Solar Wind Stream Interactions and the Wind Speed–Expansion Factor Relationship

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Abstract
Previous studies have shown that the solar wind speed observed near Earth is inversely correlated with the divergence rate of magnetic flux tubes near the Sun. We test the global validity of this relationship by employing Ulysses wind speed measurements during 1990-1997 as well as inecliptic data for 1976-1997. When the correspondence between wind speeds and expansion factors is adjusted to yield optimal agreement with the high-latitude Ulysses measurements, the model matches the overall patterns of fast and slow wind near the ecliptic but predicts too much very fast wind there. We show how this discrepancy can be resolved by taking account of wind stream interactions, where we apply a crude algorithm based on the propagation times of neighboring wind parcels; the interactions reduce the amount of very fast wind at latitudes where slow wind is present. We also test and reject an alternative model in which the wind speed is assumed to be a function only of angular distance from the heliospheric neutral sheet.