## 1 Introduction  
Ionospheric F-region irregularities, also called equatorial plasma bubbles (EPBs), refer to plasma density depletions with respect to the background ionosphere, and are generally regarded as nighttime phenomena occurring in the equatorial region within a narrow distribution of \(\pm 20^{\circ}\) magnetic latitudes [1, 2, 3, 4]. [PERSON] and [PERSON] [5] reported the first observation of ionospheric F-region irregularities, which appear as scattered echo traces in high-frequency band ionograms, a phenomenon known as spread F. It is generally accepted that the generalized Rayleigh-Taylor (RT) instability performs an important role in triggering EPBs after sunset [6; 7; 8]. The RT instability is excited in the bottom side F region, then evolves into plasma bubbles that penetrate the F peak to the topside F region [6; 7; 8]. Importantly, the growth of the RT instability can be modulated by several factors, such as the electric field, plasma density gradient, neutral wind, and gravity waves [8]. Therefore, the occurrence of EPBs shows significant seasonal, longitudinal, altitude, solar, and magnetic activity dependence [2; 9; 10; 11; 12]. EPBs are known to severely affect the performance of transionospheric radio waves, called ionospheric scintillation, leading to sharp fluctuations in the amplitude and phase of radio signals from satellite radio communication and navigation systems [12; 13; 14; 15]. Therefore, it is of great importance to study the occurrence and characteristics of the ionospheric F-region irregularities under different ionospheric background conditions. While ionospheric F-region irregularities are primarily confined in the equatorial region, large amplitude plasma depletions have occasionally been observed at midlatitudes [3; 16; 17; 18; 19; 20]. For example, [PERSON] and [PERSON] [17] detected a super plasma bubble at 30\({}^{\circ}\)N magnetic latitudes by using dense GPS receivers. In European midlatitudes, plasma bubbles of equatorial origin have been observed through the dense GNSS networks and satellite in situ measurements [18; 19]. [PERSON] et al. [20] presented a case study of midlatitude plasma bubbles after sunset over China and adjacent areas during the main phase of a storm on 8 September 2017. In the American sector, [PERSON] et al. [21] found that midlatitude plasma bubbles can be identified in the form of airglow depletions. [PERSON] et al. [8] reported that the highest latitude (altitude) of ionospheric F-region irregularities in the literature occurred at 46\({}^{\circ}\) magnetic latitudes and reached an apex altitude of 6800 km over the magnetic equator. Generally, two main mechanisms can be responsible for the formation of midlatitude plasma depletions. One reasonable interpretation is that midlatitude plasma depletions are the poleward extension of EPBs along the magnetic field lines under the additional eastward electric field. Typically, such eastward electric fields are considered to be the generated electric field (i.e., prompt penetration electric fields [PPEF] and/or disturbance dynamo electric fields [DDEF]) at high-latitude regions that penetrates to the equator and low latitudes during magnetic storms [20; 21; 22; 23; 24]. In addition, [PERSON] et al. [25] recently proposed that the sunset eastward electric field intensified by the Tonga volcano-induced effects can also produce plasma bubbles poleward extending to middle latitudes in both hemispheres. The other explanation for the generation of midlatitude plasma depletions is in association with medium-scale traveling ionospheric disturbances (MSTIDs) characterized by wave-like structures in the midlatitude ionosphere [26; 27; 28; 29; 30; 31]. By using all-sky airglow imagers, [PERSON] et al. [26] found that the evolution of mesoscale field-aligned irregularity structures was strongly associated with the intensity of the main-body MSTIDs.