Corotating Regions in the Solar Wind, Evident in Number Density Measured by a Radio-Propagation Technique


  • Thomas A. Croft


Coherent phase-modulated signals at 50 and 423 MHz are transmitted from Stanford University to specially equipped spacecraft orbiting the sun. Differential group delay and phase advance are measured; each of these parameters is approximately proportional to the average electron number density along the signal path. The experimental method enables us to discriminate between temporal and spatial variations of the solar wind by virtue of inherent spatial averaging. During the past 4.5 years, 10,000 hours of these data have been obtained from Pioneers 6, 7, 8, and 9 and Mariner 5. At times, measurements have been taken for two periods each day, by the use of first one spacecraft ahead of earth and then one behind. With such a technique, corotating flow patterns are clearly visible. Hourly samples are presented of the best data that were obtained by Pioneers 8 and 9 during the two-year period ending in February 1970. The density patterns reveal corotating regions in each of which a recognizable character is maintained during several rotations but for less than a year. This result is in contrast to results of other observers, who have seen corotating features lasting up to 1.5 years. The short duration observed here may be due to noisiness in these data or it may be typical of the active sun, whereas a longer duration is typical of quiet times. The observed density patterns are not consistent with the concept of steady corotating flows because large transients occur too fast and because identifiable features recur with nearly (but not exactly) the same period on successive rotations of the sun. These patterns could be a result of the corotation of thin steady streams that fluctuate both north-south and east-west by distances well in excess of the cross-sectional extent of the streams. It is also possible (and more likely) that the data indicate some corotating regions in which the solar wind is featureless and of low density and other regions characterized by high density and the prevalence of outward traveling plasma disturbances.