Occurrence of AE and Dst geomagnetic index levels and the selection of the quietest days in a year

Abstract

Three groups of indices were studied as indicators for quiet day geomagnetic field level determinations: one was the AE index, and the other two were the positive and negative value groups of the Dst index. A comparison of the behaviors of these three indices for 17 years showed their solar cycle changes and the differences in distribution of index levels between the times of high and low sunspot numbers. Two selections of the quietest days were made to provide at least 19 and 30 chosen days per year. These selections required the correspondence of equivalent fractional portions of the distribution of days in which all hourly values of indices were below specified levels. Lists of these days are given for the years 1958–1974. A comparison of these lists with the 5 days per month selection derived from the geomagnetic activity index Kp shows that for the same total number of quiet days per year only about one third to one half of the low‐Kp days would be quiet by AE and Dst index standards.