Contours of β₁ as a function of time for sublevel thresholds B_r = 0 to 5000 G for two ARs. Panel (a) shows ten days of evolution for SHARP 6950 (NOAA AR 12636) where day 0 is 2300 UT on 14 February 2017. No flares were observed. Panel (b) shows thirteen days of evolution for SHARP 7115 (NOAA AR 12673) where day 0 is 0900 UT on 28 August 2017. In this case, the threshold for β₁ ≳ 10 jumps up about two days before the first M-class flare (indicated by orange vertical bar), and nearly four days before the first X-class eruption (red vertical bar) that occurred at 0857 UT on 6 September 2017. The B_r strength thresholds, s₀ = 263 G, s₁ = 789 G, ..., s₈ = 4473 G and s₉ = 5000 G, correspond to the tick marks on the ordinate. The breadth of the color bar scale is the result of the large number of small-scale holes from noise in the image.