Streamer belt north-south asymmetry and its long-term evolution

The average solar wind (SW) speed at 1 AU is known to be faster (slower) in Spring than in Fall around positive (negative, respectively) helicity sunspot minima (Zieger and Mursula, 1998). While the average speeds from the two magnetic hemispheres are roughly equal, the effective latitudinal gradients of. the SW speed in the two magnetic hemispheres around the heliographic equator at 1 AU are dramatically different (Mursula et al., 2002a). There is a large effective gradient of about 5-10 km/s/deg in the southern magnetic hemisphere around each solar minimum but no statistically significant effective gradient in the northern magnetic hemisphere. Similarly, there is a large effective gradient in SW temperature of about 2700-5400 K/deg in the southern magnetic hemisphere while no significant gradient exists in the northern hemisphere (Mursula et al., 2002b). Accordingly, the streamer belt at 1 AU is systematically displaced toward the northern magnetic hemisphere. This displacement implies a new, persistent north-south asymmetry related to the solar magnetic cycle which needs to be explained by realistic solar dynamo models. We also discuss the long-term evolution of this asymmetry and show that the north-south asymmetry depicts a century-scale oscillation, being strong during high solar activity. Thus, the solar dynamo is the more asymmetric the stronger it is.