Solar cycle variation of long-duration radial IMF at 1 AU

Intervals of long-duration radial interplanetary magnetic fields (IMFs) were studied using the OMNI database for the period of approximately forty years. Long-duration radial IMF events were detected in various regimes of solar wind. It was found that long-duration radial IMF events frequently occur around solar minimum and are rarer near solar maximum. The events tend to occur in decreasing solar wind speed through solar cycle. The decreasing rates of solar wind speed associated with the events do not clearly depend on the solar activity.