The electron density profile of the outer corona and the interplanetary medium from Mariner-6 and Mariner-7 time-delay measurements.
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The round-trip propagation time delays to the Mariner-6 and Mariner-7 spacecraft were measured over a 6 month period, including the occultations by the solar corona during April and May of 1970. The interplanetary medium was sampled over a range of ray impact parameters from about 5 to 100 solar radii. Electron density models fitted to the observations show that the density profile falls off nearly exactly as the inverse square of the distance from about 10 solar radii out to the Earth's orbit. The estimated values of the slopes are : 2.05 and : 2.08 for Mariner-6 and Mariner-7, respectively. The formal standard deviation on each estimate is +- 0.24. In general, the densities are about a factor of 2 smaller than those determined by eclipse photometric measurements. The latter result agrees with recent radio ray-bending and pulsar time-delay measurements. The fluctuations in the measured time delays show that the true standard deviation of the electron density at a given point is about 100% of the local mean density. The evaluations of the estimated electron density profiles at 1 AU yield 9 +- 3 electrons cm/sup -3/ for both spacecraft experiments, which were analyzed independently. Our results are strictly correctmore » only if the general-relativistic effects are those predicted by Einstein's general theory.« less