Significant magnetospheric waves display surprising correlations

Electron cyclotron harmonic (ECH) waves and whistler-mode chorus waves, which are produced by hot electron instabilities, play important roles in Earth’s magnetosphere, causing auroral electron precipitation and accelerating relativistic electrons in the radiation belt.

However, the two are typically monitored independently, and interactions between them are rarely considered. By studying data from NASA’s Van Allen Probes, Gao et al. found a lag correlation between ECH rising tones and whistler-mode chorus rising tones.

“ECH and whistler-mode chorus waves contribute significantly to the magnetospheric dynamics,” said author Pingbing Zuo.

“How these waves are generated and evolved are always important topics,” said author Zhonglei Gao.

The group analyzed years of plasma wave data from the Van Allen Probes and used its high-resolution AC waveforms to determine the generation and effects of typical magnetospheric waves.

They found an interplay between the two waves indicative of a complex energy transfer process that can potentially have impacts on magnetospheric dynamics. The ECH waves and the chorus waves typically appear as structureless bands of constant frequency and intermittent rising or falling tones in the frequency-time spectrogram, respectively.

They both emerge as intermittent rising tones, allowing the authors to determine the lag correlation between the two. They discovered sufficiently strong whistler-mode waves have the potential to trigger ECH waves by rapidly precipitating electrons, leading to the unexpected correlation.

A full understanding of the interaction between these waves is still lacking and will require a larger data sample size in addition to further numerical studies, as the physical process governing the relationship remains unclear. The authors plan to investigate the electron scattering process by the whistler-mode chorus waves.

Source: “Lag-correlated rising tones of electron cyclotron harmonic and whistler-mode upper-band chorus waves,” by Zhonglei Gao, Xiongjun Shang, Pingbing Zuo, Zhengyang Zou, Geng Wang, Xueshang Feng, Yi Wang, Chunyi Guan, and Fengsi Wei, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/5.0008812 .