Diffuse aurora at the Earth’s high latitude regions is mainly caused by the low-energy (0.1–30 keV) electron precipitation which carries the major energy flux into the nightside upper atmosphere. Previous studies have demonstrated that combined scattering by the upper- and lower- band chorus waves act as the dominant cause of diffuse auroral precipitation [1]. However, lower-band chorus (LBC) waves often occur alone, not accompanied with upper-band chorus [2], even when diffuse auroral electron precipitation as well as the remnant pancake distribution in space have been found [3]. Spatial separation of upper- and lower- band chorus weakens the efficacy of such a mechanism, because electrons should take time to move across the spatial gap between. Therefore, we seek to find other potential alternatives to form diffuse aurora.

In our work

‘Diffuse auroral precipitation driven by lower-band chorus second harmonics’

we report that the lower-band chorus satisfying the preferred condition can generate their second harmonics so as to trigger the diffuse auroral electron precipitation. We find that the lower-band chorus alone can only cause the precipitation of electrons greater than 4 keV, while the self-consistently generated second harmonic is weak but still able to result in the electron precipitation below 4 keV. The combined effect of those modes results in the observed pancake electron distributions and the diffuse aurora. Our results clearly demonstrate an alternative but universal mechanism of chorus-driven diffuse aurora in the Earth, which may also apply to the auroral formation in other planetary magnetospheres.

[1] Thorne, R. M., Ni, B., Tao, X., Horne, R. B., & Meredith, N. P. Scattering by chorus waves as the dominant cause of diffuse auroral precipitation. Nature 467, 943-946 (2010).

[2] Teng, S., Tao, X., & Li, W. Typical characteristics of whistler mode waves categorized by their spectral properties using Van Allen Probes observations. Geophys. Res. Lett. 46, 3607–3614 (2019).

[3] Meredith, N. P. et al. ‘‘Pancake’’ electron distributions in the outer radiation belts. J. Geophys. Res. 104, 12431–12444 (1999).