Variations of geomagnetic activity with lunar phase

An analysis of 31 years of Kp data suggests a variation of geomagnetic disturbance with lunar phase. A general increase in geomagnetic activity of about 4% begins after full moon and lasts for seven days. A general decrease in geomagnetic activity of about 4% is found for the seven days preceding full moon. A study of randomized data indicates that the probability that these variations should have occurred by chance is less than 5%. The effect is found to be associated with the Kp data derived from periods of geomagnetic quiet conditions; it is not evident in the data from disturbed periods.