SPRG Seminars
February 14, 2012:
"Polar Fields in 17GHz"
Leif Svalgaard, Stanford
When Hale discovered solar magnetism he thought it evident that the Sun, like the Earth, would also have a dipolar 'General Magnetic Field' and looked for it, but what he found turned out to be spurious. The polar fields were only really observed in the 1950s by the Babcocks, and with any accuracy by us at WSO in the 1970s [and ever since]. We found that the polar fields must be strongly concentrated near the poles [B=Bp cos^8 (colatitude)] giving rise to a strong [factor of two] annual variation with the B0-angle. But observing the polar fields with a Line-of-Sight magnetograph is difficult, so detailed information as to evolution and distribution of the fields [not to speak about the physics] is hard to come by and fraught with uncertainty. There is, however, another way: The radioheliograph at Nobeyama [Japan] makes images of the Sun with a beam width of 5" at 17 GHz, corresponding to a brightness temperature of 10,000K and up.
The radio maps show the usual limb brightening as well as active region increases, but in addition one finds a number of 'bright patches' near the poles [and generally over every coronal hole, when near the limb]. The brightness temperature increases ~1500K in the patches over the background 10,000K. The number of such patches is strongly correlated with the magnetic field measured at WSO and can, actually, be used as a good proxy for the polar fields, and show strong rotational modulation. The origin of the patches is a mystery. There does not seem to be any coronal signature overlying the patches [e.g. they are not visible with AIA]. Having a mystery is always exciting, especially when it can be put to practical use as direct determinant of the polar fields used in coronal modeling.