Australian Space Science Conference 2011
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Hagen Schulte in den Baeumen

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

     Full text: Not available
     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.

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