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Shock formation at unmagnetized planets
AA. Space Rex Vol. 15, No. 8/9, pp. (8/9)415-(8/9)421, 1995 Copyright 0 1995 COSPAR Printed in Great Britain. All ri he reserved. 0273-1177(94)00126-X 0273-l 177l9P $9.50 + 0.00
SHOCK FORMATION AT UNMAGNETIZED PLANETS S. H. Brecht Berkeley Research Associates, P.O. Box 241, Berkeley, CA 94563, U.S.A.
ABSTRACT The formation of the bow shock around planets with little or no intrinsic magnetic field is a process that differs significantly from classical hydrodynamic or collisionless processes. In this paper the differences between bow shocks found at planets with strong magnetic fields, such as Earth and Jupiter, and planets with little or no field, Mars and Venus, will be reviewed. This review will be from a theoretical, observational, and simulation point of view. INTRODUCTION Bow shocks have been detected around every planet in the solar system. They have been studied since the earth’s bow shock was detected in the early 1960’s. The earth’s bow shock, as well as, those of other planets possessing an intrinsic magnetic field is truly a collisionless shock. While these shocks have been studied for over 30 years, there is still more to learn about the complexity of the formation and role they play in the dynamics of the magnetospheres of planets. The dynamics of the shocks found at unmagnetized planets is by comparison in its infancy, because of the increased complexity of the system. This paper will briefly review what is known about the bow shocks of planets having little or no intrinsic magnetic field. Of specific interest is how these shocks differ from those surrounding planets such as the earth. This paper will review some of the basic theoretical assumptions associated with various pictures of shock formation. Then it will review some of the experimental data collected from Venus and Mars; the only two planets falling into the category of unmagnetized planets. Following this section a discussion of numerical simulation tools being applied to the study of Venus and Mars will be presented. In the final section a summary of the differences in shock behavior at magnetized and unmagnetized planets will be presented. The purpose of this paper is to remind the reader of the limitations and pitfalls of various shock paradigms. Commonly used paradigms, such as the gas dynamic paradigm, are frequently used in regimes where strictly speaking they have little or no validity, for example, Mars. Still they do produce predictions of topology that are reasonably close the data. With these thoughts in mind let us examine some of the basic assumptions of the fluid dynamic paradigm of shock formation. FLUID DYNAMIC SHOCKS The equations and details of fluid dynamics shocks can be found in many text books. However, what is forgotten when considering planetary bow shocks is some of the underlying assumptions. For example, it is assumed that on any scale length of interest the dissipation necessary for shock formation occurs on a far shorter scale length. Further, the shock formation process is irreversible; entropy is created. The system is also scalar. The pressure is found to be isotropic due to rapid diffusion and heat convection. In the case of MHD propagation speeds can vary because of the added characteristics in the MHD situation. However, no information can propagate upstream of the shock. Concepts of quasi-parallel shocks do not exist in this paradigm. Finally, both the gas dynamic and the ideal MHD formulation produce a symmetric shock system due to the previous stated conditions. Russell /l/ offered an excellent description of a fluid type of shock in his paper on collisionless shocks. “Given the upstream plasma conditions and the strength of the shock, we can predict the downstream conditions almost independently of the physical processes occurring in the shock...” He goes on to point out one difficulty with the MHD or fluid picture. “In practice there is another complication in that the solar wind consists of several components not a single fluid. The Rankine-Hugoniot MHD solution does not determine how these various components are heated.”
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In the remaining parts of this paper it will be shown that when considering planets not protected by intrinsic fields, the assumptions of the fluid approach are violated. Further, the details of the physical processes do make a difference in the shock behavior. This is in stark contrast to magnetized planets. COLLISIONLESS SHOCKS There are a variety of review papers addressing the processes involved with forming and maintaining collisionless shocks. Perhaps one of the best sets of reviews is found in the AGU Geophysical Monograph 34 /2/. The collisionless shock system is described theoretically by the Vlasov equation coupled with Maxwell’s equations. These equations produce time reversible solutions. The “dissipation” in the collisionless shock is produced by wave-particle interactions and ion reflection. The “dissipation” can be viewed as occurring due to multiple mechanisms (cf. Kennel et al. /3/). The mechanisms include anamolous resistivity (wave-particle interactions), wave dispersion, and ion reflection. The latter process produces most of the dissipation for super critical shocks typically found around the planets of the solar system. The scale of the dissipation is determined by the mechanism. For example, anamo1ous resistivity and dispersion occur on scales ranging from the Debye length to the ion inertia1 length. Ion reflection and the associated electromagnetic waves occurs on scales ranging from the ion inertial length to the ion gyroradius of the lightest species. It is important to appreciate the effects of the physics occurring on the dispersion or reflection scale size. With this physics included the system is no longer constrained to be symmetric. The distribution functions do not have to be Maxwellian and particle acceleration can be expected on the tails of the plasma distribution functions. By contrast the MHD formulation requires scale lengths much larger than the ion gyroradius because the derivation of these equations assumed that the ion gyroradius effects could be averaged away. The solutions must be symmetric, the distribution function must be Maxwellian and there cannot be acceleration of segments of the plasma population. UNMAGNETIZED PLANETS The lack of an intrinsic field means that the obstacle size of Venus and Mars is significantly less than the Earth. With Mars being approximately half the radius of Venus and Earth, one has a transition in obstacle size from the Earth to Mars that is orders of magnitude. The reduction in obstacle size means that finite ion gyroradius effects become important. This is particularly true for Mars where the solar wind gyroradius typically ranges from l/2 to 1 times the planet radius. The most significant difference between the unmagnetized and magnetized planet is that the ionosphere is in direct contact with the shock system. In the case of Mars the hot oxygen corona extends beyond the subsolar shock region. Chemical processes such as electron impact ionization, photoionization, and charge exchange coupled with coulombic collisions are now part of the process. This means that the bow shock system will have multiple ion species involved. Theoretically it means that the Vlasov equation must be modified to include source terms, collisions, and multiple plasma species. At this point we are now in a regime not commonly studied or discussed when collisionless shocks are the topic. The previous paragraphs have suggested the potential for the bow shocks at Venus and Mars to behave in a decidedly non-standard fashion. In the next section a very brief review of the observational data will support these suggestions. OBSERVATIONAL FEATURES OF VENUS AND MARS There are some excellent reviews of the Venus data to be mentioned here /4,5/. The interested reader is strongly encouraged to examine these papers. Basically, there are four features of the Venus system that standout as strongly departing from the usual fluid or collisionless shock picture. They are the solar cycle dependence of the bow shock location /6/, magnetic asymmetries in the ionosphere and shock region /7,8/. the presence of O+ pickup ions /4/, and asymmetries in the shock due to the pickup ions /4,5/. Figure 1. demonstrates the solar cycle dependence as produced by Alexander and Russell 161. These features demonstrate significant departures from the fluid and collisionless shocks concepts. The asymmetries are present due to multiple species interactions and diamagnetic effects. The sun is
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Fig. 1. Comparison of the average Pioneer Venus extrapolated terminator crossing positions of the bow shock during the present solar cycle with sunspot number and F 10.7 flux (from Alexander and Russell, 1985). The average shock position measured during the Venura 9-10 missions (lower left) shows the range of positions observed over the solar cycle.
modifying the termination location of the shock via VUV ionization. It is clear that understanding of the shock formation and dynamics requires a richer formulation than provided by fluid or purely collisionless approaches. In the case of Mars stronger departure from the traditional picture of bow shocks can be expected. As mentioned earlier, the hot oxygen corona extends beyond the shock in the subsolar region. The solar wind ion gyroradius ranges from l/2 to 1 times the planet radius and the Alfven Mach number ranged from approx. 13 to 27 just during the three elliptical orbits /9/. The observations of Mars confirm our expectations. Asymmetries in the pickup O+ ions where detected (cf. /lo/). The shock region was found to be electromagnetically turbulent and highly variable /9/. Extra transition regions were detected ill/. In addition, the shock behavior was determined to be at odds with normal hydrodynamic behavior. The subsolar point is located further from the Mars than Venus although Venus has a lower Mach number. The flare angle of the Martian shock is greater than at Venus against prediction for the higher Mach number of the flow past Mars. Also, the shock did not exhibit an overshoot region 191 as expected /12/. One expects to see overshoots of order 10 or higher with the Alfven Mach numbers measured during these orbits. The dashed lines in figure 2 show the magnetic field data from the three elliptical orbits of Phobos-2 as reported by Schwingenschuh et al. /9/. Finally, the subsolar point was determined to be insensitive to solar wind ram pressure /13/. SIMULATIONS OF UNMAGNETIZED PLANETS Three types of simulations have been used to study Venus and Mars. There are: gas dynamics models /14,15,16/, MHD models /17/, and hybrid particle codes /18,19,20/. Many features of the bow shock system are recovered by these techniques and derivative uses of these codes. One of the most popular derivative use is particle tracing through the flow fields. However, the first two techniques do not recover the features being discussed in this paper. Only the hybrid codes with their kinetic treatment of the ions am capable of recovering many of the these features. Some of the features recovered are: the reflected ions, ion acceleration, asymmetry of the shocks of Venus and Mars, pickup of O+ (see Fig. 3), the electromagnetic activity of the Martian shock, the location of the bow shock of Mars (see Fig. 2 solid lines), and the reproduction of the tail lobe power dependence reported by Verigin et al. 1131.
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Fig.2. Comparison of Phobos 2 elliptical orbit data and hybrid simulation in the subsolar region. The dashed lines arc the data and the solid lines are simulations. The three plots correspond to the Feb. 1, 5, 8 orbits. The simulation correponded to the Feb. 8 solar wind conditions.
In addition the Mars simulations indicate that the shock formation may be more dependent on wave dispersion than ion reflection. The simulations indicate significant acceleration of a portion of the ion spectrum but not very much reflection of the ions. However, they do indicate a significant level of wave activity forming into a shock like structure in the plane orthogonal to the primary Phobos 2 orbital plane. The simulations have not been a complete success. All forms of simulations fail to correctly predict the subsolar point of Venus unless they are poorly resolved (see /19/). Attempts to include mass loading to push the shock location outward in accordance with the data have failed /14,17,18,19/. The notable exception to these results are found in /15,16/ which were gas dynamic simulations. The debate concerning the validity of these latter results continues because of the boundary conditions used for these simulations. The failed simulations all suggest that a stronger mass loading rate is required, it has been suggested /21/ that electron impact will provide the necessary mass loading.
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Fig. 3. Two dimensional hybrid simulations of Mars with an ionosphere loaded into the simulation. The contours are of 0” and show the pick up these ions by the electric fields associated with the solar wind interaction. Contour levels range from 0 to 3000 particles/cc in increments of 500. The results mentioned in the previous paragraphs suggest that, unlike truly collisionless shock simulations, the major challenge to the simulations is to correctly incorporate the additional ion species provided by the ionospheres of Venus and Mars. Implicit in this incorporation is the ability to simulate the finite larmor radius effects that these heavier species have. SUMMARY Data and simulation suggest that the bow shocks associated with unmagnetized planets are not like those found at planets such as the Earth or Jupiter. To properly characterize the shock and downstream behavior, one needs to know considerably more than the solar wind plasma conditions and the strength of the shock. In fact, the shock itself is significantly modified by ionospheric conditions leading to the reverse situation where the downstream conditions control the shock. From a theoretical standpoint the bow shocks of Venus and Mars are not truly collisionless: there are source and collision terms on the right hand side of the Vlasov equations. Further, the scale lengths associated with the shock structures are not properly characterized by gas dynamics or MI-ID equations. This is particularly true at Mars. Finally, Mars may not have a classic collisionless shock because the main dissipation mechanism (ion reflection) is not significant due to the large ion gyroradius of the solar wind protons. However, the simulations do indicate a significant dispersive behavior. In summary, to understand the shock system setup by the unmagnetized planet requires one to understand more than the nonlinear plasma behavior usually found in collisionless shocks. It requires knowledge and inclusion of the ionospheric behavior. This fact sets these systems apart from the magnetized planet and the usual set of theoretical and simulations tools. REFERENCES 1. C.T. Russell, Planetary Bow Shocks, Collisionless Shocks in the Heliosphere: Reviews of Current Research, p. 109, ed. R.G. Stone and B.T. Tsurutani, Geophysical Monograph 35, A.G.U., (1985). 2. Collisionless Shocks in the Heliosphere: A Tutorial Review, Ed. R.G. Stone and B.T. Tsurutani, Geophysical Monograph 34, A.G.U., (1985).
3. C.F. Kennel, J.P. Edmiston. and T. Hada, A Quarter Century of Collisionless Shock Research,
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Collisionless Shocks in the Heliosphere: A Tutorial Review, p. 1, Ed. R.G. Stone and B.T. Tsurutani, Geophysical Monograph 34, A.G.U., 1985. 4. J.G. Luhmann, The Solar Wind Interaction with Venus, Space Science Reviews, 44, 241, (1986).
5. J.L. Phillips and D.L. McComas, The Magnetosheath and Magnetotail of Venus, Space Science Reviews, 55, 1, (1991).
6. C.J. Alexander and C.T. Russell, Geophys. Res. L&f., 12, 369, (1985). 7. J.L. Phillips, J.G. Luhmann, C.T. Russell, and K.R. Moore, Finite Larmor Radius Effect on Ion Pickup at Venus, J. Geophys. Rex, 92, 9920, (1987). 8. J.L. Phillips, J.G. Luhmann, W.C. Knudsen, and L.H. Brace, Asymmetries in the Location of the Venus Ionopause, J. Geophys. Res., 93, 3927, (1988). 9.
K. Schwingenschuh, W. Riedler, H Lichtenegger, Ye. Yeroshenko, K. Sauer, J.G. Luhmann, M. Ong, and C.T. Russell, Martian bow shock: Phobos Observations, Geophys. Res. L&t., 17, 889, (1990). 10. R.A. Lundin, A. Zakharov, R. Pellinen, H. Borg, B. Hultqvist, N. Pissarenko, E.M. Dubinin, S.W. Barabasj, I. Liede, and H. Koskinen, Plasma Composition Measurements of the martian Magnetosphere Morphology, Geophys, Res. L&t., 17, 877, (1990). 11. K. Sauer, T. Roatsch, U. Motshcmann, K. Schwingenschuh, R. Lundin, H. Rosenbauer, and S. Livi, Observations of plasma boundaries and phenomena around Mars with Phobos 2, J. Geophys. Res., 97. 6227, (1992).
12. C.T. Russell, M.M. Hoppe, and W.A. Livesay, Overshoots in Planetary Bow Shocks, Nature, 296, 45, (1982). 13. M.I. Verigin, K.I. Gringauz, G.A. Kotova, A.P. Remizov, N.M. Shutte, H. Rosenbauer, S. Livi, A. Richter, W. Riedler, K. Schwingenschuh, K. Szego, I. Apathy, and M. Tatrallyay, The dependence of the Martian magnetopause and bow shock on solar wind ram pressure according to Phobos 2 TAUS ion spectrometer measurements, J. Geophys. Res., 98, 1303, (1993). 14. S.S. Stahara and J.R. Spreiter, Computer Modeling of Solar Wind Interaction with Venus and Mars, in Venus and Mars: Atmospheres, Ionospheres, and Solar Wind Interactions, p. 345, ed. J.G. Luhmann, M. Tatrallyay, and R.O. Pepin, Geophysical Monograph 66, AGU, (1992). 15. T.K. Breus, A.M. Krymmskii, V.Ya. Mitnitskii, Turbulent Pickup of New Born Ions near Venus and Mars and Problems of Numerical Modeling of the Solar Wind Interaction with these Planets, 1. Two-Fluid GD Model, Planetary Space Sci., 40, 13I, (1992). 16. O.M. Belotserkovskii, T.K. Breus, A.M. Krymskii, V. Ya Mitnitskii, A.F. Nagy, T.I. Gombosi, The Effect of the Hot Oxygen Corona on the Interaction of the Solar Wind with Venus, Geophys. Res. Lett., 14, 503, (1987).
17. J.E. McGary and D.H. Pontius, Jr., MHD simulations of boundary layer formation along the dayside Venus ionopause due to mass loading, J. Geophys. Res., 99, 2289, (1994). 18. K.R. Moore, V.A. Thomas, and D.J. McComas, A Global Hybrid Simulation of the Solar Wind Interaction with the Dayside of Venus, J. Geophys. Res., 96, 7779, (1991). 19. S.H. Brecht and J.R. Ferrante, Global Hybrid Simulation of Unmagnetized Planets: Comparison of Venus and Mars, J. Geophys. Res., 96, 11209, (1991). 20. S.H. Brecht, J.R. Ferrante, and J.G. Luhmann, Three-Dimensional Simulations of the Solar Wind
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Interaction with Mars, J. Geophys. Res., 98, 1345, (1993). 21. M.H.G. Zhang, J.G. Luhmann, A.F. Nagy, J. R. Spreiter, and S.S. Stahara, Oxygen Ionization Rates at Mars and Venus: Relative Contributions of Impact Ionization and Charge Exchange, J. Geophys. Res., 98, 3311, (1993).
SHOCK FORMATION AT UNMAGNETIZED PLANETS S. H. Brecht Berkeley Research Associates, P.O. Box 241, Berkeley, CA 94563, U.S.A.
ABSTRACT The formation of the bow shock around planets with little or no intrinsic magnetic field is a process that differs significantly from classical hydrodynamic or collisionless processes. In this paper the differences between bow shocks found at planets with strong magnetic fields, such as Earth and Jupiter, and planets with little or no field, Mars and Venus, will be reviewed. This review will be from a theoretical, observational, and simulation point of view. INTRODUCTION Bow shocks have been detected around every planet in the solar system. They have been studied since the earth’s bow shock was detected in the early 1960’s. The earth’s bow shock, as well as, those of other planets possessing an intrinsic magnetic field is truly a collisionless shock. While these shocks have been studied for over 30 years, there is still more to learn about the complexity of the formation and role they play in the dynamics of the magnetospheres of planets. The dynamics of the shocks found at unmagnetized planets is by comparison in its infancy, because of the increased complexity of the system. This paper will briefly review what is known about the bow shocks of planets having little or no intrinsic magnetic field. Of specific interest is how these shocks differ from those surrounding planets such as the earth. This paper will review some of the basic theoretical assumptions associated with various pictures of shock formation. Then it will review some of the experimental data collected from Venus and Mars; the only two planets falling into the category of unmagnetized planets. Following this section a discussion of numerical simulation tools being applied to the study of Venus and Mars will be presented. In the final section a summary of the differences in shock behavior at magnetized and unmagnetized planets will be presented. The purpose of this paper is to remind the reader of the limitations and pitfalls of various shock paradigms. Commonly used paradigms, such as the gas dynamic paradigm, are frequently used in regimes where strictly speaking they have little or no validity, for example, Mars. Still they do produce predictions of topology that are reasonably close the data. With these thoughts in mind let us examine some of the basic assumptions of the fluid dynamic paradigm of shock formation. FLUID DYNAMIC SHOCKS The equations and details of fluid dynamics shocks can be found in many text books. However, what is forgotten when considering planetary bow shocks is some of the underlying assumptions. For example, it is assumed that on any scale length of interest the dissipation necessary for shock formation occurs on a far shorter scale length. Further, the shock formation process is irreversible; entropy is created. The system is also scalar. The pressure is found to be isotropic due to rapid diffusion and heat convection. In the case of MHD propagation speeds can vary because of the added characteristics in the MHD situation. However, no information can propagate upstream of the shock. Concepts of quasi-parallel shocks do not exist in this paradigm. Finally, both the gas dynamic and the ideal MHD formulation produce a symmetric shock system due to the previous stated conditions. Russell /l/ offered an excellent description of a fluid type of shock in his paper on collisionless shocks. “Given the upstream plasma conditions and the strength of the shock, we can predict the downstream conditions almost independently of the physical processes occurring in the shock...” He goes on to point out one difficulty with the MHD or fluid picture. “In practice there is another complication in that the solar wind consists of several components not a single fluid. The Rankine-Hugoniot MHD solution does not determine how these various components are heated.”
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In the remaining parts of this paper it will be shown that when considering planets not protected by intrinsic fields, the assumptions of the fluid approach are violated. Further, the details of the physical processes do make a difference in the shock behavior. This is in stark contrast to magnetized planets. COLLISIONLESS SHOCKS There are a variety of review papers addressing the processes involved with forming and maintaining collisionless shocks. Perhaps one of the best sets of reviews is found in the AGU Geophysical Monograph 34 /2/. The collisionless shock system is described theoretically by the Vlasov equation coupled with Maxwell’s equations. These equations produce time reversible solutions. The “dissipation” in the collisionless shock is produced by wave-particle interactions and ion reflection. The “dissipation” can be viewed as occurring due to multiple mechanisms (cf. Kennel et al. /3/). The mechanisms include anamolous resistivity (wave-particle interactions), wave dispersion, and ion reflection. The latter process produces most of the dissipation for super critical shocks typically found around the planets of the solar system. The scale of the dissipation is determined by the mechanism. For example, anamo1ous resistivity and dispersion occur on scales ranging from the Debye length to the ion inertia1 length. Ion reflection and the associated electromagnetic waves occurs on scales ranging from the ion inertial length to the ion gyroradius of the lightest species. It is important to appreciate the effects of the physics occurring on the dispersion or reflection scale size. With this physics included the system is no longer constrained to be symmetric. The distribution functions do not have to be Maxwellian and particle acceleration can be expected on the tails of the plasma distribution functions. By contrast the MHD formulation requires scale lengths much larger than the ion gyroradius because the derivation of these equations assumed that the ion gyroradius effects could be averaged away. The solutions must be symmetric, the distribution function must be Maxwellian and there cannot be acceleration of segments of the plasma population. UNMAGNETIZED PLANETS The lack of an intrinsic field means that the obstacle size of Venus and Mars is significantly less than the Earth. With Mars being approximately half the radius of Venus and Earth, one has a transition in obstacle size from the Earth to Mars that is orders of magnitude. The reduction in obstacle size means that finite ion gyroradius effects become important. This is particularly true for Mars where the solar wind gyroradius typically ranges from l/2 to 1 times the planet radius. The most significant difference between the unmagnetized and magnetized planet is that the ionosphere is in direct contact with the shock system. In the case of Mars the hot oxygen corona extends beyond the subsolar shock region. Chemical processes such as electron impact ionization, photoionization, and charge exchange coupled with coulombic collisions are now part of the process. This means that the bow shock system will have multiple ion species involved. Theoretically it means that the Vlasov equation must be modified to include source terms, collisions, and multiple plasma species. At this point we are now in a regime not commonly studied or discussed when collisionless shocks are the topic. The previous paragraphs have suggested the potential for the bow shocks at Venus and Mars to behave in a decidedly non-standard fashion. In the next section a very brief review of the observational data will support these suggestions. OBSERVATIONAL FEATURES OF VENUS AND MARS There are some excellent reviews of the Venus data to be mentioned here /4,5/. The interested reader is strongly encouraged to examine these papers. Basically, there are four features of the Venus system that standout as strongly departing from the usual fluid or collisionless shock picture. They are the solar cycle dependence of the bow shock location /6/, magnetic asymmetries in the ionosphere and shock region /7,8/. the presence of O+ pickup ions /4/, and asymmetries in the shock due to the pickup ions /4,5/. Figure 1. demonstrates the solar cycle dependence as produced by Alexander and Russell 161. These features demonstrate significant departures from the fluid and collisionless shocks concepts. The asymmetries are present due to multiple species interactions and diamagnetic effects. The sun is
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Fig. 1. Comparison of the average Pioneer Venus extrapolated terminator crossing positions of the bow shock during the present solar cycle with sunspot number and F 10.7 flux (from Alexander and Russell, 1985). The average shock position measured during the Venura 9-10 missions (lower left) shows the range of positions observed over the solar cycle.
modifying the termination location of the shock via VUV ionization. It is clear that understanding of the shock formation and dynamics requires a richer formulation than provided by fluid or purely collisionless approaches. In the case of Mars stronger departure from the traditional picture of bow shocks can be expected. As mentioned earlier, the hot oxygen corona extends beyond the shock in the subsolar region. The solar wind ion gyroradius ranges from l/2 to 1 times the planet radius and the Alfven Mach number ranged from approx. 13 to 27 just during the three elliptical orbits /9/. The observations of Mars confirm our expectations. Asymmetries in the pickup O+ ions where detected (cf. /lo/). The shock region was found to be electromagnetically turbulent and highly variable /9/. Extra transition regions were detected ill/. In addition, the shock behavior was determined to be at odds with normal hydrodynamic behavior. The subsolar point is located further from the Mars than Venus although Venus has a lower Mach number. The flare angle of the Martian shock is greater than at Venus against prediction for the higher Mach number of the flow past Mars. Also, the shock did not exhibit an overshoot region 191 as expected /12/. One expects to see overshoots of order 10 or higher with the Alfven Mach numbers measured during these orbits. The dashed lines in figure 2 show the magnetic field data from the three elliptical orbits of Phobos-2 as reported by Schwingenschuh et al. /9/. Finally, the subsolar point was determined to be insensitive to solar wind ram pressure /13/. SIMULATIONS OF UNMAGNETIZED PLANETS Three types of simulations have been used to study Venus and Mars. There are: gas dynamics models /14,15,16/, MHD models /17/, and hybrid particle codes /18,19,20/. Many features of the bow shock system are recovered by these techniques and derivative uses of these codes. One of the most popular derivative use is particle tracing through the flow fields. However, the first two techniques do not recover the features being discussed in this paper. Only the hybrid codes with their kinetic treatment of the ions am capable of recovering many of the these features. Some of the features recovered are: the reflected ions, ion acceleration, asymmetry of the shocks of Venus and Mars, pickup of O+ (see Fig. 3), the electromagnetic activity of the Martian shock, the location of the bow shock of Mars (see Fig. 2 solid lines), and the reproduction of the tail lobe power dependence reported by Verigin et al. 1131.
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Fig.2. Comparison of Phobos 2 elliptical orbit data and hybrid simulation in the subsolar region. The dashed lines arc the data and the solid lines are simulations. The three plots correspond to the Feb. 1, 5, 8 orbits. The simulation correponded to the Feb. 8 solar wind conditions.
In addition the Mars simulations indicate that the shock formation may be more dependent on wave dispersion than ion reflection. The simulations indicate significant acceleration of a portion of the ion spectrum but not very much reflection of the ions. However, they do indicate a significant level of wave activity forming into a shock like structure in the plane orthogonal to the primary Phobos 2 orbital plane. The simulations have not been a complete success. All forms of simulations fail to correctly predict the subsolar point of Venus unless they are poorly resolved (see /19/). Attempts to include mass loading to push the shock location outward in accordance with the data have failed /14,17,18,19/. The notable exception to these results are found in /15,16/ which were gas dynamic simulations. The debate concerning the validity of these latter results continues because of the boundary conditions used for these simulations. The failed simulations all suggest that a stronger mass loading rate is required, it has been suggested /21/ that electron impact will provide the necessary mass loading.
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Fig. 3. Two dimensional hybrid simulations of Mars with an ionosphere loaded into the simulation. The contours are of 0” and show the pick up these ions by the electric fields associated with the solar wind interaction. Contour levels range from 0 to 3000 particles/cc in increments of 500. The results mentioned in the previous paragraphs suggest that, unlike truly collisionless shock simulations, the major challenge to the simulations is to correctly incorporate the additional ion species provided by the ionospheres of Venus and Mars. Implicit in this incorporation is the ability to simulate the finite larmor radius effects that these heavier species have. SUMMARY Data and simulation suggest that the bow shocks associated with unmagnetized planets are not like those found at planets such as the Earth or Jupiter. To properly characterize the shock and downstream behavior, one needs to know considerably more than the solar wind plasma conditions and the strength of the shock. In fact, the shock itself is significantly modified by ionospheric conditions leading to the reverse situation where the downstream conditions control the shock. From a theoretical standpoint the bow shocks of Venus and Mars are not truly collisionless: there are source and collision terms on the right hand side of the Vlasov equations. Further, the scale lengths associated with the shock structures are not properly characterized by gas dynamics or MI-ID equations. This is particularly true at Mars. Finally, Mars may not have a classic collisionless shock because the main dissipation mechanism (ion reflection) is not significant due to the large ion gyroradius of the solar wind protons. However, the simulations do indicate a significant dispersive behavior. In summary, to understand the shock system setup by the unmagnetized planet requires one to understand more than the nonlinear plasma behavior usually found in collisionless shocks. It requires knowledge and inclusion of the ionospheric behavior. This fact sets these systems apart from the magnetized planet and the usual set of theoretical and simulations tools. REFERENCES 1. C.T. Russell, Planetary Bow Shocks, Collisionless Shocks in the Heliosphere: Reviews of Current Research, p. 109, ed. R.G. Stone and B.T. Tsurutani, Geophysical Monograph 35, A.G.U., (1985). 2. Collisionless Shocks in the Heliosphere: A Tutorial Review, Ed. R.G. Stone and B.T. Tsurutani, Geophysical Monograph 34, A.G.U., (1985).
3. C.F. Kennel, J.P. Edmiston. and T. Hada, A Quarter Century of Collisionless Shock Research,
WW~
S. H. Brecht
Collisionless Shocks in the Heliosphere: A Tutorial Review, p. 1, Ed. R.G. Stone and B.T. Tsurutani, Geophysical Monograph 34, A.G.U., 1985. 4. J.G. Luhmann, The Solar Wind Interaction with Venus, Space Science Reviews, 44, 241, (1986).
5. J.L. Phillips and D.L. McComas, The Magnetosheath and Magnetotail of Venus, Space Science Reviews, 55, 1, (1991).
6. C.J. Alexander and C.T. Russell, Geophys. Res. L&f., 12, 369, (1985). 7. J.L. Phillips, J.G. Luhmann, C.T. Russell, and K.R. Moore, Finite Larmor Radius Effect on Ion Pickup at Venus, J. Geophys. Rex, 92, 9920, (1987). 8. J.L. Phillips, J.G. Luhmann, W.C. Knudsen, and L.H. Brace, Asymmetries in the Location of the Venus Ionopause, J. Geophys. Res., 93, 3927, (1988). 9.
K. Schwingenschuh, W. Riedler, H Lichtenegger, Ye. Yeroshenko, K. Sauer, J.G. Luhmann, M. Ong, and C.T. Russell, Martian bow shock: Phobos Observations, Geophys. Res. L&t., 17, 889, (1990). 10. R.A. Lundin, A. Zakharov, R. Pellinen, H. Borg, B. Hultqvist, N. Pissarenko, E.M. Dubinin, S.W. Barabasj, I. Liede, and H. Koskinen, Plasma Composition Measurements of the martian Magnetosphere Morphology, Geophys, Res. L&t., 17, 877, (1990). 11. K. Sauer, T. Roatsch, U. Motshcmann, K. Schwingenschuh, R. Lundin, H. Rosenbauer, and S. Livi, Observations of plasma boundaries and phenomena around Mars with Phobos 2, J. Geophys. Res., 97. 6227, (1992).
12. C.T. Russell, M.M. Hoppe, and W.A. Livesay, Overshoots in Planetary Bow Shocks, Nature, 296, 45, (1982). 13. M.I. Verigin, K.I. Gringauz, G.A. Kotova, A.P. Remizov, N.M. Shutte, H. Rosenbauer, S. Livi, A. Richter, W. Riedler, K. Schwingenschuh, K. Szego, I. Apathy, and M. Tatrallyay, The dependence of the Martian magnetopause and bow shock on solar wind ram pressure according to Phobos 2 TAUS ion spectrometer measurements, J. Geophys. Res., 98, 1303, (1993). 14. S.S. Stahara and J.R. Spreiter, Computer Modeling of Solar Wind Interaction with Venus and Mars, in Venus and Mars: Atmospheres, Ionospheres, and Solar Wind Interactions, p. 345, ed. J.G. Luhmann, M. Tatrallyay, and R.O. Pepin, Geophysical Monograph 66, AGU, (1992). 15. T.K. Breus, A.M. Krymmskii, V.Ya. Mitnitskii, Turbulent Pickup of New Born Ions near Venus and Mars and Problems of Numerical Modeling of the Solar Wind Interaction with these Planets, 1. Two-Fluid GD Model, Planetary Space Sci., 40, 13I, (1992). 16. O.M. Belotserkovskii, T.K. Breus, A.M. Krymskii, V. Ya Mitnitskii, A.F. Nagy, T.I. Gombosi, The Effect of the Hot Oxygen Corona on the Interaction of the Solar Wind with Venus, Geophys. Res. Lett., 14, 503, (1987).
17. J.E. McGary and D.H. Pontius, Jr., MHD simulations of boundary layer formation along the dayside Venus ionopause due to mass loading, J. Geophys. Res., 99, 2289, (1994). 18. K.R. Moore, V.A. Thomas, and D.J. McComas, A Global Hybrid Simulation of the Solar Wind Interaction with the Dayside of Venus, J. Geophys. Res., 96, 7779, (1991). 19. S.H. Brecht and J.R. Ferrante, Global Hybrid Simulation of Unmagnetized Planets: Comparison of Venus and Mars, J. Geophys. Res., 96, 11209, (1991). 20. S.H. Brecht, J.R. Ferrante, and J.G. Luhmann, Three-Dimensional Simulations of the Solar Wind
llnmagaetized Planets
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Interaction with Mars, J. Geophys. Res., 98, 1345, (1993). 21. M.H.G. Zhang, J.G. Luhmann, A.F. Nagy, J. R. Spreiter, and S.S. Stahara, Oxygen Ionization Rates at Mars and Venus: Relative Contributions of Impact Ionization and Charge Exchange, J. Geophys. Res., 98, 3311, (1993).