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such as that reported by Dolginov et al. (1973). However, for the dipole moment reported of 2.6 X 1022 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
Alfvén, H. and C. G. Falthammar, 1973, Cosmical Electrodynamics, 2nd. ed, Oxford University 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 Ionosphere by the Solar Wind," J. Geophys. Res., 74, p. 6215.
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 Atmosphere 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 Mars-3 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 Institute for Space Research, Academy of Sciences, USSR. See also 1975, Kossmicheskiye Issledovaniya, 13, p. 123.
Michel, F. C., 1971, "So1ar-Wind-Induced Mass Loss from Magnetic Field Free Planets," Planet. Space Sci., 19, p. 1580.


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