The large-scale photospheric magnetic field, measured by the Mr. Wilson magnetograph, has been analyzed in terms of surface harmonics (Pn"~(0)cosmq~ and P~m(O) sinm(o) for the years 1959 through 1972. Our results are as follows. The single harmonic which most often characterized the general solar magnetic field throughout the period of observation corresponds to a dipole lying in the plane of the equator (2 sectors, n=m=l). This 2-sector harmonic was particularly dominant during the active years of solar cycles 19 and 20. The north-south dipole harmonic (n = 1, m --0) was prominent only during quiet years and was relatively insignificant during the active years. (The derived north-south dipole includes magnetic fields from the entire solar surface and does not necessarily correlate with either the dipole-like appearance of the polar regions of the Sun or with the weak polar magnetic fields.) The 4-sector structure (n = m-2) was prominent, and often dominant, at various times throughout the cycle. A 6-sector structure (n --m = 3) occasionally became dominant for very brief periods during the active years. Contributions to the general solar magnetic field from harmonics of principal index 4 ~< n ~< 9 were generally relatively small throughout this entire solar cycle with one outstanding exception. For a period of several months prior to the large August 1972 flares, the global photospheric field was dominated by an n -5 harmonic; this harmonic returned to a low value shortly after the August 1972 flare events. Rapid changes in the global harmonics, in particular, relative and absolute changes in the contributions of harmonics of different principal index n to the global field, imply that the global solar field is not very deep or that very strong fluid flows connect the photosphere with deeper layers.
1. The General Solar Field
The discovery of polar plumes (or rays) in the corona at times of minimum solar activity led to the hypothesis that the Sun possesses a large-scale dipole magnetic field similar to that of the Earth (Bigelow, 1889;Stormer, 1911). Hale (1908) pioneered the measurement of the Zeeman splitting of photospheric iron lines and detected strong magnetic fields in sunspots. Outside sunspot regions, however, the photospheric fields are weak and the Zeeman components are not widely split. Thus Hale, using photographic spectroscopy, was not really successful in determining the strength or