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It is known that the correction of the Kyoto  Dst  index for the secular variation of the Earth's internal field produces a discontinuity in the Kyoto  Dst  index at the end of each year. We show that this secular correction also introduces a significant baseline error to the Kyoto  Dst  index that leads to an underestimate of the solar cycle variation of geomagnetic activity and of the strength of the ring current as measured by the Kyoto  Dst  index. Thus, the average value of the Kyoto  Dst  index would be approximately 13 nT more negative for the active year 2003 compared to quiet years 2006 and 2009 if the Kyoto  Dst  index properly measured the effects of the ring current and other currents that influence the  Dst  observatories. Discontinuities in the Kyoto  Dst  index at the end of each year have an average value of about 5 nT, but the discontinuity at the end of year 2002 was approximately 12 nT, and the discontinuity at the end of year 1982 may have been as large as 20 nT.

The Dst index underestimates the solar cycle variation of geomagnetic activity