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East‐west asymmetry in coronal mass ejection geoeffectiveness

This paper extends the domain of applicability of the Gosling‐McComas space weather forecast rule that applies to the postshock sheaths of fast coronal mass ejections at Earth (ICMEs). The rule is based on the draping of the sheath magnetic field around the ICME body. The original treatment considered only the radial‐from‐the‐Sun component of the preshock interplanetary magnetic field (IMF), which implied that the domain of applicability of the rule was the entire sheath region ahead of the leading face of the ICME. We show here that because of the generally prevailing Parker spiral orientation of the IMF, the domain of applicability of the rule is instead generally strongly shifted to the east side of the ICME sheath. We suggest that the eastward shift of the domain of applicability of the rule accounts for an observed greater geoeffectiveness of west hemisphere CMEs compared with east hemisphere CMEs. The approach used here to demonstrate the eastward shift of the region of potential ICME sheath geoeffectiveness, and thus to increase the accuracy of the forecast rule, is to present intensity contours of the geoeffective draping component of the IMF as computed by global MHD simulations. Since the shift depends only on a spiral magnetic field and a blunt object to drape it around, we demonstrate the generality of the principle on which the rule is based by treating both the case of the ICME and the case of Earth's magnetosphere.