(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.