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Abstract The development of an intense total electron content (TEC) depletion band over the United States during the 8 September 2017 geomagnetic storm was understood as the extension of an equatorial plasma bubble (EPB) to midlatitudes in previous studies. However, this study reports non‐EPB aspects within this phenomenon. First, the simultaneous emergence of the TEC depletion band at midlatitudes and EPBs in the equatorial region indicates that the midlatitude TEC depletion band is not initiated by an EPB. Second, the intensification of TEC depletion at midlatitudes during the decay of TEC depletion at intermediate latitudes is anomalous. Third, the location of the TEC depletion band at midlatitudes is inconsistent with the EPB location estimated from zonal plasma motion. Given ionospheric perturbations in North America from the beginning of the storm, it is plausible that the TEC depletion band was locally generated in association with these perturbations. 

Abstract Meteor radar observations provide wind data ranging from 80 to 100 km altitude, while the Michaelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) onboard the Ionospheric Connection Explorer satellite offers wind data above 90 km altitude. This study aims to generate wind profiles in the mesosphere and lower thermosphere by combining the winds derived from meteor radar and MIGHTI observations over the Korean Peninsula from January 2020 to December 2021. The wind profiles derived from the two instruments are continuous at night, but they show discrepancies during the day. The atomic oxygen 557.7 nm (green line) emission intensity measured by MIGHTI peaks at approximately 100 km during the day and 94 km at night. The vertical gradient of the airglow volume emission rate is more pronounced during the day. These differences can cause day‐night differences in the MIGHTI wind retrieval accuracy, potentially leading to discrepancies during the day. 

Abstract This study investigates the impact of vertical ionospheric drift during daytime on the evolution of predawn equatorial plasma bubbles by conducting model simulations using “Sami3 is Another Model of the Ionosphere.” The upward drift of the ionosphere transports bubbles to higher altitudes, where their lifetime is set by the atomic oxygen photoionization rate. While the bubbles generated at predawn persist into dayside, the bubbles generated shortly after sunset diminish before sunrise. Therefore, post‐sunset bubbles do not contribute to daytime electron density irregularities. Bubbles maintain their field‐aligned characteristics throughout the daytime regardless of the vertical ionospheric drift. This property allows bubbles to exist near the magnetic equator despite poleward plasma transport by the fountain process. The shift of irregularity concentration to higher latitudes over time in satellite observations is explained by the combined effect of transport of bubbles to higher altitudes and rapid refilling of depletions near the magnetic equator. 

Abstract We present a number of unique observations of ionospheric anomalies following the Hunga‐Tonga Hunga‐Ha'apai (HTHH) volcanic eruption on 15 January 2022. All are based on non‐dedicated geodetic satellite systems: Global Positioning System tracking of Low Earth Orbit (LEO) CubeSats, intersatellite tracking between two GRACE Follow‐On satellites, satellite radar altimeters to the ocean surface, and Doppler radio beacons from ground stations to LEO geodetic satellites. Their observations revealed the development of anomalously large trough‐like plasma depletions, along with plasma bubbles, in the equatorial regions of the Pacific and East Asian sectors. Trough‐like plasma depletions appeared to be confined within approximately ±20° magnetic latitude, accompanied by density enhancements just outside this latitude range. These plasma depletions and enhancements were aligned with the magnetic equator and occurred across broad longitudes. They were detected in regions where atmospheric waves from the HTHH eruption passed through around the time of the sunset terminator. We interpret these phenomena in terms of theEdynamo electric fields driven by atmospheric waves from the eruption. The uplift of the ionosphere beyond satellite altitudes, followed by subsequent plasma diffusion to higher latitudes along magnetic field lines, results in the formation of trough‐like plasma depletions around the magnetic equator and density enhancement at higher latitudes. The detection of plasma bubbles in the Asian sector during the non‐bubble season (January) is likely associated with the uplift of the ionosphere at the sunset terminator. 

Electron density irregularities in the equatorial ionosphere at night are understood in terms of plasma bubbles, which are produced by the transport of low-density plasma from the bottomside of the F region to the topside. Equatorial plasma bubbles (EPBs) have been detected by various techniques on the ground and from space. One of the distinguishing characteristics of EPBs identified from long-term observations is the systematic seasonal and longitudinal variation of the EPB activity. Several hypotheses have been developed to explain the systematic EPB behavior, and now we have good knowledge about the key factors that determine the behavior. However, gaps in our understanding of the EPB climatology still remain primarily because we do not yet have the capability to observe seed perturbations and their growth simultaneously and globally. This paper reviews the occurrence climatology of EPBs identified from observations and the current understanding of its driving mechanisms. 

Abstract This study investigates the distribution and formation mechanisms of ionization troughs inside an auroral oval (referred to as high‐latitude troughs) by analyzing Swarm observations from May–August 2014. Simultaneous measurements of plasma density, 3‐dimensional ion velocity, ionospheric radial current (IRC), and electron temperature are available during this period. Because high‐latitude troughs appear within an auroral oval while mid‐latitude troughs appear at the equatorward edge of the auroral oval, the positioning of troughs relative to the equatorward auroral boundary becomes critical for distinguishing between the two types of troughs. We ascertain the auroral boundary and the orientation of field‐aligned currents using IRC data derived from magnetic field measurements. The principal features of high‐latitude troughs identified from Swarm data include: (a) enhancements in ion velocity and electron temperature, (b) the presence of downward or absent field‐aligned current (FAC), and (c) a more frequent occurrence in the Northern (summer) Hemisphere than in the Southern (winter) Hemisphere and in the dawn and dusk sectors than in the noon and midnight sectors. The alignment of the density minimum with the velocity maximum underscores the role of high‐speed plasma convection in the formation of high‐latitude troughs; atmospheric frictional heating promotes the O⁺loss through dissociative recombination. The prevailing appearance of high‐latitude troughs at dawn and dusk sectors, coupled with downward field‐aligned currents, indicates the involvement of outward electron evacuation in trough formation.