Fig. 1. Profile of the Skylab I launch trajectory, along with the calculated linear rate of exhaust emission per kilometer of ground track. ground air), the falling gases impart substantial velocity to the air. When the air participates in the downward motion, it is less effective for slowing the descent. During that early stage, the falling velocities can be quite large, and it is important to understand the processes in some detail. These details are not included in our model. We attempt to simulate their effects by the way we set initial conditions for the numerical problem, as we describe later. 3. SKYLAB I LAUNCH The most complete set of actual data on an ionospheric depletion resulting from a rocket launch was obtained on the launch of Skylab I, as reported by Mendillo et al. (1,2). An important test for our two-dimensional models, naturally, is to run a set of computations representing that specific case and to compare the computed results with the data. The data of Mendillo et al. for ray paths penetrating the ionospheric F2 peak at a distance of 300 to 500 km from the rocket path (from an observing station at Sagamore Hill, MA), showed an abrupt drop in electron column density to below 50% of its normal value. The low electron densities persisted for about 4 h. A similar but smaller drop was observed for a ray path passing 900 to 1000 km from the rocket track (as observed from a station at London, Ontario). A definite but still less abrupt drop was observed from Goose Bay, Labrador, on a ray path 1200 km from the track at the point of second-stage engine shutdown. A small but perceptible drop was observed from Urbana, Illinois, for a ray path about 1200 km from the rocket track. A possible but very indistinct drop was seen from Narssarssuaq, Greenland, on a ray path 2000 km from the engine shutdown point. From the aggregate of the observations, one must conclude that a substantial electron depletion was produced out to a distance of about 1000 km from the rocket bum trajectory, and a small but measura-
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