title: TheDstindex underestimates the solar cycle variation of geomagnetic activity.
abstract: It is known that the correction of the KyotoDstindex for the secular variation of the Earth's internal field produces a discontinuity in the KyotoDstindex at the end of each year. We show that this secular correction also introduces a significant baseline error to the KyotoDstindex 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 KyotoDstindex. Thus, the average value of the KyotoDstindex would be approximately 13 nT more negative for the active year 2003 compared to quiet years 2006 and 2009 if the KyotoDstindex properly measured the effects of the ring current and other currents that influence theDstobservatories. Discontinuities in the KyotoDstindex 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.
authors.affiliation: Retired from Space Sciences Laboratory University of California Berkeley California USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Temerin
authors.affiliation: Laboratory for Atmospheric and Space Physics and Department of Aerospace Engineering Sciences University of Colorado Boulder Colorado USA.
authors.forename: Xinlin
authors.initials: X
authors.lastname: Li
keywords: Kyoto Dst index; geomagnetic activity; magnetic storms; solar cycle variation