On January 23, 2025, Nature published an article on the latest advancements in chorus wave generation mechanisms in space, made by Professor Cao Jinbin’s team from the School of Space and Earth Sciences at Beihang University, alongside researchers from the United States and Sweden.
In contrast to the conventional expectation that chorus waves are governed by planetary magnetic dipolar fields, the study, titled “Field-particle energy transfer during chorus emissions in space,” reports the discovery of chorus waves in an unlikely location, some 160,000 kilometres away from Earth.
Beihang Associate Professor Liu Chengming is the first author, with Associate Professor Liu Chengming and Academician Cao Jinbin serving as corresponding authors. The first affiliation is the School of Space and Earth Sciences of Beihang University.
Chorus waves are among the strongest electromagnetic emmissios in the Earth’s and planetary plasma environments, playing a key role in the acceleration of relativistic electrons in radiation belts and the generation of auroras in polar regions. These waves can accelerate low-energy electrons to high-energy levels (MeV), forming “killer electrons” that threaten satellite stability and astronaut safety. Additionally, they scatter electrons into the atmosphere, causing them to descend along the geomagnetic field lines to the upper atmosphere, where they interact with atmospheric molecules to create the stunning auroras. Chorus waves also influence the structure of radiation belts and can modulate magnetosphere-ionosphere coupling, affecting space weather dynamics.
Due to their significance, chorus waves have been a hot topic in space physics since the 1950s. Despite the decades’ long research, many important issues regarding chorus waves, such as their generation and the formation of their chirping element, remain highly debated. One main reason is that there were no direct measurements of wave–particle energy transfer inside chorus waves because (1) chorus waves were believed to be closely related to the magnetic dipolar field of planets and targeted only in near-planet space (Fig. 1), at a typical L-shell of less than 10 RE (where Earth radius RE = 6,370 km) and (2) previous satellite missions in the near-planet region cannot fully resolve three-dimensional velocity distributions of resonant electrons in sufficiently high cadence, due to the strong magnetic fields and high-energy electrons therein.
Fig. 1: Schematic showing the occurrence of whistler-mode chorus waves in geospace
In the study, the authors present observations of repetitive, rising-tone chorus waves in the terrestrial mid-tail neutral sheet (L = 26 RE), where the magnetic field is dominated by a strongly stretched magnetic field rather than a dipolar field. Using high-cadence data from NASA’s MMS mission, they present ultrafast measurements of the wave fields and three-dimensional electron distributions within the waves, which provides evidence for chorus–electron interactions and the development of electron holes in the wave phase space. The findings reveal that the waves are associated with resonant currents antiparallel to the wave magnetic field, as predicted by nonlinear wave theory. Their observations have implications for resolving long-standing controversies regarding chorus emissions and understanding of the energy transport observed in space and astrophysical environments.
Professor Richard B. Horne, a fellow of the Royal Society, commented on the study in Nature, saying that the discovery of chorus waves in an unexpected 160,000 kilometers away from Earth is a surprising result in a surprising region. He also highlighted the remarkable observation of an electron hole in the wave phase space. He noted that Liu and colleagues’ results are not just academic. Studies such as the one by Liu and colleagues enhance our understanding of these waves, which will greatly improve our forecasting ability.
Other Beihang team members who contributed to this study include master’s student Zhao Boning (second author), Associate Professor Liu Yangyang, and doctoral student Xing Xining. The work also benefited from significant support from international collaborators, including Christopher Russell from the University of California, Los Angeles, James Burch from the Southwest Research Institute, Craig Pollock from the Denali Scientific, and Per-Arne Linqvist from the KTH Royal Institute of Technology in Sweden.
This work was supported by the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities, and the young talent supporting project of the China Association for Science and Technology.
Link to the original article: https://www.nature.com/articles/s41586-024-08402-z
Link to the News & Views by Richard B. Horne: https://www.nature.com/articles/d41586-024-04211-6
Editor：Tian Zimo
