Engineering topics

Russell, C. T.

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

At least 235 records · Page 13

NASA NTRS1992

Slow mode transition in the frontside magnetosheath

Three magnetosheath passes with density enhancements in front of the magnetopause are studied with data from ISEE 1, 2, and 3. The density structure appears to be locally generated and slow mode in nature. In one pass when ISEE 1 and 2 were well separated, the motion of the density structure can be determined. The density structure appears to stand in the magnetosheath flow. Thus, it propagates upstream in the rest frame of the flow. The flow in and near the density structure appears to be closer to isothermal than adiabatic. The flow velocity decreases from super-slow to being close to the intermediate and slow mode velocities at the outer edge of the density structure. This study provides additional evidence that the density structure in front of the magnetopause is a slow mode transition in which the flow velocity decreases to the MHD slow mode velocity. The slow mode transition may consist of two wave fronts and a region with strong slow mode waves. This slow mode transition may play an important role in establishing the flow and field pattern near the magnetopause.

Song, P.↗

NASA NTRS1992

Limitations of spectral analysis of the Phobos magnetometer data in the search for an intrinsic Martian magnetic field

Both observed and synthetic time series of the magnetic field obtained in circular orbit around Mars by the Phobos spacecraft are analyzed. Of the three reported spectral peaks at 8, 12 and 24 h only the 24-h peak could be due to intrinsic sources. However, 24-h spectral peaks can also be produced in synthetic time series with no intrinsic field effects included. Hence, it is concluded that present spectral analyses of time series obtained with the Phobos magnetometers provide no constraints on the size or the existence of an intrinsic magnetic field at Mars.

Russell, C. T.↗

NASA NTRS1992

Unusually distant bow shock encounters at Venus

The location of the Venus bow shock on two days (July 6, 1981 and August 4, 1971) when the IMF was strong and the Mach number low is examined. When the solar wind Mach number approaches unity the bow shock appears to be greatly detached from the obstacle. Thus inferences from observations of shocks near planets should be treated cautiously, especially if the Mach number is small or not well known. The case of the Phobos mission to Mars is examined as an example of such a situation.

Russell, C. T.↗

NASA NTRS1992

Observations of a new class of upstream waves with periods near 3 seconds

A new class of ULF waves with periods near 3 s in the earth's upstream region is found by examining the high time resolution magnetic field data from the ISEE spacecraft. These waves are observed in the part of the upstream region which is magnetically connected to the bow shock, but only when the solar wind plasma beta is high (greater than 1). The waves are always right-handed, nearly circularly polarized in the spacecraft frame. The directions of the wave vectors are in the general direction of the average magnetic field, and the waves are convected downstream in the spacecraft frame. This study of these waves has shown that they appear to be intrinsically left-handed ion cyclotron waves in the plasma rest frame.

Le, G.↗

NASA NTRS1992

Electromagnetic ion cyclotron waves observed near the oxygen cyclotron frequency by ISEE 1 and 2

The first results of observations of ion cyclotron waves by the elliptically orbiting ISEE 1 and 2 pair of spacecraft are reported. The most intense waves (8 nT) were observed in the outer plasmasphere where convection drift velocities were largest and the Alfven velocity was a minimum. Wave polarization is predominantly left-handed with propagation almost parallel to the ambient magnetic field, and the spectral slot and polarization reversal predicted by cold plasma propagation theory are identified in the wave data. Computations of the experimental wave spectra during the passage through the plasmapause show that the spectral slots relate to the local plasma parameters, possibly suggesting an ion cyclotron wave growth source near the spacecraft. A regular wave packet structure seen over the first 30 min of the event is attributed to the modulation of this energy source by the Pc 5 waves seen at the same time.

Fraser, B. J.↗

NASA NTRS1992

Radial expansion of the tail current disruption during substorms - A new approach to the substorm onset region

A new method is used to examine the radial expansion of the tail current disruption and the substorm onset region. The expansion of the disruption region is specified by examining the time sequence (phase relationship) between the north-south component and the sun-earth component. This method is tested by applying it to the March 6, 1979, event. The phase relationship indicates that the current disruption started on the earthward side of the spacecraft, and expanded tailward past the spacecraft. The method was used for 13 events selected from the ISEE magnetometer data. The results indicate that the current disruption usually starts in the near-earth magnetotail and often within 15 RE from the earth.

Ohtani, S.↗

NASA NTRS1992

Model of the formation of the low-latitude boundary layer for strongly northward interplanetary magnetic field

A model for the formation of the low-boundary layer in which a magnetosheath flux tube reconnects in the north and south beyond the cusp when the IMF is strongly northward is presented. For northward IMF the geomagnetic field captures solar wind flux tubes through intermittent reconnection at the cusp region. These newly captured flux tubes shorten and sink into the magnetosphere while the flux tubes reorient themselves as they become assimilated into the magnetosphere. There is no significant acceleration for the particles within the flux tubes. In the magnetosphere the interchange instability disperses the flux tube azimuthally along the magnetopause to form a boundary layer. Subsequent reconnection forms sublayers of the boundary layer and different sublayers represent different ages after reconnection. The interchange instability is stable radially, which keeps sharp boundaries between sublayers and between the boundary layer and magnetosphere.

Song, P.↗

NASA NTRS1992

Discovery concepts for Mars

Two focused Mars missions that would fit within the guidelines for the proposed Discovery line are discussed. The first mission would deal with the issue of the escape of the atmosphere (Mars') to space. A complete understanding of this topic is crucial to deciphering the evolution of the atmosphere, climate change, and volatile inventories. The second mission concerns the investigation of remanent magnetization of the crust and its relationship to the ionosphere and the atmosphere.

Luhmann, J. G.↗

NASA NTRS1992

The Pioneer Venus mission

The Pioneer Venus Orbiter and probe missions have provided a wealth of in situ and remote sensing data. The probe mission sounded the clouds and lower atmosphere at four separate locations returning chemical, physical, and meteorological data on the Venus atmosphere. The orbiter observed the surface of Venus with a radar altimeter and crude radar imager. It sounded the atmosphere in the infrared and monitored the ultraviolet emissions. It also provided in situ data on the neutral and ionized environment of the planet and the magnetic fields and plasma waves in these regions. The results of the over-12-years of orbiter observations of Venus, especially those of relevance to the interpretation of data on the upper atmosphere, ionosphere and solar wind interaction with Mars are stressed.

Russell, C. T.↗

NASA NTRS1992

The intrinsic magnetic field and solar-wind interaction of Mars

The Venus-like interaction between the solar wind and the atmosphere of Mars is examined. The bow shock and magnetosheath of Mars indicate the presence of an obstacle to the solar wind that is somewhat larger than the size of the planet and its observed ionosphere, and also relatively larger than the Venus obstacle under comparable conditions. The intrinsic magnetic field of Mars must be no greater than 1.5 x 10 exp 12 T/cu m, or about 0.0001 times as strong as that of the earth to produce an obstacle of such small size. At least for solar minimum conditions, like those prevailing at the time of the Viking Landers, the ionospheric plasma (thermal) pressure is insufficient to balance the incident solar-wind pressure by itself. The ion and electron temperatures in the Martian ionosphere indicate the presence of local horizontal magnetic fields and heat sources in excess of solar radiation alone.

Luhmann, J. G.↗

NASA NTRS1991

Occurrence characteristics of VLF bursts in the nightside ionosphere of Venus

Many of the impulsive VLF signals observed by the Pioneer Venus orbiter electric field detector (OEFD) in the nightside ionosphere have been interpreted as arising from lightning in the Venus atmosphere. In order to determine the characteristics of the source, and to compare with terrestrial lightning, the normalized occurrence rate for individual bursts at Venus are determined, as opposed to previous studies of activity in 30-s intervals. Burst identification criteria are determined which take into account the decay constant of the instrument to calculate the burst rate. Under the assumption that all OEFD observations in the nightside ionosphere are due to impulsive signals, it is found that the 100-Hz channel has the highest burst rate, about 0.20/s in the altitude range 150-180 km and in the postmidnight local time sector. The burst occurrence rates for all four frequency channels decrease with increasing altitude. Burst rates at frequencies above 100 Hz are greater in the premidnight hours while 100-Hz signals peak after midnight; however, these burst rates vary from orbit to orbit. The 100-Hz low-frequency burst rate has a stronger dependence on the magnetic field strength than the higher-frequency rates. Most 100-Hz bursts are closely spaced and occur independently of signals at higher frequencies. These dependences suggest that the low- and high-frequency signals have different propagation mechanisms. The properties of the bursts are generally consistent with a lightning source, and the planet-wide burst rate at Venus may be comparable to or larger than the terrestrial lightning rate.

Ho, C.-M.↗

NASA NTRS1991

Venus ionospheric tail rays - Spatial distributions and interplanetary magnetic field control

The overall properties of Venus ionospheric tail rays (such as density, spatial extent, and distribution) and their relationship to the draped magnetic field configuration behind the planet were investigated using measurements obtained by the Pioneer Venus Orbiter Langmuir probe, a magnetometer, and a plasma-wave detector. The results suggest that tail rays are a normal feature of the steady solar wind interaction with Venus and are not generally associated with a central tail plasma sheet. The statistics of the tail rays occurrence point toward the existence of a distributed terminator ionosphere source, consistent with findings of Brace et al. (1990).

Ong, M.↗

NASA NTRS1991

Picked-up protons near Mars - Phobos observations

The measurements carried out by the plasma spectrometer, Aspera, on board the Phobos-2 spacecraft show that protons, originating in the extended hydrogen corona of Mars, were observed at altitudes not greater than 7500 km. The cyclotron instability of these pickup ions appears to generate Alfven waves observed by the Magma magnetometer. Analysis of the plasma data shows that weak pitch-angle diffusion of a ring-distribution of pickup protons occurs. The altitude profiles of pickup proton fluxes and number densities of the parent hydrogen atoms are derived.

Barabash, S.↗

NASA NTRS1991

Magnetic field studies of the solar wind interaction with Venus from the Galileo flyby

During the February 10, 1990 flyby of Venus, the Galileo spacecraft skimmed the downnstream flank of the planetary bow shock. This provided an opportunity to examine both the global and the local structure of the shock in an interval during which conditions in the solar wind plasma were quite steady. The data show that the cross section of the shock in planes transverse to the flow is smaller in directions aligned with the projection of the interplanetary magnetic field than in directions not so aligned. Ultralow-frequency waves were present in the unshocked solar wind, and their amplitude peaked when the spacecraft was downstream of the foreshock. At large distances down the tail, the Mach number of the flow normal to the shock is low, thus providing the opportunity to study repeated crossings of the collisionless shock in an interesting parameter regime. Some of the shock crossings reveal structure that comes close to the theoretically predicted form of intermediate shocks, whose existence in collisionless plasmas has not been confirmed.

Kivelson, M. G.↗

NASA NTRS1991

PC 3,4 magnetic pulsations observed simultaneously in the magnetosphere and at multiple ground stations

Periods of magnetic conjugacy between ISEE and the magnetometer array of the Institute for Geological Sciences have been examined to search for the simultaneous occurrence of Pc 3,4 magnetic pulsations. When compressional waves are seen in space, waves are also observed on the ground at the same frequency and with similar waveforms. The wave amplitude on the ground at midlatitudes is similar to that in space at ISEE but at high latitudes the amplitudes are larger than in space. The one occurrence of a purely transverse signal at ISEE was not observed on the ground. These results confirm that Pc 3,4 wave energy is most readily transported through the magnetosphere by compressional fluctuations.

Odera, T. J.↗

NASA NTRS1991

Observations of reconnection of interplanetary and lobe magnetic field lines at the high-latitude magnetopause

Results are presented of ISEE 2 observations of plasma accelerations obtained at the high-latitude (lobe) magnetopause at a time when the local magnetosheath and magnetospheric magnetic fields were nearly oppositely directed and the flow speed in the magnetosheath, V(s), was nearly equal to the local Alfven speed, V(A). The observations provide direct evidence for the rereconnection of the open field lines of the tail lobes with the IMF, when the magnetic field shear is large. It is pointed out, however, that, since V(s) was almost equal to V(A), it is unlikely that the rereconnection is associated with the strong sunward convection in the polar cap.

Gosling, J. T.↗

NASA NTRS1991

Venus ionospheric 'clouds' - Relationship to the magnetosheath field geometry

The magnetic field control of Venus plasma 'clouds' relative to the planet has been analyzed by rotating the observed locations of the clouds into a coordinate system in which all of the transverse upstream magnetic fields were aligned. The results indicate that clouds are scattered around the periphery of the planet in the terminator plane. There is no evidence of a concentration of clouds where the magnetosheath magnetic field is most strongly 'draped'. On the other hand, statistics show that the change in the orientation of the transverse upstream magnetic field between the inbound and outbound bow shock crossings, for the orbits where clouds are seen is about 30 deg greater than the average upstream change at Venus over the same time intervals.

Ong, M.↗

NASA NTRS1991

The magnetic barrier at Venus

Altitude profiles of the Venus magnetic barrier are derived here from a statistical analysis of the Pioneer Venus Orbiter magnetometer data. The outer boundary of the magnetic barrier is then compared with the obstacle expected from gasdynamic models of the bow shock, and the stagnation pressure is compared with that expected from gasdynamic theory. The magnetic barrier is strongest at the subsolar point and weakens as expected with increasing solar zenith angle. The existence of a north-south asymmetry in the barrier strength is also demonstrated. The magnetic barrier is about 200 km thick at the subsolar point and 800 km thick at the terminator. The magnetic barrier transfers most of the solar wind dynamic pressure to the ionosphere via the enhanced magnetic pressure. The convected field gasdynamic model predicts the correct bow shock location if the magnetic barrier is treated as the obstacle.

Zhang, T. L.↗