Virtual Height Variation during Plasma Bubble Evolution Associated with Plasma Blob in Low Latitude Ionosphere

The observations and theoretical analysis suggest that ionospheric plasma blob could form during the early stage of plasma bubble evolution. In this paper we perform three cases in which ionospheric plasma blob and bubble were associated, to study the virtual height variation during plasma bubble evolution with continuous observations by Digital ionosonde. The cases were over station Vanimo (Geographic 2.7°S, 141.3°E;  Geomagnetic 11.2°S, 146.2°W) and Hainan (Geographic 19.5°N, 109.1°E; Geomagnetic 9.1°N, 179.1°W), both in the low latitude region in East Asia sector while in different Hemispheres. The results show that: previous studies suggested that, 2 h before blob occurred, plasma vertical drift velocity would turn from upward (eastward electric field) to downward (westward electric field), but the feature did not show in this study. In our three cases, the turning time was 6 h before blob, or the turning direction was from downward to upward. In addition, 1 h before blob occurred, the ionospheric plasma had sudden compression trend, such as decrease of downward velocity, turn from downward to upward, or increase of upward velocity. This compression was more close to plasma blob, and not limited to the turning of drift direction.