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The strip‐like bulge is a storm‐time conjugate ionospheric plasma density enhancement, constituted by the plasmaspheric H+/He+, that extends widely (over 150° in longitude) in the zonal dimension but occupies only 1°–5° in latitude. Based on in‐situ measurements of 11 low earth orbit satellites, this study statistically investigates the bulge structures of geomagnetic storms driven by 136 interplanetary coronal mass ejections during 2000–2021. The statistical results show that the strip‐like bulges are observed at the end of the storm main phase and can persist for more than 60 hr. The spatial and temporal coverage of the strip‐like bulge varies from storm to storm. However, the bulges do exhibit occurrence preferences: stronger storms (for the ICME‐driven) during solar minimum periods, the Asian‐Pacific sector (with eastward magnetic declination), and the nightside of the dawn‐dusk terminator. A quiet time density enhancement called mid‐latitude enhancement could be recognized as a precursor of the strip‐like bulge. The evolution features of the plasmapause height exhibit similarities with the strip‐like bulge, indicating a field‐aligned downward and cross‐L inward intrusion of the plasmaspheric ions. The local net ion drifts partly support this scenario with downward/inward being the most dominant but not unique pattern, the other diverse net ion drift configurations exist but their impact on the strip‐like bulges remains unclear. A geomagnetic storm is a major disturbance of Earth's magnetic field when energy from the solar wind is effectively injected into the space environment surrounding Earth. During a storm, the ionosphere from 80 to 1,000 km is affected, and a sharp increase in ionospheric ion density with a narrow latitudinal range could be observed at low‐mid latitudes. The main ions constituting the sharp increase are H+/He+, which are the primary ions in the plasmasphere. We called this increase the “bulge.” Toward a better understanding of the principal nature as well as the formation mechanisms of the structure, we extracted a number of samples snapshotted by many satellites to conduct a statistical investigation. It is found that the bulges prefer to appear in the Asian‐Pacific region, at 18:00‐06:00 LT, during strong storms, and under solar minimum conditions. Before their appearance, a smoother density peak structure is often observed, which could be recognized as the precursor. We suggested a substantial transfer of ions from higher to lower altitudes and the storm‐induced disturbance neutral wind, causing the smoother density peak structure to evolve into the bulge. Occurrence of bulge has no strict limitation but shows some dependences on longitude, storm intensity, local time, and solar activityMidlatitude extra density peak could be recognized as a precursor of the strip‐like bulgeIon drift within the flux tube of the bulges is mainly in the field‐aligned downward/cross‐L inward pattern Occurrence of bulge has no strict limitation but shows some dependences on longitude, storm intensity, local time, and solar activity Midlatitude extra density peak could be recognized as a precursor of the strip‐like bulge Ion drift within the flux tube of the bulges is mainly in the field‐aligned downward/cross‐L inward pattern

The abundant observations and research established a detailed category of the terrestrial ionospheric irregularities, which significantly advanced our understanding of how the Earth system's complicated physical and chemical process generates the intermediate‐scale structures of the charged particles. Motivated by a future attempt at categorizing the Martian ionospheric irregularity, this study designs a method for naive classification of the plasma density depletion, enhancement, and oscillation based on the in situ measurements of the Martian ionosphere. The technique consists of several procedures: trend estimation, detrending and candidate extraction, and parameterization. The classification is achieved through a machine‐learning‐like process using some testing artificial density profiles. A preliminary credence test shows a good performance in separating the terrestrial low‐latitude Equatorial Plasma bubble (depletion) and mid‐latitude Median‐scale Traveling Ionospheric Disturbance (oscillation). Another detection experiment of the Martian plasma depletion events (collected by Basuvaraj et al. (2022a, https://doi.org/10.1029/2022je007302)) showed a recall rate (i.e., true positive) of 38% but with a high precision of 67.8%. Therefore, we believe the proposed method could convincingly extract different Martian ionospheric irregularities and help uncover the climatological characteristics in the future. Ionospheric irregularities refer to the inhomogeneous distribution of the charged particles in the ionosphere, which would affect radio wave transmission and thus pose threats to tremendous human‐made assets deployed in terrestrial outer space and on other planets such as Mars. Unlike the terrestrial ionospheric irregularities, whose categories and physical mechanisms are well‐understood, the Martian counterparts are still in poor understanding. Previous studies have indicated the diversity, but the category has yet to be built. The large data set provided by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission allows a more detailed investigation of the Martian ionospheric irregularities. Therefore, an automatic method to extract the ionospheric density enhancement, depletion, and oscillation would be a first step to figuring out the exact types of the Martian ionospheric irregularities, thus helping to extend our knowledge of how the finer ionospheric structures are formed backgrounded by the region crustal magnetic field other than the global dipole field. A modified rolling‐barrel algorithm extracts the candidate irregularitiesClassification of the candidates into depletion, enhancement, and oscillation is achieved through a few parameterization processesInitial testing shows good performance on the Martian and terrestrial ionospheric irregularities A modified rolling‐barrel algorithm extracts the candidate irregularities Classification of the candidates into depletion, enhancement, and oscillation is achieved through a few parameterization processes Initial testing shows good performance on the Martian and terrestrial ionospheric irregularities

The latitudinal location of the Equatorial Ionization Anomaly (EIA) crest has seasonal variation, and there are disagreements on the interpretation of such seasonal characteristic in previous studies. Some studies suggested that this seasonal characteristic is determined by the seasonal characteristic of the equatorial electric field. Others suggested that this seasonal characteristic is determined by the seasonal changes of the thermospheric wind. The current paper uses Total Electron Content (TEC) data and the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) to analyze the seasonal variation of the northern EIA crest in the eastern Asian sector under low solar activity. Our results show that the monthly averaged latitudinal location of the northern EIA crest has a good linear relationship (r= 0.74) with the monthly averaged Equatorial Electrojet (EEJ) intensity, which is a commonly used proxy of the eastward electric field. However, TIEGCM simulations with and without F‐region wind indicate that such a relationship might be attributed to wind effects. Additionally, the linear relationship between the EEJ intensity and the northern‐southern EIA crest distance is not significant (r= 0.47) in the eastern Asian sector. Our results suggest that a good correspondence between the eastward electric field and the latitudinal location of the EIA crest is not assured annually, as the seasonally varying F‐region wind significantly influences EIA evolution. Variations of the EIA crest latitude in the east Asian sector are analyzed with TEC data and TIEGCM simulationEEJ intensity and northern EIA crest latitude have a good linear relationship but might be attributed to F‐region meridional wind effectWind effect on the EIA evolution cannot be neglected on an annual timescale Variations of the EIA crest latitude in the east Asian sector are analyzed with TEC data and TIEGCM simulation EEJ intensity and northern EIA crest latitude have a good linear relationship but might be attributed to F‐region meridional wind effect Wind effect on the EIA evolution cannot be neglected on an annual timescale

In situ electron density measurements by the CHAllenging Minisatellite Payload and the Defense Meteorological Satellites Program F17 satellites show that the midlatitude ionization at altitudes of ∼350 and 850 km is enhanced in the late evening. The enhancements increase to maximum around midnight and are clearly observed till early morning as the equatorial ionization decays to minimal level. They appear in the winter hemisphere during June and December solstices and in both hemispheres during equinox. The enhancements are well confined between ±30° and ±50° magnetic latitude, with the magnetic flux tubes of L= 1.3 − 2.4 connecting to the plasmasphere. Furthermore, coincident longitudinal variations exist in both the ionospheric enhancements and the plasmaspheric total electron content, especially during the solstice months. The coincidence may suggest essential plasma transport between the ionosphere and the plasmasphere. These facts support the idea that the plasmasphere provides extra plasma to the midlatitude ionosphere through downward plasma influx along the magnetic field lines to form the nighttime ionization enhancements when the sunlight is absent. Satellites reveal global pictures of nighttime enhancements in the midlatitude ionosphere at altitude of ∼350 and 850 kmWe find correlation of longitudinal dependence between the ionospheric enhancements and the plasmasphereThe plasmasphere is suggested to feed and dominate the formation of the nighttime enhancements

A new type of total electron content (TEC) variation was observed when an interplanetary shock impacted on the Earth's magnetosphere on 17 March 2015. With hundreds of ground‐based Global Positioning System (GPS) receivers, instantaneous TEC impulses were detected in the signals from four GPS satellites cruising in the dayside equatorial magnetosphere. Despite the small amplitude (0.3 TECU), the impulses were synchronously registered by receivers spreading from low to middle latitudes on the ground; they lasted for several minutes, during which period the interplanetary magnetic field (IMF) kept northward. The TEC impulses therefore discriminate themselves from usual traveling ionospheric disturbances and exclude possible effects from the southward IMF‐driven magnetic storm. We suggest that the TEC variation is caused by shock‐induced magnetospheric compression, which moves plasma earthward in the dayside plasmasphere. As a result, some plasma outside of GPS orbit (4.2 RE) is moved to the inside and contributes to the plasma content traversed by GPS raypath. The GPS TEC technique thus exhibits an unprecedented capability to capture small tremor of the magnetosphere, and with the dense receiver network on the ground it can be a feasible tool for remote sensing of the plasma dynamics around 4.2 RE. Sudden increase of GPS TEC is observed in response to shock‐induced magnetospheric compressionMagnetospheric plasma moving earthward in dayside equatorial plane contributes to the TEC increaseGPS TEC technique shows potential capability to remotely sense the radial plasma motion around 4 RE(GPS orbit)

The change of solar EUV irradiance between the 1996 and 2008 minima is still in active debate due to a possible trend in the data from the Solar EUV Monitor (SEM) on board the Solar and Heliospheric Observatory (SOHO). For the period of years 2002–2012, i.e., the descending phase of solar cycle 23 (SC23) and the ascending phase of SC24, concurrent observations from the Solar EUV Experiment (SEE) on board the Thermospheric Ionospheric Mesospheric Energy and Dynamics spacecraft are available. We compare the data from the two instruments in terms of an expected linear relation between the EUV irradiance and the ionospheric peak electron density (NmF2). For the SEE data the NmF2‐EUV function is solar cycle invariant, but for the SEM data the function is found to be different between SC23 and SC24, which is hard to explain at present. An attempt is therefore made to obtain a unbiased SEM data set by using the SEE data as a calibration source. As a result, a temporal sensor drift coefficient of ∼−2%/yr is fitted, and the trend causing the unnatural solar cycle different NmF2‐EUV function is removed from the data. A new estimation of the interminima change of EUV irradiance based on the adjusted SEM data is ∼10% reduction in 2008 compared to 1996, much smaller than previous results. After adjustment the SOHO SEM could continue to be a reliable data source for solar EUV and aeronomic studies over both SC23 and SC24. Ionospheric NmF2data are used to confirm an instrumental trend existing in the SEM EUV dataThe trend is estimated to be ∼−2%/yr and removed from the SEM data using the TIMED SEE observation as a referenceThe adjusted SEM data suggest a solar EUV reduction of 10% in 2008 compared to 1996

We revisited a typical interplanetary (IP) shock event on 7 November 2004, with high time resolution (1 s) total electron content (TEC) observations. The IP shock impinged on the dayside magnetopause at ∼18:27 UT, and launched a pulse into the magnetosphere. Using the multi‐spacecraft timing method, the pulse was estimated to propagate at an angular velocity of ∼0.1 MLT/s. While the in situ spacecraft observations only provided a first‐order estimation of the propagation speed, a more precise monitoring of the propagating pulse was achieved by the densely distributed network of the Global Navigation Satellite System (GNSS). Following the shock arrival, GNSS receivers throughout the equatorial region recorded the associated TEC impulses. The signatures were sequentially observed from the sub‐solar to the nightside via both flanks, which in turn demonstrated the concrete propagation pattern of the pulse. In addition to a consistent angular velocity, a ∼40 s difference was systematically found between the TEC measurements and the in situ spacecraft observations, revealing the timescale of the shock‐induced magnetospheric‐ionospheric coupling. Global Positioning System total electron content (TEC) impulses associated with the interplanetary shock impingement were systematically observed in the equatorial regionThe TEC observations showed an azimuthal propagation velocity of ∼0.1 MLT/s and a time delay of ∼40 s between the spacecraft observationsThe TEC technique achieved a monitoring of the shock‐induced pulse and revealed the timescale for the pulse to couple to the ionosphere Global Positioning System total electron content (TEC) impulses associated with the interplanetary shock impingement were systematically observed in the equatorial region The TEC observations showed an azimuthal propagation velocity of ∼0.1 MLT/s and a time delay of ∼40 s between the spacecraft observations The TEC technique achieved a monitoring of the shock‐induced pulse and revealed the timescale for the pulse to couple to the ionosphere

Following the extreme solar minimum during 2008–2009, solar activity keeps low in solar cycle 24 (SC24) and is making SC24 the weakest one of recent cycles. In this paper, using observations from Earth-orbiting satellites, we compare the solar extreme ultraviolet (EUV) irradiance between SC23 and SC24 and investigate the solar cycle change of linear dependence of EUV on the P ((F10.7?+?F10.7A)/2) and Mg II core-to-wing ratio indices. The Bremen composite Mg II index is strongly correlated with P over the two solar cycles, while this is not the case for the Laboratory for Atmospheric and Space Physics (LASP) composite Mg II index, so we focus on the different dependence of EUV on the P and LASP Mg II indices. As a result we find that three coronal emissions (Fe XV at 28.4?nm and 41.7?nm and Fe XVI at 33.5?nm) brighten in SC24 relative to P; i.e., the magnitude of irradiance is higher than in SC23 at the same level of P. But relative to the LASP Mg II index, these emissions show no appreciable solar cycle differences. By contrast, the H I Lyman a at 121.6?nm dims in SC24 relative to the LASP Mg II but shows identical dependence on P in the two solar cycles. This result seems to contradict a well-accepted fact that chromospheric and transition region emissions are better represented by the Mg II index and coronal lines by F10.7. For the different solar cycle variability of EUV in SC24, whether it is caused by source changes on the Sun is still unclear, but we suggest that it needs to be considered in proxy modeling of the EUV irradiance and aeronomic studies. Dependence of EUV on proxies (P and Mg II) are examined in solar cycles 23 and 24Three coronal emissions are stronger in SC24 relative to P, while Lyman alpha is weakened relative to LASP Mg IIDifferent selection of proxy or adopted proxy-EUV relation is necessary in SC24

Using data from ground‐based magnetometers and HF Doppler sounder, we study ultralow frequency (ULF) waves excited during the storm sudden commencement (SSC) on 8 March 2012 and find possible evidence on the link between ULF waves and ionospheric Doppler shifts. Pc1–Pc2 ULF waves are observed from 11:04 to 11:27 UT after the SSC by ground stations of L shell ranging from 1.06 to 2.31, mapping to the topside ionosphere. There are weak responses in this frequency range in the power spectra of ionospheric Doppler shift. From 11:19 to 11:23 UT, oscillations of magnetic field in a lower frequency range of Pc3–Pc4 are observed and are well correlated with the trace of Doppler shift. It is thus suggested that ionospheric Doppler shift can response to ULF oscillations in magnetic field in various frequency ranges, especially in the frequency range of Pc3–Pc4 and below. This paper demonstrates a new mechanism of magnetosphere‐ionosphere coupling. ULF waves observed on the ground within L shell ranging from 1.06 to 2.31 during SSCIonospheric Doppler shifts present a bulge and Pc3‐Pc4 pulsations during SSCULF waves induced during SSC propagate deeply into the low‐latitude ionosphere

The ionosphere exhibits enhanced semidiurnal lunitidal (M2) perturbations during sudden stratospheric warming (SSW) events, of which the manifestation and mechanism are not well documented and understood. This paper studies the latitudinal and inter‐hemispheric variations of the ionospheric M2 perturbations during the 2021 major Arctic SSW with total electron content (TEC) data in the American, Asian‐Australian, and African‐European sectors. Results show that the M2 perturbations in the three sectors all enhanced during the SSW, with the strongest M2 perturbations in the Northern and Southern Hemispheres at about 10°∼15°N and 25°∼30°S geomagnetic latitudes, respectively. Also, clear M2 perturbations in the three sectors all extended to middle latitudes during the SSW only in the Southern Hemisphere. Comparatively, the inter‐hemispheric asymmetry of the magnitude of the M2 perturbations during the SSW was more complex. The inter‐hemispheric asymmetry of the crest locations of the low‐latitude M2 perturbations in the three sectors during the SSW was probably due to the F‐region meridional wind effect on the diffusion process of the low‐latitude plasma fountain. Meanwhile, the longitudinal differences were also obvious. The TEC M2 amplitude in the American sector was obviously larger than those in the Asian‐Australian and African‐European sectors during the SSW. The M2 characteristics in the southern middle latitude in the American sector were significantly different from those in the other two sectors during the SSW, which may be related to the differences of the combined effects of the local wind and geomagnetic configuration. In addition, a moderate geomagnetic activity occurred during the 2021 SSW, which brings difficulties to the analysis of the SSW‐related TEC variation. The latitudinal differences of the ionospheric semidiurnal lunitidal (M2) perturbations during the 2021 sudden stratospheric warming were studied with total electron contentThe low‐latitude M2 perturbations exhibited similar latitudinal and inter‐hemispheric variations in the American, Asian and African sectorsThe M2 perturbation in the southern middle latitude was obviously different in the American from those in the Asian and African sectors The latitudinal differences of the ionospheric semidiurnal lunitidal (M2) perturbations during the 2021 sudden stratospheric warming were studied with total electron content The low‐latitude M2 perturbations exhibited similar latitudinal and inter‐hemispheric variations in the American, Asian and African sectors The M2 perturbation in the southern middle latitude was obviously different in the American from those in the Asian and African sectors

Gravity waves (GWs) are significant dynamical processes in planetary atmospheres due to their efficient transport of momentum and energy from the lower to the upper atmosphere. Recent observations from in‐situ measurements by spacecraft like Mars Atmosphere and Volatile Evolution (MAVEN) also reveal ubiquitous GW activity in the Martian thermosphere. As a critical parameter for characterizing GWs, the wavelength spectrum has been studied for years. However, most of the current observations for the Martian thermosphere can only determine the apparent GW wavelength because they usually take one‐dimensional measurements within a typical GW period. In this work, we adopt a full‐wave GW model to statistically investigate the upward propagation of the topographic‐generated GWs and how their wavelength spectrum apparently looks in the thermosphere when observed by a spacecraft like MAVEN. We find that the wind significantly modifies the vertical wavelengths of GWs in the Martian atmosphere. Considering the GWs generated from topographic sources, we calculate their wavelength distribution in the thermosphere, with a most probable apparent vertical wavelength of around 14 km, consisting with the MAVEN observation. However, our theoretical wavelength spectrum is mainly concentrated in the low wavelength part (10–20 km), while the observed result contains a long‐tailed component in the long wavelength part. The mismatch between our model and observations indicates that at least 72% of the observed GWs in the thermosphere are not generated from topographic sources. Gravity waves are a common and important fluctuation phenomenon in the atmosphere of a planet like Earth and Mars. They can transfer energy from the lower atmosphere to the upper atmosphere. Recent observations of the Martian atmosphere by many spacecrafts have also seen this fluctuating activity in the Martian upper atmosphere. The wavelength distribution, that is, a feature representing the wave pattern's spatial scale and traveling speed, is very important for studying the characteristics of gravity waves. But most current observations of the Martian upper atmosphere have no way to determine the actual wavelength distribution effectively. In this work, we employ a gravity wave model to study the propagation of gravity waves generated from the topography and their wavelength distribution in the Martian upper atmosphere. We found that atmospheric winds can significantly alter the vertical wavelength of gravity waves. Our results indicate that these gravity waves are most likely to be observed in the upper atmosphere at a wavelength of about 14 km. However, our theoretically calculated wavelength distribution also partly differs from the observations. These differences suggest that at least 72 percent of the gravity waves observed in the upper atmosphere are not topographically generated. We use a full wave model to simulate the upward propagation of topographic gravity waves to the Martian thermosphereThe apparent vertical wavelength spectrum of the topographic gravity waves in the Martian thermosphere peaks at around 14 kmAt least 72% of the observed gravity waves in the thermosphere are not generated from topographic sources according to our theoretical analysis We use a full wave model to simulate the upward propagation of topographic gravity waves to the Martian thermosphere The apparent vertical wavelength spectrum of the topographic gravity waves in the Martian thermosphere peaks at around 14 km At least 72% of the observed gravity waves in the thermosphere are not generated from topographic sources according to our theoretical analysis

Previous works have shown that coseismic ionospheric disturbances (CIDs) after the tsunamigenic 2011 Tohoku earthquake (Tohoku EQ, Mw9.1) covered a vast area and were observed thousands of kilometers away from the epicenter. For the purpose of making a comprehensive comparison between powerful oceanic and inland EQs, we conduct a retrospective investigation of CIDs and geomagnetic responses to the 2008 Wenchuan EQ (Mw7.9) using a combination of techniques, total electron content, HF Doppler, and ground magnetometer. It is the very first study to present CIDs recorded by different techniques at co‐located sites and profiled with regard to changes of both ionospheric plasma and current (geomagnetic field) simultaneously. The integrated observation also shows that (1) in the Wenchuan case, most of the ionospheric and geomagnetic disturbances were observed within 1000 km distance which is far less than the Tohoku case; (2) two groups of CIDs were found with maximum amplitudes in the direction of azimuth 150° and 135°, respectively; and (3) the geomagnetic changes were only registered by three magnetometers located to the east and southeast of the epicenter. All the facts indicate that the main directional lobe of Wenchuan EQ energy propagation is to southeast and perpendicular to the direction of the fault rupture, but this kind of directivity is not that distinct in the Tohoku case. We suggest that the different fault slip (inland or submarine) affecting the way of couplings of lithosphere with atmosphere may contribute to the discrepancies between the two events. Ionospheric and geomagnetic changes caused by a major inland earthquakeStrong directivity of CID distribution related to fault rupture directionDifferent features in CIDs of inland case compared with oceanic case

During the geomagnetic storm on 3–6 November 2021, the in‐situ plasma density and the gridded total electron content (TEC) maps registered a persistent presence of the strip‐like bulges (electron density enhancement, also known as the shoulders in previous literatures) at lower‐middle latitudes. The observed strip‐like bulge resided in the Pacific‐America‐Atlantic sector and lasted from 07:00 UT on 4 November to 20:00 UT on 6 November. For the first time, the temporal evolution of the 2‐D strip‐like bulge was recorded by gridded TEC maps, though observations were limited over the North American continents. The TEC maps showed a continuous shrinking of the nightside midlatitude plasma band structure right before the presence of the narrow strip‐like bulge. Simultaneous measurements from the Ionospheric Connection Explorer satellite (ICON) revealed an equatorward turning of the field‐aligned ion transportation driven by the disturbance equatorward neutral winds. However, the enhanced fountain effect at low latitudes was not observed during the entire formation phase of the strip‐like bulge. We propose that the storm‐induced enhanced equatorward thermospheric neutral winds push the plasma equatorward along the field lines, and the plasma band structure was developed into the strip‐like bulge. In addition, the ion composition of both the plasma band structure and the strip‐like bulges are dominated by H+, and the maintenance of the strip‐like bulge could be due to the compensation of the downward plasmaspheric content flux. The temporal evolution of the 2‐D strip‐like bulge is recorded on gridded total electron content (TEC) mapsThe strip‐like bulge is a consequence of the narrowing midlatitude band structure driven by the disturbance neutral windThe enhanced equatorial fountain effect is not involved during the formation of the strip‐like bulge The temporal evolution of the 2‐D strip‐like bulge is recorded on gridded total electron content (TEC) maps The strip‐like bulge is a consequence of the narrowing midlatitude band structure driven by the disturbance neutral wind The enhanced equatorial fountain effect is not involved during the formation of the strip‐like bulge

The equatorial ionization anomaly (EIA) crest derived from GPS observations, F2‐layer peak height (hmF2), critical frequency (foF2), and equatorial electrojet (EEJ) in China low latitude are used to study the lunar tidal effect on EIA region for the years from 2006 to 2015. A predominant 14.76‐day periodic component in EIA crest, hmF2, and EEJ concurrently appears in some seasonal intervals, which coincides with the half of the lunar revolution period (29.53 days) and the lunar phase, indicating that the 14.76‐day periodic oscillation in EIA region is modulated by the lunar tide. This 14.76‐day periodic oscillation occurs in all Northern Hemisphere (NH) winter and corresponds to their respective sudden stratospheric warming (SSW) period from 2006 to 2015. In addition, this 14.76‐day periodic oscillation also occurs in other seasons for some years, for example, May and August, in which wave power is sometimes comparable to that in SSW period. The occurrence of the 14.76‐day periodic oscillation and its wave power during non‐SSW seasons in the solar maximum years (2012–2015) is more frequent and higher than that in the solar minimum years (2008–2010). Our results suggest that, besides the NH SSW condition, the solar activity and some other unclear factors are also responsible for the lunar tide enhancement in EIA region. A consistent 14.76‐day periodic variation is revealed in equatorial ionization anomaly (EIA) location, hmF2, and equatorial electrojet in China EIA regionThis 14.76‐day periodic oscillation occurs not only in Northern Hemisphere winter (sudden stratospheric warming) but also in other seasonsThis 14.76‐day periodic oscillation is related to the semidiurnal lunar tides (M2) with a 12.42‐hr period A consistent 14.76‐day periodic variation is revealed in equatorial ionization anomaly (EIA) location, hmF2, and equatorial electrojet in China EIA region This 14.76‐day periodic oscillation occurs not only in Northern Hemisphere winter (sudden stratospheric warming) but also in other seasons This 14.76‐day periodic oscillation is related to the semidiurnal lunar tides (M2) with a 12.42‐hr period

Ionospheric plasma density data from the Planar Langmuir Probe onboard the CHAllenging Minisatellite Payload (CHAMP) are used to investigate the latitudinal profile of the ionospheric plasma density. Along with the well‐known profiles with features of the equatorial ionization anomaly and equatorial plasma bubbles, a third type is observed as smooth profiles with large‐scale deep equatorial trough (DET) where the plasma density is extremely low. A global survey focusing on magnetically quiet conditions reveals that the DET profiles are generally a post‐sunset phenomenon, and their appearance depends on longitude and season. During equinoxes in active solar years, the strong pre‐reversal enhancement electric field is believed to uplift the ionosphere and produce the DET profiles. While around June solstice, the DET profiles are clustered at 45°W–0°E longitude, where the zonal electric field is weak but the magnetic meridional component of the neutral wind is most significant. The SAMI2 model is then employed to simulate the ionospheric plasma distribution in the magnetic meridional plane with enhanced neutral wind, which successfully reproduced the main features of the observed DET profile. The results indicate that the neutral wind can obviously affect the equatorial ionosphere by transporting plasma in the magnetic flux tube through neutral‐ion interaction. The neutral wind is previously known to work with the inclined magnetic field at mid‐latitude, while this study emphasizes its effect around the magnetic equator especially when the electric field forcing (E×B) is weak or absent. Deep equatorial trough (DET) occurs at night under weak upward plasma drift as well as under strong prereversal enhancementThe DET profiles is accompanied by higher equatorial hmF2, when either the E×Bdrift or strong magnetic meridional wind is presentModel simulation verifies that field‐aligned plasma transport driven by the neutral wind leads to the extremely low density at the equator Deep equatorial trough (DET) occurs at night under weak upward plasma drift as well as under strong prereversal enhancement The DET profiles is accompanied by higher equatorial hmF2, when either the E×Bdrift or strong magnetic meridional wind is present Model simulation verifies that field‐aligned plasma transport driven by the neutral wind leads to the extremely low density at the equator

Few observations investigated the interaction between an electrical medium‐scale traveling ionospheric disturbance (EMSTID) and an equatorial plasma bubble (EPB). This paper presents another interaction between a southwestward propagating EMSTID and an eastward drifting EPB in the equatorial ionization anomaly (EIA) crest region of China. When the EMSTID and the EPB touched each other, several depletions of the EMSTID (EPB) showed the eastward (westward) velocity disturbances of the EPB (EMSTID) depletions. Besides, phase elongations of the EPB depletions contrarotated as the EMSTID propagated southwestward. However, of important finding is that the interaction of the EMSTID and the EPB could have polarized one depletion of the postmidnight EPB that should have become a fossilized bubble. Inside that polarized EPB depletion were meter‐scale irregularities that caused activated radar echoes and enhanced ranged spread F(RSF). The interaction occurred in descending ionosphere and the lower density regions got filled up with an enhanced density plasma. We propose that the EMSTID and the EPB could have electrically coupled with each other, causing an enhanced polarization electric field (PEF) that polarized that EPB depletion; the E× Bgradient drift instability (Kelley, 1989) could have caused the meter‐scale irregularities when that enhanced PEF was imposed on that reactivated EPB depletion surrounded by that enhanced density plasma. This study provides observational evidence that how an electrical couple of EMSTID and EPB events can activate a postmidnight EPB depletion that should become a fossilized structure. Otsuka et al. (2012, https://doi.org/10.1029/2012GL052286) provided the first report indicating the disappearance of an equatorial plasma bubble (EPB) airglow depletion after interaction with an electrical medium‐scale traveling ionospheric disturbance (EMSTID) airglow enhancement. For comparison, the current study reports a postmidnight airglow event in which an electrical coupling of an EMSTID and an EPB could have polarized one EPB airglow depletion that caused the activated radar echoes of very high frequency (VHF) radar and the enhanced ranged spread F(RSF) in descending ionosphere filled with an enhanced density plasma. Interaction between an electrical medium‐scale traveling ionospheric disturbance (EMSTID) and an equatorial plasma bubble (EPB) in the equatorial ionization anomaly (EIA) crest region over ChinaElectrical coupling of the EMSTID and the EPB polarized one EPB depletion that caused activated radar echoes and enhanced ranged spread F(RSF)The interaction occurred in descending ionosphere and the lower density regions got filled up with an enhanced density plasma Interaction between an electrical medium‐scale traveling ionospheric disturbance (EMSTID) and an equatorial plasma bubble (EPB) in the equatorial ionization anomaly (EIA) crest region over China Electrical coupling of the EMSTID and the EPB polarized one EPB depletion that caused activated radar echoes and enhanced ranged spread F(RSF) The interaction occurred in descending ionosphere and the lower density regions got filled up with an enhanced density plasma

During sudden stratospheric warming events, the ionosphere exhibits phase‐shifted semidiurnal perturbations, which are typically attributed to vertical coupling associated with the semidiurnal lunar tide (M2). Our understanding of ionospheric responses to M2 is limited. This study focuses on fundamental vertical coupling processes associated with the latitudinal extent and hemispheric asymmetry of ionospheric M2 signatures using total electron content data from the American sector. Our results illustrate that the asymmetry maximizes at 15°N and 20°S magnetic latitudes. In the Southern Hemisphere, the M2‐like signatures extend deep into midlatitude and encounter the Weddell Sea Anomaly. The time evolution of the Anomaly exhibits a distortion, which is attributed to an M2 modulation. The hemispheric asymmetry of M2 signatures in the low latitude can be primarily explained by the transequatorial wind modulation of the equatorial plasma fountain. Other physical processes could also be relevant, including hemispheric asymmetry of the M2 below the F‐region, the ambient thermospheric composition and ionospheric plasma distribution, and the geomagnetic field configuration. Sudden stratospheric warming event is a meteorological phenomenon occurring mainly during the boreal winter, during which the polar temperature enhances and the polar vortex breaks. Recently, scientists have found that during such an event at stratospheric altitude, the ionosphere, which is tens and hundreds of kilometers higher than the stratosphere, exhibits featured perturbations in large ranges of longitude, latitude, and altitude. With electron density observations derived from the Global Positioning System during three boreal winter periods, this work studies the latitudinal variation and hemispheric asymmetry of ionospheric perturbations with semidiurnal lunitidal signatures, which are not well investigated in previous research works. Based on our observations, we specifically focus on analyzing how the lunar tide from the lower atmosphere affects up to the upper ionosphere. It is found that the ionospheric semidiurnal lunitidal signatures lean southward in the low latitudes and extend up to middle latitudes only in the Southern Hemisphere. We demonstrate that the low‐latitude asymmetry is mainly attributed to the wind that blows from the summer (Southern) to the winter (Northern) Hemisphere. The cause of the perturbations in the southern middle latitude is more complicated. We also find that an ionospheric phenomenon called the Weddell Sea Anomaly may also be influenced. Ionospheric semidiurnal lunitidal signatures in the American sector exhibit hemispheric asymmetry during sudden stratospheric warmingsSuch asymmetry at low latitudes is due to the wind‐modulated fountain effect, and lunitidal signatures extend to southern midlatitudesThe in situ lunitidal wind potentially modulates the ionospheric Weddell Sea Anomaly Ionospheric semidiurnal lunitidal signatures in the American sector exhibit hemispheric asymmetry during sudden stratospheric warmings Such asymmetry at low latitudes is due to the wind‐modulated fountain effect, and lunitidal signatures extend to southern midlatitudes The in situ lunitidal wind potentially modulates the ionospheric Weddell Sea Anomaly

In this paper, we first investigate the effects of a postevening Weddell Sea Anomaly (WSA)‐like plasma patch on a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID) over Xinglong, China (40.4°N, 117.6°E; Mlat. ~30.4°N). We found that some of this kind of WSA‐like plasma patches moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they traveled westward into China. During 2012–2016, these plasma patches frequently occurred during the period of May–August with a monthly occurrence rate of over 20%, causing nighttime plasma density enhancements (NPDEs) in midlatitudes of China. Over 50% of these structures were accompanied by concurrent MSTID and Es. We propose that an intense polarization electric field (PEF) associated with an MSTID/Esfrom the more northern regions of EIA could frequently drive these plasma patches poleward. On the night of 27 June 2014 (Kp< 2−), either an MSTID and an intense Eslayer (the maximum foEs>12 MHz) occurred at the onset of the poleward motion of a WSA‐like plasma patch, lending credence to this possibility. The observed plasma patch was pushed poleward to interact with the observed MSTID, causing some poleward extending C‐shaped airglow depletions/enhancements of the MSTID in a transition region where the ionosphere changed from a collapse region to an uplifted one. We attributed those poleward extending airglow depletions to the interaction between the MSTID and the plasma patch because of the secondary gradient drift instability and those of airglow enhancements to the redistribution of plasma. This study found that postevening Weddell Sea Anomaly (WSA)‐like patchy plasma structures frequently occurred during May–August of 2012–2016 in the East Asian regions. Among these plasma patches, a small portion moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they moved westward into China. On the magnetically quiet night of 27 June 2014 (Kp< 2−), such a plasma patch already moved westward and poleward to the midlatitude region of China, affecting the evolution/propagation of a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID). This paper, therefore, investigates the interaction between a midlatitude MSTID and a poleward moving WSA‐like plasma patch. We mainly investigate the possible physical mechanisms behind this kind of plasma patches and how they could affect the propagation/evolution of MSTIDs over midlatitudes of China. Poleward extending C‐shaped airglow depletions/enhancements of MSTID were caused by interaction between an MSTID and a postevening WSA‐like plasma patchWestward and northward moving WSA‐like plasma patches frequently caused nighttime midlatitude plasma enhancements over Xinglong, ChinaAn intense PEF associated with an MSTID/Esfrom the more northern regions of EIA could frequently cause poleward motion of the WSA‐like plasma patch

The morphological difference of the northern equatorial ionization anomaly between the eastern Asian and American sectors is statistically studied with ground‐based total electron content data from 2000 to 2011. The intensity (Ic), latitudinal location (Lc), and occurrence time (Tc) of the daytime equatorial ionization anomaly crest are derived from daytime peak total electron content in time‐latitude plots. The main results are as follows. Lcin the two sectors exhibits an apparent difference, especially in solar minimum, during which Lcin the American sector shows an annual variation that is more poleward in northern summer and more equatorward in northern winter, while Lcin the eastern Asian sector shows a semiannual variation that is more poleward around equinoxes and more equatorward around solstices. Ictends to be stronger in the eastern Asian sector than that in the American sector in all seasons, and this difference increases with the increase of the solar flux index. Tctends to be earlier in northern winter and later in northern summer in both sectors and shows dependence on solar activity. We demonstrate that the seasonal variation of Lcin the American sector is not dominated by the ionospheric equatorial zonal electric field, and suggest that the difference of Lcbetween the two sectors is probably related to different meridional wind effects due to the displacement of geographic and magnetic equators. The Icdifference is probably related to the longitudinal wave number‐3 and ‐4 structures driven by tidal forcing from the 5lower atmosphere. The seasonal variation of the northern EIA crest location is distinctly different between the eastern Asian and American sectorsSuch difference is probably related to the different meridional wind effects due to the displacement of geographic and magnetic equatorsThe EIA crest intensity tends to be stronger in the eastern Asian sector in all seasons that may be due to the nonmigrating tides

Variations of the low‐latitude ionosphere in the eastern Asian sector during the 2017 sudden stratospheric warming (SSW) were studied with multi‐instrumental observations that correspond to different altitudes. Specifically, the variations of the topside ionosphere as high as over 830 km were investigated with data from the Swarm B satellite (~500 km), the Defense Meteorological Satellite Program F15 satellite (~830 km), and the Meteorological Operational satellite‐A (~830 km). The main results are as follows. (1) Simultaneous variations were detected in the ionospheric parameters from the Eregion to the topside ionosphere around the first peak warming of the 2017 SSW. (2) These variations exhibited similar quasi‐16‐day (Q16D; ~14.5‐day) periods, and these periods were most significant just before the first peak warming. (3) These simultaneous Q16D variations were not due to the solar extreme ultraviolet irradiance or geomagnetic activities but were probably driven from below, by planetary waves or semidiurnal lunar tides that enhanced during the SSW. (4) The Q16D variation of the topside ionospheric electron density at ~830 km was probably driven not only by the upward mapping electric field modulated in the Eregion dynamo but also by the electric field modulated in the Fregion dynamo. Multi‐instrumental observations at different altitudes were used to study the low‐latitude ionospheric variations during the 2017 SSWSimultaneous ~14.5‐day periodic variations were detected at different ionospheric heights from the Eregion to the topside ionosphereThe ~14.5‐day ionospheric variations at ~830 km were probably driven by not only the fountain effect but also the Fregion dynamo process

Yearly averaging is a basic preprocessing of ionospheric data to smooth out the short‐term, cyclic variations in the observations. However, we find that the use of yearly (running) average method probably leads to a biased evaluated relation between the ionospheric parameters (e.g., F2 peak electron density, NmF2) and the solar extreme ultraviolet (EUV) irradiance. The bias is essentially related to the annual/seasonal variations of the ionosphere, which means that the ionospheric sensitivity to the solar EUV irradiance varies with season and the same solar input can produce different ionization levels in different months/seasons. The yearly method simply averages the observations of different seasons, leading to a biased yearly NmF2‐EUV relation, which will further affect the estimation of the ionospheric hysteresis (different NmF2‐EUV relation in the solar rising and declining phase). Also, the extraction of long‐term trends in the ionosphere will be more uncertain if using a regression approach based on the biased NmF2‐EUV relation. In conclusion, we suggest to do the analysis month by month (using monthly average method) to properly deal with the annual/seasonal variations of the ionosphere, and to obtain accurate estimation of the hysteresis and long‐term trends. Though both the yearly and monthly methods were used in precedent works, the bias introduced by the yearly average method has not been fully recognized prior to this study. Improper use of yearly average method causes uncertainty in the determined ionosphere‐EUV relationEvaluation of ionospheric hysteresis and long‐term trends is probably biased due to the uncertaintyMonthly analysis is a better method to avoid the uncertainty/bias essentially related to strong seasonal variations of the ionosphere