## 1 Introduction  
In the past decades, the global navigation satellite system (GNSS) has been widely used around the world, and countless works have been conducted to improve the positioning accuracy and signal reliability of the GNSS. The ionosphere, which has served as a key medium for the GNSS radio signal transmission, would show a great impact on the reliability of the GNSS applications. Specifically, the drastic changes in the ionosphere, which are normally referred to as plasma density irregularities, would directly result in signal attenuation or interruption. Therefore, ionospheric research is an essential aspect of mitigating GNSS error. The presence of plasma density irregularities in the ionosphere would cause rapid random fluctuations in the amplitude and phase of transionospheric radio waves [1; 2]. This phenomenon is called scintillation, which would lead to the reduction of signal quality, and even cause the GNSS satellite's signal tracking failure for receivers. This process is usually called \"loss of lock\", which may lead to an increase in navigation inaccuracies or even failures [3]. When the fading depth of the signal caused by amplitude scintillation exceeds the fade margin of the receiver, the signal could be covered by background noise; or the shift in frequency caused by phase scintillation exceeds the phase lock loop bandwidth, cycle slips could occur, and the signal could even be lost [4]. The scintillations occur most commonly near the geomagnetic equator at night during solar maximum and at high latitudes [5]. The generation and development of ionospheric density irregularities in different regions correspond to different plasma physical processes and mechanisms. At low latitudes, the most common ionospheric irregularities include equatorial plasma depletion (EPD) and plasma blob. EPD is also referred to as equatorial plasma bubble (EPB) or equatorial spread F (ESF), which characterizes the decrease in plasma density relative to ambient plasma. EPB is the result of Rayleigh-Taylor (R-T) instability at the bottom-side F layer, and it can penetrate through the topside ionosphere [6]. Another low-latitude irregularity is the plasma blob, a high-density structure compared with the surrounding region, but its generation is still unclear [7]. In addition to the above irregularities, there are also sporadic E layers (Es layers) and F region irregularities generated through the Perkins instability at low latitudes [8]. Although the variations of low-latitude irregularities (mainly EPBs) with season, longitude, latitude, local time (LT), and solar activity have been studied, whether the phase scintillation could present similar characteristics and how the low-latitude irregularities modify the phase scintillation are still not clear. Due to the low occurrence rate of scintillation at middle latitudes, previous studies of ionospheric irregularities have been less concentrated at middle latitudes. However, the medium-scale traveling ionospheric disturbances (MSTIDs) can be observed frequently at nighttime in the midlatitude ionosphere, whose horizontal wavelength is 100-300 km and whose phase velocity is 50-300 m/s. The generation of MSTIDs was originally explained by the Perkins instability. Previous studies have shown that atmospheric gravity waves (AGWs) and sporadic E layer (Es) instability induce the development of the Perkins instability, which in turn drives the formation of nighttime MSTIDs [9; 10; 11]. Besides, at middle latitudes, there is a band structure with plasma density enhancement at the nighttime ionosphere, which is often observed around \(\pm 40^{\circ}\) geomagnetic latitudes from 23 to 05 LT at the topside ionosphere. The middle-latitudinal band structure at the topside ionosphere may be mainly derived from the ambipolar diffusion of the plasmosphere and the upward plasma drift caused by the equatorward neutral wind along the geomagnetic field line [12].