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An Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia was recently deployed. Using ionospheric total electron content (TEC) from the two crossed Beidou geostationary satellite receiver chains of the network along 110 degrees E and 23 degrees N and Doppler velocity measurements from the Sanya (18.3 degrees N, 109.6 degrees E) portable digital ionosonde, we report first observations of low latitude TEC oscillations synchronized over a wide longitude range in East/Southeast Asia, which occur at nighttime, after the main phase of the geomagnetic storm on 20 April 2018. A comparison among TEC and Doppler velocity and interplanetary magnetic field (IMF) Bz component shows that the periodic TEC enhancements correlate with F region downward plasma drifts and IMF Bz southward turnings. The results suggest that the quasiperiodic southward turnings of IMF Bz could produce multiple short-lived westward prompt penetration electric fields, which contribute to driving the nighttime low latitude TEC oscillations simultaneously over the wide longitude range.

Plain Language Summary Regional-scale positive/negative ionospheric storm effects have been widely investigated using GPS total electron content (TEC) and ionosonde observations. Here we report significant nighttime TEC oscillations detected at latitudes lower than the equatorial ionization anomaly crest in the longitudes of 95-120 degrees E by the Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia during geomagnetic storm. Periodic TEC enhancements are nearly synchronized over the wide longitude range, in close association with downward plasma drifts at low latitude. The good consistence between the changes of plasma vertical drift and of interplanetary magnetic field Bz polarity indicates that multiple short-lived westward prompt penetration electric fields could drive the periodic downward plasma drifts, which are very likely to cause low-pressure region of topside ionosphere at low latitude and subsequent flowing of plasma from higher latitudes (equatorial ionization anomaly crest) to the low-pressure region. The extra plasma sourced from higher latitudes, which is against the loss of increased recombination due to F region height decrease, could contribute to the TEC oscillations. Compared with the TEC measurements by GPS receiver network that are affected by satellite motion, the Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia, which measures TEC at fixed ionospheric pierce points along the same longitude/latitude by receiving Beidou geostationary satellite signals, provides a unique means for fine-scale observations of ionospheric perturbations not previously possible.