The latitude of ephemeral regions as an indicator for solar-cycle strength

Digitized images of full-disk Ca K spectroheliograms from two solar observatories were used to study the cycle variation of ephemeral regions (ERs) over the ten solar cycles 14–23. We calculate the monthly averaged unsigned latitude of ERs and compare it with the annual sunspot number. We find that the average latitude of ERs can be used as a predictor for the strength of a solar cycle. For a short-term prediction (dT ∼ 1–2 years), the maximum latitude of ERs (in the current cycle) defines the amplitude of that cycle (the higher the latitude of ERs, the larger the amplitudes of the sunspot cycle). For a long-term prediction (dT ∼ 1.5 solar cycles), the latitude of ERs during the declining phase of the nth cycle determines the amplitude of the (n + 2)th cycle (the lower the latitude of ERs, the stronger is the sunspot cycle). Using this latter dependency, we forecast the amplitude of sunspot cycle 24 to be at W = 92 ± 13 (in units of annual sunspot number).