The Floor in the Solar Wind Magnetic Field Revisited

Abstract

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 ≈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 ≈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 ≈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 – 23 and B _MIN at their preceding minima. These correlations suggest that at 11-year minima, B consists of i) a floor of ≈2.8 nT, and ii) a component primarily due to the solar polar fields that varies from ≈0 nT to ≈3 nT. The solar polar fields provide the “seed” for the subsequent sunspot maximum. Removing the ≈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 – 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 ≈70% of the time. We propose that the floor corresponds to a baseline (non-cyclic or ground state) open solar flux of ≈8×10¹³ Wb, which originates in persistent small-scale (supergranular and granular) field.