The diurnal variation ofKindices of geomagnetic activity on disturbed days in 1949-1957

To extend recent work that covered the nine years 1940–1948, the diurnal variation of the K indices from six observatories in moderately low latitudes has been computed for the succeeding nine years, 1949–1957. From the eight daily K numbers for the five disturbed days of each month, local-time and universal-time components of the diurnal variation have been obtained. The nine-year average local-time component for these nine years shows a rather close resemblance to that for the preceding nine years. The universal-time component, which is less regular than the local-time component, is of smaller amplitude than that for the preceding nine years, but exhibits some similarities to it. The amplitude of the local-time component, in the nine separate yearly averages, changes over these years much as it did in the previous nine, decreasing from 1953 to 1957, as it did from 1943 to 1947 in the preceding sunspot cycle. It is suggested that this change and also a change found earlier in the quiet days may involve an effect in the atmosphere (in the lower ionosphere), the amplitudes of the diurnal variations changing with the sunspot cycle as conditions in the ionosphere change.