Traveling regions of high solar wind density observed in early August 1972

Summary. During the first 2 weeks of August 1972 the spatial average of solar wind electron concentration along the path from earth to Pioneer 9 was observed by the dual-frequency radio technique to vary in an unusual manner. The spacecraft was trailing earth in a heliocentric orbit, and its distances from earth and the sun were each approximately ¾ AU. A large increase in the concentration to ≈40/cm³ on August 3 was followed by a rapid decrease to an anomalously low value (under 1/cm³) on August 6. Another transient increase to ≈15/cm³ on August 9 was again followed by a region of low concentration. Others have measured number densities of 40/cm³ or 1/cm³ at points in the solar wind, but it is unusual to observe such densities as a spatial average over so long a path, because averaging tends to preclude extreme values. These observations permit discrimination between temporal and spatial flow patterns by virtue of the fact that any steady state flow during early August must have affected the measured spatial average for more than 4 days (the corotation delay). Because the high concentrations on August 3 and 9 each lasted only 1 day or less, it can be seen on this basis alone that the causative phenomena were not steady flows from the sun; rather, there must have been outward-traveling regions of unusually dense plasma. It is surmised that only the trailing edge of the first such region was still in the radio path when its geophysical manifestations were observed on August 4. If this interpretation is correct, the trajectory of the region of the most dense plasma probably appeared between earth and Pioneer 9.