The Polar Ionosphere-Magnetosphere Coupling group of Shandong University's Solar Eruptions and Their Impact on the Planetary Space Environment Expedition Team has made new progress in Aurora research, revealing the formation and evolution mechanism of "Horse-collar aurora". The merging of the auroral inner edges and the disappearance of the polar cap indicate that the magnetosphere has shrunk to an unusually small and nearly closed configuration. The results were presented on February 13 in a paper entitled "Unusual shrinkage and reshaping of Earth’s magnetosphere under a strong northward interplanetary magnetic field", which is published online in Communications Earth & Environment. Professor Zhang Qinghe, the leader of the Ionosphere-Magnetosphere Coupling research group, is the corresponding author. PhD student Wang Xiangyu is the first author. Wang Chi and his team from the National Space Science Center of the Chinese Academy of Science and Zhang Yongliang from Johns Hopkins University made important contributions.

Aurora is a natural luminous phenomenon occurring in the northern and southern polar regions, which is formed when energetically charged particles from the solar wind interact with the upper atmosphere. The auroral oval usually presents an elliptic zonal region distributed in 60°-75° geomagnetic latitude (as shown in Figure 1) and varies with the intensity of geomagnetic activity. The region within the inner edge of the oval is called the polar cap, where the Earth's magnetic field lines are open to interplanetary space. When the interplanetary magnetic field is northward for a long time under quiet geomagnetic activity, the auroral oval will shrink towards high latitude due to the interaction between the solar wind and magnetosphere. Its inner boundary will move poleward from both sides of dawn and dusk and form a “Horse-collar” aurora pattern (as shown in Figure 1). As the inner edges of the Horse-collar aurora move further poleward and merge, the polar cap nearly disappears. The communication, navigation and remote sensing in polar regions will be affected by the formation and evolution of horse-collar aurora. However, the formation and evolution mechanism of horse-collar aurora and which solar wind-magnetosphere coupling process is reflected are still challenging problems.

Figure 1. The illustrations and observations of the auroral oval and “Horse-collar” aurora. a. The forecast auroral oval given by NOAA’s OVATION aurora model; b. The schematic diagram of Horse-collar aurora (Hones et al., 1989); c & d. Observations of Horse-collar aurora from space and ground (Hosokawa et al., 2020).

To solve these problems, the Ionosphere-Magnetosphere Coupling research group of Shandong University has carried out systematic research with a series of advanced observations and numerical simulations in cooperation with domestic and foreign researchers. These observations and simulations include ionospheric, magnetospheric spacecrafts and SuperDARN radar networks including high-frequency radars at the Chinese Meridian Project's Zhongshan Station in Antarctica, as well as high spatial-temporal resolution three-dimensional solar wind-magnetosphere-ionosphere coupled magnetohydrodynamics simulations of related events by the Chinese Academy of Sciences Space Center.

On April 10, 2015, the aurora imager on board the U.S. Defense Meteorological Satellite Program (DMSP) observed a clear process of the formation, evolution and merging of the Horse-collar aurora almost simultaneously over the North and South polar regions (as shown in Figure 2). The polar cap almost disappears and results in obvious energy particle precipitations in most polar regions, which can greatly affect communication, navigation and other high-tech activities in related regions.

To understand the mechanism behind this phenomenon the research group in collaboration with the team of Wang Chi, an academician at the National Space Science Center of the Chinese Academy of Sciences, simulated the whole process using the high-resolution magnetohydrodynamics model (PPMLR-MHD). The results show that the relatively stable dual-lobe reconnections continuously consume the open and even closed magnetic field lines of the Earth's magnetic tail and form closed magnetic field lines on the dayside under the long-term strong northward IMF conditions. These magnetic field lines captured the solar wind particles moving tailward along the flanks of magnetosphere, forming the cold and dense plasma sheets (CDPS) on both sides and a nearly completely closed dipole magnetosphere with a short tail of only 28 earth radii. The charged particles are accelerated by the flow shears and precipitated into the polar ionosphere, which resulted in the Horse-collar aurora and the disappearance of the polar cap.

The results show that the open flux in the magnetosphere may be closed under long-term strong northward interplanetary magnetic field conditions, forming a nearly completely closed quasi-dipole magnetosphere and leading to large-scale aurora and particle precipitations in the polar region, greatly increasing the risk of spacecraft exposure to solar wind and reducing the quality of communication and navigation. This study not only updates people's understanding of the interaction between the solar wind and magnetosphere under the condition of quiet geomagnetic activity but also provides important scientific support for the modeling and forecast of the polar ionosphere.

Figure 2. Aurora observations and in-situ plasma measurements of the merging of “Horse-collar” aurora.

Figure 3. The simulation results of the “Horse-collar” aurora case given by PPMLR-MHD and schematic diagram of the mechanism for the interaction between solar wind and magnetosphere. a, c, e are the results of PPMLR-MHD, and b, d, f are the schematic diagram.

In recent years, the Ionosphere-Magnetosphere Coupling research group of the Space Science Expedition team has actively participated in major national missions such as the Meridian Project, focusing on frontier scientific issues such as the ionosphere-magnetosphere couplings in the polar region and its space weather effect, and has made a series of research achievements. More than 100 academic papers have been published in Science, Nature Physics, PNAS, Nature Communications, GRL and JGR, many of which have been selected as the research highlights or covers of Science, Nature and JGR. The group was also invited to write two monograph chapters of American Geophysical Union (AGU). The research was supported by the National Natural Science Foundation of China and the Meridian Project.

Link to the paper: https://www.nature.com/articles/s43247-023-00700-0