Uneven weighting of stations in the Dst index

Abstract

We note in this paper that the average disturbances of the four Dst stations are systematically different and, therefore, the stations contribute to the Dst index by unequal weights. This is an important problem, e.g., for the estimated longitudinal asymmetries of the ring current and the long-term averages of the Dst index where the contribution of the most dominant station (HON) is twice as large as the weakest station (KAK). We use an extended network of stations to demonstrate that the averaged local Dst indices are ordered according to the station's geographic longitude, with westernmost stations depicting the largest disturbances and contributions to the Dst index and easternmost the smallest. We show that the problem is related to the way that the quiet days are treated in the Dst recipe. We modify the recipe so that UT-fixed quiet days are used in all stations, whence the corrected local Dcx indices have equal weights at all stations. This gives strong support for using the corrected Dcx index instead of the Dst index.