Engineering topics

Russell, C. T.

Publications and source records attributed to Russell, C. T..

At least 397 records · Page 22

NASA NTRS1985

ISEE-1 and 2 observations of field-aligned currents in the distant midnight magnetosphere

Magnetic field measurements obtained in the nightside magnetosphere by the co-orbiting ISEE-1 and 2 spacecraft have been examined for signatures of field-aligned currents (FAC). Such currents are found on the boundary of the plasma sheet both when the plasma sheet is expanding and when it is thinning. Evidence is often found for the existence of waves on the plasma sheet boundary, leading to multiple crossings of the FAC sheet. At times the boundary layer FAC sheet orientation is nearly parallel to the X-Z GSM plane, suggesting 'protrusions' of plasma sheet into the lobes. The boundary layer current polarity is, as expected, into the ionosphere in the midnight to dawn local time sector, and outward near dusk. Current sheet thicknesses and velocities are essentially independent of plasma sheet expansion or thinning, having typical values of 1500 km and 20-40 km/s respectively. Characteristic boundary layer current densities are about 10 nanoamps per square meter.

Elphic, R. C.↗

NASA NTRS1985

Evidence for mass-loading of the Venus magnetosheath

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the -V(bar)XB(bar) acceleration process, stochastic pickup of ionospheric ions, and J(bar)XB(bar) force 'scavenging' at the ionopause all occur at various times.

Luhmann, J. G.↗

NASA NTRS1985

Electron densities and temperatures in the Venus ionosphere Effects of solar EUV, solar wind pressure and magnetic field

The Venus ionosphere is influenced by variations in both solar EUV flux and solar wind conditions. On the dayside the location of the topside of the ionosphere, the ionopause, is controlled by solar wind dynamic pressure. Within the dayside ionosphere, however, electron density is affected mainly by solar EUV variations, and is relatively unaffected by solar wind variations and associated magnetic fields induced within the ionosphere. The existence of a substantial nightside ionosphere of Venus is thought to be due to the rapid nightward transport of dayside ionospheric plasma across the terminator. Typical solar wind conditions do not strongly affect this transport and consequently have little direct influence on nightside ionospheric conditions, except on occasions of extremely high solar wind dynamic pressure. However, both nightside electron density and temperature are affected by the presence of magnetic field, as in the case of ionospheric holes.

Elphic, R. C.↗

NASA NTRS1985

On the source region of flux transfer events

An examination has been conducted of the region of occurrence of flux transfer events for three distinct orientations of the interplanetary magnetic field: nearly horizontal in the solar magnetospheric equator, diagonally southward at 45 deg to the magnetospheric equator and nearly due south. For horizontal IMF conditions the FTE's occur in a horizontal band about + or - 6 earth radii wide. For diagonally southward IMF conditions, the FTE's occur in a diagonal swath about + or - 6 earth radii wide passing through the subsolar point. For duskward but nearly due southward IMF conditions, the observations reveal FTE's throughout the northern morning quadrant. These observations are consistent with a near equatorial source for flux transfer events and hence with component merging and not anti-parallel merging. These observations also help understand the energetic ion anisotropies seen in these events.

Russell, C. T.↗

NASA NTRS1985

Patchy reconnection and magnetic ropes in astrophysical plasmas

The formation of flux ropes in the magnetosphere of Venus due to velocity shear is examined in order to infer the properties of flux ropes in the earth magnetosphere. Observational data obtained by the International Sun-Earth Explorer (ISEE) spacecraft for the geometry of directed flows in the earth magnetosheath are compared with magnetometer measurements carried out on board Pioneer Venus Orbiter, and the results are discussed in detail. It is shown that the velocity shear is the dominant element shaping the magnetic structure of the magnetosheaths.

Russell, C. T.↗

NASA NTRS1985

Dependence of Venus ionopause altitude and ionospheric magnetic field on solar wind dynamic pressure

The shape of the dayside Venus ionopause, and its dependence on solar wind parameters, is examined using Pioneer Venus Orbiter field and particle data. The ionopause is defined here as the altitude of pressure equality between magnetosheath pressure and ionospheric thermal pressure; its typical altitudes range from about 300 km near the subsolar point to about 900 km near the terminator. A strong correlation between ionopause altitude and magnetosheath magnetic pressure is demonstrated; correlation between magnetic pressure and the normally incident component of solar wind dynamic pressure is also evident. The data support the hypothesis of control of the ionopause altitude by solar wind dynamic pressure, manifested in the sheath as magnetic pressure. The presence of large scale magnetic fields in the ionosphere is observed primarily when dynamic pressure is high and the ionopause is low.

Phillips, J. L.↗

NASA NTRS1985

Mass-loading and the formation of the Venus tail

Despite its lack of intrinsic magnetic field Venus has a well defined magnetotail, containing about 3 megawebers of magnetic flux in a tail about 4 Venus radii across with perhaps a slightly elliptical cross section. This tail arises through the mass-loading of magnetic flux tubes passing by the planet. Mass-loading can occur due to charge exchange and photoionization as well as from the diffusion of magnetic field into the ionosphere. Various evidence exists for the mass-loading process, including the direct observation of the picked up ions with both the Venera and Pioneer Venus plasma analyzers.

Russell, C. T.↗

NASA NTRS1985

Current-driven plasma instabilities and auroral-type particle acceleration at Venus

Above the ionosphere of Venus, several instruments on the Pioneer Orbiter detect correlated wave, field and particle phenomena suggestive of current-driven anomalous resistivity and auroral-type particle acceleration. In localized regions the plasma wave instrument measures intense mid-frequency turbulence levels together with strong field-aligned currents. Here the local parameters indicate that there is marginal stability for ion acoustic waves, and the electron temperature probe finds evidence that energetic primaries are present. This suggests an auroral-type energy deposition into the upper atmosphere of Venus. These results appear to be consistent with the direct measurements of auroral emissions from the Pioneer-Venus ultraviolet imaging spectrometer.

Scarf, F. L.↗

NASA NTRS1985

Pioneer Venus observations during Comet Halley's inferior conjunction

On Feb. 4, 1986, Halley passed through inferior conjunction with Venus but was at high latitudes. Not all data for this time period have been received. However, the data that are available suggest that at most only weak effects associated with Halley were seen at Pioneer Venus. The data during this time, however, are useful for correlating with the behavior of the plasma tail.

Russell, C. T.↗

NASA NTRS1985

Planetary bow shocks

Planetary bow shocks provide insight into both the behavior of collisionless shocks and the nature of the planetary obstacle responsible for creating those bow shocks. This review paper first presents a survey of the microstructure of planetary bow shocks using data obtained at Mercury, Venus, the earth, Mars, Jupiter, and Saturn. When it examines the correspondence of the behavior of the shock jumps with that predicted by theory, and the differences between gas dynamic and magnetohydrodynamic solutions. Finally, it examines the information available from observations of the location of the bow shock about the nature of the solar wind-planetary interaction.

Russell, C. T.↗

NASA NTRS1984

Growth and maintenance of large-scale magnetic fields in the dayside Venus ionosphere

Observations from the Pioneer Venus orbiter magnetometer reveal the presence of large-scale magnetic fields in the dayside ionosphere during or after periods of high solar wind dynamic pressure. Various hypotheses have been proposed concerning the spatial and temporal evolution of these field structures. The hypothesis that the field is produced by a diffusion/convection process rather than by currents driven by electric fields resulting from the solar wind interaction is examined. Dynamic pressure variations occur on various time scales at Venus, producing transient and quasi-steady magnetization features. A one-dimensional diffusion/convection calculation is performed, using typical ionopause field and pressure values for a variety of altitudes. The Venus ionopause is considered to be the altitude at which ionospheric thermal pressure is equal to the magnetosheath magnetic pressure, which in turn is well correlated with the normal component of solar wind dynamic pressure. A subsolar model of downward plasma velocity in the 140-290 km range is used. The calculated growth of the ionospheric magnetic field, and the resulting quasi-steady altitude profiles, compare favorably with the observed profiles. The majority of the observed magnetic structures are best explained as the quasi-steady effects of the prevailing solar wind dynamic pressure, which determines the altitude and magnetic field strength of the ionopause boundary.

Phillips, J. L.↗

NASA NTRS1984

Interplanetary field enhancements in the solar wind Statistical properties at 0.72 AU

A new class of disturbance in the interplanetary magnetic field has been discovered. This disturbance consists of an enhancement in the magnetic field strength lasting tens of minutes to hours. The strength of the enhancement is variable ranging up to over double the background field strength. The peak field pressure can be as high as 10 percent of the solar wind dynamic pressure. These events occur randomly with respect to the position of the spacecraft relative to Venus but not randomly with respect to Venus solar ecliptic longitude. There is a significant tendency for these events to cluster near certain ecliptic longitudes. The field distortion is often greater in the direction perpendicular to the solar wind flow rather than along it. These characteristics suggest that the source of the disturbances are weakly outgassing objects, possibly dispersed along their orbits such as in meteor streams.

Russell, C. T.↗

NASA NTRS1984

Interplanetary magnetic field enhancements and their association with the asteroid 2201 Oljato

Comparison of the times of occurrence of a newly discovered type of disturbance in the interplanetary magnetic field at 0.72 astronomical unit with the passage of the small Venus-crossing asteroid, 2201 Oljato, reveals a possible association, but the source of these disturbances appears to be associated with outgassing material in the Oljato orbit some distance behind the asteroid and not with the asteroid itself. This suggested association can account for one quarter of the total number of events seen in eight Venus years.

Russell, C. T.↗

NASA NTRS1984

A possible observation of a cometary bow shock

In February, 1982, an unusual enhancement of the IMF was observed by the Pioneer Venus orbiter at 0.72 AU. The three discontinuities within this large disturbance, previously suggested to be due to the possible passage of a comet, are examined in view of the alternative possibility of their representing the first observation to date of a cometary shock. It is noted that number density, temperature, and velocity changes, as well as the magnetic structure of the discontinuities, are consistent with this hypothesis.

Russell, C. T.↗

NASA NTRS1984

On the proper Mach number and ratio of specific heats for modeling the Venus bow shock

Observational data from the Pioneer Venus Orbiter are used to investigate the physical characteristics of the Venus bow shock, and to explore some general issues in the numerical simulation of collisionless shocks. It is found that since equations from gas-dynamic (GD) models of the Venus shock cannot in general replace MHD equations, it is not immediately obvious what the optimum way is to describe the desired MHD situation with a GD code. Test case analysis shows that for quasi-perpendicular shocks it is safest to use the magnetospheric Mach number as an input to the GD code. It is also shown that when comparing GD predicted temperatures with MHD predicted temperatures total energy should be compared since the magnetic energy density provides a significant fraction of the internal energy of the MHD fluid for typical solar wind parameters. Some conclusions are also offered on the properties of the terrestrial shock.

Tatrallyay, M.↗

NASA NTRS1984

The structure and dynamics of the magnetosphere: Progress in the IMS

Knowledge of the magnetosphere before the International Magnetospheric Study (IMS) is reviewed, and advances due to IMS are outlined. Post IMS studies are summarized. The GEOS spacecraft continuously monitored the state of the interior of the magnetosphere. The ISEE-1 and 2 satellites measured the velocities of magnetospheric boundaries. The ISEE-3 monitored solar wind input to the magnetosphere. The spacecraft provided data on magnetopause thickness, reconnection, electric field configuration, and plasma currents. Post IMS projects examined the ionosphere, auroras, and the deep geomagnetic tail.

Russell, C. T.↗

NASA NTRS1984

Magnetic field observations at the dayside magnetopause: ISEE-1 and -2 contributions during the IMS

Dayside magnetopause macroscopic characteristics such as velocities and thicknesses of the boundary and internal magnetic field structure of the current layer were studied by ISEE-1 and 2. The ISEE results suggest that the magnetopause is much thicker than previously thought. A current sheet thicker than several proton gyroradii rules out classical kinetic theories of a magnetopause current layer of the order of the ion gyroradius. Such a thick boundary may explain the failure of theories which predict a unique polarization for the field rotation inside the current sheet.

Berchem, J.↗

NASA NTRS1984

The near Earth current sheet: ISEE-1 and -2 case studies

Two crossings of the near Earth current sheet are examined using magnetic field and plasma data from ISEE-1 and 2. The current sheet thickness is measured to be 0.9 Re using a 2 satellite technique which greatly reduces the ambiguity of such measurements. Approximate pressure balance is found to exist between the diamagnetically dominated current sheet and surrounding, already high beta, plasma sheet. The accurate calculation of the sheet normal velocity with the two satellite technique allows the calculation of the current density distribution in the sheet. The crosstail current density is found to peak about an ion-gyroradius on both sides of the center of the sheet. A simple model consistent with these results is discussed.

Mccomas, D. J.↗