The Floor in the Solar Wind Magnetic Field Revisited

Svalgaard and Cliver (Astrophys. J. Lett. 661, L203, 2007) proposed that the solar-wind magnetic-field strength [B] at Earth has a "floor" value of a parts per thousand 4.6 nT in yearly averages, which is approached but not broached at solar minima. They attributed the floor to a constant baseline solar open flux. In both 2008 and 2009, the notion of such a floor was undercut by annual B averages of a parts per thousand 4 nT. Here we present a revised view of both the level and the concept of the floor. Two independent correlations indicate that B has a floor of a parts per thousand 2.8 nT in yearly averages. These are i) a relationship between solar polar-field strength and yearly averages of B for the last four 11-year minima (B (MIN)), and ii) a precursor relationship between peak sunspot number for cycles 14 -aEuro parts per thousand 23 and B (MIN) at their preceding minima. These correlations suggest that at 11-year minima, B consists of i) a floor of a parts per thousand 2.8 nT, and ii) a component primarily due to the solar polar fields that varies from a parts per thousand 0 nT to a parts per thousand 3 nT. The solar polar fields provide the "seed" for the subsequent sunspot maximum. Removing the a parts per thousand 2.8 nT floor from B (MIN) brings the percentage decrease in B between the 1996 and 2009 minima into agreement with the corresponding decrease in solar polar-field strength. Based on a decomposition of the solar wind (from 1972 -aEuro parts per thousand 2009) into high-speed streams, coronal mass ejections, and slow solar wind, we suggest that the source of the floor in B is the slow solar wind. During 2009, Earth was in slow solar-wind flows a parts per thousand 70% of the time. We propose that the floor corresponds to a baseline (non-cyclic or ground state) open solar flux of a parts per thousand 8x10(13) Wb, which originates in persistent small-scale (supergranular and granular) field.