On mapping flux transfer events to the ionosphere

Abstract

If a flux transfer event is modeled as a round patch of open field lines on an otherwise closed magnetopause, it maps to the ionosphere as an elongated shape pointing radially away from the cusp. We demonstrate this distortion by superposing a cylindrical flux rope field with the shielded Chapman‐Ferraro magnetospheric field. The axial component of the flux rope is normal to the planar magnetopause and decreases to zero at the edge of the rope. Field lines mapped from the edge nearest the cusp trace along the magnetopause surface to the cusp and then follow the singular cusp field line down to the ionosphere. However, field lines traced from the far edge must pass through the flux rope, where the axial component is strong. Consequently, they extend deeper into the magnetosphere, where the field is less cusp‐configured and more dipolar, and map down to the ionosphere away from the cusp, elongating the footprint of the round patch. Elongation increases with displacement of the patch from the subsolar region of the magnetopause. A series of patches mapped from distances successively further from the subsolar point forms a pattern of striations radiating from the cusp that resembles an observed pattern of auroral arcs in the midday oval, previously interpreted as signatures of magnetosheath plasma injection at the cusp by an unspecified mechanism. The mapping suggests that the mechanism could be patchy reconnection at distances which can be considerably removed from the cusp.