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Edited by Lennard A. Fisk, University of Michigan, Ann Arbor, MI, and approved May 26, 2020 (received for review January 11, 2020)
Significance
Colorful and dynamic aurora has attracted human’s attention since the dawn of time. However, mystery remains in understanding a distinct class of aurora, transpolar auroral arc (TPA) (or “theta” aurora) which occurs in extremely high latitude of the Earth polar regions when interplanetary magnetic field (IMF) is northward. Previous theories are unable to explain why multiple TPAs often occur. Our comprehensive observations in the ionosphere and magnetotail as well as a three-dimensional magnetosphere modeling shed insight on how multiple TPAs form. Our study offers clues how solar wind energy and mass transfer into the magnetosphere and ionosphere under a northward IMF that occurs nearly half of the time.
Abstract
A distinct class of aurora, called transpolar auroral arc (TPA) (in some cases called “theta” aurora), appears in the extremely high-latitude ionosphere of the Earth when interplanetary magnetic field (IMF) is northward. The formation and evolution of TPA offers clues about processes transferring energy and momentum from the solar wind to the magnetosphere and ionosphere during a northward IMF. However, their formation mechanisms remain poorly understood and controversial. We report a mechanism identified from multiple-instrument observations of unusually bright, multiple TPAs and simulations from a high-resolution three-dimensional (3D) global MagnetoHydroDynamics (MHD) model. The observations and simulations show an excellent agreement and reveal that these multiple TPAs are generated by precipitating energetic magnetospheric electrons within field-aligned current (FAC) sheets. These FAC sheets are generated by multiple-flow shear sheets in both the magnetospheric boundary produced by Kelvin–Helmholtz instability between supersonic solar wind flow and magnetosphere plasma, and the plasma sheet generated by the interactions between the enhanced earthward plasma flows from the distant tail (less than −100 RE) and the enhanced tailward flows from the near tail (about −20 RE). The study offers insight into the complex solar wind-magnetosphere-ionosphere coupling processes under a northward IMF condition, and it challenges existing paradigms of the dynamics of the Earth’s magnetosphere.
Footnotes
- ↵1To whom correspondence may be addressed. Email: zhangqinghe{at}sdu.edu.cn.
Author contributions: Q.-H.Z. and Y.-L.Z. designed and performed research; C.W. and B.-B.T. contributed new reagents/analytic tools; Q.-H.Z., Y.-L.Z., H.-G.Y., Z.-Y.X., K.O., and Y.-Z.M. analyzed data; Q.-H.Z., Y.-L.Z., C.W., M.L., H.-G.Y., B.-B.T., Z.-Y.X., K.O., L.R.L., Y.-Z.M., Q.-G.Z., J.I.M., and L.-D.X. participated in the scientific discussions; and Q.-H.Z., Y.-L.Z., M.L., K.O., and L.R.L. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Data deposition: The 3D MHD simulation data are available on the website of https://zenodo.org/record/3777265# with a separate DOI of 10.5281/zenodo.3777265. The time sequence of 557.7-nm aurora images from the all-sky imager at the Chinese Antarctic Zhongshan Station (ZHS) are available on the website of https://zenodo.org/record/3778095# with a separate DOI of 10.5281/zenodo.3778095.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2000614117/-/DCSupplemental.
- Copyright © 2020 the Author(s). Published by PNAS.
This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
Multiple transpolar auroral arcs reveal insight about coupling processes in the Earth’s magnetotail
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- Earth, Atmospheric, and Planetary Sciences
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