Cusp width and Bz: Observations and a conceptual model

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Abstract
Because a southward turning of the interplanetary magnetic field Bz enhances day side merging, leading to an equatorward motion of the equatorward edge of the cusp, one might suppose that the cusp latitudinal width would be larger for a southward Bz condition. The Defense Meteorological Satellite Program F7 observations confirm two previous case studies showing that the cusp low‐altitude latitudinal extent is actually narrower when Bz is southward than when Bz is northward. The earlier studies dealt with intense geomagnetic storms; examples presented here show that it is routine to observe a narrowing of the cusp width following a southward turning of Bz. In addition, we present a simple conceptual model that shows how enhanced tailward convection of the newly opened field lines during periods of southward Bz can account for the observed narrowness.

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