Origins of the semiannual variation of geomagnetic activity in 1954 and 1996

We investigate the cause of the unusually strong semiannual variation of geomagnetic activity observed in the solar minimum years of 1954 and 1996. For 1996 we separate the contributions of the three classical modulation mechanisms (axial, equinoctial, and Russell-McPherron) to the six-month wave in the aa(m) index and find that all three contribute about equally. This is in contrast to the longer run of geomagnetic activity (1868-1998) over which the equinoctial effect accounts for similar to70% of the semiannual variation. For both 1954 and 1996, we show that the Russell-McPherron effect was enhanced by the Rosenberg-Coleman effect (an axial polarity effect) which increased the amount of the negative (toward Sun) [positive (away from Sun)] polarity field observed during the first [second] half of the year; such fields yield a southward component in GSM coordinates. Because. this favourable condition occurs only for alternate solar cycles, the marked semiannual variation in 1954 and 1996 is a manifestation of the 22-year cycle of geomagnetic activity. The 11-year evolution of the heliospheric current sheet (HCS) also contributes to the strong six-month wave during these years. At solar minimum, the streamer belt at the base of the HCS is located near the solar equator, permitting easier access to high speed streams from polar coronal holes when the Earth is at its highest heliographic latitudes in March and September. Such an axial variation in solar wind speed was observed for 1996 and is inferred for 1954.