Photospheric magnetic field distribution around AR 8100, taken during Carrington rotations from 1929 to 1933, adapted from the synoptic maps provided by the Wilcox Solar Observatory. The neutral field line is drawn as a black solid line, while positive and negative contours are drawn as colored solid and dashed lines. The greenish squares in panel a) are the footpoints of the open magnetic field lines. The CH boundaries are based on He i 10 830 Å line observations from the Kitt Peak telescope and are shown as a gray long-dashed contour.