Unexpected Secondary Enhancement of Daytime Scintillations Near Sunrise in Low‐Latitude Regions

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Abstract Ionospheric scintillations are typically considered as a nighttime phenomenon at equatorial and low latitudes. However, this study reports unexpected daytime scintillation enhancements near the sunrise terminator during geomagnetic storm periods. Multi‐instrument observations over Hainan revealed post‐midnight equatorial plasma bubbles persisting into the daytime. These plasma bubbles were triggered by eastward overshielding penetration electric fields induced by a sudden northward turning of the interplanetary magnetic field (IMF) Bz. While sunrise‐driven photoionization generally leads to the dissipation of plasma bubbles and a decline in scintillation, secondary daytime enhancements were observed. Analysis shows that subsequent southward turnings of IMF Bz caused the descent of the ionospheric layer, creating pronounced upward plasma density gradients at the upper boundaries of the bubbles near sunrise. Meanwhile, the strengthening of the eastward equatorial electric field enhanced the upward E × B drift. The combination of these factors triggered gradient drift instability, generating small‐scale irregularities and ultimately leading to a secondary enhancement of daytime scintillation. The results highlight the dynamic coupling between photoionization and irregularities driven by plasma instabilities, advancing our understanding of scintillation generation and evolution in the post‐midnight to dawn sector.

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