Astrophysics and Space Sciences Transactions www.astrophys-space-sci-trans.net 1810-6528 1810-6536 4 2 2008 10.5194/astra-4-41-2008 http://www.astrophys-space-sci-trans.net/4/41/2008/ http://www.astrophys-space-sci-trans.net/4/41/2008/astra-4-41-2008.html http://www.astrophys-space-sci-trans.net/4/41/2008/astra-4-41-2008.pdf 41 45 2008-07-18 Correct normalization of the <i>Dst</i> index K. Mursula kalevi.mursula@oulu.fi L. Holappa A. Karinen Department of Physical Sciences, P.O.Box 3000, FIN-90014, University of Oulu, Finland permanently at: Institute of Occupational Health, Helsinki, Finland The <i>Dst</i> index has been one of the most important solar-terrestrial indices for decades, and it is used in numerous studies as a measure of the temporal development and intensity of magnetic storms and the ring current. Here we discuss two issues related to the relative and absolute normalization that are problematic to the <i>Dst</i> index. We show for the first time quantitatively that the magnetic disturbances at the four <i>Dst</i> stations are ordered according to the latitudinal projection of an equatorial disturbance upon the local horizontal component of the geomagnetic field. Therefore, the disturbances observed at each station should be first normalized by the cosine of the geomagnetic latitude of the station before they are averaged to form the <i>Dst</i> index. Perhaps surprisingly, the recipe to calculate the <i>Dst</i> index does not include this normalization and, therefore, must be revised on this part. We also discuss the effects of correcting the quiet-time seasonal variation, the so called "non-storm component" in the <i>Dst</i> index. This correction is seasonally varying, being largest around equinoxes and smallest at solstices, leading to an average correction (increase) of about 6 nT, i.e. about 25–30%, for annual averages of the <i>Dst</i> index. This increase also leads to significantly improved correlations between the corrected <i>Dst</i> index, the so called <i>Dcx</i> index, and many other indices of solar-terrestrial disturbance. We show here in detail that the correlation between the geomagnetic <i>Ap</i> index and the <i>Dcx</i> index (cc = 0.83) is much higher than between <i>Ap</i> and <i>Dst</i> (cc = 0.60). These results give further evidence that the <i>Dcx</i> index is a more truthful measure of magnetic storminess than the original <i>Dst</i> index. Cliver, E. W., Kamide,~Y., Ling,~A G., and Yokoyama,~N.: Semiannual variation of the geomagnetic \emphDst index: Evidence for a dominant nonstorm component, J. Geophys. Res., 106, 21 297–21 304, 2001. Karinen, A. and Mursula,~K.: A new reconstruction of the \emphDst index for 1932–2002, Ann. Geophys., 23, 475–485, 2005. Karinen, A. and Mursula,~K.: Correcting the \emphDst index: Consequences for absolute level and correlations, J. Geophys. Res., 111, A08207, doi:10.1029/2005JA011299, 2006. Mursula, K. and Karinen,~A.: Explaining and correcting the excessive semiannual variation in the \emphDst index, Geophys. Res. Lett., 32, L14107, doi:10.1029/2005GL023132, 2005. Sugiura, M.: IAGA Resolution 2, in IAGA Bulletin, Madrid, Spain, 27, 123, 1969. Sugiura, M. and Kamei,~T.: Equatorial \emphDst index 1957–1986, in IAGA Bull., 40, edited by A. Berthelier and M. Menvielle, ISGI Publ. Off., Saint-Maur-des-Fossés, France, 1991. WDC-C2, Definition of the \emphDst index. The World Data Center for Geomagnetism (WDC-C2), http://swdcdb.kugi.kyoto-u.ac.jp, Kyoto, Japan, 2004.