The Floor in the Solar Wind Magnetic Field Revisited
Abstract
Svalgaard and Cliver (Astrophys. J. Lett. 661, L203, 2007) proposed that the solarwind 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 (BMIN), and ii) a precursor relationship between peak sunspot number for cycles 14 23 and BMIN 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 BMIN 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 07, 2012
- Accession Number
- ADA562846
Entities
People
- A. G. Ling
- Edward W. Cliver
Organizations
- Air Force Research Laboratory