Page:Solar wind.djvu/5

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SHAPE OF THE OBSTACLE—SYMMETRIES AND ASYMMETRIES

Although the flow patterns calculated by Spreiter et al. (1966, 1968, 1970) are axisymmetric, there are clearly asymmetries between opposite (polar) hemispheres in the altitude distribution of planetary ions added to the flow external to the ionopause owing to the oppositely-directed electric fields as seen from the planetary atmosphere. Cloutier et al. (1974) 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 ~O 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 pattern 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 terminators 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.[1] 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 corresponds to a larger effective obstacle polar diameter than equatorial diameter. One contributing effect is the loss in efficiency in the acceleration of planetary photoions by the flow at low (equatorial) latitudes due to decreasing angle between v and B. At the equator, the angle between v 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. Measurements in the Earth's magnetosheath indicate an average transverse noise component of


  1. Wolf, R. A., private communication, 1975.

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