Solar corona electron density distribution

Three and one-half months of single-frequency (f=02.2 x 109 Hz) time delay data (earth-to-spacecraft and return signal travel time) were acquired from the Helios 2 spacecraft around the time of its solar occupation (May 16, 1976). Following the determination of the spacecraft trajectory the excess time delay due to the integrated effect of free electrons along the signal's ray path could be separated and modeled. An average solar corona, equatorial, electron density profile, during solar minimum, was deduced from time delay measurements acquired within 5--60 solar radii (R/sub S/) of the sun. As a point of reference, at 10 R/sub S/ from the sun we find an average electron density of 4500 el cm-3. However, there appears to be an asymmtry in the electron density as the ray path moved from the west (preoccultation) to east (post-occulation) solar limb. This may be related to the fact that during entry into occulation the heliographic latitude of the ray path (at closes approach to the sun) was about 6 0, whereas during exit it became -7 0. The Helios electron density model is compared with similar models deduced from a variety of different experimental techniques. Within 5--20 R/sub S/ of the sun the models separate according to solar minimum or maximum conditions; however, anomalies are evident