A Large‐Scale Magnetospheric Line Radiation Event in the Upper Ionosphere Recorded by the China‐Seismo‐Electromagnetic Satellite.

This paper reports a large‐scale magnetospheric line radiation (MLR) event during a moderate geomagnetic storm on 11 September 2018, which was well recorded by the China‐Seismo‐Electromagnetic Satellite (CSES) in the upper ionosphere. The event shows a symmetrical propagation feature at the conjugated locations between the two hemispheres, exhibiting a large spatial extension roughly from the latitudes 54°N to 53°S. The parallel structures are visible both in the electric and magnetic fields at a frequency band ranging from the local proton cyclotron frequency to ∼1.6 kHz. The wave intensity of parallel spectral lines was primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings between neighboring spectral lines roughly vary from ∼80 to 110 Hz at the high latitudes and ∼80–130 Hz at the low latitudes, suggesting a slight variation feature with latitude. The parallel spectral structures of MLR drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. The wave vector analysis shows that the MLR waves are right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, where the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. The other large‐scale MLR events all exhibit similar parallel structures and polarization characteristics, suggesting the universality of such a phenomenon. However, the azimuthal angles differ among different events, showing complex features. Plain Language Summary: Magnetospheric line radiation (MLR) is a unique electromagnetic wave distinguished by parallel spectral lines. This study reports a large‐scale MLR event that occurred in the dayside ionosphere. The event shows a symmetrical propagation feature, with a large spatial extension between latitudes 54°N and 53°S. The parallel structures are visible both in the electric and magnetic spectrogram, ranging from the local proton cyclotron frequency to ∼1.6 kHz. The MLR structures were primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings of MLR roughly vary from ∼80 to 110 Hz in the high latitudes and from ∼80 to 130 Hz in the mid‐low latitude region, slightly varying with latitude. The MLR structures drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. This MLR event is right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, and the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. However, the azimuthal angles differ among different events, indicating the complexity of the wave propagation feature. Key Points: A large‐scale magnetospheric line radiation (MLR) event shows a symmetrical propagation feature in two hemispheres, presenting a distinct V‐shaped structureBoth the frequencies of the parallel spectral lines and their frequency spacings slightly drift with latitudesThe MLR waves are right‐hand polarized, obliquely propagating toward the Earth and azimuthal direction [ABSTRACT FROM AUTHOR]