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The most significant and fundamental feature of the ionospheric variations is the diurnal pattern of the ionosphere, which has very interesting and complicated phenomenon. In this brief review, we will introduce some works related with the ionospheric diurnal variations. Our topics includes the sunrise ionosphere with a focus on the enhancement of equatorial electric field around sunrise hours, the phenomenon of the daytime double peaks or noontime bite-out, and the nighttime enhancements in electron density, especially the summer mid-latitude nighttime anomalies represented by the Weddell Sea anomaly. We further discuss some points which are still under debate and remain elusive.