Behavior of the interplanetary and magnetospheric electric fields during very intense storms

The role of a positive (northward) component of the interplanetary magnetic field (IMF) Bz in triggering large magnetic storms is investigated. Storms with Kp values equal to or greater than 7(o) which are preceded by a Sudden Commencement (Ssc) are selected, taking into account the period from 1964 to 1978. On the basis of a case-by-case study and also on a statistical basis, a correlation is established between the geomagnetic index Km and the three-hourly averaged Bz. The statistical relation obtained is Bz (gamma) = -2.58 Km + 8.70 with a correlation coefficient of 0.47. It is found that storms associated with Bz greater than 0 are less intense than those associated with Bz less than 0. However, major storms can also be triggered by solar wind events associated with a northward IMF.