4.8 Active latitudes

While Spörer’s name is often attached to the concept of sunspot zones and their drift toward the equator, it appears that Carrington was the first to discover it. Carrington (1858) noted that the sunspots prior to the “minimum of frequency in February 1856” were confined to an equatorial band well below 20° latitude. He went on to note that after that date two new belts of sunspots appeared at latitudes between 20° and 40° latitude in each hemisphere. The RGO USAF/NOAA sunspot area and position data plotted in Figure 8View Image was used by Hathaway et al. (2003Jump To The Next Citation Point) to investigate the nature of this equatorward drift. The individual sunspot cycles can be separated near the time of minimum by the latitudes of the emerging sunspots (and more recently by magnetic polarity data as well). The centroid positions of the sunspot areas in each hemisphere are then calculated for each solar rotation. While Hathaway et al. (2003) investigated the latitude positions as functions of time relative to the date of maximum for each cycle, the data show far less scatter when plotted relative to the time of minimum. These centroid positions are plotted as functions of time relative to the date of minimum in Figure 28View Image. The area weighted averages of these positions in 6-month intervals are shown with the colored lines for different amplitude cycles. Near minimum the centroid position of the sunspot areas is about 25° from the equator. The equatorward drift is more rapid early in the cycle and slows late in the cycle – eventually stopping at about 7° from the equator.
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Figure 28: Latitude positions of the sunspot area centroid in each hemisphere for each Carrington Rotation as functions of time from cycle minimum. Three symbol sizes are used to differentiate data according to the daily average of the sunspot area for each hemisphere and rotation. The centroids of the centroids in 6-month intervals are shown with the red line for large amplitude cycles, with the green line for medium amplitude cycles, and with the blue line for the small amplitude cycles.

Cycle-to-cycle variations in this equatorward drift have been reported and latitudes of the sunspot zones have been related to cycle amplitudes. Vitinskij (1976) used the latitudes of sunspot near minimum as a predictor for the amplitude of cycle 21. Separating the cycles according to size now suggests that this is a poor indicator of cycle amplitude. Regardless of amplitude class all cycles start with sunspot zones centered at about 25°. This is illustrated in Figure 28View Image where the latitude positions of the sunspot area centroids are shown for small amplitude cycles (cycles 12, 13, 14, and 16) in blue, for medium amplitude cycles (cycles 15, 17, 20, and 23) in green, and for large amplitude cycles (cycles 18, 19, 21, and 22) in red.

Becker (1954) and Waldmeier (1955) noted that in large cycles, the latitudes of the sunspot zones are higher at maximum than in small cycles. This is indicated in the data plotted in Figure 28View Image. The sunspot zones in large amplitude cycles tend to stay at higher latitudes than in medium or small cycles from about a year after minimum to about five years after minimum. Since large amplitude cycles reach their maxima sooner than do medium or small amplitude cycles (the Waldmeier Effect – Section 4.6), the latitude differences are increased further.

The latitudinal width of the sunspot zones also varies over the cycle and as a function of cycle amplitude. This is illustrated in Figure 29View Image where the latitudinal widths of the sunspot zones are plotted for each hemisphere and Carrington rotation. The active latitude bands are narrow at minimum, expand to a maximum width at about the time of maximum, and then narrow again during the declining phase of the cycle. Larger cycles achieve greater widths than do smaller cycles. At all cycle phases and for all cycle amplitudes the active latitudes are fairly symmetrically centered with no systematic skew in the latitude distribution of sunspot areas.

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Figure 29: Latitudinal widths of the sunspot area centroid in each hemisphere for each Carrington Rotation as functions of time from cycle minimum. Three symbol sizes are used to differentiate data according to the daily average of the sunspot area for each hemisphere and rotation. The centroids of the centroids in 6-month intervals are shown with the red line for large amplitude cycles, with the green line for medium amplitude cycles, and with the blue line for the small amplitude cycles.

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