On the winding of the IMF spiral for slow and fast wind within the inner heliosphere

Magnetic field and plasma hourly data, recorded within the inner heliosphere from 1975 through mid 1981, have been used to test the interplanetary magnetic field configuration as predicted by the Parker theory against observed interplanetary magnetic field spiral configuration. Following this theory, magnetic field lines should always be aligned with wind flow direction when observed in a reference system corotating with the Sun. Our observations showed a general tendency for the field lines to be more tightly wound with respect to the predicted spiral configuration in agreement with similar studies performed around and beyond 1 AU by other authors. If we set the corotation radius b = 20R(5), we obtain an average overwinding of about 2.7 degrees +/- 0.3 degrees. Our results also showed that field lines associated with fast and slow wind have different winding offsets with respect to the predicted Parker spiral. A possible explanation might be in a different value of the corotating radius b depending on the speed of the wind. A value of b = 20R, for slow wind would suggest a b=12R(5), for fast wind.