HR: 11:15h
AN: SA12A-04    [Abstracts]
TI: Rederivation of Dst Index
AU: * Svalgaard, L
EM: leif@leif.org
AF: ETK, 6927 Lawler Ridge, Houston, TX 77055 United States
AB: Derivation of the Dst geomagnetic index involves removing the quiet diurnal variation (Sq) caused by thermal winds and tides in the ionosphere maintained by solar EUV radiation. The standard procedure has been to fit average, smoothed curves to the Sq computed from the five internationally quietest days of each month and to use these curves on all days to remove Sq from the observed values of the horizontal component of the geomagnetic field for three stations in the northern hemsphere and one station in the southern hemisphere, then to average the resulting values (weigthed by latitude) into a global Dst index. This procedure is problematic because 1) the day-to-day variability of the regular diurnal variation (Sr) is as large as the variation itself and 2) an annual variation of the level of horizontal component is not properly compensated for by using an unequal number of northern and southern stations. We show how these problems can be avoided by using only nighttime values and by using an equal number of stations in both hemispheres and calculate a new index Dsv back to 1930 (potentially back to 1900 or beyond). Using this new index, the long-term trend, semiannual variation, and average storm signature (superposed epoch around SSCs) are studied.
DE: 2199 General or miscellaneous
DE: 2778 Ring current
DE: 7954 Magnetic storms (2788)
SC: SPA-Aeronomy [SA]
MN: Fall Meeting 2005