Improving the predictions of solar wind speed and interplanetary magnetic field at the Earth

The Wang-Sheeley-Arge (WSA) model, an advanced version of the potential field source surface (PFSS) model, is widely used to predict the solar wind speed (SWS) and the interplanetary magnetic field (IMF) polarities at the Earth. The results, however, do not always match the observations. To improve the predictive capability of this model we made the following changes: (1) We used the high resolution magnetograms produced by the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observer (SOHO) spacecraft. We properly calibrated the magnetic field strength of the MDI observations using the Mt. Wilson (MWO) FeI magnetograms so that each MDI level 1.8 magnetogram can be converted to the same basis as the saturation-corrected long-duration MWO Fel magnetogram. (2) The WSA model requires a map of full solar surface magnetic field, and traditionally a synoptic chart is used. However, the synoptic chart does not represent the full solar surface at a particular time. Therefore, we suggest to use a new format called heliospheric (or snapshot) map in the model. (3) We implement a better estimate of the polar field that is not observable during some part of the year due to the solar tilted angle B0. The magnetic field near the solar poles is very important because it may be the dominant part of the solar magnetic field far away from the Sun, especially during the period of solar minimum. (4) The WSA model assumes that the solar photospheric magnetic field is nearly radial, so that its radial component can be obtained directly from the line-of-sight (LOS) of the observed field. This approach produces very strong radial magnetic field near the solar poles. We solve this problem by first obtaining the spherical harmonic coefficients directly from the LOS magnetic data and then reconstructing the radial magnetic chart. (5) Finally, changing the radius of the source surface, rss, in the PFSS analysis strongly affects the predicted SWS and IMF at the Earth. Our results show that the smaller the value of rss, the stronger the IMF is computed at 1 AU, and also rss may be a function of the solar cycle.