A Floor in the Solar Wind Magnetic Field

Abstract

Long-term (∼130 years) reconstruction of the interplanetary magnetic field (IMF) based on geomagnetic indices indicates that the solar wind magnetic field strength has a "floor," a baseline value in annual averages that it approaches at each 11 yr solar minimum. In the ecliptic plane at 1 AU, the IMF floor is ∼4.6 nT, a value substantiated by direct solar wind measurements and cosmogenic nuclei data. At high heliolatitudes, Ulysses measured a constant radial field with a magnitude (normalized to 1 AU) of ∼3.2 nT during solar minimum conditions in ∼1995 when the observed solar polar fields were ∼100 mT and in 2006 when the polar fields were ∼60 mT, as well as for solar maximum conditions in 2001 when the polar fields were close to zero. We identify the floor with a constant (over centuries) baseline open magnetic flux at 1 AU of ∼ Wb, corresponding 14 4 # 10 to a constant strength (∼10 11 A) of the heliospheric current. Solar cycle variations of the IMF strength ride on top of the floor. The floor has implications for (1) the solar wind during grand minima-we are given a glimpse of Maunder minimum conditions at every 11 yr minimum; (2) current models of the solar wind-both source surface and MHD models are based on the assumption, invalidated by Ulysses, that the largest scale fields determine the magnitude of the IMF; consequently, these models are unable to reproduce the high-latitude observations; and (3) the use of geomagnetic input data for precursor-type predictions of the coming sunspot maximum-this common practice is rendered doubtful by the observed disconnect between solar polar field strength and heliospheric field strength.

The floor in the solar wind: status report

“…Arnab Choudhuri: In our paper of 2007(Jiang et al, MNRAS, 381, 1527 we made some effort to use the polar faculae as a proxy for the next activity cycle. We found two outliers.…”

“…2. Cliver and Ling therefore suggested that the floor had a value of ∼2.8 nT, significantly below the Svalgaard et al (2007) value of ∼4.6 nT and also under Crooker and Owens' revised value of ∼3.7 nT. The reduced floor value of 2.8 nT has several advantages.…”

“…Because the floor of ∼4.6 nT in the solar wind proposed by Svalgaard and Cliver (2007) was thought to apply to the Maunder Minimum, the precursor relation in Fig. 1(a) implies that this value, which was approached at every 11-yr minimum during the space age, is relatively independent of the solar polar field strength.…”

“…This misconception was based in part on Ulysses observations through November 2006 (Balogh & Smith, 2006) which showed little change in the radial field strength between fast latitude scans near the minima following solar cycles 22 and 23. As Svalgaard and Cliver (2007) wrote, "While the radial IMF strengths [B R ] are essentially identical for these minima, the solar polar magnetic fields are ∼40% weaker at present (60 µT vs. 100 µT for ∼1995; Svalgaard et al (2005) and current data from Wilcox Solar Observatory [WSO]). It seems hard to escape the conclusion that the polar fields do not determine the magnitude of the IMF at solar minimum."…”

“…The concept of a floor in the solar wind magnetic field strength (B) was introduced separately by Svalgaard and Cliver (2007) and Owens et al (2008). Svalgaard and Cliver posited a floor in B of ∼4.6 nT based on their long-term geomagnetic-based reconstruction of B and the correlation of B with the sunspot number (SSN ) while Owens et al (2008) obtained a B value of ∼4 nT at solar minimum when the rotation-averaged coronal mass ejection rate was extrapolated to zero.…”