PC 1 micropulsations observed by Magsat in the ionospheric F region

Large‐amplitude (5‐30 nT) Pc 1 micropulsations were detected near the plasmapause by a low‐altitude satellite, Magsat, in the ionospheric F region. Both left‐ and right‐hand‐polarized waves were observed in a latitudinally narrow (<100 km) region. The sense of polarization tends to switch around the amplitude peak, suggesting the coupling of fast mode and transverse mode waves and the propagation of the incident wave not only along the ambient magnetic field but also in the east‐west direction across the magnetic field. On the ground, periodic hm emissions having nearly the same frequencies as those at Magsat were observed at the stations 1000‐3000 km from Magsat orbit, and the amplitude on the ground was more than 2 orders of magnitude smaller than that observed by Magsat.