Nonzero azimuthal magnetic fields at the solar source surface: Extraction, model, and implications
Hagen Schulte in den Baeumen
School of Physics, University of Sydney
Iver H. Cairns
Peter A. Robinson
Last modified: July 26, 2011
A recent two-dimensional (radial distance and solar longitude) model for the solar wind is driven using 1-hour average data from the Wind spacecraft. We extend the treatment of the Sun's magnetic field to allow a nonzero azimuthal component Bφ at the source surface, assumed to be at the photosphere where r = Rs, in addition to the radial component Br. We find nonzero azimuthal magnetic fields at the source surface with important consequences for the distant polar heliospheric magnetic field. The averages |Bφ(1Rs)| and |Br(1Rs)| inferred over solar cycle 23 were 440 ± 480 nT (4.4 ± 4.8 mG) and 110 ± 80 μT (1.1 ± 0.8 G) at the photosphere, respectively. Both components vary with time by more than an order of magnitude, with |Bφ(1Rs)| ≤ |Br(1Rs)|. While the surface magnetic field is closely radial on average it is sometimes 20 degrees from radial. The azimuthal component varies smoothly on periods of order a day with evidence for relatively narrow current sheets. Both Bφ(1Rs) and Br(1Rs) vary with the solar cycle: Br(1Rs) is correlated with the sunspot number, but with a time lag of 20 months, while Bφ(1Rs) has a two-level behaviour, decreasing near solar maximum and increasing near solar minimum. Our results and model can account naturally for non-Parker-like magnetic field directions at 1 AU since the azimuthal components inferred at the source surface lead to the radial and azimuthal components at 1 AU having similar average magnitudes and large variability.