Possible north–south asymmetry related to the mean Bz of interplanetary coronal mass ejections

Abstract

The annual/semiannual behaviour of monthly mean of B z component of interplanetary magnetic field (IMF) separated by positive and negative B y components were studied. The study was confined to the ascending phases of the four recent sunspot cycles when interplanetary counterparts of coronal mass ejections (ICMEs) dominate among the sources of geoeffectiveness. Definite differences were found between the annual variations of the monthly mean B z values of geoeffective ( Kp > 3) ICMEs. When the solar dipole is parallel to the terrestrial one, the Russell–McPherron effect is detectable in the opposite annual variations of the mean GSM B z as is expected. However, when the solar and terrestrial dipoles are antiparallel, the mean B z does not exhibit the Russell–McPherron effect in the GSM (Geocentric Solar Magnetospheric) system because there are strong inverse annual variations in the GSE (Geocentric Solar Ecliptic) system. This kind of smaller minima of the curves of mean GSE B z during the antiparallel years indicate that the ICMEs may have much stronger negative GSE B z values during these years. The southern excess may come from the large-scale north–south asymmetries of the heliospheric magnetic field or from the characteristics of magnetic clouds (direction of the axial field or polarities of the leading and trailing fields).