For a long time it has been known that the Dst index has a small diurnal variation which has been ascribed to various physical reasons.

Recently it has been shown that the form of the diurnal variation is mainly due to the partial and asymmetric spatial distribution of the Dst stations. Moreover, it was shown in [1] that the Dst index exhibits an exceptionally large UT variation in 1971. This conspicious variation was suggested to be due to an erroneous weighting of the stations in the Dst for this year.

In the present paper we study the UT variation of the Dst index, especially in order to compare the year 1971 with other times. We calculate the autocorrelation function of the Dst index and the diurnal UT variation by the superposed epoch method from the hourly Dst values. Both methods verify the exceptionally strong UT variation in 1971, as observed in [1]. Following the original derivation of the Dst, we have recalculated the Dst index (to be called the model Dst index). We compare this model index with the original Dst index in order to study the differences in particular during the exceptional year 1971.

We conclude that the large UT variation in 1971 originates from an erroneous derivation of the original Dst index and suggest for an improved Dst index for that time.

Reference:

[1] Takalo, J., and K. Mursula, A model for the diurnal UT variation of the Dst index, J. Geophys. Res., 2000.