Predicting the Solar Wind Speed and Interplanetary Sector Structure during Solar Cycles 21–25

Abstract We use potential-field source-surface (PFSS) extrapolations of photospheric field data to predict the solar wind speed and interplanetary sector structure at Earth during solar cycles 21–25, and compare the results with in situ measurements. The wind speed is taken to be inversely correlated with the rate of coronal flux-tube expansion, which is derived using photospheric synoptic maps from the Wilcox Solar Observatory, the National Solar Observatory, the Global Oscillation Network Group, and the Helioseismic and Magnetic Imager. The extrapolations tend to predict too much fast wind near sunspot minimum, when the heliospheric current sheet is located near the ecliptic and the latitudinal gradients in the wind speed are very steep; the predictions are generally best during the declining phase of each cycle (with correlation coefficients of order 0.35–0.5), when large low-latitude extensions of the polar coronal holes give rise to recurrent high-speed streams. The overprediction of fast wind during 2009 and 2019 is associated with an overestimate of the areal sizes of remnant coronal holes and of the warping of the streamer belt. These in turn may be caused by an underestimate of the Sun’s axial dipole or polar field strength and by the omission of the effect of the low-latitude sheet currents beyond 2.5 R ⊙ on the near-coronal field. The latter currents act to decrease the sizes of open field regions relative to the PFSS model, with the result being similar to that of moving the source surface outward from 2.5 to 3.0 R ⊙ .