The annual and daily variations of the Dst index

Summary. This investigation concerns both the annual variation and the daily variation of the Dst index for 18 years. The data display a 6-month wave, a 12-month wave and a 24-hr wave the phase of which apparently does not vary in the course of the year. This contrasts with the variations observed with planetary indices derived from subauroral K indices, which undergo a modulation in sin²Ψ_M (Ψ_M is the angle between the solar wind and the dipole axis) with the well-known change in phase of the daily variation from one solstice to another. The origin of the 6-month wave is attributed to the mechanism suggested by Malin & Isikara: the 6-month variation of the mean latitude of the ring current. The 12-month wave may be due to the northern latitude of most of the Dst observatories. The daily variation is probably due to the asymmetric part of the ring current which is not averaged out because of the non-uniform longitudinal distribution of the Dst observatories.