosheathlike plasma on northward magnetic field lines. The ISEE 1/2 observations were originally interpreted as evidence for strong diffusion of magnetosheath plasma across the magnetopause and the Kelvin-Helmholtz instability at the inner edge of the low-latitude boundary layer. An alternate explanation, in terms of magnetic field merging and flux transfer events, has also been advocated. In this paper, a third interpretation is proposed in terms of quasi-periodic magnetopause motion which causes the satellites to repeatedly exit the magnetosphere and observe draped northward magnetosheath magnetic field lines in the plasma depletion layer.

Sibeck, D. G.↗
NASA NTRS1989

Elemental composition of low energy Galactic cosmic rays

This paper describes new measurements (at about 100 MeV/n) of elemental ratios in cosmic rays, made from Voyager 2 at about 22.5 AU. These data are characterized by a very low level of solar modulation, which results from the combination of the 1986-1987 solar minimum period and the large heliocentric distance. The data were obtained from one of the two HET telescopes on board Voyager 2, collected during 1986 and 1987; the charges were derived from a double dE/dx vs E analysis. The results on the secondary/primary elemental ratios are presented along with previous results obtained at 1 AU by IMP-8 and ISEE-3.

Ferrando, P.↗
NASA NTRS1990

Relationship of solar wind parameters to continuous, dayside, high latitude traveling ionospheric convection vortices

The results of a statistical study of the occurrence of steady traveling ionospheric convection vortices are presented. The upstream solar wind parameters observed by the IMP 8 spacecraft are studied as well. It is suggested that this class of pulsations is most likely to be detected post local noon and when the solar wind speed is low. A correlation between the frequency of dayside pulsations and the solar wind speed is found.

Mchenry, Mark A.↗
NASA NTRS1991

A census of the asteroid belt

Observations obtained by the Infrared Astronomical Satellite (IRAS) during its ten month mission in 1983 were originally processed by the Asteroid Data Analysis System (ADAS) to search for 3453 asteroids with known orbital elements as of September 1985. A total of 1811 had one or more observations of sufficient reliability to be accepted. These results were released in October 1986. Recently IRAS data were reprocessed to increase both the number of recognized asteroid observations and their reliability. As input 7311 asteroids were used with known orbital elements as of December 1990. This processor is referred to as the IRAS Minor Planet Survey (IMPS). As of April 1991 approximately 3000 asteroids had been identified with one or more acceptable observations. These results were used to derive the total number of asteroids with diameters greater than 1 km. In addition to being an interesting piece of information in itself these size-frequency distributions produce bias-correction factors which, for example, will be used in investigations of the physical properties of asteroid dynamical families and to estimate the distribution of the taxonomic classes as a function of heliocentric distance.

Tedesco, E. F.↗
NASA NTRS1991

Trajectories of inner and outer heliospheric spacecraft: Predicted through 1999

Information is presented in tabular and graphical form on the trajectories of the international fleet of spacecraft that will be probing the far reaches of the heliosphere during the 1990s. In particular, the following spacecraft are addressed: Pioneer 10 and 11, Pioneer Venus Orbiter (PVO), Voyager 1 and 2, Galileo, Ulysses, Suisei, Sakigake, Giotto, International Cometary Explorer (ICE), and Interplanetary Monitoring Platform 8 (IMP 8). Yearly resolution listing of position information in inertial space are given for Pioneer and Voyager spacecraft from the times of their launches in the 1970s. One series of plots shows the radial distances, latitudes, and longitudes of the Pioneers and Voyagers. The solar ecliptic inertial coordinate system is used. In this system, the Z axis is normal to the ecliptic plane and the X axis is towards the first point of Aries (from Sun to Earth on the vernal equinox).

Parthasarathy, R.↗
NASA NTRS1990

Cosmic-ray gradient measurements and modulation beyond the inner solar wind termination shock

Data provided by the IMP, Voyager, and Pioneer spacecraft for the cosmic-ray particles with E greater than 60 MeV show the existence of integral radial cosmic-ray-density gradients which were nearly constant from 1977 to 1982. In this paper, these measurements are explained using a model in which significant modulation occurs in the turbulent shocked plasma flow between the inner solar wind termination and the outer contact discontinuity separating the interstellar medium flow from the heliospheric plasma.

Quenby, J. J.↗
NASA NTRS1991

A study of the relationship between micropulsations and solar wind properties

A year-long comparison between daily averages of solar wind parameters obtained from the MIT experiment on IMP-8 and micropulsation measurements made by the Universita dell'Aquila has shown a correlation between solar wind speed and micropulsation power with peaks of the correlation coefficient greater than 0.8 in the period range from 20 to 40 s. Different behavior observed for different period bands suggests that the shorter period activity tends to precede the highest values of the solar wind speed while the longer period activity tends to persist for longer intervals within high velocity solar wind streams. A comparison with simultaneous interplanetary magnetic field measurements supports the upstream origin of the observed ground pulsations.

Yedidia, B. A.↗
NASA NTRS1991

Prompt arrival of solar energetic particles from far eastern events - The role of large-scale interplanetary magnetic field structure

Intensity-time profiles of solar energetic particle enhancements generally show an asymmetry with respect to the heliolongitude of the associated solar event. Particles arrive promptly from events to the west of an observer because of good magnetic connection, whereas particle enhancements from poorly connected eastern source regions usually show much slower onsets. However, some 15 percent of eastern events do show prompt onsets. Two prompt particle enhancements associated with eastern solar events are studied using data from the Goddard Space Flight Center instruments on the ISEE 3 and IMP 8 spacecraft. In both events the prompt particle onset was observed when the spacecraft were in a postshock plasma region, apparently within a magnetic bottle. It is suggested that the magnetic bottle extended back to the sun and served as a channel for fast particle propagation to the spacecraft. Particles accelerated at an expanding coronal shock initiated by the eastern event could be injected onto field lines in the foot of the bottle.

Richardson, I. G.↗
NASA NTRS1991

Solar cycle variations in the size and shape of the magnetopause

The 10 years of the ISEE 1 and 2 mission covering much of solar cycle 21 and the beginning of solar cycle 22 make it possible to study the position, shape, and motion of the magnetopause throughout the course of changing solar activity. The size and shape of the magnetopause were determined for each observing season using the ISEE 1 and 2 magnetometer data IMP 8 data were used to monitor the solar wind changes with the solar cycle. During the 1979-1980 season, at solar maximum, the solar wind dynamic pressure was at its lowest values, and, at solar minimum, the solar wind pressure was at its largest values, more than double the value in the 1979-1980 season. During this solar cycle, the magnetopause was about 0.5 R(E) farther when the interplanetary magnetic field (IMF) was strongly northward, than when strongly southward. Both standoff distance values are fround to be smaller than the value found by Fairfield (1971). The standoff distance of the magnetopause for northward IMF is anticorrelated with the solar wind pressure. However, the standoff distance for southward IMF seems relatively insensitive to solar wind dynamic pressure.

Petrinec, S. P.↗
NASA NTRS1991

Magnetic properties of the high-latitude tail boundary - Draping of magnetosheath field lines and tail-aligned current

Magnetic properties of the high-latitude tail boundary are examined with IMP 8 magnetometer data. The high-latitude tail boundary separates the tail lobe from the magnetosheath. Magnetic fields are stable in the tail lobe, but very irregular in the magnetosheath. Boundary crossings are marked by the rotation of magnetic components parallel to the boundary plane. The magnetic component normal to the boundary, if any, is very small in comparison to this rotational change. Despite large magnetic fluctuations in the magnetosheath, the magnetosheath-side field orientation is consistent with the draping of the IMF against the magnetotail. The boundary current has a component parallel to the lobe field (tail-aligned current), as well as a circumferential component. The IMF orientation controls the sheath-side B(Y), while the lobe-side field has a more rigid configuration flaring antisunward.

Ohtani, S.↗
NASA NTRS1991

The charge state of the anomalous component of cosmic rays

The ionic charge state of anomalous cosmic-ray oxygen has been determined by comparing measurements obtained inside the magnetosphere on a series of Cosmos satellite flights with simultaneous observations outside the magnetosphere from IMP 8 and ICE. A mean charge state of 0.9 + 0.3, - 0.2 is found for abount 10 MeV/nucleon anomalous oxygen, consistent with the model of Fisk et al. (1974), in which the anomalous cosmic rays originate from the neutral component of the local interstellar medium. This same approach gives results consistent with a mean charge state of +7 for solar energetic oxygen ions.

Adams, J. H., Jr.↗
NASA NTRS1991

ULF waves upstream of the Venus bow shock - Properties of one-hertz waves

Pioneer Venus Orbiter data are used here to study the properties of a class of ULF upstream waves with relatively high observed frequencies. These waves show significant similarity to 'one-Hz' waves identified at earth in the ISEE 1 and 2 observations and the whistler waves identified earlier by IMP 6 observations. The waves appear almost immediately after the spacecraft crosses the magnetic field tangent line to the bow shock surface into the region of connected field lines. The wave amplitude decreases with distance from the shock measured along the magnetic field line. Group velocities calculated using the cold plasma dispersion relation indicate that the waves have sufficient upstream velocities to propagate form the shock into the solar wind. The totality of observations seem best explained by a source of right-handed whistler mode waves at the bow shock.

Orlowski, D. S.↗
NASA NTRS1990

Distant solar wind plasma - View from the Pioneers

Temporal variation and spatial gradients over nearly 50 AU in heliocentric distance and about 17 deg of latitude are investigated on the basis of observations from Pioneer 10-11, Voyager 2, IMP 8, and the Pioneer Venus Orbiter. The average solar wind velocity was found to vary somewhat over the solar cycle, but at any given epoch the average velocity is essentially independent of heliocentric distance. No indication of a terminal shock or signature of interaction with the interstellar medium was found. The stream structure commonly observed in the inner heliosphere diminishes with increasing heliocentric distance, reflecting the processing of material in the stream-stream heliosphere, both at times of high sunspot activity and disturbed solar conditions, and near the solar minimum when the interplanetary current sheet is flattened.

Barnes, Aaron↗
NASA NTRS1991

Evolution on large-scale plasma structures in comets: Kinematics and physics

The disconnection event (DE) consists of the periodic loss of a comet's entire plasma tail and the growth of a new one. This spectacular phenomenon is not understood. The strategy was to assemble a data base of specific events studied in detail, determine the solar wind conditions responsible for DEs, and develop a consistent physical model. Analysis is complete for the sequence of DEs that took place during 13 to 18 April 1986. The first DE correlates well with a sector boundary crossing for the comet and a group of DEs that occurred approximately a day later could have produced by polarity reversals seen in the IMP-8 data. Thus, these DEs are consistent with the frontside, magnetic reconnection mechanism.

Brandt, John C.↗
NASA NTRS1991

Structure and evolution of compound streams at not greater than 1 AU

Two cases of compound stream formation and evolution at heliocentric distances of not greater than 1 AU have been studied using simultaneous data from both multipoint in situ and global sources. The sequences of events analyzed occurred around December 16, 1978, and February 19, 1979, respectively. The sources of the multipoint observations were Helios A, Helios B, IMP 8, and ISEE 3. The global observations were obtained by interplanetary scintillation analysis of radio sources. It was found that the radio and in situ data complemented each other and that both were needed for a unique interpretation of complex, time-dependent flows. The cases studied show that a single corotating stream can interact with different transient flows at different times, and a single transient flow can interact with different corotating streams at different times. In each instance, a compound stream is formed. This work further shows that the cosmic ray intensity at a point depends on the global compound stream configuration in the vicinity of the point.

Burlaga, L. F.↗