Skip to main content

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 ≈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.

This is a preview of subscription content, access via your institution.


Download references

Author information

Authors and Affiliations


Corresponding author

Correspondence to E. W. Cliver.

Additional information

The Sun – Earth Connection near Solar Minimum

Guest Editors: M.M. Bisi, B.A. Emery, and B.J. Thompson

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Cliver, E.W., Ling, A.G. The Floor in the Solar Wind Magnetic Field Revisited. Sol Phys 274, 285–301 (2011).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


  • Solar wind
  • Slow solar wind
  • Floor
  • Cycle 24