Engineering topics

Russell, C. T.

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

At least 325 records · Page 18

NASA NTRS1988

Multipoint measurements of upstream waves

Two-wave MHD populations are seen at collisionless shocks: precursor waves standing in the shock ramp which form an integral part of the shock and upstream waves which are usually attempting to propagate upstream but are carried back toward the shock by the solar wind flow. Both types of waves are observed at interplanetary shocks and planetary bow shocks. The difficulty in studying interplanetary shocks is that the shock normal is hard to determine accurately but multiple spacecraft measurements are of some assistance in this regard. Two types of multispacecraft studies have been used, closely spaced ones such as with ISEE-1 and -2 and more distantly separated ones such as with ISEE and UKS. These studies suggest that the paradigm proposed here for the evolution of large amplitude or 'fully developed' turbulence needs some revision.

Russell, C. T.↗

NASA NTRS1988

ISEE studies of the quasi-parallel bow shock

ISEE-1 and-2 plasma and magnetic field observations are used to examine several encounters with brief magnetic pulsations in the quasi-parallel region of the earth's bow shock. The two-spacecraft timing of the magnetic field signature is examined to see if the pulsation encounters are nested (as for back and forth motion of the shock over the spacecraft) or simply time-shifted (as for a structure convected anti-sunward across the spacecraft). Examples of both types of signatures are found, some within minutes of each other, suggesting that at least some pulsations probably originate from the growth and steepening of upstream waves but may eventually be able to stand in the upstream flow, perhaps becoming part of the shock surface itself. The possibility that some of the pulsations may simply be brief encounters with the moving shock is also not ruled out.

Thomsen, M. F.↗

NASA NTRS1988

Multi-spacecraft observations of magnetopause surface waves - ISEE 1 and 2 determinations of amplitude, wavelength and period

The multispacecraft ISEE mission made it possible to study propagation of surface waves on the magnetopause. One case studied in detail was near local noon at a latitude of 24.4 deg. The inferred instantaneous magnetopause normal vectors oscillated about the model magnetopause normal vector in a manner expected for a wave propagating along the magnetopause from the subsolar region. By assuming a model monochromatic wave and a constant magnetopause thickness, the best-fit that yields the three wave parameters( amplitude, wavelength, and period) is determined.

Song, PU↗

NASA NTRS1988

An ISEE-1/2 spacecraft study of an unusual flux transfer event

Using the ISEE-1/2 spacecraft, an unusual FTE was studied. The perturbation in the direction normal to the magnetopause reached a maximum size of 64 nT, exceeding the field amplitude in the adjacent magnetosphere. The observation was made at 1500 Local Time in the Southern Hemisphere (10 deg below the magnetic equator). From two point observations, it was found that the structure has a scale length of 2R(E) in the north-south direction in boundary normal coordinates. The velocity of the structure in that direction was about 50 km/sec. The dimension and the velocity in the east-west direction could not be established. The FTE signature recurred after about 9 minutes.

Zhu, X. M.↗

NASA NTRS1988

Radial Alfven velocity profiles in the magnetosphere and their relation to ULF wave field-line resonances

The radial Alfven velocity, V(A), in the magnetosphere was investigated using magnetic field and plasma data from the ISEE-1 spacecraft. The harmonic frequency structure calculated from these data was compared with the observed resonant wave structure in the frequency range 5-100 mHz and over the region L = 3-9. It was found that the frequency of these resonances depend on the A(V), which is a function of the magnetic field strength and plasma density. The toroidal mode resonance frequency structure was modeled for the first four harmonics, with excellent agreement between the modeled and observed harmonics for a passs which does not show a plasmapause gradient. In the presence of a steep plasmapause gradient, the model was found to predict a correspondingly steep increase in the resonance frequencies at the plasmapause, by about 5 mHz for the first harmonic and by 20 mHz for the fourth harmonic. Special consideration is given to the effect of heavy ion mass loading on the modeling.

Fraser, B. J.↗

NASA NTRS1988

The magnetosphere of Mercury

Data provided by the Mariner-10 spacecraft on the properties of Mercurian magnetosphere are examined. These observations indicate that the Mercurian magnetosphere has a magnetopause and a bow shock which are quite similar to their terrestrial counterparts, although much smaller. However, due to the absence of any significant atmosphere or ionosphere, the flow of current in the Mercurian magnetosphere is different from the patterns at the earth. Many questions regarding the intrinsic magnetic field properties of Mercury remain unanswered, such as the existence of radiation belts, magnetic storms, the size of auroral regions, the mechanism of global magnetospheric convection, and the source of plasma.

Russell, C. T.↗

NASA NTRS1987

Cusp displacement at the magnetopause for large IMF Y component

The magnetic field orientation just earthward of the dayside magnetopause from 95 ISEE and 11 HEOS crossings with /By/ greater than /Bz/ in the adjacent magnetosheath indicate a statistically significant-1-earth-radius shift of the cusps toward dawn (dusk) in the Northern Hemisphere and dusk (dawn) in the Southern Hemisphere for positive (negative) By. This small shift near the magnetopause and the contrasting large shifts of some reported ionospheric cusp signatures are explained in terms of a finite thickness magnetopause model through which the cusps shift gradually from the inner to the outer edge.

Crooker, N. U.↗

NASA NTRS1987

Distribution of whistler mode bursts at Venus

Several thousand impulsive whistler mode noise bursts were detected by the Pioneer Venus wave instrument during the first 10 seasons with nightside traversals at low altitudes. The altitude distribution for these events shows that essentially all of the bursts were detected when the orbiter was less than 2000 km above the planet, suggesting that the varying plasma conditions could not maintain coherent whistler mode field-aligned guidance over greater distances. Within the 2000-km range, the distribution of the number of events versus altitude shows that there are two distinct subregions. These results are interpreted in terms of two types of whistler mode propagation from sources below the ionosphere.

Scarf, F. L.↗

NASA NTRS1987

Effect of possible passage through Halley's magnetic tail on geomagnetic activity

It is found that there occurred a geomagnetic storm on May 18/19, 1910 that cannot be dismissed as a recurrent storm. The period of storminess is close to the period of time at which the magnetic and plasma tail of Halley should have passed over the earth. The signature is worldwide and of the form and magnitude expected if the solar wind were shielded from the earth by the comet. Comparison with computer simulations applicable closer to the comet suggests that this interpretation is feasible. If this interpretation is correct, the 'lobes' of the Halley tail have deeper lows at 24 x 10 to the 6th km than in the present model at 7 x 10 to the 6th km downstream and are at least twice as wide.

Russell, C. T.↗

NASA NTRS1987

An examination of possible solar wind sources for a sudden brightening of Comet IRAS-Araki-Alcock

Terrestrial and near-earth measurements are examined in order to investigate possible solar wind sources for the sudden global brightenings noted in Comet IRAS-Araki-Alcock. These brightenings are not found to be associated with increases in the solar wind momentum flux, quantity of solar energetic particles, or solar activity, in contradiction to the proposal of Lutz and Wagner (1986). If a radial alignment of the IMF seen at IMP-8 after about 0800 UT was not responsible for the cometary brightening, then it is suggested that the brightening must have been intrinsic to the comet.

Russell, C. T.↗

NASA NTRS1987

Finite Larmor radius effect on ion pickup at Venus

The interaction of the solar wind with Venus is influenced by the pickup of newly born exospheric oxygen ions by the convecting magnetosheath plasma. The flow and field configuration of the magnetosheath plasma, together with the large gyroradius of the pickup ions, cause mass loading to occur preferentially on one side of the magnetosheath. The observed hemispherical asymmetry in the magnetic field in the near-planet magnetosheath, attributed to this pickup process, is confirmed by direct observation of the picked-up planetary particles. Test particle calculations show that a current system created by ion pickup has the appropriate location and magnitude to account for the magnetic field asymmetry. The results indicate that a fluid treatment of the Venus mass-loading problem is not entirely appropriate; a hybrid or kinetic model is necessary to incorporate the finite Larmor radius of the pickup particles which produces the observed asymmetry.

Phillips, J. L.↗

NASA NTRS1987

Magnetotails at unmagnetized bodies - Comparison of Comet Giacobini-Zinner and Venus

It is found that the near ionopause environs play a crucial role in the tail formation process at both Venus and G-Z and that draping at the two very different sized bodies occurs on ionopause scale sizes. On the other hand, ion densities, downtail mass fluxes, tailward J x B forces, and lobe betas are factors of about 10,000, 50, 100, and 20 times greater in the G-Z tail than in Venus', while bulk flow speeds and ion temperatures are factors of about 15 and 240 times lower. These large quantitative differences in the properties within the two magnetotails are attributable to the significantly greater upstream mass loading of the solar wind by the extended neutral atmosphere at G-Z (comets in general) compared to the gravitationally bound atmosphere of Venus.

Mccomas, D. J.↗

NASA NTRS1987

Characteristics of the Marslike limit of the Venus-solar wind interaction

The response of the Venus ionosphere to the solar wind when the latter exceeds the peak thermal pressure in the Venus ionosphere is characterized on the basis of observations from the Pioneer Venus Orbiter. At these times, the Venus ionosphere becomes increasingly magnetized until the total ionospheric pressure matches that of the solar wind. The ionopause becomes less distinct and asymptotically approaches a limiting altitude of about 220 km for extremely high pressures. When the ionopause is low, the hot regions of electrons which occurs within the ionopause gradient extends to lower altitudes. The ion temperatures show little change, and there is no discernible increase in plasma wave activity at the ionopause. The Venus observations at high dynamic pressure provide a framework for reassessing the available Mars observations. In particular, this study shows that solar wind standoff without sufficient ionospheric plasma pressure cannot by itself be used as evidence for a planetary magnetic field.

Luhmann, J. G.↗

NASA NTRS1987

Evidence of small-scale reconnection in the magnetosphere

Flux transfer events (FTE's) at the magnetopause, and brief, intense flows in the magnetotail plasma sheet are discussed. Nearly simultaneous observations of FTE's at the dayside magnetopause by the AMPTE-UKS and IRM spacecraft near the equator, and by ISEE-1 and 2 at the same local time but 5 R sub e to the south are shown. In the magnetotail, ISEE-1 and 2 data show that 2 to 10 min intense (200 to 1000 km/sec) flows occur at the neutral sheet. The flows appear to be consistent with short bursts of enhanced reconnection within the plasma sheet, and can transport the plasma between 10 and 30 R sub e earthward or tailward, i.e., the entire distance between the earth and ISEE apogee. The FTE-like structures are also associated with these events.

Elphic, R. C.↗

NASA NTRS1987

Upstream waves simultaneously observed by ISEE and UKS

Measurements obtained in the solar wind by ISEE-2 and the United Kingdom Subsatellite (UKS) have been examined for observations of upstream waves. These data reveal that the waves in the foreshock region are enhanced at all frequencies from at least 0.003 Hz to 0.5 Hz. The wave spectra generally have a spectral peak, but this peak is usually broad and the peak frequency depends on the position of the spacecraft. Generally, the spectra seen at the two spacecraft are most similar at high frequencies and least similar at low frequencies. The geometry of the interaction is displayed in the plane containing the magnetic field, the solar wind velocity, and the spacecraft location. However, this coordinate system does not order all the observed wave properties. It does not clearly explain or order the handedness of the waves, or their direction of propagation. It is clear that the upstream region is inherently three-dimensional. The position-dependent nature of the upstream waves indicates that comparisons between ground-based measurements and in-situ observations must be undertaken with some caution.

Russell, C. T.↗

NASA NTRS1987

Magnetic field draping in the Comet Halley coma - Comparison of Vega observations with computer simulations

During the Vega-1 encounter with Comet Halley, the magnetometer observed draping and compression of the interplanetary magnetic field. These are reproduced well by a three-dimensional MHD simulation of the cometary interaction. Rotations in the magnetic field similar to those at closest approach are also observed 2.75 hours earlier. It is suggested that both rotations correspond to the same IMF interval and that the spacecraft had overtaken the plasma and encountered 'older' magnetic field as it penetrated the coma. Analysis of the MHD model indicates that it should take about 3 to 5 hours for a solar wind parcel to pass from the unperturbed solar wind to Vega-1 at closest approach. A simulated magnetic field profile composed of nested sections for different IMF orientations closely resembles the observations. This result supports the hypothesis of layered magnetic orientations in the coma.

Schwingenschuh, K.↗

NASA NTRS1987

Mirror instability in the magnetosphere of Comet Halley

High resolution Vega 1 and 2 magnetic measurements in the cometary magnetosphere and magnetosheath of Halley reveal the presence of fluctuations with the signature expected for the mirror instability. This instability is a mechanism by which homogeneously produced cometary ions with a large perpendicular temperature anisotropy can be concentrated into discrete linear features. Thus, the mirror instability may provide a new mechanism for the generation of cometary rays. To our knowledge the presence of this instability in the cometary plasma was not predicted.

Russell, C. T.↗

NASA NTRS1987

Interplanetary magnetic field enhancements - Further evidence for an association with asteroid 2201 Oljato

In 1980 and 1983 the asteroid 2201 Oljato passed inside the orbit of Venus, respectively 65 and 21 days prior to the passage of Venus, apparently causing disturbances in the interplanetary magnetic field, possibly through the interaction of its debris trail with the solar wind. In July 1986 the asteroid passed through the inner solar system 25 days after Venus and it is expected to find similar disturbances if the debris trail extended in front of the asteroid. The expected disturbances were observed. All data from the longitude range of the disturbances have been examined for all Venus years as well as from a control period over the same solar longitudes but at different ecliptic longitudes. No enhancements in the rate of occurrence of events were seen in the control periods during 1980, 1983 and 1986, and the rate was uniformly low at all other times. The new data reaffirm the association of solar wind disturbances with the asteroidal passage and imply a debris trail that extends perhaps 1 AU both in front of and behind the asteroid.

Russell, C. T.↗