A model for the diurnal universal time variation of the Dst index

Using a simple model for the asymmetric ring current, we show that the main reason for the diurnal universal time (UT) variation of Dst index is the uneven distribution of the Dst network stations. The model takes into account the four Dst stations and the strong disturbance due to the partial ring current in the 1800 LT sector. In agreement with the equinoctial theory the diurnal variation is found to be larger in local summer than winter for Hermanus (HER). However, while Hermanus is the only Southern Hemisphere Dst station, this leads to a larger UT variation in the Dst index in the Northern Hemisphere summer. Moreover, we find evidence that the diurnal LT minimum is shifted by a couple of hours closer to the midnight during local winter, supporting the Malin-Isikara effect. We also note that the Russell-McPherron (RMP) effect is in phase (out of phase) with the modeled diurnal variation of the Dst index around vernal (autumnal) equinox, this increasing (decreasing) the diurnal variation in the Dst index in spring (fall) by 20-30%. However, the RMP effect is not responsible for the overall diurnal UT variation of the Dst index.