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 μT and in 2006 when the polar fields were ~60 μT, 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 ~4 × 1014 Wb, corresponding to a constant strength (~1011 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.