ULF Fluctuation of Low-Latitude Ionospheric Electric Fields During Sudden Commencements

We use the C/NOFS satellite observations to provide the direct evidence for ultra-low-frequency (ULF) fluctuations in the low-latitude ionospheric electric field during the sudden commencement (SC) on 8 March 2012. The meridional plasma drift shows a sudden downward motion with a large amplitude after SC onset. This initial westward electric field signature is followed by ULF fluctuations with periods of 28-100 s. Further, C/NOFS observations show that the ionospheric zonal electric field and compressional component of geomagnetic field present the similar ULF fluctuations with a similar to 90 degrees phase difference, indicating the existence of a standing compressional mode wave. The fluctuations in the ionospheric electric field and horizontal component of the geomagnetic field on the ground also exhibit a high correlation and the frequency of the ground-based geomagnetic field shows no dependence with latitudes. These results suggest that the ULF waves in the ionospheric electric field may be associated with a cavity mode. We also show the ULF waves in ionospheric electric fields in other cases. The ULF wave electric fields provide important information on the ionospheric studies and ionosphere-magnetosphere coupling in low latitudes.