Multiple transpolar auroral arcs reveal insight about coupling processes in the Earth’s magnetotail Qing-He Zhanga,1, Yong-Liang Zhangb, Chi Wangc, Michael Lockwoodd, Hui-Gen Yange, Bin-Bin Tangc, Zan-Yang Xinga, Kjellmar Oksavikf,g, Larry R. Lyonsh, Yu-Zhang Maa, Qiu-Gang Zongi, Jøran Idar Moenj,g, and Li-Dong Xiaa aShandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, Shandong, 264209, China; bThe Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723; cState Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, 100190, China; dDepartment of Meteorology, University of Reading, Reading, RG6 6BB, United Kingdom; eMinistry of Natural Resources Key Laboratory of Polar Science, Polar Research Institute of China, Shanghai, 200136, China; fDepartment of Physics and Technology, Birkeland Centre for Space Science, University of Bergen, Bergen, N-5020, Norway; gArctic Geophysics Department, The University Centre in Svalbard, Longyearbyen, N-9171, Norway; hDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095; iSchool of Earth and Space Sciences, Peking University, Beijing, 100871, China; and jDepartment of Physics, University of Oslo, Blindern, Oslo 0371, Norway Edited by Lennard A. Fisk, University of Michigan, Ann Arbor, MI, and approved May 26, 2020 (received for review January 11, 2020) A distinct class of aurora, called transpolar auroral arc (TPA) (in twisting of the magnetotail due to IMF variations (3, 21, 22). some cases called “theta” aurora), appears in the extremely high- These processes could result in plasma sheet bifurcation, fila- latitude ionosphere of the Earth when interplanetary magnetic mentary extent into the lobes, or hot plasma trapped in the tail field (IMF) is northward. The formation and evolution of TPA of- lobes (1, 7, 23–25). None of the suggested processes can explain fers clues about processes transferring energy and momentum all of the observed characteristics, and especially multiple TPAs. from the solar wind to the magnetosphere and ionosphere during It has also been difficult to investigate the phenomenon due to a northward IMF. However, their formation mechanisms remain constraints on obtaining simultaneous observations at multiple poorly understood and controversial. We report a mechanism points across the solar wind–magnetosphere–ionosphere system. identified from multiple-instrument observations of unusually Here, we present ground-breaking multiple-instrument obser- bright, multiple TPAs and simulations from a high-resolution vations of multiple TPAs structures seen simultaneously both in three-dimensional (3D) global MagnetoHydroDynamics (MHD) the ionosphere and the distant magnetotail. The observations are model. The observations and simulations show an excellent agree- EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES interpreted using a global three-dimensional (3D) magnetohy- ment and reveal that these multiple TPAs are generated by pre- cipitating energetic magnetospheric electrons within field-aligned drodynamic (MHD) simulation, giving for a discussion of the current (FAC) sheets. These FAC sheets are generated by multiple- formation of multiple TPAs from a global perspective. On 7 September 2017, long-lasting, relatively stable northward flow shear sheets in both the magnetospheric boundary produced > by Kelvin–Helmholtz instability between supersonic solar wind IMF conditions (IMF Bz 0 for more than 8 h) occurred with – flow and magnetosphere plasma, and the plasma sheet generated comparable dawn dusk components (IMF By) and roughly sta- by the interactions between the enhanced earthward plasma ble solar wind conditions, except for some IMF variations and flows from the distant tail (less than −100 RE) and the enhanced tailward flows from the near tail (about −20 RE). The study offers Significance insight into the complex solar wind-magnetosphere-ionosphere coupling processes under a northward IMF condition, and it chal- Colorful and dynamic aurora has attracted human’s attention lenges existing paradigms of the dynamics of the Earth’s since the dawn of time. However, mystery remains in un- magnetosphere. derstanding a distinct class of aurora, transpolar auroral arc (TPA) (or “theta” aurora) which occurs in extremely high lati- aurora | solar–terrestrial interaction | magnetosphere | polar ionosphere | tude of the Earth polar regions when interplanetary magnetic transpolar auroral arcs field (IMF) is northward. Previous theories are unable to ex- plain why multiple TPAs often occur. Our comprehensive ob- ranspolar arcs (TPAs) are the large-scale auroral forms that servations in the ionosphere and magnetotail as well as a three- Tstretch almost entirely across the polar cap and connect the dimensional magnetosphere modeling shed insight on how dayside and nightside auroral oval (1–5). TPAs can appear as a multiple TPAs form. Our study offers clues how solar wind energy single or multiple arcs (6–8). Previous theories and observations and mass transfer into the magnetosphere and ionosphere under indicated that TPAs are related to the magnetospheric cusp as a northward IMF that occurs nearly half of the time. their dayside ends join together in the cusp region (called “cusp- ” Author contributions: Q.-H.Z. and Y.-L.Z. designed and performed research; C.W. and aligned auroral arcs) (9, 10), and are generated by the solar B.-B.T. contributed new reagents/analytic tools; Q.-H.Z., Y.-L.Z., H.-G.Y., Z.-Y.X., K.O., wind–magnetosphere–ionosphere coupling processes as a whole 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., system during northward interplanetary magnetic field (IMF) L.R.L., Y.-Z.M., Q.-G.Z., J.I.M., and L.-D.X. participated in the scientific discussions; and and quiet geomagnetic conditions (2–5). TPAs are sometimes Q.-H.Z., Y.-L.Z., M.L., K.O., and L.R.L. wrote the paper. seen simultaneously in both polar caps (11) and have been The authors declare no competing interest. inferred to be on both open and closed magnetic field lines (12). This article is a PNAS Direct Submission. TPA formation processes have long been debated, in partic- Published under the PNAS license. ular whether they are driven by dayside or nightside processes Data deposition: The 3D MHD simulation data are available on the website of https:// (5). The main candidates for dayside processes have been day- 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 side reconnection at the low-latitude magnetopause or high- Zhongshan Station (ZHS) are available on the website of https://zenodo.org/record/ latitude lobe regions (13–17) and plasma instabilities (e.g., 3778095# with a separate DOI of 10.5281/zenodo.3778095. Kelvin–Helmholtz instability, interchange instability) near the 1To whom correspondence may be addressed. Email: [email protected]. magnetospheric low and high-latitude boundary layers (8, This article contains supporting information online at https://www.pnas.org/lookup/suppl/ 18–20). The main candidates for nightside processes have been doi:10.1073/pnas.2000614117/-/DCSupplemental. proposed as nightside reconnection in the magnetotail (4, 7) and www.pnas.org/cgi/doi/10.1073/pnas.2000614117 PNAS Latest Articles | 1of6 Downloaded by guest on September 26, 2021 dynamic pressure increases around 12:30 UT (Fig. 1 A–C). The electrons that have been accelerated to more than 1 keV. Most northward IMF reached around 8 nT and was roughly stable, of the TPAs were not associated with observable ion pre- with the By component around 4 nT, and a high solar wind speed cipitation, but may be associated with upward ion beams ob- of around 520 km/s (see gray shading in Fig. 1 indicating the served by Cluster satellites in other events (31, 32). The electron interval of interest). These conditions are favorable for triggering energy fluxes (Fig. 2G) also reveal several smaller arcs that were of the Kelvin–Helmholtz instability between the solar wind and not visible in the DMSP/SSUSI images with a limited spatial magnetosphere (26–29) and multiple cusp-aligned auroral arcs in resolution and/or sensitivity (Fig. 2A). The formation of such the polar cap (9, 10). The auroral electrojet AL and AU indices, multiple/filamentary TPAs has remained an intriguing and un- the smallest (AL) and largest (AU) values of geomagnetic vari- resolved puzzle for many years. ations in the horizontal component observed at 11 selected ob- Using the measured interplanetary conditions as input, a high- servatories along the auroral zone in the northern hemisphere, resolution 3D global MHD code called Piecewise Parabolic show nonsubstorm conditions after 15:30 UT (Fig. 1D), indi- Method with a Lagrangian Remap to MHD (PPMLR-MHD) cating a quiet auroral oval. (33, 34), was run to investigate the formation of multiple TPAs. Auroral observations from the Southern Hemisphere are An electrostatic ionosphere shell with height-integrated conduc- presented in Fig. 2 A–E. Poleward of a normal auroral oval at tanceisimbeddedintheMHDcode,allowing for the electrostatic lower latitudes, Fig. 2A shows at least six bright TPAs in the coupling and calculation of FACs between the ionosphere, which is otherwise empty polar cap (poleward of −78° MLAT, and near the Earth, and the model’s magnetospheric inner boundary. highlighted by gray shading in Fig. 2 F–H) from Defense Mete- Fig. 3A shows a two-dimensional extract from a movie (Movie S1) orological Satellite Program (DMSP) F17 Special Sensor UV of simulated FACs and plasma velocity vectors in the Geocentric Spectrographic Imager (SSUSI) data. In addition, there are Solar Magnetosphere (GSM) X-Y plane [Z ∼−1RE for better several radially aligned arcs both within the dawnside (−70 capturing the phenomenon in the Southern Hemisphere (35)]. The to −78° MLAT) and duskside (−70 to −77° MLAT) auroral oval.