Micropulsation records obtained at California stations during a 6‐year period, from August 1, 1960, to July 30, 1966, have been analyzed for a possible full‐moon effect on hydromagnetic (hm) emission ...activity. The data, which had previously been digitized into the form of a daily hm emission activity index W, were considered in both their original form and then according to several selection criteria. For example, geomagnetically quiet intervals (ΣKp < 30 for each day) containing a full moon were analyzed, as well as similar intervals when the moon's ecliptic latitude, β, was restricted to values β ≤ 3.5°. Also analyzed were days when a full moon occurred during a well‐defined geomagnetic storm. In all cases there was an indication of an enhancement of hm emission activity commencing the day before full moon and, at least in the case of storm days, extending into the full moon day. A full‐moon effect on the Kp index has been previously suggested but, as in the present case, there were insufficient statistics to establish the effect beyond doubt. This new result for hydromagnetic emissions provides additional evidence for a small lunar disturbance of the magnetosphere around the time of the full moon.
