Geomagnetic indices to quantify long-term external field variations (2) aa – (Mayaud, 1972; 1980) ● widely used to depict long-term variations of the geomagnetic activity; ● long-term similarity between aa and R in terms of 11- year running averages (Feynman & Crooker, 1978; Feynman, 1982; Silver, 1992; Cliver et al., 1998) ● heliospheric conditions at 1 AU (IMF strength, SW speed & density) account for the upward drift in the 20th century (Stamper et al., 1997) ● calibration problems – (Svalgaard et al., 2004) The average, for each 3-hour period, of the K indices from two near-antipodal stations after the transformation of K into amplitudes; cancellation of local diurnal ionospheric effects
{}