Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh13b1547s&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SH13B-1547
Physics
2134 Interplanetary Magnetic Fields, 2169 Solar Wind Sources, 7509 Corona, 7524 Magnetic Fields, 7529 Photosphere
Scientific paper
Source-surface models of the solar corona and heliosphere typically entail spherical-harmonic expansions of scalar magnetic potentials that extend to rather high degree and order so as to accommodate localized magnetic fields associated with sunspots. Here it is proposed to approximate sunspot magnetic fields in the corona by placing a few magnetic monopoles beneath the Sun's surface (i.e., inside the Sun), so as to reduce the number of internal spherical harmonics required for a good fit. (Magnetic monopoles are permitted in such a construction if they add-up to zero magnetic charge and reside outside the region of physical interest.) Magnetic image charges placed outside the source surface can serve to nullify the tangential component of the resulting magnetic field on a sphere of any desired radius (e.g., rs = 2.5rsun). More generally, however, the source surface should be non-spherical (typically prolate) in a way that reflects the underlying structure of the Sun's main B field. A source surface that seems to work well in this respect [Schulz, Ann. Geophysicae, 15, 1379-1387, 1997] is a surface of constant F = r- kB, where B is the scalar strength of the Sun's main magnetic field (including any sunspot fields thus modeled) and k (~ 1.4) is a shape parameter. If the region surrounded by the source surface is regarded as current-free, then the source surface itself should be made (as nearly as possible) an equipotential surface for the corresponding magnetic scalar potential (expanded, for example, in a combination of spherical harmonics and offset-monopole terms). More generally, the mean-square tangential component of the coronal magnetic field on the source surface should be minimized [Schulz et al., Solar Phys., 60, 83-104, 1978] with respect to any adjustable parameters (e.g., external spherical-harmonic expansion coefficients) of the field model. Solar wind should then be regarded as flowing not quite radially, but rather in a straight line along the outward normal to the source surface, and the heliospheric B field should follow a corresponding generalization of Parker's spiral [Levine et al., Solar Phys., 77, 363-392, 1982].
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