The recurrence tendency and forecasts of magnetic activity

The recurrence tendency in magnetic character figures is evaluated by auto‐correlation coefficients for various intervals up to 83 days for each year from 1890 to 1944, The strength of the 27‐day recurrence pulse varies during the 55 years, being generally strong from about two years after sunspot maximum through the minimum year. Evidence for persistent recurrence intervals other than multiples of about 27 days is very weak. The magnitudes of the correlation coefficients for short intervals indicate that the time unit of geomagnetic activity is two to three days. Formulas for the prediction of magnetic character figures by the method of least squares are based on the mean auto‐correlation coefficients for intervals of 1, 26, 27, 28, and 54 days for stages of the sunspot cycle with weak and strong recurrence conditions. A prediction formula with a non‐linear term is also derived.