Oxygen Ion Percentage Inferred From the Cutoff Frequencies of Ionospheric ELF Hiss Emissions

The lower cutoff frequency of hiss emissions observed in the ionosphere is controlled by local plasma composition and density. This study identifies 6,340 ionospheric hiss wave events based on observations from the China Seismo-Electromagnetic Satellite (CSES-01) from 1 August 2018 to 31 July 2020. Their cutoff frequencies are evaluated using the Canny edge-detection method. The O⁺ ion percentage is subsequently calculated from the cold plasma dispersion relation. It is shown that the average O⁺ percentage at CSES-01 altitude (∼510 km) is ∼92%, which is consistent with the prediction of IRI model. Detailed correlation and superposed epoch analyses indicate that the O⁺ percentage increases with storm intensification and reaches the peak value about 2 hr after the minimum of the Dst index. Our study provides an effective way from the wave aspect to investigate ion composition variations in the upper ionosphere.