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Luhmann, J. G.

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

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

A model of the ion wake of Mars

Observations from the Phobos spacecraft suggest that, for the most part, the Martian magnetotail is induced instead of intrinsic. Test particle techniques are used to determine how the associated ion wake might appear to detectors behind Mars if it is produced primarily by planetary ions picked up by the solar wind in the dayside exosphere. The results suggest that low energy pickup ions populate the inner wake, while a more energetic component is present in the outer magnetosheath and adjacent solar wind.

Luhmann, J. G.↗
NASA NTRS1990

Upstream waves at Mars - Phobos observations

The region upstream from the Mars subsolar bow shock is surveyed for the presence of MHD wave phenomena using the high temporal resolution data from the Magma magnetometer. Strong turbulence is observed when the magnetic field is connected to the Mars bow shock in such a way as to allow diffuse ions to reach the spacecraft. Also weak waves are observed at the proton gyro frequency. These waves are left-hand elliptically polarized and may be associated with the pick-up of protons from the Mars hydrogen exosphere.

Russell, C. T.↗
NASA NTRS1990

A model of the energetic ion environment of Mars

Because Mars has a weak intrinsic magnetic field and a substantial atmosphere, instruments on orbiting spacecraft should detect a population of energetic heavy planetary ions which result from comet-like ion pickup in the solar wind and magnetosheath convection electric fields, in addition to those that might result from processes internal to a Martian 'magnetosphere.' Although this ion exosphere has been previously discussed in the literature, detailed predictions that might be directly applied to the interpretation of data are not available. Here a test particle model is used to construct a global picture of Martian pickup ions in the Mars environment. The model makes use of the recent Nagy and Cravens (1988) model of the Martian exosphere and Spreiter and Stahara's (1980) gas dynamic model of the magnetosheath. The pickup of ions originating at Phobos is also considered. Notable properties of the resulting ion distributions include their near-monoenergetic spectra, pancake pitch angle distributions, and large gyroradii compared to the planetary scale.

Luhmann, J. G.↗
NASA NTRS1990

Solar cycle variations in the neutral exosphere inferred from the location of the Venus bow shock

Solar UV and EUV varies significantly during the solar cycle. Pioneer Venus can measure this variation both directly and indirectly. A direct measure of the EUV is obtained from the photoelectron current of the Langmuir probe when the spacecraft is in the solar wind. The indirect measure is by monitoring the location of the Venus bow shock. The UV and EUV both heat the upper atmosphere and ionize it. When solar activity is high, the upper atmosphere should be ionized more rapidly. This effect adds a greater number of planetary ions to the magnetosheath plasma as it flows by Venus. It is this increase in mass flow that causes the Venus bow shock to move away from its solar minimum location. Pioneer Venus has now monitored the location of the bow shock for an entire solar cycle. The bow shock location is well correlated with the variation in EUV flux as measured by the Langmuir probe. The bow shock is farther from Venus than expected from the sunspot number or 10.7 cm solar radio flux, indicating that solar UV radiation may be even stronger at the present time than would be predicted from the relationships determined during the previous solar cycle.

Russell, C. T.↗
NASA NTRS1990

Remote sensing of Mars' ionosphere and solar wind interaction - Lessons from Venus

Although the Phobos spacecraft will make some limited in situ measurements of Mars' upper ionosphere during its transfer orbits, the major part of the mission will be limited to remote sensing by radio occultation and topside sounding methods. To 'calibrate' the former as a means of studying the ionosphere and solar wind interaction, the Pioneer Venus Orbiter radio occultation experiment results are examined in light of what is known from in situ plasma and magnetic field data. This calibration can be used to reassess the data from previous Mars missions and to provide a basis for interpreting data from the upcoming Phobos mission.

Luhmann, J. G.↗
NASA NTRS1990

An assessment of the conditions for critical velocity ionization at the weakly magnetized planets

It has been proposed that critical velocity ionization (CVI) may occur in the magnetosheaths of weakly magnetized planets where the solar wind flows through the planets' upper atmospheres. One can examine this possibility for Venus and Mars by using magnetosheath flow and exosphere models to determine whether the criteria for CVI are met. These criteria include Brenning's empirical condition on the cross-field flow velocity (it must not exceed the local magnetosonic velocity) and the 'Townsend condition' requiring that the integrated probability of impact ionization by a hot electron be greater than 1 along a streamline past the planet. Application of the Spreiter-Stahara gas dynamic flow model and the Nagy and coworkers' models for the exospheres lead to the conclusion that the conditions can be met in a limited region near Venus. However, evidence for CVI has not been identified in in-situ data. Since key details of the planet-solar wind interaction can be explained in terms of photoionization alone, other signatures of the process must be sought in the observations if the presence of CVI is to be positively inferred.

Luhmann, J. G.↗
NASA NTRS1990

The solar wind interaction with unmagnetized planets - A tutorial

The interaction of the solar wind with the Venus ionosphere induces currents which can substantially exclude the solar wind and IMF from the dayside ionosphere beneath the 'ionopause', where ionosphere thermal pressure equals incident solar wind dynamic pressure. The field then diffuses through the ionopause with increasing speed at decreasing altitudes, and is weakest in the subpolar region. Once within the ionopause, the magnetic field is redistributed by ionospheric convection, and then decays at low altitudes via collisional dissipation of the associated currents. The maximum ionospheric field magnitudes observed, of about 150 nT, furnish magnetic pressures exceeding the ionospheric thermal pressure by a factor of about 3.

Luhmann, J. G.↗
NASA NTRS1990

'Wave' analysis of Venus ionospheric flux ropes

The behavior of the general field fluctuations observed by the Pioneer Venus Orbiter magnetometer in the Venusian ionosphere is approached via the application of wave-analysis techniques to their compressional amplitude, ellipticity, and polarization. The most marked change in these properties occurs within about 15 deg of the terminator; ellipticity and compressional power plummet, implying that flux ropes either drastically change in character there or are confined to the dayside hemisphere, within about 75 percent of the subsolar point. Attention is given to a heuristic model which attempts to unify dayside flux-rope and 'terminator wave' interpretations.

Luhmann, J. G.↗
NASA NTRS1989

Electron heat flux dropouts in the solar wind - Evidence for interplanetary magnetic field reconnection?

An examination of ISEE-3 data from 1978 reveal 25 electron heat flux dropout events ranging in duration from 20 min to over 11 hours. The heat flux dropouts are found to occur in association with high plasma densities, low plasma velocities, low ion and electron temperatures, and low magnetic field magnitudes. It is suggested that the heat flux dropout intervals may indicate that the spacecraft is sampling plasma regimes which are magnetically disconnected from the sun and instead are connected to the outer heliosphere at both ends.

Mccomas, D. J.↗
NASA NTRS1989

Small scale irregularities in Comet Halley's plasma mantle - An attempt at self-consistent analysis of plasma and magnetic field data

VEGA-1 measurements of the plasma density and magnetic field in the coma of Comet Halley show characteristic signatures over a significant portion of the outbound pass. It is found that the assumption that there is a balance between the thermal and magnetic pressures in these features can be used to obtain estimates of the plasma temperature as a function of distance from the nucleus. These estimates indicate that the ions cool from about 1.5 x 10 to the 6th K at 10 to the 5th km to 2 x 10 to the 5th K at 5 x 10 to the 4th km. The technique used here represents a novel approach whereby temperature measurements can be made in situations where only plasma density and magnetic field data are available.

Vaisberg, O. L.↗
NASA NTRS1989

The visual appearance of comets under varying solar wind conditions

Three-dimensional MHD simulations have been performed for four different sets of solar wind conditions and cometary outgassing rates appropriate to the Halley encounters. Even though the simulations are single fluid calculations, it is possible to separate the solar wind and cometary ions using the divergenceless nature of the solar wind ions. The cometary ion density is then integrated along the line-of-sight from the observer through the comet to determine how the comet would look to a distant observer under these different conditions. In general, comet tails appear longer when the interplanetary magnetic field lies in the plane of the sky rather than along the line-of-sight. Also, the tail shrinks as the speed of the solar wind increases and/or the mass loading rate decreases.

Russell, C. T.↗
NASA NTRS1989

A search for the solar roots of the most disturbed interplanetary field intervals of solar cycle 21

During the course of the Pioneer Venus Orbiter mission, fairly continuous interplanetary plasma and magnetic field data were obtained which span the interval from prior to the last solar maximum to the current solar minimum recovery. Within this nearly complete solar cycle interval, several periods of exceptional disturbance of the interplanetary field stand out. The available solar data have been examined to determine what features, if any, distinguish these periods. Neither flare nor coronal mass ejection reports show particularly unusual behavior. However, these periods appear to occur in conjunction with marked changes in the interplanetary sector structure. This suggests that heliospheric current sheet reconfiguration is an indicator of the level of interplanetary disturbance distinct from the more traditional solar activity data.

Luhmann, J. G.↗
NASA NTRS1988

Solar and interplanetary control of the location of the Venus bow shock

The Venus bow shock location has been measured at nearly 2000 shock crossings, and its dependence on solar EUV, solar wind conditions, and the interplanetary magnetic field determined. The shock position at the terminator varies from about 2.14 Venus radii at solar minimum to 2.40 Venus radii at solar maximum. The location of the shock varies little with solar wind dynamic pressure but strongly with solar wind Mach number. The shock is farthest from Venus on the side of the planet in which newly created ions gyrate away from the ionosphere. When the interplanetary magnetic field is perpendicular to the flow, the cross section of the shock is quite elliptical. This effect appears to be due to the anisotropic propagation of the fast magnetosonic wave. When the interplanetary magnetic field is aligned with the flow, the bow shock cross section is circular and only weakly sensitive to changing EUV flux.

Russell, C. T.↗
NASA NTRS1988

A three-dimensional diffusion/convection model of the large scale magnetic field in the Venus ionosphere

A three-dimensinal diffusion/convection model of the dayside Venus ionosphere magnetic field was developed on the basis of previously published one-dimensional diffusion/convection models, and assuming that the field and flow at the upper boundary (in the magnetic barrier) as well as the ionospheric plasma velocity are known. The results indicate that the low-altitude magnetosheath field draping may be distorted by the interaction with the ionosphere in such a manner that there is an apparent 'focusing' of the field toward the subsolar point, caused by the shear in the horizontal velocity between the magnetosheath and ionospheric flows. A comparison of published magnetic-field observations with the present results indicates that the simple nesting of external and internal velocity fields may be a good approximation to global plasma flows near Venus under normal conditions.

Luhmann, J. G.↗
NASA NTRS1988

Asymmetries in the location of the Venus ionopause

Four large data sets of Venus ionopause observations, based on different definitions and observations by Pioneer Venus Orbiter field and particle instruments, are compared. Regardless of the definition chosen, the ionopause is observed to be higher on average at dawn than at dusk. The results of an empirical ionosphere model and a data analysis technique which minimizes orbital bias are combined to show that most or all of this asymmetry results from the effects of orbital bias and uneven sampling of solar wind conditions. However, an additional asymmetry is found in which the ionopause is displaced in the direction opposite to the motional electric field of the solar wind. This effect is attributed to asymmetric pickup of exospheric ions by the electric and magnetic fields in the magnetosheath. This ion pickup interpretation can be understood in terms of a previous observed magnetosheath magnetic field asymmetry. Quantitative analysis indicates that the transverse momentum imparted by the solar plasma to pickup particles in their initial gyration is transmitted by the magnetosheath magnetic field to the ionosphere boundary.

Phillips, J. L.↗
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.↗