Prof. Zhigang Yuan and His Team Found a New Channel for Diffuse Aurora Generation

On January 27, Prof. Zhigang Yuan and his team, from School of Electronic Information in Wuhan University, have published their new findings on the Earth’s diffuse aurora generation in Nature Communications, titled as “Diffuse auroral precipitation driven by lower-band chorus second harmonics”. Dr. Xiongdong Yu is the first author, Prof. Yuan is the corresponding author, and the first affiliations is School of Electronic Information in Wuhan University.
As the monitor of particle and energy transports from the magnetosphere into the atmosphere, diffuse aurora is the focus of research on different sphere coupling in the Earth for decades. It is commonly thought that the combined scattering effects of upper- and lower- band chorus waves drive magnetospheric electrons into their loss cones and cause them to precipitate into the atmosphere. However, these two types of waves do not always occur simultaneously, in which case spatial separation of them weakens the efficacy of their scattering on electron loss. Therefore, other potential alternatives are needed to form diffuse aurora.
Based on simultaneous observations of Van Allen Probes and NOAA satellites, this work finds that when the diffuse aurora electron precipitation occurred, the traditional upper-band chorus is absent. Instead, lower-band chorus waves are accompanied with their second harmonics. Different to the traditional two band chorus which are independent to each other, second harmonics occur with lower-band chorus in a one-by-one manner, and causal relation has been demonstrated between them. As demonstrated in previous studies of Prof. Yuan and his team (Geophys. Res. Lett. 2021, 48，e2020GL091762;  Geophys. Res. Lett. 2022,49，e2021GL097143;Geophys. Res. Lett. 2022, 49，e2022GL098166; Geophys. Res. Lett. 2022,49, e2022GL099152), second harmonics are excited by the nonlinear current density resulted from the interaction of lower-band chorus and plasmas. This casual relation leads to the one-by-one occurrence in space, which provides a great opportunity for combined scattering on electrons.
Moreover, this work has proposed the preferred condition for the second harmonic generation of chorus waves, which is verified with a statistical survey of in-situ high-resolution satellite data. The result provides a solid theoretical and observational basis for evaluating the role of second harmonics playing in the evolution of magnetospheric electrons. Furthermore, using the quasilinear resonance theory and Fokker-Planck simulation, this work confirms that second harmonics, playing the role of traditional upper-band chorus demonstrated in previous models, can work together with lower-band chorus waves to drive diffuse auroral electron precipitations. It provides one new channel of the diffuse aurora generation and new idea to understanding the different sphere coupling in the Earth.
For this work, Dan Deng, a PhD candidate in School of Electronic Information, implement the statistical survey and participate in the derivation of the preferred condition, while Prof. Jiang Yu from Sun Yat-Sen University, Research Scientist Dr. Dedong Wang from GFZ German Research Center for Geosciences in Germany, and Prof. H. O. Funsten from ISR Division, Los Alamos National Laboratory in USA make contributions to the simulation and satellite data guarantee. This work is supported by the National Natural Science Foundation of China (41925018, 42104148, 41874194).