Density fluctuations in different types of solar wind flow at 1 AU and comparison with results from Doppler scintillation measurements near the Sun

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Abstract
Density fluctuations with periods in the range 10 min to 1 hour are investigated using ISEE 3 plasma measurements of solar wind flows at 1 AU. Coronal hole, interstream, plasma sheet, coronal mass ejection, and interaction region flow types are considered. The ISEE 3 results are consistent with the interpretation of large‐scale variations in density fluctuations observed by Doppler scintillation measurements inside 0.2 AU. The highest absolute and fractional density fluctuations are found ahead of and within the plasma from coronal mass ejections, with the maximum values occurring between the associated interplanetary shocks and the driver gas. For the quasi‐stationary solar wind, density and fractional density fluctuations are highest and most variable in the plasma sheet in which the heliospheric current sheet is embedded, and lowest and least variable in the high‐speed coronal hole flow. A superposed epoch analysis of heliospheric current sheet crossings shows that the regions of enhanced density fluctuations previously identified in Doppler scintillation measurements near the Sun persist to 1 AU and are broader there. The results of this study confirm the advantages of using density fluctuations rather than density as a tracer of solar wind flows of differing origins at the Sun and as a detector of propagating interplanetary disturbances.