tokens: 750 and HARP 1028. These flare-productive active regions produced at least one flare of intensity greater than M5. Middle panel: power index evolution for HARP 1464 and HARP 1621. These flare-productive active regions produced flares of intensity between M1 and M5. Lower panel: power index evolution for HARP 1019 and HARP 1318. These active regions showed no flare productivity. Vertical lines in the graphs indicate the occurrence of flares with intensity greater than M1. In addition to exhibiting, on average, a lower value for the power index, flare-quiet regions tend to show much more variability in the temporal evolution of the index. Figure 5 (bottom) shows one such trend—where the index tends to drop off over several days. A second trend is shown in Figure 7 , where NOAA 11274 shows periods when the power index rises to 1. However, these periods of time are short. In contrast, there is no sudden spike in the index value for any of the flare-productive regions as they track across the solar disk. Instead, flare-productive regions exhibit a large power index for many days, both before and after they produce solar flares. This suggests that the flare potential of any active region could be determined a few days in advance of the production of a large solar flare and the occurrence of solar flares does not reduce the future flare potential. Figure 6 shows the results in the form of histograms for all 53 HARPs. The range of index values (1.0–2.5) is similar to that in previous studies (Abramenko Yurchyshyn 2010 ). The dotted histogram shows the power index values taken by flare-quiet active regions. The power index distribution for these flare-quiet regions is quite broad and peaks around 1.6. The dashed histogram indicates the results for flare-productive active regions that produced flares of magnitude M1–M5. This distribution is much narrower, with fewer values below 1.5, and peaks around 1.8. However, the high-value tail does overlap with the flare-quiet distribution. Finally, the solid histogram shows the power index values taken by flare-productive active regions that produced at least one flare of intensity greater than M5. In this case, the power index mostly lies above 2 with a maximum at around 2.2. Figure 6. Histograms of power index values for all 53 HARPs. The dotted line indicates a histogram for 25 flare-quiet active regions. The dashed line shows a histogram for 19 flare-productive active regions that produced flares of intensity between M1 and M5. The solid line indicates a histogram for nine flare-productive active regions that produced at least one flare with intensity greater than M5. Figure 7. Power index evolution of a non-flaring active region HARP 803 (NOAA 11274). Flare-quiet regions tend to show much variation in the power index values. The above classification of HARPs was performed based on the maximum intensity of any flare from the region. However, the flare productivity can be measured more carefully in terms of
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