4.10 Active longitudes

Sunspots and solar activity also appear to cluster in “active longitudes.” Early observers had noted that sunspot groups often emerge at the same positions as earlier groups. Bumba and Howard (1965) and Sawyer (1968) noted that new active regions grow in areas previously occupied by old active regions. Bogart (1982) found that this results in a periodic signal that is evident in the sunspot number record.

Figure 32View Image illustrates the active longitude phenomena. In Figure 32View Imagea the sunspot area in 5° longitude bins averaged over 1805 solar rotations since 1878 and normalized to the average value per bin is plotted as a function of Carrington longitude. The 2σ uncertainty in these values is represented by the dotted lines. This 2σ limit is reached at several longitudes and significantly exceeded at two (85° – 90° and 90° – 95°). Figure 32View Imageb shows similar data for each individual cycle with the normalized value offset in the vertical by the sunspot cycle number. There are many peaks at twice the normal value and one, in cycle 18 at 85° – 90°, at three times the normal value. Some of these peaks even appear to persist from one cycle to the next, a result that has been noted by many authors including Bumba and Henja (1991), Miklailutsa and Makarova (1994), and Bai (2003). Henney and Harvey (2002) noted the persistence of magnetic structures in the northern hemisphere at preferred longitudes (drifting slightly due to the latitude) for two decades but also noted that (as seen in Figure 32View Imageb) that the sunspot records suggests that two decades is about the limit of such persistence.

Another interesting aspect of this phenomenon concerns the hemispheric differences. Berdyugina and Usoskin (2003) found that the active longitude in the northern hemisphere tends to be shifted by 180° in longitude from that in the southern hemisphere. This effect requires significant processing of the data to discern.

View Image

Figure 32: Active longitudes in sunspot area. The normalized sunspot area in 5° longitude bins is plotted in the upper panel (a) for the years 1878 – 2009. The dotted lines represent two standard errors in the normalized values. The sunspot area in several longitude bins meets or exceeds these limits. The individual cycles (12 through 23) are shown in the lower panel (b) with the normalized values offset in the vertical by the cycle number. Some active longitudes appear to persist from cycle to cycle.

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