4.9 Active hemispheres
Comparisons of the activity in each solar hemisphere show significant asymmetries. Spoerer (1889) and
Maunder (1890, 1904) 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 (1971) 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 30 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 (4) is shown in Figure 31. This indicates that north–south asymmetry can persist for
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.
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 31 where
cycle 12 is the first cycle shown.) Waldmeier (1957, 1971) 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