Effect of Sun's magnetic field polarity on interplanetary magnetic field B z

The results of a statistical study of interplanetary magnetic field IMF B z are presented for two minima of solar activity for the periods of 1985–1987 and 1995–1997, related to negative and positive polarity of the Sun's magnetic field, respectively, for low sunspot numbers and relatively quiet solar wind conditions. The negative and positive polarities correspond to conditions when the Sun's magnetic field in the northern hemisphere is directed toward and outward the Sun, respectively. We found that IMF B z is dependent on the magnitude of IMF horizontal ( B x ) component, IMF orientation, and Sun's magnetic field polarity. The correlation between IMF B z and ∣ B x ∣ is positive for positive polarity and negative for negative polarity. Another interesting result is the fact that positive and negative IMF B z tend to occur predominantly during IMF orientations significantly different from the Parker spiral at the angle ϕ* ≈ ±60° where ϕ* is the acute angle measured counterclockwise from Parker spiral direction to IMF vector in the horizontal plane. The effect of Sun's magnetic field polarity is especially evident for large IMF ∣ B x ∣ > 4 nT. In this case, IMF B z are predominantly negative for negative polarity and positive for positive polarity, and the difference between average IMF B z for positive and negative Sun's magnetic field polarities is ∼2 nT. Correlation coefficient characterizing the dependence of IMF B z on Sun's magnetic field polarity is about 0.5 and increases with increasing IMF ∣ B x ∣. The observed features of IMF B z may be caused by decreasing solar wind speed near the HCS. As a result, portions of disturbed magnetic field lines slow down while approaching the HCS. This leads to the rotation of IMF vector in the vertical and horizontal planes. The observed dependences of IMF B z on IMF ∣ B x ∣, IMF orientation, and Sun's magnetic field polarity are qualitatively consistent with this hypothesis.