4.9 Active hemispheres

Comparisons of the activity in each solar hemisphere show significant asymmetries. Spoerer (1889) and Maunder (1890Jump To The Next Citation Point1904) noted that there were often long periods of time when most of the sunspots were found preferentially in one hemisphere and not the other. Waldmeier (1971Jump To The Next Citation Point) found that this asymmetry extended to other measures of activity including faculae, prominences, and coronal brightness. Roy (1977) reported that major flares and magnetically complex sunspot groups also showed strong north–south asymmetry. Simply quantifying the asymmetry itself is problematic. Taking the difference between hemispheric measures of activity (absolute asymmetry) produces strong signals around the times of maxima while taking the ratio of the difference to the sum (relative asymmetry) produces strong signals around the times of minima. Figure 30View Image shows the absolute asymmetry (North – South) of several key indicators. It is clear from this figure that hemispheric asymmetry is real (it consistently appears in all four indicators) and is often persistent – lasting for many years at a time. The absolute asymmetry in the RGO USAF/NOAA sunspot area smoothed with the 24-month Gaussian filter given by Equation (4View Equation) is shown in Figure 31View Image. This indicates that north–south asymmetry can persist for years.
View Image

Figure 30: Absolute north–south asymmetry (North – South) in four different activity indicators. Sunspot area is plotted in black. The Flare Index scaled by 146 is shown in red. The number of sunspot groups scaled by 443 is shown in green. The Magnetic Index scaled by 234 is plotted in blue.
View Image

Figure 31: Smoothed north–south asymmetry in sunspot area. The hemispheric difference is shown with the solid line while the total area scaled by 1/10 is shown with the dotted line.

Systematic variations over the course of a solar cycle or as a function of cycle amplitude have been suggested but these variations have invariably been found to change from cycle to cycle (See Section 5). For example, Newton and Milsom (1955) showed that the northern hemisphere dominated in the early phases of cycles 12 – 15 with a switch to dominance in the south later in each cycle while the opposite was true for cycles 17 – 18. (This can be seen in Figure 31View Image where cycle 12 is the first cycle shown.) Waldmeier (19571971) noted that a significant part of these variations can be accounted for by the fact that the two hemispheres are not exactly in phase. When the northern hemisphere activity leads that in the southern hemisphere, the north will dominate early in the cycle while the south will dominate in the declining phase. Carbonell et al. (1993) examined the relative asymmetry in sunspot areas with a variety of statistical tools and concluded that the signal is dominated by a random (and intermittent) component but contains a component that varies over a cycle and a component that gives long-term trends. The variation in the strength of the asymmetry over the course of an average cycle is strongly dependent upon how the asymmetry is quantified (strong at minimum for relative asymmetry, strong at maximum for absolute asymmetry). The variation from cycle-to-cycle will be discussed in Section 5.

  Go to previous page Go up Go to next page