Theta Aurora: Transpolar Arcs Linked to a Folded Magnetotail Neutral Sheet

S. C. Hill; T. I. Pulkkinen; A. Kullen; A. Brenner; M. Ala-Lahti; Q. Al Shidi; A. Mukhopadhyay; S. Zou; M. Liemohn; A. N. Jaynes

doi:10.1029/2025JA034825

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Theta Aurora: Transpolar Arcs Linked to a Folded Magnetotail Neutral Sheet

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Theta Aurora: Transpolar Arcs Linked to a Folded Magnetotail Neutral Sheet

S. C. Hill, T. I. Pulkkinen, A. Kullen, A. Brenner, M. Ala-Lahti, Q. Al Shidi, A. Mukhopadhyay, S. Zou, M. Liemohn and A. N. Jaynes

Journal of geophysical research : Space physics (2013 - Present), Vol.131(8), e2025JA034825

08/20/2026

DOI: 10.1029/2025JA034825

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Abstract

Nightside‐originating transpolar arcs (TPAs) represent a distinct mechanism of magnetosphere‐ionosphere energy transfer during northward IMF conditions. Nightside‐originating TPAs originate from the nightside auroral oval and grow across the polar cap to connect to the dayside, appearing to cross multiple topological boundaries as they develop. We study the magnetospheric conditions required for such a phenomenon by modeling the 10 January 1997 nightside‐originating TPA event observed by Polar UVI with a high resolution SWMF Geospace simulation. Our simulation produces a large central fold in the magnetotail neutral sheet that protrudes into the northern lobe. When magnetically mapped into the ionosphere, the development and subsequent dynamics of the folded neutral sheet show remarkable agreement to the observed nightside‐originating TPA. The folded neutral sheet bifurcates the northern lobe in the mid‐tail, around X=−30REX=-30R_(E) , and is located within a region of field‐aligned current (FAC) generation. We suggest that the large central fold in the neutral sheet forms as a response to multiple opposing twists of the magnetotail, driven by the changes in the IMF BYB_(Y)direction. We discuss the formation and decay of the neutral sheet fold during northward and southward IMF, and the relationship of the neutral fold to the ionospheric convection. Our results suggest that a neutral sheet fold with subsequent FAC generation is needed for transpolar arc activity, rather than just an extended closed field line region poleward of the main auroral oval. Key Points Using SWMF Geospace simulation, we associate the evolution of an observed transpolar arc with a large fold in the magnetotail neutral sheet The large neutral sheet fold is formed by dynamic IMF BYB_(Y) , twisting the magnetotail to bifurcate the northern lobe The neutral sheet fold maps to the location of the observed theta aurora, and is co‐located with a field‐aligned current generation region

Ionosphere

aurora

magnetosphere‐ionosphere coupling

magnetotail

neutral sheet

transpolar arc

Details

Title: Subtitle: Theta Aurora: Transpolar Arcs Linked to a Folded Magnetotail Neutral Sheet
Creators: S. C. Hill - University of Iowa
T. I. Pulkkinen - University of Michigan
A. Kullen - KTH Royal Institute of Technology
A. Brenner - Goddard Space Flight Center
M. Ala-Lahti - University of Helsinki
Q. Al Shidi - West Virginia University
A. Mukhopadhyay - University of Alberta
S. Zou - University of Michigan
M. Liemohn - University of Michigan
A. N. Jaynes - University of Iowa
Resource Type: Journal article
Publication Details: Journal of geophysical research : Space physics (2013 - Present), Vol.131(8), e2025JA034825
DOI: 10.1029/2025JA034825
ISSN: 2169-9380
eISSN: 2169-9402
Publisher: AMER GEOPHYSICAL UNION
Number of pages: 18
Grant note: National Aeronautics and Space Administration (80NSSC22K1846) National Science Foundation (2033563; 2420675; 2349872) Swedish National Space Agency (DNR‐2021‐00134)
Language: English
Date published: 08/20/2026
Academic Unit: Physics and Astronomy; University College Courses
Record Identifier: 9985221447002771

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