Large-scale solar wind density enhancement and its boundaries: Helios 1, 2 and IMP 8 observations

In the present paper, we investigate a large pulse in the solar wind density observed on April 1–2, 1977 by the Helios 1, 2 probes and 2 days later by the IMP 8 spacecraft in the “line-up” condition (all three spacecraft had the positions along the Sun–Earth line). In this pulse the strong enhancement in density (∼50% relative to the undisturbed level) was not accompanied by significant changes in other main solar wind parameters. The outcomes of detailed analysis of physical properties as well as of geometrical features of the observed plasma pulse structure are discussed. One of the characteristic peculiarities of this pulse was its sharp boundaries (with durations of several minutes only). The main scientific result of our study is that the trailing edge of the pulse was found to be very stable (conserved its shape and duration) propagating over as large distance as 0.6 AU (90×106 km) or during 2.3 days of the solar wind motion.