Predictions of Solar Wind Speed and IMF Polarity Using Near-Real-Time Solar Magnetic Field Updates

1. Purpose & Basis Purpose of Model: The Wang-Sheeley model is an empirical model that predicts the background solar wind speed and the interplanetary magnetic field (IMF) polarity at Earth. Basic Hypothesis: That the magnetic flux tube expansion factor (fs) and solar wind speed are inversely correlated. Small fs fast solar wind Large fs slow solar wind fs= (R/Rs)2[BP(R)/BP(Rs)] = Rate at which magnetic flux tubes expand between the photosphere and the source surface (Rs=2.5R). Photospheric & Coronal Fields Photospheric Magnetic Field Synoptic map of the photospheric magnetic field distribution Br. Statistical comparison of WSO, MWO, and MWOp predictions over the entire 5-year data set. Each Dot represents the statistical comparison between 4-day-advanced model predictions and the WIND or ACE satellite data for a temporal bin spanning 3 full Carrington rotations. The TOP and MIDDLE panels show the statistical comparison between the predicted and observed velocities using, respectively, the average fraction deviation and the correlation coefficient. The BOTTOM panel is the fraction of correct IMF polarity predictions for each method. The green sinusoidal lines are the solar b angle. Source Surface Map Source surface field Br , where R = 2.5 R. Uncorrected (open circles) and corrected (filled diamonds) WSO (left) and MWO (right) northern and southern polar magnetic field strengthsas a function of time (late 1993 to late 1999) for periods when the magnitude of the solar b angle is greater 5o. (Fig. from Schatten, 1971) Comparison of the WSO, MWO, and MWOp (i.e., MWO with polar field corrections) 4-day-advanced solar wind speed predictions with WIND or ACE satellite velocity observations (solid black lines) for Carrington rotations 1901 (top) and 1921 (bottom). Comparison of the WSO, MWO, and MWOp 4-day-advanced IMF polarity predictions with WIND or ACE satellite observations (solid black line) for Carrington rotations 1901 (top) and 1921 (bottom). Solar Wind Speed & Coronal Hole Area Solar Wind Speed (at the source surface) v(km/s) color > 560 red 560-490 white 490-420 yellow 420-350 green < 350 blue Inferred Coronal Hole Areas Photospheric footpoints (colored dots) of open field lines, with different colors used to indicate the expansion factors fs associated with the flux tubes. Areas shaded light and dark gray are closed field lines . Predictions of Solar Wind Speed and IMF Polarity Using Near-Real-Time Solar Magnetic Field Updates C. “Nick” Arge University of Colorado/CIRES & NOAA/SEC Vic Pizzo NOAA/SEC Modifications Made to the Original Model Abstract Reliable predictions of the background solar wind speed and interplanetary magnetic field (IMF) polarity can be directly used to improve forecasts of high-speed streams, current sheet crossings, and many other space weather applications. Our application of the Wang & Sheeley model provides solar wind speed and IMF polarity based on current daily photospheric magnetic field measurements (modified by a number of correction techniques) from multiple ground based solar observatories. Using WIND and ACE data for comparisons, we find that the model makes successful predictions of background solar wind conditions from about a year before the 1996 solar minimum to at least 3 years into the ascending phase of the solar cycle. • 1) The solar wind velocities are first assigned at source surface using the following empirical function relatingV withfs: • For WSO: • V(fs) = 285 + 650/(fs)5/9 (km s-1) • or • If fs 1.9, then V(fs) = 725 km s-1 • For MWO: • V(fs) = 250 + 675/(fs)1/2.2 (km s-1) • or • If fs 2.3, then V(fs) = 725 km s-1 • 2) The solar wind is then propagated from source surface to the Earth using radial propagation and a simple method to account for stream interactions. Verification Polar Field Correction Solar Wind Speed and IMF Polarity Predictions Summary Our modified version of the Wang & Sheeley model makes successful predictions of the background solar wind speed and IMF polarity during the first three years of the ascending phase of the solar cycle, extending the period of validity of the model beyond the minimum phase. The statistical results presented here provide only a lower limit to the actual agreement between the predictions and "true" background parameters, since transients where not taken into account. Quality control of the input magnetic data is essential for improving the predictive success of the model. Joint use of data from multiple solar observatories can (potentially) improve the predictive reliability of the model.