Tracking a high-latitude corotating stream for more than half a solar rotation

A sky survey of interplanetary scintillation has been used to map a major corotating structure which extended to high helio-latitudes during 17 July–5 August 1979. Observations of 900 sources enabled the feature to be tracked for 20 days as it approached and passed over the Earth, and its 3-dimensional structure was determined by comparing the data with theoretical maps computed for simple models. The main features of the disturbance, including both its shape and development with radial distance, could be explained by a sector of enhanced density followed by a wider sector of below-average density, in accordance with the structure usually associated with a high-speed solar wind stream. Our observations and deductions are in broad agreement with spacecraft measurements of the solar plasma near Earth, although the high-speed flow was less prominent than expected. This may have been due, in part, to the location of the main disturbance above the ecliptic but our observations indicated significant temporal evolution and it is likely that the speed of the stream was decreasing as it passed the Earth.