Engineering topics

Gazis, P. R.

Publications and source records attributed to Gazis, P. R..

35 records · Page 2

NASA NTRS1993

Pioneer Venus and IMP 8 observations of the latitude dependence of the solar wind

Solar wind speeds and magnetic field data from the Pioneer Venus Orbiter (0.7 AU) and IMP 8 (1 AU) have been compared to infer the latitudinal structure of the solar wind in the inner heliosphere between 1984 and 1987. The solar wind in the inner heliosphere was found to exhibit an unusual structure during the last solar minimum (1986-1987). High-speed streams were excluded from the vicinity of the solar equator, and the solar wind at low heliographic latitudes (less than 3 deg) was characterized by low-speed solar wind with irregular fluctuations in velocity. At higher latitudes the solar wind had a conventional stream structure with two high-speed streams per solar rotation. While the velocities of these high-speed streams did not appear to vary significantly with latitude, the latitudinal gradients at the equatorward boundaries of high-speed streams were high.

Gazis, P. R.↗

NASA NTRS1992

Global properties of the plasma in the outer heliosphere. I - Large-scale structure and evolution

Pioneers 10 and 11, and Voyager 2, have active plasma analyzers as they proceed through heliocentric distances of the order of 30-50 AU, facilitating comparative studies of the global character of the outer solar wind and its variation over the solar cycle. Careful study of these data show that wind ion temperature remains constant beyond 15 AU, and that there may be large-scale variations of temperature with celestial longitude and heliographic latitude. There has thus far been no indication of a heliospheric terminal shock.

Barnes, A.↗

NASA NTRS1992

Solar wind temperature observations in the outer heliosphere

The Pioneer 10, Pioneer 11, and Voyager 2 spacecraft are now at heliocentric distances of 50, 32 and 33 AU, and heliographic latitudes of 3.5 deg N, 17 deg N, and 0 deg N, respectively. Pioneer 11 and Voyager 2 are at similar celestial longitudes, while Pioneer l0 is on the opposite side of the sun. The baselines defined by these spacecraft make it possible to resolve radial, longitudinal, and latitudinal variations of solar wind parameters. The solar wind temperature decreases with increasing heliocentric distance out to a distance of 10-15 AU. At larger heliocentric distances, this gradient disappears. These high solar wind temperatures in the outer heliosphere have persisted for at least 10 years, which suggests that they are not a solar cycle effect. The solar wind temperature varied with heliographic latitude during the most recent solar minimum. The solar wind temperature at Pioneer 11 and Voyager 2 was higher than that seen at Pioneer 10 for an extended period of time, which suggests the existence of a large-scale variation of temperature with celestial longitude, but the contribution of transient phenomena is yet to be clarified.

Gazis, P. R.↗

NASA NTRS1992

The structure of the inner heliosphere from Pioneer Venus and IMP observations

The IMP 8 and Pioneer Venus Orbiter (PVO) spacecraft explore the region of heliographic latitudes between 8 deg N and 8 deg S. Solar wind observations from these spacecraft are used to construct synoptic maps of solar wind parameters in this region. These maps provide an explicit picture of the structure of high speed streams near 1 AU and how that structure varies with time. From 1982 until early 1985, solar wind parameters varied little with latitude. During the last solar minimum, the solar wind developed strong latitudinal structure; high speed streams were excluded from the vicinity of the solar equator. Synoptic maps of solar wind speed are compared with maps of the coronal source surface magnetic field. This comparison reveals the expected correlation between solar wind speed near 1 AU, the strength of the coronal magnetic field, and distance from the coronal neutral line.

Gazis, P. R.↗

NASA NTRS1991

Physics of the outer heliosphere

Major advances in the physics of the outer heliosphere are reviewed for the 1987-1990 time frame. Emphasis is placed on five broad topics: the detailed structure of the solar wind at large heliocentric distances, the global structure of the interplanetary field, latidudinal variations and meridional flows, radial and temporal variations, and the interaction of the solar wind with the local interstellar medium.

Gazis, P. R.↗

NASA NTRS1989

Pioneer and Voyager observations of the solar wind at large heliocentric distances and latitudes

Data obtained from the electrostatic analyzers aboard the Pioneer 10 and 11 spacecraft and from the Faraday cup aboard Voyager 2 were used to study spatial gradients in the distant solar wind. Prior to mid-1985, both spacecraft observed nearly identical solar wind structures. After day 150 of 1985, the velocity structure at Voyager 2 became flatter, and the Voyager 2 velocities were smaller than those observed by Pioneer 11. It is suggested that these changes in the solar wind at low latitudes may be related to a change which occurred in the coronal hole structure in early 1985.

Gazis, P. R.↗

NASA NTRS1987

Solar wind stream structure at large heliocentric distances Pioneer observations

Time profiles and histograms of plasma data from Pioneers 10 and 11 are examined for the period between 1975 and 1983. During this time, Pioneer 10 traveled between a heliocentric distance of 8.7 and 30.4 AU. The velocity structure of the solar wind at these heliocentric distances is found to have one of two distinct forms: approximately 70 percent of the time the solar wind has a nearly flat velocity profile. Occasionally, this flat velocity profile is accompanied by quasi-periodic variations in density and in thermal speed consistent with the concept that the 'corotating interaction regions' which are produced by the interaction of high- and low-speed streams at intermediate heliocentric distances are replaced by 'pressure regions' in the outer heliosphere. The remaining 30 percent of the time the solar wind is marked by large (50-200 km/s) long-term (30-120 days) shifts in the average solar wind velocity.

Gazis, P. R.↗

NASA NTRS1985

Shock evolution in the outer heliosphere - Voyager and Pioneer observations

Observations are reported of 35 interplanetary shocks detected at heliocentric distances between 6.5 and 9.4 AU in mid-1980 by the Voyager 1, Voyager 2, and Pioneer 11 spacecraft. These shocks were all evidently generated by the interaction of corotating streams. Measurements of the pre- and postshock plasma parameters are used to determine the shock normals and speeds for each shock. Twelve of these 35 events are observed at all three spacecraft. The shock parameters at each spacecraft are compared to determine the time history for the 12 shocks. The single-spacecraft determinations of shock normal and speed are compared wih the results of several techniques for determining shock velocities using all three spacecraft. It is found that shocks undergo significant evolution as they travel past the three spacecraft. Every shock seen by both end spacecraft was also seen by the middle spacecraft. Shock formation possibly may be occurring at a heliocentric distance as great as 7 AU. It appears that the shock surfaces have a complex three-dimensional structure with 'ripples' on a scale of 0.001-1.0 AU.

Gazis, P. R.↗

NASA NTRS1984

The Solar Wind at 20-30 AU

Pioneer 10 sampled the interplanetary plasma over the range 20 to 30 astronomical units, during the period 1979-1983. The median flow speed is about 400 km/s, and at 20 AU the median density, proton temperature and dynamic pressure are, respectively, 0.025 cm-3, 10(4) K, and 6x10-11 dyne cm-2. It is shown that the average solar wind flow speed does not vary significantly with increasing heliocentric distance, and the density falls off as R-2, as predicted by simple solar wind models. The day-to-day variations in solar wind parameters are smaller at larger distance. Very large shocks however, were detected beyond 25 AU. Comparison of Pioneer 10 and 11 observations at similar distances but different phases of the solar activity cycle shows that solar wind dynamic pressure varies over a wider range during epochs of high solar activity. The variation near 20 AU is likely to be smaller at Voyager 2 Uranus encounter than observed by Pioneer 10 in the 1979-80 period.

Barnes, A.↗

NASA NTRS1984

Stream dynamics between 1 AU and 2 AU: A detailed comparison of observations and theory

A radial alignment of three solar wind stream structures observed by IMP-7 and -8 (at 1.0 AU) and Voyager 1 and 2 (in the range 1.4 to 1.8 AU) in late 1977 is presented. It is demonstrated that several important aspects of the observed dynamical evolution can be both qualitatively and quantitatively described with a single-fluid 2-D MHD numerical model of quasi-steady corotating flow, including accurate prediction of: (1) the formation of a corotating shock pair at 1.75 AU in the case of a simple, quasi-steady stream; (2) the coalescence of the thermodynamic and magnetic structures associated with the compression regions of two neighboring, interacting, corotating streams; and (3) the dynamical destruction of a small (i.e., low velocity-amplitude, short spatial-scale) stream by its overtaking of a slower moving, high-density region associated with a preceding transient flow. The evolution of these flow systems is discussed in terms of the concepts of filtering and entrainment.

Burlaga, L. F.↗

NASA NTRS1984

Observations of plasma bulk parameters and the energy balance of the solar wind between 1 and 10 AU

The solar wind bulk velocity, the product of the mass flux and the square of the heliocentric distance, and the product of the density and the square of the heliocentric distance, are all found not to vary with heliocentric distance in spacecraft observations of the radial evolutions of several solar wind bulk parameters in the 1-10 AU range. Density's anticorrelation with velocity vanishes at greater distances from the sun than 1 AU, while solar wind bulk velocity and thermal speed are correlated over the entire 1-10 AU range. Thermal process heating, bulk kinetic energy conversion to thermal energy, and electron heat conduction, are considered as mechanisms accounting for the radial variation of the solar wind proton temperature over 1-10 AU. It is found that the addition of a heat flux due to heat conduction is a persuasive alternative.

Gazis, P. R.↗

NASA NTRS1984

Shocks in the solar wind between 1 and 8.5 AU: Voyager 1 observations

A survey was made of all interplanetary shocks detected by the plasma science experiment aboard the Voyager 1 spacecraft between 1.2 and 8.5 AU. Shock normals and shock velocities are determined. The variation of shock frequency and various shock parameters with heliocentric distance is discussed. The results indicate that beyond 1.2 AU, the vast majority of shocks were associated with interaction regions between high and low speed streams; of 95 events, only 1 was clearly associated with a transient event. Forward shocks were more numerous and seemed to form closer to the sun than reverse shocks. Forward shocks were stronger than reverse shocks. The energy balance of three shocks is examined. A close agreement is found between the measured and the predicted pressure ratios across these shocks. The contribution of shocks to the global energy balance is discussed. Shocks are found to have a significant effect in heating the solar wind.

Gazis, P. R.↗

NASA NTRS1983

The radial evolution of the solar wind, 1-10 AU

The interplanetary plasma and magnetic field observations from 1 to 10 AU are reviewed. Over this distance no clear reduction in average speed is seen. The range of wind speeds becomes smaller though high speed streams are still observed. The density, temperature and magnetic field profiles become dominated by the large values seen in the co-rotating interaction regions. The temperature falls more slowly than would be expected from a simple, adiabatic model. Co-rotating shocks appear beyond approximately 3 AU in Voyager data as opposed to beyond approximately 1.5 AU in the Pioneer data. Reverse shocks appear later than forward shocks; reverse shocks do not begin to appear until approximately 4 AU; reverse shocks appear to decay more rapidly than forward shocks. No clear effect due to interaction with the interstellar medium was seen in this radial range.

Gazis, P. R.↗

NASA NTRS1983

A Pioneer-Voyager study of the solar wind interaction with Saturn

Voyager 1/2 observations are used to confirm and quantify the unusual bluntness of the dayside Saturn magnetopause suggested by the earlier Pioneer 11 measurements. Once corrected for variations in solar wind pressure, the Saturn observations are found to indicate a magnetospheric radius in the terminator plane equal to 1.8 times the nose distance. This ratio is 30% greater than the terrestrial value and is corroborated by the very blunt shape of the Saturn bow shock and large width of the forward magnetosheath. Possible causes and ramifications of these results for the solar wind interaction with Saturn are discussed.

Slavin, J. A.↗

NASA NTRS1982

Voyager observations of solar wind proton temperature - 1-10 AU

Simultaneous measurements are made of the solar wind proton temperatures by the Voyager 1 and 2 spacecraft, far from earth, and the IMP 8 spacecraft in earth orbit. This technique permits a separation of radial and temporal variations of solar wind parameters. The average value of the proton temperature between 1 and 9 AU is observed to decrease as r (the heliocentric radius) to the -(0.7 + or - 0.2). This is slower than would be expected for adiabatic expansion. A detailed examination of the solar wind stream structure shows that considerable heating occurs at the interface between high and low speed streams.

Gazis, P. R.↗

NASA NTRS1982

Plasma observations near Saturn - Initial results from Voyager 2

Results of plasma measurements made by Voyager 2 in the vicinity of Saturn are discussed and compared with those made by Pioneer 11 and Voyager 1 in a more limited range of latitudes. The initial bow shock crossing on the inbound trajectory closely agreed with the shock position inferred from the external ram pressure in the solar wind, although boundaries on the outbound pass were much further out than expected. Magnetospheric plasma observations reveal the presence of (1) shocked solar wind plasma in the magnetosheath between 30 and 22 Saturn radii; (2) a variable density region between 17 Saturn radii and the magnetopause; (3) an extended thick plasma sheet between 17 and 7 Saturn radii; and (4) an inner plasma torus probably originating from local sources. The ratio of heavy to light ions was observed to vary with distance to the equatorial plane in the dayside magnetosphere, with the heavy ions, probably O(+), more closely confined to the equatorial plane. The plasma data also account for the observed inner boundary of the neutral hydrogen torus discovered by Voyager 1.

Bridge, H. S.↗

NASA NTRS1981

Plasma observations near Saturn - Initial results from Voyager 1

The Voyager 1 encounter with Saturn and its satellites yielded extensive measurements of magnetospheric low-energy plasma electrons and positive ions, both heavy and light, probably of hydrogen and nitrogen or oxygen. At radial distances between 15 and 7 Saturn radii on the inbound trajectory, the plasma appears to corotate with a velocity within 20% of that theoretically expected for rigid corotation. The Titan data, taken while the moon was inside the Saturn magnetosphere, shows a clear signature characteristic of the interaction between a subsonic corotating magnetospheric plasma and the atmospheric or ionospheric exosphere of Titan.

Bridge, H. S.↗