Center for Space Physics, Boston University, Boston, Massachusetts

Continuous solar wind data from WIND reveals a new recurrence pattern which implies that speed variations contribute to the 22-year cycle of geomagnetic activity. Near December 1994 solstice, in keeping with expectation, a four-sector interplanetary magnetic field pattern was accompanied by four streams. As the season advanced toward March equinox, however, the streams in the two sectors with away polarity diminished, leaving a strikingly unusual two-stream, four-sector pattern until late April. Since the magnetic field pointed toward the sun in both streams, the polarity effect of Russell and McPherron [1973] combined with the high-speed flow resulted in a recurrent pattern of sustained geomagnetic activity during these sector passages. The solar wind pattern is consistent with Earth's excursion to southern heliographic latitudes at March equinox enabling WIND to sample high-speed flow from only the southern coronal hole. The WIND data imply that the 22-year variation in geomagnetic activity results not only from longer immersion in toward sectors in March and away sectors in September during even solar cycles, as proposed by Russell and McPherron [1973], but also from higher flow speeds in those sectors.

Geophys. Res. Lett., 23, 1275-1278, 1996