John R. Spreiter

Solar wind flow past nonmagnetic planets - Venus and Mars

Abstract The hydromagnetic theory of solar wind flow past the Earth has been extended and modified so as to be applicable to nonmagnetic planets, such as Venus and Mars, that have a sufficient ionosphere to deflect the solar plasma around the planet and its atmosphere. The principal difference in the analysis stems from the fact that the current sheath that bounds the solar wind away from the planet is formed by interaction with the ionosphere rather than with the geomagnetic field as in the case of the Earth. After stating the principal assumptions and equations, the theory is applied to determine the shape of the ionosphere boundary, or ionopause, across which the Newtonian approximation for the pressure of the flowing plasma is balanced by the pressure of a stationary ionosphere. Specific numerical results are given for a wide range of ionospheric parameters representative of those proposed for Venus and Mars. The location of the bow wave and the properties of the flow field are then calculated using magnetohydrodynamic and gasdynamic considerations in a manner similar to that employed for the flow of the solar wind plasma past the Earths magnetosphere. Examination of the results reveals a correspondance rule that enables results presently available for the location of the bow wave and the properties of the flow about the Earths magnetosphere to be converted rapidly into those for a nonmagnetic planet by a simple relabeling of the scales. The results are shown to be in general accordance with observations made by Mariner 5 as it flew past Venus, although certain differences near the theoretical location of the ionopause suggest the presence of a thick boundary layer. A similar analysis of the data from Mariners 4,6 and 7 indicates that Mars has a sufficient ionosphere for the theory to be applicable. Further comparison is relatively uninformative beyond revealing no inconsistencies with the theoretical results, however, because Mariner 4 did not approach close enough, and Mariners 6 and 7 were not equipped to make the necessary measurements.

Oxygen ionization rates at Mars and Venus: Relative contributions of impact ionization and charge exchange

Oxygen ion production rates above the ionopauses of Venus and Mars are calculated for photoionization, charge exchange, and solar wind electron impact ionization processes. The latter two require the use of the Spreiter and Stahara (1980) gas dynamic model to estimate magnetosheath velocities, densities, and temperatures. The results indicate that impact ionization is the dominant mechanism for the production of O+ ions at both Venus and Mars. This finding might explain both the high ion escape rates measured by Phobos 2 and the greater mass loading rate inferred for Venus from the bow shock positions.

Aligned magnetohydrodynamic solution for solar wind flow past the earth's magnetosphere

Abstract Exact numerical solutions of the magnetohydrodynamic equations for a perfect dissipation-less gas with aligned magnetic field are given for conditions representative of steady supersonic solar wind flow past an axisymmetric model of the earths magnetosphere. The solution is based on use of a transformation that relates without approximation the equations of magnetohydrodynamics to those of gasdynamics of a pseudo gas that has an unusual equation of state. The results confirm the applicability of the previously existing gasdynamic solutions for the typically modest intensities of the interplanetary magnetic field that lead to Alfven Mach numbers of about 10 or greater. For smaller values, however, significant effects are indicated with the flanks of the bow wave moving away from the earth and the nose moving toward the earth. The results are consistent with direct observations in space.

On the processes in the terrestrial magnetosheath 1. Scheme development

We propose a new method to study the structure of the magnetosheath and thereby determine the underlying processes that create this structure. This method provides a systematic means of separating perturbations due to the solar wind variations from those generated within the magnetosheath. As a result, we are able to study the magnetosheath processes as well as the dynamic solar wind-magnetopause interaction. We use the solar wind measurements from an upstream monitor as the input to the gasdynamic convected field model and then compare the model output with the in situ magnetosheath observations. We introduce three parameters to scale the model prediction to match the timings of the magnetopause crossing, bow shock crossing, and upstream variations. With this procedure the relationship between the upstream measurements and the magnetosheath observations and the location of the magnetosheath satellite relative to the magnetopause and bow shock boundaries are highly constrained. We then introduce a series of normalization procedures that provide the means to remove the effects of the solar wind variations. The systematic differences between the model prediction and observation indicate physical processes that are not included in the gasdynamic model. An application of this approach is presented in a companion paper.

The Martian bow wave—Theory and observation

Abstract The relationship between the trajectory of Mariner 4 in its fly-by of Mars and the calculated location of a proposed Martian bow arising from interaction of the solar wind and the ionosphere has been refined by inclusion of aberration effects of the planets motion about the Sun. The modified theory indicates that the bow wave was crossed twice during a 3-hr interval following the time of closest approach of Mariner 4 to Mars, instead of being missed slightly by the spacecraft as indicated in our previous paper in which the theory of this interaction was presented, and the solar wind was considered, for simplicity, to flow along the Sun-Mars line. It is shown that the observed magnetic field changed abruptly at virtually the precise times of the shock crossings indicated by the theory. Although malfunction of the plasma probe prevents observational confirmation of these as shock crossings, this coincidence supports the recent suggestion of the magnetometer experimenters that a bow wave may have been traversed. It also supports the interpretation that the bow wave results from interaction of the solar wind with the ionosphere, and not with a weak planetary magnetic field.

On the processes in the terrestrial magnetosheath: 2. Case study

We test a new scheme to study the magnetosheath. The scheme uses the solar wind measurements as the input into the gasdynamic convected field model, and the model output is compared with magnetosheath observations. In our four test cases there is a significant overall success in the model prediction. This scheme works better than other methods in magnetosheath studies and is potentially useful for space weather forecasts and nowcasts. The direction of the magnetic field is modeled most accurately. The prediction of the size of the magnetosphere is accurate within a few percent. The predicted thickness of the magnetosheath is accurate up to 90%. With a double-normalization procedure developed in this study, we are able to separate the processes intrinsic in the magnetosheath from those due to large-scale upstream temporal variations. The test cases confirm the existence of a compressional front one third of the distance from the magnetopause to the bow shock near the stagnation streamline. The magnetosheath density profile near the stagnation streamline is consistent with the models that add a compressional front between the two depletion processes described by the plasma depletion model. A major unexpected feature is that the magnetosheath flow pattern is very different from that described by the model and maybe by most other models, including MHD models. The magnetosheath flow near the stagnation streamline does not slow down gradually toward the stagnation point. It moves rapidly until reaching a very small region near the magnetopause.

Gasdynamic modeling of the Venus magnetotail

A gasdynamic, convected magnetic field model of the solar wind interaction with Venus is used for the first time to model the steady state Venus magnetotail. Model results are directly compared with observations. The flow obstacle surface is approximated as a tangential discontinuity. The obstacle shape is an input parameter to this model. An initial obstacle shape, accurate on the dayside, is defined by balancing a hydrostatic equilibrium approximation for the internal plasma pressure with an external flow pressure approximation. These pressure approximations produce a cylindrical obstacle in the distant tail. A refined obstacle shape that attempts to balance this same internal pressure with the calculated external flow pressure tapers inward toward the tail axis downstream of the terminator. Cold fluid (photoionized planetary oxygen) is added to the flow about the tapered model obstacle. The resultant bulk plasma flow and magnetic field properties compare well with experimentally observed average proton velocity and magnetic field components in the magnetotail. The added oxygen plasma has significant number densities only within 1 Rv of the tail axis in the distant tail. The model predicts central magnetotail oxygen plasma number densities of about 0.2 cm−3 and temperatures on the order of 106° K, flowing tailward at speeds as low as 200 m/s. These properties are consistent with the flat, featureless Pioneer Venus Orbiter plasma analyzer spectra observed in the deep central tail. Pickup ions, in the test particle limit, match direct observations of tail pickup ions. These steady state model results suggest that the mass addition at Venus originating above the dayside ionopause is predominantly fluidlike and produces the slowed flows and severe field draping observed in the central distant tail. Oxygen ions produced higher above the ionopause on the dayside, at much lower number densities, behave more as test particles. Their large gyroradii produce an asymmetric population in the distant outer tail and sheath.

Large scale structures in the magnetosheath: Exogenous or endogenous in origin?

Observations of the solar wind and the interplanetary magnetic field from ISEE-3 are used as input to the gasdynamic convected field model, as implemented in a new space weather forecast model. Then the model output, for three case studies, is compared with the magnetosheath quantities observed at ISEE-2 in order to identify the sources of the observed variations of the magnetosheath. It is found that some variations in the magnetosheath plasma and magnetic field are well correlated with corresponding variations in the solar wind and hence have their sources in the solar wind. However, some variations in the magnetosheath magnetic field correlate well with those in the solar wind but not variations in plasma density. Finally, we find that other variations in both plasma and magnetic field in the magnetosheath do not have appreciable correlations with variations in the solar wind. Most of these latter variations occur in the inner magnetosheath, indicating that they are endogenous in origin. Our results show that the forecast model can provide an accurate estimate of the timeshift from the solar wind monitor to the magnetosheath, of the instantaneous locations of the bow shock and magnetopause, and of the properties of the plasma and magnetic field in the outer and middle magnetosheath.

The effects of finite Larmor radius on the perturbation flow mixing of a collisionless plasma

Collisionless plasma theory modification to include effects of finite Larmor radius of ion and electron on perturbation flow mixing

Transonic Wind Tunnel Interference Assessment- Axisymmetric Flows

A wind tunnel interference assessment concept that presents a rational predictive means of wall interference analysis is evaluated. The procedure consists of employing as an outer boundary condition an experimentall y measured pressure distribution along a convenient control surface located inward from the actual tunnel walls. Attention has been focused on axisymmetric flows in the transonic regime, where tunnel interference is high and where the experimentally measured conditions on the control surface are of mixed subsonic/supersonic type. Based on the transonic small-disturbance equation, results for surface and near-flow field pressure distributions are presented for a variety of different slender-body shapes. These calculations indicate both the accuracy of the procedure as well as its ease of implementatio n. The procedure relates directly to the correctable-interference wind-tunnel concept recently suggested.