Seasonal and diurnal variation of geomagnetic activity: Russell-McPherron effect during different IMF polarity and/or extreme solar wind conditions

The Russell‐McPherron (R‐M) effect is one of the most prevailing hypotheses accounting for semiannual variation of geomagnetic activity. To validate the R‐M effect and investigate the difference of geomagnetic activity variation under different interplanetary magnetic field (IMF) polarity and during extreme solar wind conditions (interplanetary shock), we have analyzed 42 years interplanetary magnetic field and geomagnetic indices data and 1270 SSC (storm sudden commencement) events from the year 1968 to 2010 by defining the R‐M effect with positive/negative IMF polarity (IMF away/toward the Sun). The results obtained in this study have shown that the response of geomagnetic activity to the R‐M effect with positive/negative IMF polarity are rather profound: the geomagnetic activity is much more intense around fall equinox when the direction of IMF is away the Sun, while much more intense around spring equinox when the direction of IMF is toward the Sun. The seasonal and diurnal variation of geomagnetic activity after SSCs can be attributed to both R‐M effect and the equinoctial hypothesis; the R‐M effect explains most part of variance of southward IMF, while the equinoctial hypothesis explains similar variance of ring current injection and geomagnetic indices as the R‐M effect. However, the R‐M effect with positive/negative IMF polarity explains the difference between SSCs with positive/negative IMFByaccurately, while the equinoctial hypothesis cannot explain such difference at the spring and fall equinoxes. Thus, the R‐M effect with positive/negative IMF polarity is more reasonable to explain seasonal and diurnal variation of geomagnetic activity under extreme solar wind conditions. Key Points IMF polarity is very important for semiannual variation of geomagnetic activity Semiannual variation after SSCs is due to R‐M effect and equinoctial hypothesis R‐M effect under different IMF polarity explains more variation after SSCs