The diurnal variation ofKindices of geomagnetic activity on quiet days in 1940-1948

Local- and universal-time components of the diurnal variation of the K indices have been computed from the eight daily K numbers for the 10 quiet days of each month in the 9 years 1940–1948 from six observatories in moderately low latitudes and fairly evenly distributed in longitude. Because the quiet days were universal time days, the universal time component contains a statistical curvature effect. The magnitude of this has been estimated. The universal time component is less regular than the local time component. When the local time component is examined for each of the 9 years separately, a pronounced change in form of much apparent regularity is exhibited over this 9-year interval. While it is possible that the variability of the amplitude and form of the quiet-day daily variation of the field contributed appreciably to the K numbers for these quiet days, it seems probable that this change of form of the local-time component, particularly marked from 1943 to 1947, arises mainly from disturbance and represents a change in the relative amounts of disturbance having different forms of diurnal variation. It may represent a change of this distribution with the solar cycle.