Relationship between PC index and magnetospheric substorms observed under conditions of northward IMF

The substorms listed by Lee et al. (2010) as extraordinary events occurring under influence of the persistent northward IMF were tested in their relation to coupling function EKL and the polar cap magnetic activity PC index. It turned out that the EKL and PC values in all events were in excess of the threshold level ∼1.5–2 mV/m typical of ordinary substorms (Janzhura et al., 2007). This excess is attained at the expense of extremely large IMF azimuthal component which is taken into account by coupling function EKL, the latter being unexamined by Lee et al. (2010). The PC index characterizing the polar cap magnetic activity affected by EKL demonstrates even better relationship with substorm behavior than EKL function itself. The conclusion is made that substorms considered by Lee et al. (2010), as unusual events, turn out to be the ordinary phenomena, if the proper coupling function and the corresponding PC index are taken into examination.

► Substorms occurring under conditions of northward IMF are examined.
► Coupling function (EKL) and PC index testify that these substorms are quite ordinary events.
► Large azimuthal IMF component is responsible for substorms occurrence.