Planetary encounters by numerous spacecraft have furnished information concerning the solar wind interaction with the planets Mercury, Venus, Mars, and Jupiter. While direct measurements have indicated a wide range of atmospheric densities and intrinsic magnetic field strengths, the data seem to indicate that the flow pattern around nonmagnetized or weakly magnetized planets with atmospheres optically thick at ionizing wavelengths is basically the same as that around a strongly magnetized planet's magnetosphere, such as the earth's. The planetary ionosphere apparently presents a hard obstacle to the flow, with bow shock formation required in the supersonic, super-Alfvenic flow to slow and direct most of the solar wind plasma around the planetary ionosphere. Various aspects of the interaction are examined in the context of theoretical models in an attempt to explain observed details of the interaction regions of Venus and Mars

Cloutier, P. A.

English

NASA Technical Reports Server

SOLAR-WIND INTERACTION WITH PLANETARY IONOSPHERES 
P. A. Cloutier 
Department o f  Space Physics and Astronomy 
Rice University 
Houston, Texas 
INTRODUCTION 
Planetary encounters by numerous spacecraft launched by the USA and USSR have fur- 
nished information concerning the solar-wind interaction with the planets Mercury, Venus, 
Mars, and Jupiter. While direct measurements have indicated a wide range of atmospheric 
densities and intrinsic magnetic field strengths, the data seem to indicate that the flow pat- 
tern around nonmagnetized or weakly-magnetized planets with atmospheres optically thick 
at ionizing wavelengths (that is, with well-developed ionospheres above the planetary sur- 
face) is basically the same as that around a strongly-magnetized planet's magnetosphere, such 
as the Earth's. The planetary ionosphere apparently presents a hard obstacle to the flow, 
with bow-shock formation required in the supersonic, super-Alfvgnic flow to slow and direct 
most of the solar-wind plasma around the planetary ionosphere. In this paper, various as- 
pects of the interaction are examined in the context of theoretical models in an attempt to 
explain observed details of the interaction regions of Venus and Mars. 
NATURE OF THE OBSTACLE-MECHANICS AND ELECTRODYNAMICS 
In order to understand the nature of the obstacle presented to the flow by a planetary iono- 
sphere and to be able to predict the details in the flow field around the planet, two basic 
boundary conditions must be considered. The first condition specifies the behavior of the 
magnetic field at the lower boundary of the planetary ionosphere, and the second specifies 
the penetrating solar-plasma flux into a defined upper boundary (ionopause) of the planetary 
ionosphere. As will be shown, determination of these conditions allows estimation of the 
parameters of the flow around the planet and of the dynamic behavior of the planetary 
ionosphere. 
The first condition is that, in the absence of an intrinsic planetary magnetic field, the inter- 
planetary magnetic field must become vanishingly small in the lower atmosphere below the 
ionosphere. This may be simply understood in terms of magnetic diffusion through the 
highly-conducting dayside ionosphere. The solar-wind flow around the sides of the planet 
outside the atmosphere carries interplanetary field lines back at a higher rate than the flow 
through the ionosphere can carry these same field lines (Dessler, 1968). The result is retarda- 
tion of flux tubes in the ionosphere, with a given flux tube reaching the bottom of the iono- 
sphere behind the segments of the same flux tube carried by the flow around the ionosphere. 
https://ntrs.nasa.gov/search.jsp?R=19760018046 2020-03-22T14:07:22+00:00Z
The tension of the field lines will result in rapid straightening and high velocity of the field 
lines (v % c) through the neutral atmosphere and planetary body, with very low magnetic 
flux density there (B S 0). 
The second condition, predicted by theoretical calculations and supported by observational 
data, is that the planetary ionosphere is nearly impenetrable to the solar-wind plasma. 
Cloutier et al. (1969) have shown that simultaneous conservation of mass, momentum, and 
energy in a gas flowing through a region in which cold gas particles are being created (equiva- 
lent to planetary photoions being added to the solar wind) is only possible for gas addition 
rates below a certain critical value. Above this critical value, the flow can no longer accom- 
modate the added mass by adjustment of its velocity, pressure, and density, and a distur- 
bance must propagate upstream to alter the incident flow. Calculations in a one-dimen- 
sional model with realistic photoionization rates and solar-wind parameters have placed an 
upper limit of 1 km/s on the penetration velocity of the solar wind into the atmosphere. 
This in turn requires bow-shock formation in the solar wind, with less than one percent of the 
incident flow actually penetrating into the atmosphere, and most of the flow being directed 
around the planet in the post-shock region (Michel, 1971). The flow field around the planet 
may therefore be described to an excellent approximation by the aerodynamic models of 
Spreiter et al. (1966, 1968, 1970) with an impenetrable boundary surface between the solar 
wind and the ionosphere. A qualitative model of the flow region is shown in figure 1. 
The combination of the two boundary conditions described above also allows calculation of 
the flow field within the ionosphere and of the electric and magnetic field configuration 
there. From the first boundary condition, the total ionospheric current magnitude required 
to reduce the shock-compressed interplanetary field to zero at the base of the ionosphere 
can be estimated. For an interplanetary field of 5 y, the height-integrated current magnitude 
is roughly 3 X ampslm length along the equator (Cloutier and Daniell, 1973). Using 
model atmospheres for Venus and Mars to calculate the tensor conductivity profiles within 
the atmosphere, and using the dynamo _v X B electric field from Spreiter's models, it is pos- 
sible to estimate the subsolar height of the ionopause boundaries for these planets at which 
the cancellation currents are obtained. These height ranges are shown for Venus in figure 2 
and for Mars in figure 3. The effects of any intrinsic magnetic field on Mars are omitted to 
demonstrate the effects of the purely ionospheric obstacle. The higher apparent obstacle 
height reported by Gringauz et al. (1975) at Mars may be due to the effects of an intrinsic 
Martian dipole reported by Dolginov et al. (1973). The height of the obstacle and of the 
shock will be discussed in greater detail in a later section. 
The volume distribution of currents within the ionospheres may be determined by minimiza- 
tion of the volume integral of Joule heating by a variational technique. Recent calculations 
for Venus* have indicated a total potential difference between polar terminators at the 
*Daniell, R. E., Jr., private communication, 1975. 
Figure 1. Plasma flow geometry in the vicinity of a nonmagnetic planet. 
ionopause boundary of roughly 40 V. This corresponds to less than 0.1 percent of the solar- 
wind plasma incident on the planet penetrating the ionopause. The volume current distri- 
bution together with the conductivity allows determination of electric and magnetic fields 
everywhere within the ionosphere, and hence specifies the convection pattern of ionization 
within the ionosphere. A quantitative understanding of the distribution of ionization must 
clearly include these convective effects with diffusive and photochemical mechanisms. 
Integrated Current Density (amps/meter) 
Figure 2. Total ionospheric current magnitude induced in the Venusian ionosphere by the mlar wind. The 
vertical dashed line indicates the current magnitude required to cancel the shock-compressed interplanetary 
magnetic field. The various curves shown are labelled with ratio H/r, of scale height to planetary radius and 
the percentageof He upper limit corresponding to 10' cmJ at 200 km altitude. Above 300 km H/ro should 
be 0.06. Standoff altitudes are given a t  the subsolar point. 
Figure 3. Total ionospheric current magnitude induced in the Martian ionosphere by the solar wind. H/ro 
= 0.01 is appropriate for O*, which is expected to be the dominant topside ion. 
SHAPE OF THE OBSTACLE-SYMMETRIES AND ASYMMETRIES 
Although the flow patterns calculated by Spreiter et al. (1966, 1968, 1970) are axisymme- 
tric, there are clearly asymmetries between opposite (polar) hemispheres in the altitude dis- 
tribution of planetary ions added to the flow external to the ionopause owing to the oppo- 
sitely-directed electric fields as seen from the planetary atmosphere. Cloutier et al. (1 974) have 
considered this effect and concluded that characteristically-different ion densities and energy 
spectra would be detectable in the two hemispheres. In the hemisphere in which the induced 
electric field is outward, the ion distribution of a given species is nearly constant between the 
ionopause and a height of two gyroradii above the ionopause, and decreases exponentially 
above two gyroradii with roughly the species neutral scale height. In this hemisphere, all 
ions are accelerated to the average flow velocity. In the opposite hemisphere, in which the 
electric field is directed inward, the ion distributions are concentrated much closer to the 
ionopause surface and fall off rapidly with height above it. In this hemisphere, the average 
drift velocity of ions is less than the flow velocity within two gyroradii of the ionosphere, 
and varies from -0 at the boundary to the flow velocity at a height of two gyroradii. If the 
ionopause altitudes in both hemispheres were equal, resulting in equal total mass addition to 
the flow in the two hemispheres, then clearly the density and velocity differences will result 
in a difference in momentum transfer from the flow to  the photoions. The drag to  the flow 
will be less in the hemisphere in which the electric field is directed inward, and the flow pat- 
tern symmetry axis should shift toward this hemisphere. Calculations of the total drag for 
equal and symmetric ionopause altitudes in both hemispheres show that the altitude of equal 
drag differs by 300 km at the polar terminators of Venus and 1000 km at the polar termina- 
tors of Mars. However, the drag may be equalized by changing the ionopause altitudes 
slightly to increase the total mass added in one hemisphere and decrease it in the other.* 
The required height differences between hemispheres at the polar terminators are 140 km 
for Venus and 25 km for Mars, with comparable height differences expected in the shock 
altitudes. 
Another asymmetry may be produced by a combination of two effects. This asymmetry cor- 
responds to a larger effective obstacle polar diameter than equatorial diameter. One contri- 
buting effect is the loss in efficiency in the acceleration of planetary photoions by the flow 
at low (equatorial) latitudes due to decreasing angle between y and B. At the equator, the 
angle between y and B is very small, and the v_ X B electric field is much less than at the poles. 
If the shock-compressed interplanetary field is relatively free of significant fluctuations 
transverse to the average B, then at low latitudes the momentum transfer from the flow to 
the planetary photoions, and hence the drag on the flow, will be much less than at the poles. 
It may be argued, however, that the interplanetary field is not completely noise-free. Meas- 
urements in the Earth's magnetosheath indicate an average transverse noise component of 
*Wolf, R. A., private communication, 1975. 
order 10-I of the steady field. Acceleration of planetary photoions at low latitudes may 
occur in this case by a magnetic pumping mechanism (Alfv6n and Falthammar, 1973), with 
the average ion velocity approaching the flow velocity v, according to the relation: 
where t is the average time from creation of an ion in the flow, and T is the characteristic 
acceleration time given by: 
with AB the average transverse field noise component, B the average field magnitude, and 
Twc = 21rm/eB the cyclotron period of the ion species of mass, m. Calculation of the result- 
ing average ion velocities at the equatorial terminator for Hef on Venus and Of on Mars in- 
dicate that these ions have only been accelerated to a few percent of the flow velocity as 
they cross the terminator. Reduction of ionopause heights at the equator t o  achieve equal- 
ity in equatorial and polar drag factors requires a reduction in effective equatorial obstacle 
radius relative to polar obstacle radius of 650 krn for Venus and 250 km for Mars. 
In addition to this possible asymmetry in the ionopause height from pole to equator, there 
may be a difference in the height of the shock above the polar and equatorial ionopause 
boundaries caused by a difference in plasma compressibility. For flow around the ionopause 
in the equatorial plane, the steady field is nearly parallel to the flow velocity, and the com- 
pression and expansion of the plasma along the streamlines is not strongly affected by the 
magnetic field. Moreover, any transverse magnetic noise component will tend to isotropize 
the pressure, and instabilities in the flow with _v II B will enhance this effect. The gas in this 
case should act as an ideal gas with ratio of specific heats y = 513. However, for flow over 
the poles, the streamlines are nearly orthogonal to the magnetic field along the flow tube, 
and compression and expansion of the plasma involves the magnetic pressure directly. In 
this case, the ratio y may be closer to 2. This difference in compressibility will cause a sig- 
nificant asymmetry in the shock altitudes from pole to equator even for a completely sym- 
metry ionopause, and the ionopause height asymmetry described above will add to the total 
shock asymmetry. 
The shock positions corresponding to several different Mach numbers for y = 513 and 2 have 
been calculated by Spreiter et al. (1966) for the Earth's magnetosphere, and are shown in 
figure 4. Direct scaling of this figure to Mars and Venus without inclusion of the ionopause 
height asymmetries shows that the shock distances at the terminator could differ from pole 
to equatorial terminator by 1000 krn for Mars and 1500 krn for Venus due to  the compres- 
sibility effect alone. The subsolar shock distances extrapolated from the terminator shock 
distances could differ by 500 krn for Mars and 750 krn for Venus, leading to obstacle height 
estimates differing by 350 km for Mars and 500 km for Venus. 
Figure 4. Differences in bow-shock position for y = 513 and 7 = 2. The magnetospheric boundary 
should be scaled to the planetary ionopause surface (from Spreiter e t  al., 1966). 
DISCUSSION 
This paper has reviewed a theoretical model for the obstacle presented by a planetary iono- 
sphere to the solar wind. Basic features of this model include small flux penetration of the 
ionosphere by the solar wind and a weak magnetic field below the dayside ionosphere. The 
model also predicts that large asymmetries may be found in the ionopause height and shock 
height from pole to equatorial terminator. The variations in shock height due to these asym- 
metries are comparable to the variations reported by Gringauz et al. (1974) for Mars, although 
the variability could also be explained in terms of an asymmetric rotating planetary dipole* 
*Ness, N. F. and S. J. Bauer, 1974, "USSR Mas Observations; The Case for an Intrinsic Planetary Magnetic Field," GSFC 
X-690-74-69, Greenbelt, Maryland. 
such as that reported by Dolginov et al. (1973). However, for the dipole moment reported 
of 2.6 X loz2 G cm3, the atmospheric asymmetries may still dominate any effects due to 
the dipole rotation and control the dynamics of shock formation (Rassbach et al., 1974). 
The reported observation of constant magnetic field direction within the Martian nightside 
obstacle boundary offers a stronger argument in support of an intrinsic Martian field than 
the observations of shock variability. A further argument for at least a weak intrinsic field 
is the absence of evidence of downward ionospheric convection (scale height depression by 
solar-wind pressure) in the Martian dayside atmosphere such as that proposed by Bauer and 
Hartle (1974) to explain Mariner-10 observations of Venus. 
ACKNOWLEDGMENTS 
The author is grateful to R. A. Wolf, F. C. Michel, R. E. Daniell, Jr., and K. I. Gringauz for 
their pertinent comments and suggestions. This work was supported by the Atmospheric 
Sciences Division of the National Science Foundation, N.S.F. Grant DES74-19668. 
REFERENCES 
Alfv6n, H. and C. G. Falthammar, 1973, Cosmical Electrodynamics, 2nd. ed, Oxford Univer- 
sity Press, London, p. 64. 
Bauer, S. J. and R. E. Hartle, 1974, "Venus Ionosphere: An Interpretation of Mariner 10 
Observations," Geophys. Res. Lett., 1, p. 7. 
Cloutier, P. A., M. B. McElroy, and F. C. Michel, 1969, "Modification of the Martian Iono- 
sphere by the Solar Wind," J. Geophys. Res., 74, p. 62 1 5. 
Cloutier, P. A. and R. E. Daniell, Jr., 1973, "Ionospheric Currents Induced by Solar Wind 
Interaction with Planetary Atmosphere," Planet. Space Sci., 21, p. 463. 
Cloutier, P. A., R. E. Daniell, Jr., and D. M. Butler, 1974, "Atmospheric Ion Wakes of Venus 
and Mars in the Solar Wind," Planet. Space Sci., 22, p. 967. 
Dessler, A. J., 1968, "Ionizing Plasma Flux in the Martian Upper Atmosphere," The Atmo- 
sphere of Venus and Mars, Brandt, J. C. and M. B. McElroy, eds., Gordon and Breach, 
New York, p. 241. 
Dolginov, Sh. Sh., Ye. G. Yeroshenko, and L. N. Zhuzgov, 1973, "Magnetic Field in the Very 
Close Neighborhood of Mars According to  Data from Mars-2 and Mars3 Spacecraft," J. 
Geophys. Res., 78, p. 4779. 
Gringauz, K. I., V. V. Bezrukikh, M. I. Verigin, and A. P. Remizov, 1974, "On Electron and 
Ion Components of Plasma in the Antisolar Part of Near Martian Space," Report of Insti- 
tute for Space Research, Academy of Sciences, USSR. See also 1975, Kossmicheskiye 
Issledovaniya, 13, p. 123. 
Michel, F. C., 1971, "Solar-Wind-Induced Mass Loss from Magnetic Field Free Planets," 
Planet. Space Sci., 19, p. 1580. 
Rassbach, M. E., R. A. Wolf, and R. E, Daniell, Jr., 1974, "Convection in a Martian Magneto- 
sphere," J. Geophys. Res., 79, p. 1 125. 
Spreiter, J. R., A. L. Summers, and A. Y. Alksne, 1966, "Hydromagnetic Flow Around the 
Magnetosphere," Planet. Space Sci., 14, p. 223. 
Spreiter, J. R., A. Y. Alksne, and A. L. Summers, 1968, "External Aerodynamics of the Mag- 
netosphere," Physics of the Magnetosphere, Caravillano, R. L., J. F. McClay, and H. R. 
Radoski, eds., Reidel, Dordrecht, Holland, p. 301. 
Spreiter, J. R., A. L. Summers, and A. W. Rizzi, 1970, "Solar Wind Flow Past Nonmagnetic 
Planets-Venus and Mars," Planet. Space Sci., 18, p. 128 1. 
Cloutier/Galeev: The asymmetry of the ionopause which results from the difference in the 
drift of photoions across the ionopause in the cases of inward and outward interplanetary 
electric fields will be diminished by two effects: 
1. The collective interaction of photoions and solar-wind ions could prevent the 
photoions' acceleration up to solar-wind velocity and they will not experience a 
complete cycloidal trajectory. 
2. If the ionopause is diffuse, then the difference between the inward and outward 
electric field cases wig be smaller. 
CloutierlNess: I point out that the last figure in your talk (see figure 2(a) in Rassbach et al., 
1974) is not a quantitative model but rather a schematic suggestion which does not bear a 
close relationship to the geometry of the observed conditions at Mars by the Mars-2, -3, and 
-5 spacecraft. That is, you indicate a very wide magnetosphere and tailward region which 
does not conform to the experimental observations of a much narrower tail region. 
Cloutierlvaisberg: Do you think azimuthal asymmetry of an obstacle necessarily leads to 
asymmetry of the bow shock, since an azimuthal component of the flow may develop due 
to the differences of an obstacle from a figure of rotation? 
Cloutier: Yes, I think shock asymmetry will result from obstacle asymmetry. 


Solar-wind interaction with planetary ionospheres

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