Engineering topics

Luhmann, J. G.

Publications and source records attributed to Luhmann, J. G..

At least 91 records · Page 5

NASA NTRS1987

Characteristics of cometary picked-up ions in a global model of Giacobini-Zinner

Energetic ions observed during the International Cometary Explorer (ICE) spacecraft flyby of comet Giacobini-Zinner provide information about both the constitution of comets and the plasma physical processes associated with their interaction with the solar wind. In this investigation the details of ion 'pickup,' in the limit where small-scale fluctuations in the plasma and magnetic field are neglected, are modeled by following the motion of a large number of initially cold, heavy (mass 18) ions in a global magnetohydrodynamic model of the local plasma and magnetic field. The results indicate how the background or macroscopic velocity and magnetic field structure of the comet can affect the average spatial and spectral characteristics of the observed cometary ions. These effects, which occur by virtue of forces associated with the compression and the curvature of the magnetic field in the presence of the stagnating plasma flow, can explain the double maxima in the time series of the energetic ion flux observed along the ICE trajectory.

Kimmel, C. D.↗

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

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

Solar wind mass-loading at Comet Halley - A lesson from Venus?

Recent observations at Comet Halley show that the region within which cometary ions become the dominant component lies outside of the magnetic field-free cavity. This behavior resembles that found at Venus under conditions where the incident solar wind dynamic pressure exceeds the ionospheric pressure. On these occasions the magnetosheath magnetic field is found well inside of the region where planetary ions are observed. Although scaling and the details of formation of the inner boundary of the magnetic field are different for these two objects, the processes by which the interplanetary magnetic field penetrates into the ionospheres at Venus and at Comet Halley are in many ways analogous.

Breus, T. K.↗

NASA NTRS1987

On the role of the quasi-parallel bow shock in ion pickup - A lesson from Venus?

Previous observations at Venus show convincing evidence of planetary O(+) ion pickup by the largescale motional -V x B electric field in the magnetosheath when the interplanetary magnetic field is perpendicular to the solar wind flow. However, the presence of magnetic field fluctuations in the magnetosheath downstream from the quasi-parallel bow shock should allow pickup to occur even when the upstream magnetic field B and plasma velocity V are practically coaligned. Single-particle calculations are used to demonstrate the convecting magnetic field fluctuations similar to those observed in the Venus magnetosheath when the subsolar bow shock is quasi-parallel can efficiently accelerate cold planetary ions by means of the electric field associated with their transverse components. This ion pickup process, which is characterized by a spatial dependence determined by the bow shock shape and the orientation of the upstream magnetic field, is likely also to occur at Mars and may be effective at comets.

Luhmann, J. G.↗

NASA NTRS1987

Studies of the configuration of the Venus ionospheric magnetic field

The dayside ionospheric magnetic field of Venus has been modeled from two different points of view. The Cloutier et al. electrodynamic model makes specific predictions about the behavior of the global magnetic field configuration that can be compared with those expected from the alternate diffusion/convection model. Although the diffusion/convection model is currently only one-dimensional, it is found that it is consistent with the observations in several areas where the three-dimensional electrodynamic model is not.

Luhmann, J. G.↗

NASA NTRS1987

Waves on the subsolar ionopause of Venus

The subsolar ionopause of Venus is expected to be stable to both the Kelvin-Helmholtz and flute instabilities. However, magnetic profiles obtained in the subsolar region indicate that the surface of the ionopause contains large amplitude corrugations, perhaps incipient flux ropes. A possible mechanism for destabilizing the boundary is suggested by the observation that the ion density does not drop abruptly at the ionopause but continues to decrease smoothly into the magnetosheath.

Russell, C. T.↗

NASA NTRS1986

The Venus ultraviolet aurora - Observations at 130.4 nm

Continuous and highly variable emissions at 130.4 nm are observed on the night-side of Venus by the Pioneer Venus Orbiter ultraviolet spectrometer. The emissions have a typical intensity of 10-20 R, occasionally exceed 100 R, and have a dawn-dusk symmetry in brightness during periods of unusual intensity. Possible energy source for this 'aurora' is the precipitation of suprathermal electrons. Field and particle observations suggest an additional source for the brightest emissions which is associated with solar flares and/or interplanetary shocks.

Phillips, J. L.↗

NASA NTRS1986

Interplanetary field control of the location of the Venus bow shock - Evidence for comet-like ion pickup

The location of the Venus bow shock is found to be sensitive to both the angle of the IMF to the solar wind flow and the phase of the solar cycle. The former dependence is attributed to the effect of planetary ion pickup by the magnetosheath convection electric field analogous to cometary ion pickup by the solar wind electric field. The latter dependence is attributed to the solar-cycle variation in the density of exospheric neutrals in the magnetosheath analogous to the cometary response to distance from the sun. Both a north-south and a pole-equator asymmetry in the location of the bow shock are also found, controlled by the direction of the IMF in the plane perpendicular to the solar wind flow.

Alexander, C. J.↗

NASA NTRS1986

Magnetic configuration of the Venus magnetosheath

A data set consisting of nearly six Venus years of Pioneer Venus Orbiter magnetometer data is analyzed in order to describe the configuration of the magnetosheath. A coordinate system is developed which isolates the effects of the solar wind flow and the interplanetary magnetic field (IMF). The data indicate that the magnetosheath field magnitude is responsive to solar wind dynamic pressure and that the compression of the upstream field is controlled by magnetosonic Mach number. The draping of the field is similar to that predicted by gasdynamic modeling; specific draping features include distinct dependence on the radial and the transverse components of the IMF, and a tendency of the field to encircle the planet at low altitude. Certain features of the processes of mass loading by magnetosheath fluctuations and by the motional electric field are examined. Magnetic fluctuations can dominate the magnetosheath field for periods of low interplanetary cone angle and in general for the magnetosheath regions near the quasi-parallel part of the bow shock. The magnetosheath magnetic field magnitude displays a hemispherical asymmetry; the sense of this asymmetry is controlled by the cross-flow component of the IMF in a manner which indicates that ion pickup by the convective electric field occurs preferentially in one hemisphere. The results of this survey indicate that mass loading is a significant process which should be incorporated into computational models but that a fluid treatment of the planetary ion component may not be appropriate.

Phillips, J. L.↗

NASA NTRS1986

A gas dynamic magnetosheath field model for unsteady interplanetary fields - Application to the solar wind interaction with Venus

The steady state gas dynamic model of magnetosheath magnetic fields previously developed by Spreiter and Stahara (1980) is generalized for the common situation of a temporally varying interplanetary field orientation. Examples for the particular case of Venus in the solar wind illustrate the application of the model to the passage of rotational discontinuities and MHD waves through planetary magnetosheaths. The results of this model illustrate how the field structure near a planetary magnetopause or ionopause can be affected by interplanetary field variations rather than by local processes because of the 'pile up' of magnetosheath fields from a sequence of upstream fields. Changes in the spectrum of interplanetary waves on their transmission to the magnetopause are also indicated.

Luhmann, J. G.↗

NASA NTRS1986

Spatial distributions of magnetic field fluctuations in the dayside magnetosheath

In a study that tests the hypothesis that magnetosheath magnetic fields are disturbed on plasma streamlines which are connected to the quasi-parallel bow shock, magnetometer observations from the ISEE 2 and IMP 8 spacecraft are used to investigate the dayside spatial distributions of fluctuating magnetosheath fields for different interplanetary field orientations. The results suggest that although other sources such as Kelvin-Helmholtz instabilities and flux transfer processes probably contribute to the fluctuations in the magnetosheath field, the quasi-parallel shock source is an important contributor in the dayside region.

Luhmann, J. G.↗

NASA NTRS1986

Simultaneous observation of upstream waves with ISEE and AMPTE

Measurements obtained by ISEE-2 and the UKS spacecraft upstream of the earth's bow shock are examined. Simultaneous observations show that upstream waves are excited over a broad frequency range. Even when peaked spectra occur the peaks can occur at different frequencies in different regions of space. Generally the spectra seen at the two locations are most similar at high frequencies and least similar at low frequencies. 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 NTRS1986

Interplanetary magnetic field control of the Venus magnetosheath field and bow shock location

Six Venus years of Pioneer Venus Orbiter (PVO) data are analyzed in a coordinate system which isolates magnetic effects. The field draping pattern features two lobes easily identifiable even in the near planet region. The magnetosheath magnetic magnitude has a hemispherical asymmetry controlled by the IMF orientation in a manner suggesting preferrential ion pickup in one hemisphere. This result complements recent findings that the bow shock position is responsive to IMF direction. Examination of the ionopause position to evaluate ionospheric effects on the overall asymmetry of thfe Venus-solar wind interaction produces negative but not conclusive results.

Phillips, J. L.↗

NASA NTRS1986

Interplanetary flux enhancements - Comparison with cometary models and observations

Interplanetary field enhancements (IFE's) are unusual nearly symmetric increases in the strength of the interplanetary magnetic field lasting tens of minutes to hours. Examples of interplanetary field enhancements are compared with MHD models and with the data obtained by the ICE spacecraft at Giacobini-Zinner. These comparisons suggest that the varying properties of IFE's are due to the fact that some events are due to passages in front of the nucleus, others in the near tail and yet others in the distant tail.

Russell, C. T.↗

NASA NTRS1986

The solar wind interaction with Venus

The relation between Venus and the solar wind is analyzed. The effects of the intrinsic field, neutral atmosphere, and ionosphere of Venus on the solar wind are examined. The solar wind interaction phenomena is studied; consideration is given to the free-stream solar wind, bow shock, magnetosheath, boundary layer, ionospheric features, neutral atmosphere features, and wake and magnetotail. Further research on the boundary layer and tail formation, global models, and high-dynamic pressure interaction is proposed.

Luhmann, J. G.↗

NASA NTRS1985

The Pioneer Venus Orbiter event of February 11, 1982 - Of cometary of solar origin?

On February 11, 1982, an unusual cusp-shaped temporal variation in the interplanetary magnetic field was detected by the Pioneer Venus orbiter. While variations of the helium content of the solar wind were detected on the preceding day, these changes had no obvious relationship to the occurrence of the cusp-shaped temporal variation in the IMF on February 11. In fact the solar wind the content was quite nominal on February 11. The magnetic variations were also quite unlike those previously reported to be magnetic clouds. Moreover, the scale size of the disturbance had to be smaller than 4 x 10 to the 6th km and thus of cometary dimensions rather than of dimensions usually found in solar initiated events. Thus, there seems to be no need to alter the original interpretation that the observations of Pioneer Venus on February 11, 1982 were consistent with the passage of a comet close to Venus.

Russell, C. T.↗

NASA NTRS1985

On the dynamo generation of flux ropes in the Venus ionosphere

Small scale magnetic field structures or 'flux ropes' observed in the ionosphere of Venus can be interpreted as the result of a kinematic dynamo process acting on weak seed fields. The seed fields result from the prevailing downward convection of magnetic flux from the vicinity of the ionopause, while small scale fluctuations in the velocity of the ionospheric plasma, which can be caused by collisional coupling to gravity waves in the neutral atmosphere, provide the mechanism by which the field is twisted and redistributed into features of similar scale. This mechanism naturally explains some of the average properties of flux ropes such as the variation of their characteristics with altitude and solar zenith angle. It also elucidates the relationship between the large scale and small scale ionospheric magnetic fields.

Luhmann, J. G.↗