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. During the storm recovery phase on 27 August 2018, the ChinaSeismo-Electromagnetic Satellite (CSES) detected Pc1 wave activities in both the Northern Hemisphere and Southern Hemisphere in the high-latitude, post-midnight ionosphere with a central frequency of about 2 Hz. Meanwhile, the typical Pc1 waves were simultaneously observed for several hours by the Sodankyla Geophysical Observatory (SGO) stations on the ground. In this paper, we study the propagation characteristics and possible source regions of those waves. Firstly, we find that the Pc1 waves observed by the satellites exhibited mixed polarisation, and the wave normal is almost parallel with the background magnetic field. The field-aligned Poynting fluxes point downwards in both hemispheres, implying that the satellites are close to the wave injection regions in the ionosphere at about L=3 . Furthermore, we also find that the estimated position of the plasmapause calculated by models is almost at L=3 . Therefore, we suggest that the possible sources of waves are near the plasmapause, which is consistent with previous studies in that the outward expansion of the plasmasphere into the ring current during the recovery phase of geomagnetic storms may generate electromagnetic ion cyclotron (EMIC) waves, and these EMIC waves propagate northwards and southwards along the backgroundmagnetic field to the ionosphere at about L=3 . Additionally, the ground station data show that Pc1 wave power attenuates with increasing distance from L=3 , supporting the idea that the CSES observes the wave activities near the injection region. The observations are unique in that the Pc1 waves are observed in the ionosphere in nearly conjugate regions where transverse Alfven waves propagate down into the ionosphere.

Ionospheric Pc1 waves during a storm recovery phase observed by the China Seismo-Electromagnetic Satellite