(a) Daily (black) and 30-day smoothed (red) sunspot number (ISN, v2.0) over cycle 21, including the adjacent mimima. The average values during the activity maximum (SN1 = 217) and minimum (SN2 = 17) of cycle 21 are marked by the horizontal dashed lines. (b) Evolution of the monthly-smoothed power-law slope m over the same period, the horizontal dashed lines mark m1 = −2.5954 corresponding to SN1 = 217 and m2 = −2.7846 corresponding to SN2 = 17.
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