The Solar Activity Cycle and Sun-As-A-Star Variability in the Visible and Infrared
Karen L. Harvey, Solar Physics Research Corporation

3. Variations in Spatial Distribution of Solar Activity

The emergence and evolution of solar activity follows characteristic patterns at the surface, giving rise to a well-defined sequence of events that occur during a solar cycle and, as discussed in the previous section, result in variations in solar irradiance. Two spatial patterns of activity are discussed in this section: activity pulses and the latitude distribution as a function of the solar cycle.

3.1. Activity Pulses and Complexes of Activity

As discussed in Section 2, solar activity occurs in pulses of between 2 to 12 months. These pulses result strictly from active region emergences, as shown in Figure 10. In this Figure, the variation of total magnetic flux in active regions is compared to that in the quiet sun determined from the NSO/KP synoptic magnetic field maps. The separation of these two components is done by setting a simple threshold of 25 Mx/cm2 (Harvey, 1994). Figure 10 illustrates that most if not all of the increase in magnetic flux during an activity pulse occurs in the active region component, as for example the pulse indicated by an arrow. There is an increase in the quiet component. It is slightly delayed, however, and results from the weakening active region flux as it disperses across the surface.

FIGURE 10: A comparison of total magnetic flux within `active regions' and the `quiet sun' defined in the NSO/KP synoptic rotation magnetic maps as >25 and < 25 Mx/cm2. The arrow indicates a pulse of activity in the spring of 1978.

The spatial distribution of activity during these pulses of activity can be traced to the strong tendency for active regions to preferentially emerge within already existing active regions (Castenmiller et al., 1986; Brouwer and Zwaan, 1990; Harvey and Zwaan, 1993). This tendency gives rise to spatial and temporal clustering of active regions in one dominant activity complex, particularly in the rise and decline of a cycle, or many activity complexes spread throughout the activity belts in the maximum phase of a cycle. Figure 11 illustrates one such activity complex, studied extensively by Gaizauskas et al. (1983), that dominated the pulse of activity indicated by the arrow in Figure 10.

FIGURE 11: Series of NSO/KP synoptic rotation magnetic maps for 12 Carrington rotations: 1660 --- 1673. In these grey-scale images, white indicates areas of positive polarity and black negative polarity. The latitude range of each panel extends from 0o to N90o; this scale is in sine latitude.

The tendency for active regions to emerge within existing regions plays an important role in perpetuating the large-scale pattern of unipolar magnetic flux apparent on the solar surface at all but the minimum phase of a cycle. These large-scale patterns are produced by the dispersion of the active region magnetic by the supergranular flows, differential rotation, and meridional motions which ultimately carry the flux to the polar regions. Only a small portion (< 30%) of the magnetic flux that originally emerges in active regions participates in this process, and by the time the dispersing magnetic flux reaches the poles, only about 1% remains at the surface.

3.2.Butterfly Diagram: Sunspots and Magnetic Flux

Over the long-term, active regions emerge at the surface forming a pattern of what is described as butterfly wings in a latitude-time plot; this type of plot is known as a butterfly diagram. As described in Section 1 and shown in Figure 12, the first appearance of new cycle active regions occurs in the latitude range of 20-30° . Over the course of a cycle, this latitude range expands toward the equator and higher latitudes in the rise and maximum phase. The largest range for solar activity is reached during maximum when regions are observed from the equator up to latitudes of 30° to 45° . This latitudinal extent is also larger for higher amplitude cycles than lower amplitude cycles; compare, for example, the latitudinal extent of regions between cycles 19 and 20 in Figure 12. Following the maximum phase, the higher latitude bound of active regions moves progressively to lower latitudes. Figure 12 also shows the temporal overlap of activity belonging to two solar cycles: the outgoing at lower latitudes and the incoming at higher latitudes.

FIGURE 12: The butterfly diagram latitude-time plot of sunspot regions from 1942 to the present. Cycle membership is based on latitude and polarities of regions; o's are the odd-numbered cycles and +'s even-number cycles. The times of cycle minimum are shown by the light, dotted vertical bars.

Figure 13 is a butterfly diagram of magnetic flux constructed from the NSO/KP synoptic rotation magnetic maps. For each rotation, the net magnetic flux (the arithmetic sum of the positive and negative flux corrected for geometric effects) was summed across all longitudes for each latitude bin. Each rotation represents a column in the diagram shown in Figure 13.

FIGURE 13: A latitude-time grey-scale image of net magnetic flux based on the NSO/KP synoptic rotation magnetic maps from April 1975 to the present. White depicts positive polarities and black negative polarities.

This butterfly diagram clearly shows the activity belts in the northern and southern hemisphere that coincide with the sunspot butterfly diagram. Of note, however, is the overall polarity pattern; the follower polarity of active region occurs along the poleward side of the activity belts and the leader the equatorward side. This pattern is reversed between the northern and southern hemispheres and in successive cycle in concert with the reversal of the polarities of the leader and follower portions of active regions between hemispheres and with each new cycle. Also seen are a succession of long-lived patterns of alternating unipolar patterns of magnetic flux extending from the activity belts to the polar regions. Those streams of the follower polarity ultimately lead to the reversal of the polar fields in each hemisphere during the maximum phase of a solar cycle. Each of the streams can be traced to the emergence of large active regions or complexes of activity, indicating that they result from the dispersal of active region magnetic flux.

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