The interaction between transpolar arcs and cusp spots

TL;DR

This paper reports observations of the interaction between transpolar arcs and cusp spots, revealing that lobe reconnection can open closed magnetotail flux, challenging previous assumptions about their independence.

Contribution

It provides the first observational evidence of transpolar arcs intersecting cusp spots, demonstrating that lobe reconnection can open closed magnetotail flux.

Findings

Lobe reconnection can open closed magnetotail flux.

Transpolar arcs can intersect cusp spots during their lifetime.

Such interactions may be more common than previously thought.

Abstract

Transpolar arcs and cusp spots are both auroral phenomena which occur when the interplanetary magnetic field is northward. Transpolar arcs are associated with magnetic reconnection in the magnetotail, which closes magnetic flux and results in a "wedge" of closed flux which remains trapped, embedded in the magnetotail lobe. The cusp spot is an indicator of lobe reconnection at the high-latitude magnetopause; in its simplest case, lobe reconnection redistributes open flux without resulting in any net change in the open flux content of the magnetosphere. We present observations of the two phenomena interacting--i.e., a transpolar arc intersecting a cusp spot during part of its lifetime. The significance of this observation is that lobe reconnection can have the effect of opening closed magnetotail flux. We argue that such events should not be rare.