Stephen S. Stahara

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.

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.

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.

Heliospheric termination shock motion due to fluctuations in the solar wind upstream conditions: Spherically symmetric model

Large-scale fluctuations in the solar wind plasma upstream of the heliospheric termination shock (TS) will cause inward and outward motions of the shock. Using numerical techniques, we extend an earlier, strictly one-dimensional (planar) analytic gasdynamic model [Barnes, 1993] to spherical symmetry to investigate the features of global behavior of shock motion. Our starting point is to establish a steady numerical solution of the gasdynamic equations describing the interaction between the solar wind and the interstellar medium. We then introduce disturbances of the solar wind dynamic pressure at an inner boundary and follow the subsequent evolution of the system, especially the motion of the termination shock. Our model solves spherically symmetric gasdynamic equations as an initial-boundary value problem. The equations in conservative form are solved using a fully implicit total variation diminishing (TVD) upwind scheme with Roe-type Riemann solver. Boundary conditions are given by the solar wind parameters on an inner spherical boundary, where they are allowed to vary with time for unsteady calculations and by a constant pressure (roughly simulating the effect of the local interstellar medium) on an outer boundary. We find that immediately after the interaction, the shock moves with speeds given by the earlier analogous analytic models. However, as the termination shock propagates, it begins to slow down, seeking a new equilibrium position. In addition, the disturbance transmitted through the TS, either a shock or rarefaction wave, will encounter the outer boundary and be reflected back. The reflected signal will encounter the TS, causing it to oscillate. The phenomenon may be repeated for a number of reflections, resulting in a “ringing” of the outer heliosphere.

A new computational model for the prediction of mass loading phenomena for solar wind interactions with cometary and planetary ionospheres

The modified gasdynamic convected-magnetic-field MHD model developed by Spreiter and Stahara (1980) to simulate the supersonic flow of the solar wind past planetary magnetoionospheres is extended to account for cases (such as Venus and comets) in which significant numbers of neutral atmospheric atoms become ionized in the surrounding flow and add to its momentum, energy, and mass. The mathematical model and the solution procedures for the nose and tail regions are explained; typical computational grids are shown; and numerical results for a comet and for the Venus ionosheath and bow shock are presented graphically. It is found that the bow shock weakens and moves further upstream of the obstacle as mass loading is increased and the flow upstream of the bow shock becomes more compressed.

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.

On the spatial range of validity of the gas dynamic model in the magnetosheath of Venus

In the past the global solar wind interaction with Venus has been treated principally with gas dynamic models. While this gas dynamic treatment has proven successful in modeling some of the global characteristics of the interaction, this model does not include the magnetic barrier in a self-consistent manner. This magnetic barrier is formed in the inner magnetosheath where it transfers solar wind momentum flux to the obstacle via magnetic pressure. In this study we examine the extent to which the gas dynamic fluid approximation describes the magnetic field in the dayside Venus magnetosheath by comparing with two gas dynamic models, one which matches the observed ionopause location and one which matches the bow shock location. We find that each model predicts the field profile reasonably well in the vicinity of the matched bow shock or ionopause but neither model provides an adequate model over the entire range from the ionopause to the bow shock.

Transonic Flow Past Axisymmetric and Nonaxisymmetric Boatt ail Projectiles

The development of a predictive method for investigating the steady inviscid aerodynamic behavior of ballistic projectiles having various axisymmetric and nonaxisymmetric boattail shapes is reported. These shapes include the now standard conical boattail as well as a variety of nonaxisymmetr ic shapes. The theoretical procedure employs the classical transonic equivalence rule and a new transonically corrected apparent mass loading method. Theoretical results for surface pressures, loadings, and static aerodynamic characteristics are presented throughout the transonic range for a variety of projectiles having different boattail geometries. Comparisons with results of both experiment and other theoretical methods demonstrate the accuracy of the procedure. YPICAL projectiles in current use by the Army are slender, spin-stabilized bodies of revolution. The boattail configuration that has become the standard is a conical shape with a relatively shallow cone angle ~(5-»10 deg). The primary purpose of any boattail is to increase the projectile range by reducing drag from what it would be if the projectile afterbody were cylindrical. While drag reduction is ac- complished, an associated detrimental result is the creation of negative lift on the boattail. This tends to increase further the destabilizing pitching moment produced by positive lift on the nose, and thereby to reduce additionally the gyroscopic stability of the projectile. At transonic flight speeds, which usually occur near ballistic trajectory apex, the negative loading on the boattail is strongly augmented by the development and movement of shock waves on the boattail. This results in a rapid peaking of the destabilizing pitching moment at flight Mach numbers just below one. That and related changes of other aerodynamic characteristics can result in the projectile becoming unstable. In an effort to reduce the adverse transonic behavior of ballistic projectiles, the Army has recently investigated ex- perimentally1 a series of nonaxisymmetr ic boattail shapes. Some of these were found to improve significantly the aerodynamic characteristics over those of the conical con- figuration. In particular, it was found that increased gyroscopic and dynamic stability and decreased drag could be attained simultaneously. These findings are of considerable importance because they showed for the first time that projectiles designed with such shapes would have both in- creased range and improved stability compared with projectiles employing the standard boattail. The present work describes the development of a theoretical method for predicting the transonic static aerodynamic characteristics of these projectiles. The theoretical analysis for determining the nonlinear three-dimensional projectile flowfields is based on the classical transonic equivalence rule (TER). A new loading calculation method based on apparent mass concepts and which makes use of nonlinear TER flow solutions is used to predict the static aerodynamic coef- ficients. Theoretical results for surface pressures, loadings, and static aerodynamic coefficients are presented for a variety of projectiles with different boattail geometries at Mach numbers throughout the transonic range. Comparisons are made insofar as possible with both other theoretical methods and experimental results.