The simple index described clearly shows periods of current geomagnetic behavior. The index is based on half-day values of the geomagnetic aa index for 1868 through 1983, which reveal rather lengthy periods when geomagnetic activity patterns are faithfully repeated every 27 days. It is shown that these recurrent periods occur at the end of each sunspot cycle; that they end fairly abruptly with the beginning of a new sunspot cycle; that they tend to last significantly longer when associated with even-numbered sunspot cycles; and that the effect is clearly centered on a 27-day repetition rate. The 27-day recurrence index indicates those regularly occurring times when the solar wind structure remains relatively stable boyond 1 AU for periods of 1 1/2 to 5 years. Correlations have been reported between the index and auroral heights and even the global albedos of Titan and Neptune.