Large-Scale Magnetic Field

Abstract The structures of the solar wind and the interplanetary magnetic field vary systematically over a period of 11 years in relation to the solar activity cycle. The solar activity cycle is traditionally measured by the sunspot number. The sunspot number from 1962 to 1993 is shown in Fig. 2.1. Four phases of the solar cycle can be identified: solar minimum, the ascending phase, solar maximum, and the descending phase. The three solar minima observed during the space age to date are shown in Fig. 2.1: 1964-65, 1976, and 1986. All the minima have similar values for the sunspot number. Three solar maxima observed during the space age to date are also shown in Fig. 2.1: a broad maximum centered around 1969, a larger maximum in 1979, and a comparable maximum in 1989. An ascending phase extends from a solar minimum to the following solar maximum, and the descending phases extends from a solar maximum to the following solar minimum.