In conclusion, we make a few remarks about ionospheric irregularities. We now know of two distinct types of plasma instabilities that^ actually lead to irregularity growth in the ionosphere. They are the ExB Gradient Drift instability, which is responsible for striations in barium clouds, and the Gravitational Rayleigh-Taylor instability, which is responsible for the growth of Equatorial Spread F. Both of these instabilities have been well studied and are reasonably well understood and both may be associated with the ionospheric morphology produced by SPS operations. However, it cannot now be asserted whether that morphology will lead to growth or damping of irregularities. The Current Convective instability has recently been proposed as also occurring in the natural ionosphere. It could also be associated with SPS ionospheric morphology but has not yet been well studied. In summary, there are a number of plasma instabilities that might cause irregularities, but the morphology associated with the SPS scenario is insufficiently well known to state with some confidence which, if any, might actually apply. 3.3 EFFECTS ON SATELLITE DRAG (Curtis) The drag force on a satellite is given by whe^e v is the satellite velocity, S is its projected surface area normal_ to v, C^ is the drag coefficient (dimensionless number of order one) and p is the mean atmospheric mass density Here n^ is the total number density of the thermosphere and m^ and n^ are the mass and number density of the i-th constituent. The change in effective satellite altitude, z, is related to the density through the relation (Cook, et al., 1960; King-Hele, 1962; Cook and King-Hele, 1963) Thus one obtains a given change in altitude. Az for a circular orbit of period T which lies within a density enhancement Ap for a time AT: The enhanced change in z is then
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