The correlation of the mean solar magnetic field and the interplanetary magnetic field reported by Wilcox et al. (1969) and Severny et al. (1970) has been interpreted by comparing the relationship of the measurement of the mean solar field with the physics involved in the formation of the interplanetary field. The high correlation observed is thus interpreted as a fortuitous correspondance between two integrals. The high correlation thus provides further support for the source surface model involved in these calculations. A new method is then suggested for observing the 'mean solar field' that might improve the correlations slightly.
Recently Wilcox et aL (1969) have reported a correlation of the daily mean magnetic field of the sun (seen as a star) with the polarity of the interplanetary magnetic field observed 42 ~-days later near the earth for the interval March-June 1968. They report an almost complete agreement. These observations are quite surprising since the mean solar field is an average of the sun's field over an entire hemisphere (13 89 days by solar rotation) whereas the interplanetary field is thought to be a direct extension of the solar field, thus not correlating with the mean field from an entire solar hemisphere. This paper attempts to explain these observations using the 'source surface' model of Schatten et al. (1969).
In this model a potential field exists close to the sun. Beyond some distance, about 0.6 solar radii above the photosphere, the solar wind plasma begins to carry the magnetic field outward. This model has been tested by comparisons of solar eclipse structure from 1-3 solar radii, of Faraday rotation measurements of the coronal field from 4-12 solar radii, and of interplanetary magnetic field observations near the earth at 1 AU with computations from photospheric field observations; see Schatten et al. (1969), Stelzried et aL (1970( ), Schatten (1969( , 1970) ) and Smith and Schatten (1970).
Figure 1 illustrates the manner in which the source surface model suggests the mean solar field-interplanetary field correlation. The mean solar field represents an average of the photospheric field over the solar disk with an appropriate weighting factor. This factor is a function of the spherical angle from that portion of the photosphere to the subsolar point. The main contributions to this factor are an area projection factor due to the difference between the magnetograph measuring the line-of-sight magnetic field and the angular distribution of the direction of the photospheric field (perhaps radial on the average). Limb darkening and effects of sunspots, not seen by the magnetograph, are also contributing factors.
The 'source surface' model states that the interplanetary field near the earth results
Solar Physics 15 (1970) 499-503.