The lunar period, the solar period, andKp

The variation of geomagnetic activity with the phase of the moon has been tested for statistical significance. The fluctuations in Kp, at the lunar period are typical of the periods near it and in fact are within one standard deviation of the average fluctuation amplitude. Furthermore, these Kp fluctuations appear to be due to the sun. The so-called 27-day peak due to the sun (the well-known M-region effect) has a width of about a week and thus is not a sharp peak but contributes to periods between about 24 and 31 days. Any narrow-banded detection system, such as the superposed epoch analysis, will sense the solar effect anywhere within this broad interval. The 29.5-day lunar period falls within the interval; Kp analyses at the lunar period should take into account the nonrandom variation in Kp caused by the broad solar peak. It is shown that previous analyses concluding that there is a significant lunar-Kp effect have treated the data so as to disregard the possible solar origin of the effects found. When the solar peak is included in the analyses, the hypothesized lunar effect falls well within the limits of chance occurrence. We conclude that the available data do not indicate any statistically significant lunar influence on geomagnetic activity; the effect at 29.53-days periodicity that had been attributed to the moon is actually due to the sun.