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The impact of meridional winds on the onset and evolution of equatorial plasma bubbles (EPBs) is reviewed. The conventional wisdom had been that transequatorial meridional winds have a stabilizing effect on the development of EPBs during equatorial spread F (ESF). However, this result is based on a uniform transequatorial meridional wind. Subsequently, it was demonstrated that a non-uniform meridional wind could have a stabilizing or destabilizing effect on EPB formation depending on the direction of wind gradient. The destabilization of EPBs associated with equatorward flowing meridional winds has recently been investigated during a midnight temperature maximum event and a geomagnetic storm. Although the neutral wind is a direct destabilizing influence in these cases, the large decrease in the Pedersen conductance caused by meridional equatorward winds is the primary reason for the large increase in the growth rate of the generalized Rayleigh-Taylor instability. We review the theoretical and modeling studies of this topic as well as observational studies that have been made to assess the relationship between meridional winds and ESF. [ABSTRACT FROM AUTHOR]

This study uses satellite measurements of plasma densities and thermospheric winds to analyze the effects of the November 2021 geomagnetic storm on ionospheric pre‐midnight topside plasma bubbles over South America. Using observations from the Ionospheric Connection Explorer (ICON) and the Global‐scale Observations of the Limb and Disk (GOLD) satellites, we find that pre‐midnight topside plasma bubbles were inhibited over eastern South America during the recovery phase of the storm. This is particularly notable because of the otherwise high occurrence rate of plasma bubbles at these longitudes during this season. This inhibition coincided with the recovery phase of the geomagnetic storm, marked by a northward turning of the z‐component of the interplanetary magnetic field (IMFBz) and quiet‐time values of the SuperMAG Auroral Electrojet Index (SME). We observed a westward turning of the zonal wind before the bubble inhibition, so we conclude the inhibition of topside plasma bubbles is likely related to a westward disturbance dynamo electric field (DDEF) causing a downward E×B $\mathbf{E}\times \mathbf{B}$ drift and suppress the growth of the instability responsible for bubble development. Contrary to theoretical predictions, we do not observe notable changes to the meridional wind during the event. These results provide new insights into the ionosphere‐thermosphere system's response to geomagnetic storms and highlight the role of wind patterns in inhibiting ionospheric irregularities, contributing to better predictive models for these phenomena. Plain Language Summary: Geomagnetic storms negatively affect communication and navigation systems by producing unpredictable changes in ionospheric densities. Plasma bubbles are irregularities in the ionosphere that occur mainly in the equatorial region and may be affected by geomagnetic storms. Studying plasma bubbles is crucial because they can significantly reduce the performance of communication and navigation systems, leading to signal loss or degradation. We conducted a study on how a geomagnetic storm in November 2021 affected the occurrence of pre‐midnight plasma bubbles over South America. Using data from two satellites (ICON and GOLD), we observed that these plasma bubbles disappeared during the storm's recovery phase. Our analysis suggests that this inhibition is likely linked to changes in wind patterns in the ionosphere. Specifically, a westward wind created conditions that prevented the generation of plasma bubbles. This study offers new insights into how winds during geomagnetic storms affect the ionosphere and helps improve predictions for communication and navigation systems affected by these disruptions. Key Points: Pre‐midnight topside plasma bubbles are inhibited over eastern South America during the recovery phase of a geomagnetic stormZonal wind measurements show evidence of wind‐driven electric fields inhibiting plasma bubble formationAlthough theoretical predictions also anticipate changes in the meridional wind, they are not observed [ABSTRACT FROM AUTHOR]

The relationship between the post‐sunset equatorial Plasma Bubbles (EPBs) and the Rayleigh‐Taylor instability growth rates γRT $\left({\gamma }_{RT}\right)$ was studied by means of the occurrence probabilities. EPB observations were obtained through the Rate of Total Electron Content Index (ROTI) measured by ground‐based Global Navigation Satellite System (GNSS) receivers in the South American equatorial region. Data from the period 2013 to 2022 were utilized. The γRT ${\gamma }_{RT}$ calculations were based on F‐layer vertical drift Vp $\left({V}_{p}\right)$ measurements obtained from the São Luís ionosonde (2.33°S, 44.2°W, dip angle: −0.5°), the International Reference Ionosphere model (IRI‐2016), Horizontal Wind Model (HWM‐14), and Spectrometer Incoherent Scatter Model‐2,000 (NRLMSISE‐00). The EPB occurrence probability increased from 0% to 100% as γRT ${\gamma }_{RT}$ increased from 0 to 2.2×10−3s−1 $2.2\times 1{0}^{-3}\hspace*{.5em}{\mathrm{s}}^{-1}$. Additionally, the occurrence probability exhibited seasonal variations, with a gradual increase during the equinox months, minimal occurrence during the June solstice, and a sudden increase during the December solstice. We found that the occurrence probability varied with day‐to‐day, season and solar activity for the same γRT ${\gamma }_{RT}$. Plain Language Summary: Equatorial plasma bubble (EPB) in the earth's equatorial and low latitude ionosphere is a large‐scale plasma depletion producing a large‐scale irregularity, which causes problems in the space based telecommunications and Global Navigation Satellite System (GNSS) positioning systems. Prediction and forecasting of such ionospheric irregularities are an important issue in the space technology applications. Present work tries to visualize probability of EPB occurrence at a certain location and time of the year. For this purpose, we calculated the occurrence probability using past observed data (temporal variabilities of Total Electron content (TEC) and ionospheric layer height) and ionospheric models. Our results show that the occurrence probability increases gradually when the irregularity index of the ionosphere increases. We also found that the gradual increase has a seasonal dependency, very low probability during June‐July, and high probability during December–January. Key Points: The relationship between the Equatorial Plasma Bubbles and the R‐T instability growth rate was studied by probability of occurrenceProbability gradually rises with growth rate during equinox seasons, but experiences a sudden surge during the December solsticeFor the same growth rate, the probability varies with the seasons and solar cycle [ABSTRACT FROM AUTHOR]

Equatorial plasma irregularities in the ionospheric F‐region proliferate after sunset, causing the most apparent radio scintillation "hot‐spot" in geospace. These irregularities are caused by plasma instabilities, and appear mostly in the form of under‐densities that rise up from the F‐region's bottomside. After an irregularity production peak at sunset, the amplitude of the resulting turbulence decays with time. Analyzing a large database of irregularity spectra observed by one of the European Space Agency's Swarm satellites, we have applied a novel but conceptually simple statistical analysis to the data, finding that turbulence in the F‐region tends to decay with a uniform, scale‐independent rate, thereby confirming and extending the results from an earlier case study. We find evidence for two regimes, one valid post‐sunset (1.4 hr decay rate) and one valid post‐midnight (2.6 hr). Our results should be of utility for large‐scale space weather modeling efforts that are unable to resolve turbulent effects. Plain Language Summary: After sunset in the equatorial region of Earth, GPS devices frequently experience service interruption due to space weather. The signal disruptions that cause these interruptions are in turn caused by plasma turbulence in Earth's ionosphere, a layer of ionized gas that covers Earth like a blanket of electrical currents. The growth of such turbulence has been studied for decades, but little is still known about how such plasma irregularity structures decay with time. We elucidate the topic, showing that turbulence cause structures to decay at the same rate regardless of size. This important result will have consequences for large‐scale space weather modeling efforts, since such models rarely have the capability to resolve turbulence. Turbulence is an enigmatic chaotic behavior often that is often present in astrophysical processes, but also on Earth's oceans and in its atmosphere. Key Points: Turbulence forces equatorial irregularities to decay with a scale‐independent rateEquatorial irregularities of scale‐sizes between 500 m and 75 km are not dissipating by chemical recombination or perfect ambipolar diffusionDecay rates depend on solar local time, with post‐sunset decay rates around 1.4 hr, increasing to 2.6 hr post‐midnight [ABSTRACT FROM AUTHOR]

In this study, the evolutional features and underlying driving mechanisms of the postmidnight‐to‐dawn equatorial plasma bubble (EPB) irregularities during the weak geomagnetic activity period on 13 November 2015 were investigated based on the multiple satellite and ground‐based observations. By using the coherent scatter radar operating at very high frequency at Fuke (19.5°N, 109.1°E, dip latitude 14.4°N), China, it was found that the freshly developed field‐aligned irregularities (FAIs) occurred within the radar's field of view around ∼04:37 LT and sustained for more than 40 min. The remarkable EPB‐related density and total electron content (TEC) depletions measured by the satellite and GNSS receivers were also observed, which indicates the persistence of EPB irregularities until ∼06 LT. The significant elevation of bottomside F‐layer's virtual height obtained by the Digisonde at Fuke as well as the upward vertical F‐layer plasma drifts derived from a nearby Digisonde at Sanya (18.4°N, 109.6°E, dip latitude 13.1°N) both imply the existence of strong eastward perturbation electric fields after local midnight. These findings suggest that the collective effects of eastward overshielding penetration electric field (PEF) resulted from the substorm onset and rapid northward turning of the interplanetary magnetic field (IMF) Bz, surpassed the role of westward undershielding prompt penetration electric field (PPEF) induced by the southward turning of IMF Bz. Thus, the former predominated in modulating the equatorial/low‐latitude zonal electric fields and raised the F‐layer considerably, which consequently boosted the growth of R‐T instability and created the favorable conditions for the postmidnight‐to‐dawn EPBs development. Key Points: Freshly developed field‐aligned irregularities last for 40 min within the radar's field‐of‐view and the equatorial plasma bubble (EPBs)‐related density depletions show the EPBs sustaining until ∼06 LTThe competition process among different perturbation electric fields for triggering the postmidnight‐to‐dawn EPB irregularities is examinedThe eastward overshielding penetration electric field resulted from substorm onset and interplanetary magnetic field Bz northward turning provide favorable conditions for EPB development [ABSTRACT FROM AUTHOR]

An expression of Rayleigh‐Taylor (R‐T) instability growth rate based on the field‐line integrated theory is newly established. This expression can be directly utilized in ionosphere models with magnetic flux tube structure based on Modified Apex Coordinates. The R‐T instability growth rates are calculated using the thermospheric and ionospheric conditions based on the coupled Whole Atmosphere Model and Ionosphere Plasmasphere Electrodynamic model (WAM‐IPE). The parameters used in this calculation include the field‐line integrated conductivities and currents, which consider the Quasi‐Dipole Coordinates and the modifications to the equations of electrodynamics. Detailed description of the new formulas and comprehensive analyses of diurnal, longitudinal, and seasonal variations of the R‐T instability growth rate are carried out. The dependencies of growth rates on pre‐reversal enhancement (PRE) vertical drifts and solar activity are also examined. The results show that pronounced R‐T growth rates are captured between 18 and 22 local time (LT) when strong PRE occurs in the equatorial ionosphere. The simulated R‐T growth rate increases with increasing solar activity levels and demonstrates strong correlations with the angle between the sunset terminator and the geomagnetic field line. These results are consistent with plasma irregularity occurrence rates shown in various satellite observations, suggesting that the newly developed R‐T growth rate calculation can effectively capture the probability of irregularities by considering the changes along magnetic flux‐tubes in the ionosphere. Since the WAM‐IPE is running in operation at National Oceanic and Atmospheric Administration Space Weather Prediction Center, the new calculations can be potentially implemented in the near future to provide forecasted information of the R‐T growth rate. Key Points: A new expression incorporating Modified Apex coordinates for calculating the Rayleigh‐Taylor (R‐T) instability growth rate has been establishedThe simulation of R‐T growth rate using Whole Atmosphere Model and Ionosphere Plasmasphere Electrodynamic model closely resembles the bubble occurrence observed by satellitesThe R‐T instability is primarily driven by the eastward electric field, which is correlated with the pre‐reversal enhancement vertical drift [ABSTRACT FROM AUTHOR]

We report results from a global simulation of the September 2017 geomagnetic storm. The global model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM‐X. We show that a train of large‐scale EPBs form in the Pacific sector during the storm recovery phase on 8 September 2017. The EPBs are associated with storm‐induced modification of the zonal and meridional winds. These changes lead to an eastward electric field which in turn causes an upward E × B drift in the post‐midnight sector. A large decrease in the Pedersen conductance caused by meridional equatorward winds leads to an increase in the growth rate of the generalized Rayleigh‐Taylor instability that causes EPBs to develop. Interestingly, several EPBs reach altitudes above 3,000 km. Plain Language Summary: The uppermost layer of the atmosphere, the thermosphere, is heated at high latitudes during geomagnetic storms by energy inputs from the magnetosphere. This heating significantly modulates the thermosphere winds on a global scale that results in the modification of the electrodynamics of the ionosphere at low‐ to mid‐latitudes. Using the coupled SAMI3/WACCM‐X model, we show that equatorial plasma bubbles (EPBs) (large‐scale depletions of the electron density in the ionosphere) can develop because of these stormtime changes to the winds and electric field. This is significant because EPBs can adversely impact space‐based communication and navigation systems by degrading the reception of electromagnetic signals that pass through them. Key Points: Stormtime modulation of the zonal and meridional winds increase the eastward electric field at night in the Pacific sectorEquatorial plasma bubbles subsequently develop in the Pacific sector during the September 2017 storm on September 8Several equatorial plasma bubbles rise to over 3,000 km with upward velocities exceeding 300 m/s [ABSTRACT FROM AUTHOR]

We argue the merits of having a simplified method to simulate equatorial plasma bubble (EPB) plume structures for practical usage. The capability to realistically model EPB plume structures in simulations would be advantageous when assessing the severity of ionospheric threats. Such advantages would arise because a realistic model of EPB plume structures could allow nonstationary scintillation signals to be simulated. Although EPB plume structures can be modeled via first-principle physics-based models, these models tend to be computationally demanding. High-performance computing facilities might be able to offer some remedy, but serious handicaps would remain for those without access to such advanced facilities. We investigated multiple options that utilize the diffusion-limited aggregation (DLA) fractal process to generate bifurcating structures that resemble typical EPB plume structures. We combined the DLA algorithm with the International Reference Ionosphere model to simulate EPBs in three dimensions. Initial tests of this modeling approach indicate promising results. [ABSTRACT FROM AUTHOR]

The impact of meridional winds on the onset and evolution of equatorial plasma bubbles (EPBs) is reviewed. The conventional wisdom had been that transequatorial meridional winds have a stabilizing effect on the development of EPBs during equatorial spread F (ESF). However, this result is based on a uniform transequatorial meridional wind. Subsequently, it was demonstrated that a non-uniform meridional wind could have a stabilizing or destabilizing effect on EPB formation depending on the direction of wind gradient. The destabilization of EPBs associated with equatorward flowing meridional winds has recently been investigated during a midnight temperature maximum event and a geomagnetic storm. Although the neutral wind is a direct destabilizing influence in these cases, the large decrease in the Pedersen conductance caused by meridional equatorward winds is the primary reason for the large increase in the growth rate of the generalized Rayleigh-Taylor instability. We review the theoretical and modeling studies of this topic as well as observational studies that have been made to assess the relationship between meridional winds and ESF.

Abstract In this study, the evolutional features and underlying driving mechanisms of the postmidnight‐to‐dawn equatorial plasma bubble (EPB) irregularities during the weak geomagnetic activity period on 13 November 2015 were investigated based on the multiple satellite and ground‐based observations. By using the coherent scatter radar operating at very high frequency at Fuke (19.5°N, 109.1°E, dip latitude 14.4°N), China, it was found that the freshly developed field‐aligned irregularities (FAIs) occurred within the radar's field of view around ∼04:37 LT and sustained for more than 40 min. The remarkable EPB‐related density and total electron content (TEC) depletions measured by the satellite and GNSS receivers were also observed, which indicates the persistence of EPB irregularities until ∼06 LT. The significant elevation of bottomside F‐layer's virtual height obtained by the Digisonde at Fuke as well as the upward vertical F‐layer plasma drifts derived from a nearby Digisonde at Sanya (18.4°N, 109.6°E, dip latitude 13.1°N) both imply the existence of strong eastward perturbation electric fields after local midnight. These findings suggest that the collective effects of eastward overshielding penetration electric field (PEF) resulted from the substorm onset and rapid northward turning of the interplanetary magnetic field (IMF) Bz, surpassed the role of westward undershielding prompt penetration electric field (PPEF) induced by the southward turning of IMF Bz. Thus, the former predominated in modulating the equatorial/low‐latitude zonal electric fields and raised the F‐layer considerably, which consequently boosted the growth of R‐T instability and created the favorable conditions for the postmidnight‐to‐dawn EPBs development.

This work investigates the ionospheric response to the March 2023 geomagnetic storm over American and Asian sectors from total electron content (TEC), rate of TEC index, ionospheric heights, Swarm plasma density, radio occultation profiles of Formosat-7/Cosmic-2 (F7/C2), Fabry-Perot interferometer driven neutral winds, and E region electric field. During the storm's main phase, post-sunset equatorial plasma bubbles (EPBs) extend to higher latitudes in the western American longitudes, showing significant longitudinal differences in the American sector. Over the Indian longitudes, suppression of post-sunset irregularities is observed, attributed to the westward prompt penetration electric field (PPEF). At the early recovery phase, the presence of post-midnight/near-sunrise EPBs till post-sunrise hours in the American sector is associated with the disturbance of dynamo-electric fields (DDEF). Additionally, a strong consistency between F7/C2 derived amplitude scintillation (S4) ≥ 0.5 and EPB occurrences is observed. Furthermore, a strong eastward electric field induced an increase in daytime TEC beyond the equatorial ionization anomaly crest in the American region, which occurred during the storm's main phase. Both the Asian and American sectors exhibit negative ionospheric storms and inhibition of ionospheric irregularities at the recovery phase, which is dominated by the disturbance dynamo effect due to equatorward neutral winds. A slight increase in TEC in the Asian sector during the recovery phase could be explained by the combined effect of DDEF and thermospheric composition change. Overall, storm-time ionospheric variations are controlled by the combined effects of PPEF and DDEF. This study may further contribute to understanding the ionospheric responses under the influence of storm-phase and LT-dependent electric fields. [ABSTRACT FROM AUTHOR]

We formulate a numerical data detrending technique that can be used to help reveal large‐scale equatorial plasma bubble (EPB) structures in 2‐dimensional data from the Global‐scale Observations of the Limb and Disk (GOLD) mission. This GOLD data detrending technique is inspired by and is a generalization of a previous rolling‐barrel data detrending method for 1‐dimensional total electron content (TEC) observations on individual global positioning system (GPS) satellite passes. This 2‐dimensional GOLD data detrending technique treats the observed 135.6 nm radiance as a function of longitude and latitude as an uneven terrain, where EPBs appear as deep but narrow elongated valleys. The unperturbed background radiance is inferred by rolling a ball on the 2‐dimensional terrain to skip over the EPB valleys. The two degrees‐of‐freedom possessed by the rolling ball allow it to smoothly trace the edges of EPB depletions, without falling into the deep valleys. Surface interpolation of radiance values at the ball's contact points onto the whole domain produces the baseline radiance. Subtracting the baseline from the original radiance data yields the net detrended radiance. As a result of the detrending, sharper contrast is present between EPB depletions and the ambient surroundings. As such, this new 2‐dimensional GOLD data detrending may potentially open the door to the development of other more advanced techniques for automated EPB detection and tracking, or data assimilation into low‐latitude space domain awareness (SDA) information ecosystems. Key Points: A 2‐dimensional data detrending method based on mechanical analogy of rolling a spherical ball on rough and uneven surface is formulatedThe data detrending method may be effective for revealing large‐scale equatorial plasma bubble structures in 135.6 nm GOLD observation dataEnhanced equatorial plasma bubble structures in nighttime GOLD images may be useful for development of more advanced practical applications [ABSTRACT FROM AUTHOR]

This study investigates the day-to-day variability of equatorial plasma bubbles (EPBs) over the Atlantic–American region and their connections to atmospheric planetary waves during the sudden stratospheric warming (SSW) event of 2021. The investigation is conducted on the basis of the GOLD (Global Observations of the Limb and Disk) observations, the ICON (Ionospheric Connection Explorer) neutral wind dataset, ionosonde measurements, and simulations from the WACCM-X (Whole Atmosphere Community Climate Model with thermosphere–ionosphere eXtension). We found that the intensity of EPBs was notably reduced by 35% during the SSW compared with the non-SSW period. Furthermore, GOLD observations and ionosonde data show that significant quasi-6-day oscillation (Q6DO) was observed in both the intensity of EPBs and the localized growth rate of Rayleigh–Taylor (R-T) instability during the 2021 SSW event. The analysis of WACCM-X simulations and ICON neutral winds reveals that the Q6DO pattern coincided with an amplification of the quasi-6-day wave (Q6DW) in WACCM-X simulations and noticeable ∼6-day periodicity in ICON zonal winds. The combination of these multi-instrument observations and numerical simulations demonstrates that certain planetary waves like the Q6DW can significantly influence the day-to-day variability of EPBs, especially during the SSW period, through modulating the strength of prereversal enhancement and the growth rate of R-T instability via the wind-driven dynamo. These findings provide novel insights into the connection between atmospheric planetary waves and ionospheric EPBs. [ABSTRACT FROM AUTHOR]

Abstract We report results from a global simulation of the September 2017 geomagnetic storm. The global model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM‐X. We show that a train of large‐scale EPBs form in the Pacific sector during the storm recovery phase on 8 September 2017. The EPBs are associated with storm‐induced modification of the zonal and meridional winds. These changes lead to an eastward electric field which in turn causes an upward E × B drift in the post‐midnight sector. A large decrease in the Pedersen conductance caused by meridional equatorward winds leads to an increase in the growth rate of the generalized Rayleigh‐Taylor instability that causes EPBs to develop. Interestingly, several EPBs reach altitudes above 3,000 km.

• Scintillation is concentrated in early hours of the night. • Deep fading events are more likely to occur around 21:00 LT at all stations. • The fading events are deeper in the early evening hours. Several works investigated aspects of amplitude scintillation over low-latitude regions. Some of these studies also addressed characteristics and consequences of fading events in receivers and systems relying on information from satellites. It is now known that amplitude scintillation severity in transionospheric signals is stronger around the region of the Equatorial Ionization Anomaly and depends on the signal propagation path. However, a temporal characterization of the occurrence of amplitude scintillation and fading events over distinct latitudes is still needed. In this work this temporal analysis was done by using data from 4 ground-based scintillation monitor stations with distinct dip latitudes over the southern hemisphere (Brazilian region). The period of observation covers 5 months during the peak of the last solar cycle, and the hours considered are from 19:00 LT up to 23:30 LT for every station. The analyses revealed that the occurrence of scintillation for all the stations peaks at 21:00 LT, with weaker/stronger scintillation levels for stations closer to the dip equator/Equatorial Ionization Anomaly, respectively. More importantly, the results demonstrated that deep fading events are concentrated in the early hours of the night for all levels of scintillation. The probabilities of deep fading events peaks at 21:00 LT, decaying considerably for later hours. The results also revealed that the deepest fading experienced occurs in the earlier hours and that the depth of the fading decreases as later hours are considered. The probabilities of a given depth for the deepest fading also decay after peaking at 21:00 LT. All these findings provide a temporal characterization that can be insightful to users of information from satellites whose links are susceptible to problems such as cycle slips and loss-of-lock, especially in the earlier hours of the night. [ABSTRACT FROM AUTHOR]

Abstract The Hunga Tonga–Hunga Ha’apai (HTHH) undersea volcanic eruption that occurred at 04:15 UT on 15 January 2022 is one of the most explosive events in the modern era, and a vertical plume reached approximately 55 km, corresponding to a height of the lower mesosphere. The intense explosion and subsequent plume generated acoustic and atmospheric gravity waves detected by ground-based instruments worldwide. Because a global-scale atmospheric and ionospheric response to the large volcanic eruption has not yet been observed, it provides a unique opportunity to promote interdisciplinary studies of coupling processes in lithosphere–atmosphere–ionosphere with ground-based and satellite observations and modeling. Further, this event allows us to elucidate the propagation and occurrence features of traveling ionospheric disturbances, the generation of equatorial plasma bubbles, the cause of electron density holes around the volcano, and the magnetic conjugacy of magnetic field perturbations. The most notable point among these studies is that the medium-scale travelling traveling ionospheric disturbances (MSTIDs) have magnetic conjugacy even in the daytime ionosphere and are generated by an external electric field, such as an E-region dynamo field, due to the motions of neutrals in the thermosphere. This advocates a new generation mechanism of MSTIDs other than the neutral oscillation associated with atmospheric gravity waves and electrified MSTIDs, which are frequently observed during daytime and nighttime, respectively. This paper reviews the recent studies of atmospheric and ionospheric disturbances after the HTHH volcanic eruption and summarizes what we know from this extreme event analysis. Further, we analyzed new datasets not shown in previous studies to give some new insights to understanding of some related phenomena. As a result, we also found that 4-min plasma flow oscillations caused by the acoustic resonance appeared with the amplitude of approximately 30 m/s in the northern hemisphere a few hours before the initial arrival of the air pressure waves. The propagation direction was westward, which is the same as that of the daytime MSTIDs with a magnetic conjugate feature. This result suggests that the 4-min oscillations are generated by an external electric field transmitted to the northern hemisphere along magnetic field lines. Graphical Abstract

Abstract High-Frequency Doppler (HFD) sounders at low-latitudes often detect characteristic oblique spreading Doppler traces in the spectrogram, known as Oblique Spread Structure (OSS). OSS has been expected to be generated by the dispersion of radio wave reflection due to equatorial plasma bubbles (EPBs). However, it has not yet been confirmed whether OSS is surely a manifestation of EPB by conducting simultaneous observations of EPB and OSS with different observational techniques. Additionally, it remains unclear what kinds of properties of EPB are reflected in the fine structure of OSS. In this study, we investigated three cases of OSSs and EPBs simultaneously observed by a HFD sounding system and an all-sky airglow imager in Taiwan. For the three cases presented here, the timing of OSS occurrence in the HFD data well coincided with that of the EPB appearance in the airglow data. The frequency shift of OSS is quantitatively explained assuming a radio wave reflection at 250–300 km altitudes. These results strongly indicate that OSS is formed by electron density variations at F-region altitudes accompanying EPB; thus, OSS is a manifestation of EPB in the HFD observations. Furthermore, it was suggested that the fine structure of OSS reflected the branching structure of EPB when the multiple branches of EPB reached the intermediate reflection point of the HFD observation. The detection of EPB occurrence and its fine structure using HFD observation enables monitoring of EPB regardless of weather conditions, which will contribute to monitoring the space weather impact of EPBs, for example, on GNSS navigation, in a wide area. Graphical Abstract

Abstract It has long been known that field-aligned irregularities within equatorial plasma bubbles (EPBs) can cause long-range propagation of radio waves in the VHF frequencies such as those used for TV broadcasting through the so-called forward scattering process. However, no attempt has been made to use such anomalous propagations of VHF radio waves for wide-area monitoring of EPBs. In this study, we investigated the feasibility of monitoring of EPBs using VHF radio waves used for aeronautical navigation systems such as VHF Omnidirectional radio Range (VOR). There are 370 VOR stations in the Eastern and Southeastern Asian region that can be potentially used as Tx stations for the observations of anomalous propagation. We have examined the forward scattering conditions of VHF waves using the magnetic field model and confirmed that it is possible to observe the EPB-related anomalous propagation if we set up Rx stations in Okinawa (Japan), Taiwan, and Thailand. During test observations conducted in Okinawa since 2021, no signal has been received that was clearly caused by anomalous propagation due to EPBs. This is simply because EPBs have not developed to high latitudes during the observation period due to the low solar activity. In March 2023, however, possible indications of EPB-related scattering were detected in Okinawa which implies the feasibility of observing EPBs with the current observation system. We plan to conduct pilot observations in Taiwan and Thailand in future to further evaluate the feasibility of this monitoring technique. Graphical Abstract

The Hunga Tonga–Hunga Ha'apai (HTHH) undersea volcanic eruption that occurred at 04:15 UT on 15 January 2022 is one of the most explosive events in the modern era, and a vertical plume reached approximately 55 km, corresponding to a height of the lower mesosphere. The intense explosion and subsequent plume generated acoustic and atmospheric gravity waves detected by ground-based instruments worldwide. Because a global-scale atmospheric and ionospheric response to the large volcanic eruption has not yet been observed, it provides a unique opportunity to promote interdisciplinary studies of coupling processes in lithosphere–atmosphere–ionosphere with ground-based and satellite observations and modeling. Further, this event allows us to elucidate the propagation and occurrence features of traveling ionospheric disturbances, the generation of equatorial plasma bubbles, the cause of electron density holes around the volcano, and the magnetic conjugacy of magnetic field perturbations. The most notable point among these studies is that the medium-scale travelling traveling ionospheric disturbances (MSTIDs) have magnetic conjugacy even in the daytime ionosphere and are generated by an external electric field, such as an E-region dynamo field, due to the motions of neutrals in the thermosphere. This advocates a new generation mechanism of MSTIDs other than the neutral oscillation associated with atmospheric gravity waves and electrified MSTIDs, which are frequently observed during daytime and nighttime, respectively. This paper reviews the recent studies of atmospheric and ionospheric disturbances after the HTHH volcanic eruption and summarizes what we know from this extreme event analysis. Further, we analyzed new datasets not shown in previous studies to give some new insights to understanding of some related phenomena. As a result, we also found that 4-min plasma flow oscillations caused by the acoustic resonance appeared with the amplitude of approximately 30 m/s in the northern hemisphere a few hours before the initial arrival of the air pressure waves. The propagation direction was westward, which is the same as that of the daytime MSTIDs with a magnetic conjugate feature. This result suggests that the 4-min oscillations are generated by an external electric field transmitted to the northern hemisphere along magnetic field lines. [ABSTRACT FROM AUTHOR]

Airglow images obtained with the aid of a ground-based Airglow Imager (AGI) located at Abuja, Nigeria (Geographic: 7.39°E, 8.99°N; Dip latitude: −1.60°) have been used to study the possible relationship between the occurrence of Atmospheric Gravity Waves (AGWs) and Equatorial Plasma Bubbles (EPBs) to investigate the role AGW plays in the occurrence of EPB. Airglow images covering over two and half years (nighttime only), from June 2015 to August 2018 were used. Three filters of the airglow imager were used in this study, taking observation over 652 days (nights). Results show that AGW occurrence is random with no regular trend and very rare at the station compared with EPB. EPB is found to occur more frequently with local time and seasonal dependence. Peak occurrences of EPB are observed during the equinoctial months. Although concurrent occurrences of the two phenomena are very infrequent, results further show that whenever both phenomena occur simultaneously, the duration of occurrences tends to suggest there exists a relationship between them. The concurrent occurrence of the two phenomena is observed more during post-sunset than post-midnight hours whenever it happens. Although EPB occurs independently of AGW, the results show that on nights where we have concurrent occurrences, EPB occurrence tends to take place earlier than days of EPB occurrence without AGW. We, therefore, conclude from the results obtained in this study that, AGW is not the only factor that contributes to the occurrences of EPB at the station since the AGW observed might not have consistently propagated to the bottomside of the F layer due to its dissipation. However, whenever it occurs, AGW tends to influence the conditions that trigger the occurrence of EPB. [ABSTRACT FROM AUTHOR]

On 12 October 2020 and 26 December 2021, NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission observed differently shaped equatorial plasma bubbles (EPBs) simultaneously within ∼10° longitude, near the subsatellite point and over the Atlantic, respectively which is unusual. On 12 October 2020, three EPBs with differing curvatures were observed in a ∼12° longitude sector. The westside EPB was curved toward the east, in a C‐shape. The middle was straight. The eastside EPB was curved westward, in a reversed C‐shape. In the second case, 26 December 2021, in a smaller longitude range of ∼6° adjacent C‐shaped and reversed C‐shaped EPBs were observed. EPBs' zonal drift velocities at the magnetic equator and both equatorial ionization anomaly crests were compared. These occurrences of oppositely shaped EPBs simultaneously in a narrow longitude may indicate that small‐scale longitudinal variations in the E‐region density, electric field, neutral wind variations, or a combination of them were present. Plain Language Summary: The post‐sunset ionosphere becomes conducive to the formation of plasma irregularities associated with depleted plasma densities. In the images obtained from space or ground, these plasma depleted regions appear as latitudinally elongated dark bands, which are known as "equatorial plasma bubbles (EPBs)." Satellite communication and navigation systems are adversely affected when the trans‐ionospheric radio signals travel through the EPBs. Thus, investigations of EPBs' formation and development are important. In 2‐D airglow images, the EPBs appear to be straight (aligned along the magnetic field lines). However, sometimes the poleward extensions of the EPBs can be tilted eastward or westward from the magnetic field line resembling either a C‐shape or reversed C‐shape, respectively. These differently shaped EPBs have been observed before. But, NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission observed them simultaneously within ∼12° and 6° longitudes on 12 October 2020 and 26 December 2021 which is unusual. Such occurrence of opposite‐shaped EPBs within a small longitude range indicates small‐scale longitudinal variations in the E‐region density, neutral winds, electric fields, or a combination of them. We present detailed observations of these two unique events with possible explanations. Key Points: Differently shaped equatorial plasma bubbles are observed simultaneously within a small longitude of ∼10°Observations indicate longitudinal variations in EPBs' zonal drift velocities at the magnetic equator and equatorial ionization anomaly (EIA) crestsIndication of the effect of small spatial scale E‐region density, electric field, neutral wind variations, or a combination of them [ABSTRACT FROM AUTHOR]

The Communications/Navigation Outages Forecast System satellite mission was designed to investigate the ionospheric conditions that lead to the formation of irregularities. Here, we have studied the effect of magnetic storms on the formation and evolution of plasma bubbles during the satellite's lifetime (2008–2015). During this period encompassing solar minimum and maximum conditions, many magnetic storms of varying intensity developed, producing a unique and rich data set of 248 storms (14 intense, 69 moderate, and 165 weak) that occurred during the same timeframe to examine the role of external magnetospheric drivers in the production and dynamics of equatorial plasma bubbles. We have used the Planar Langmuir Probe and Ion Velocity Meter instruments to elucidate the role of magnetic storm intensity on the bubble's depth, internal speed, width, occurrence, and lifetime. The pre‐reversal enhancement (PRE) tends to increase during the main phase and when BZ is southward. New bubbles occur during large excursions of the PRE value. The bubble lifetime extends and remains active during the main and part of the recovery phase. The plasma velocity within the bubbles increases and typically becomes over 100 m/s during significant PRE and BZ negative times. The depth of bubbles reaches values close to 100% during intense storms. In general, the intensity of the storms seems to control and augment the plasma bubbles' depth, width, and internal velocity. Plain Language Summary: Plasma irregularities, known as plasma bubbles, have been studied for several decades. Ongoing studies of plasma bubbles include the investigation of background conditions and events such as geomagnetic storms (disturbances in the upper atmosphere caused by southward IMF) that lead to the production and growth of plasma bubbles. Our work focuses on the formation of plasma bubbles that developed during several geomagnetic storms of varying intensities that occurred within 7 years, encompassing solar minimum and maximum. Changes in the production time, count, structure, and lifetime of bubbles were observed during the most important period of storms accompanying the increased evening‐time upward speed of the background plasma. These changes are consistent with studies based on the impact of geomagnetic storms on the upper atmosphere. The results provide an insight into the timeframe for bubble production, structural growth, and decay during the duration of the storms. Key Points: We analyzed bubble characteristics during magnetic storms that occurred between May 2008 and April 2014New bubbles that develop during the initial, main, or early recovery phase of storms don't decay until dawnBubble depths and vertical bubble velocities enhance during storms' initial, main, or early recovery phase [ABSTRACT FROM AUTHOR]

High-Frequency Doppler (HFD) sounders at low-latitudes often detect characteristic oblique spreading Doppler traces in the spectrogram, known as Oblique Spread Structure (OSS). OSS has been expected to be generated by the dispersion of radio wave reflection due to equatorial plasma bubbles (EPBs). However, it has not yet been confirmed whether OSS is surely a manifestation of EPB by conducting simultaneous observations of EPB and OSS with different observational techniques. Additionally, it remains unclear what kinds of properties of EPB are reflected in the fine structure of OSS. In this study, we investigated three cases of OSSs and EPBs simultaneously observed by a HFD sounding system and an all-sky airglow imager in Taiwan. For the three cases presented here, the timing of OSS occurrence in the HFD data well coincided with that of the EPB appearance in the airglow data. The frequency shift of OSS is quantitatively explained assuming a radio wave reflection at 250–300 km altitudes. These results strongly indicate that OSS is formed by electron density variations at F-region altitudes accompanying EPB; thus, OSS is a manifestation of EPB in the HFD observations. Furthermore, it was suggested that the fine structure of OSS reflected the branching structure of EPB when the multiple branches of EPB reached the intermediate reflection point of the HFD observation. The detection of EPB occurrence and its fine structure using HFD observation enables monitoring of EPB regardless of weather conditions, which will contribute to monitoring the space weather impact of EPBs, for example, on GNSS navigation, in a wide area. [ABSTRACT FROM AUTHOR]

It has long been known that field-aligned irregularities within equatorial plasma bubbles (EPBs) can cause long-range propagation of radio waves in the VHF frequencies such as those used for TV broadcasting through the so-called forward scattering process. However, no attempt has been made to use such anomalous propagations of VHF radio waves for wide-area monitoring of EPBs. In this study, we investigated the feasibility of monitoring of EPBs using VHF radio waves used for aeronautical navigation systems such as VHF Omnidirectional radio Range (VOR). There are 370 VOR stations in the Eastern and Southeastern Asian region that can be potentially used as Tx stations for the observations of anomalous propagation. We have examined the forward scattering conditions of VHF waves using the magnetic field model and confirmed that it is possible to observe the EPB-related anomalous propagation if we set up Rx stations in Okinawa (Japan), Taiwan, and Thailand. During test observations conducted in Okinawa since 2021, no signal has been received that was clearly caused by anomalous propagation due to EPBs. This is simply because EPBs have not developed to high latitudes during the observation period due to the low solar activity. In March 2023, however, possible indications of EPB-related scattering were detected in Okinawa which implies the feasibility of observing EPBs with the current observation system. We plan to conduct pilot observations in Taiwan and Thailand in future to further evaluate the feasibility of this monitoring technique. [ABSTRACT FROM AUTHOR]

Equatorial plasma bubble (EPB) development during different phases of the geomagnetic storm of 3–4 November 2021 (SYMHmin = −118 nT) was examined using observations and simulations. The initial phase of the storm coincided with postsunset (about 30 min after sunset) at Fortaleza (FZ) and São Luís (SL) with longitudes of ∼38.45°W and ∼44°W respectively on November 3 while the recovery phase of the storm started at 12:45 UT on November 4. GOLD shows the longest (shortest) extension of EPBs on November 3 (4) compared to days before and after November 3 and 4, including quiet days. This indicates an intensification (weakening) of EPBs on November 3 (4). From ionosondes at FZ and SL, a strong (weak) range spread F (SSF (RSF)) was observed on November 3 (4). The postsunset peak F layer height on November 3 reached 450 km and exceeded the preceding and succeeding days by ∼50–100 km at SL indicating the presence of a Prompt Penetration Electric Field (PPEF) which enhanced EPB development via the favorable postsunset vertical E x B and Rayleigh‐Taylor instability (RTI) mechanisms on November 3. The lower‐than‐quiet time F layer height observed on November 4 during Pre‐reversal enhancement (PRE) indicates the presence of a westward‐oriented Disturbance Dynamo Electric Field (DDEF) that undermined RTI growth and led to the weakening of EPB development. Simulation results confirm that the storm‐time electric fields modified the evening‐time ionosphere and influenced the magnitude of vertical E x B drift required for the development of EPBs. Key Points: Intensification (weakening) of equatorial plasma bubble (EPB) is observed by GOLD during the initial (recovery) phase of a stormStorm‐origin electric fields considerably altered the pre‐reversal electric field and influenced EPB developmentBoth observations and simulations confirm that disturbance electric fields modified the plasma distribution and instability growth rate [ABSTRACT FROM AUTHOR]

This study investigates the day-to-day variability of equatorial plasma bubbles (EPBs) over the Atlantic–American region and their connections to atmospheric planetary waves during the sudden stratospheric warming (SSW) event of 2021. The investigation is conducted on the basis of the GOLD (Global Observations of the Limb and Disk) observations, the ICON (Ionospheric Connection Explorer) neutral wind dataset, ionosonde measurements, and simulations from the WACCM-X (Whole Atmosphere Community Climate Model with thermosphere–ionosphere eXtension). We found that the intensity of EPBs was notably reduced by 35% during the SSW compared with the non-SSW period. Furthermore, GOLD observations and ionosonde data show that significant quasi-6-day oscillation (Q6DO) was observed in both the intensity of EPBs and the localized growth rate of Rayleigh–Taylor (R-T) instability during the 2021 SSW event. The analysis of WACCM-X simulations and ICON neutral winds reveals that the Q6DO pattern coincided with an amplification of the quasi-6-day wave (Q6DW) in WACCM-X simulations and noticeable ∼6-day periodicity in ICON zonal winds. The combination of these multi-instrument observations and numerical simulations demonstrates that certain planetary waves like the Q6DW can significantly influence the day-to-day variability of EPBs, especially during the SSW period, through modulating the strength of prereversal enhancement and the growth rate of R-T instability via the wind-driven dynamo. These findings provide novel insights into the connection between atmospheric planetary waves and ionospheric EPBs.

We report results from a self‐consistent global simulation model in which a large‐scale equatorial plasma bubble (EPB) forms during a midnight temperature maximum (MTM). The global model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM‐X. We consider solar minimum conditions for the month of August. We show that an EPB forms during an MTM in the Pacific sector and is caused by equatorward neutral wind flows. Although this is consistent with the theoretical result that a meridional neutral wind (V) with a negative gradient (∂V/∂θ < 0) is a destabilizing influence [Huba & Krall, 2013, https://doi.org/10.1002/grl.50292] (where a northward meridional neutral wind V is positive and θ is the latitude and increases in the northward direction), we find that the primary cause of the EPB is the large decrease in the Pedersen conductance caused by the equatorward winds. Plain Language Summary: The equatorial ionosphere often develops electron density irregularities at night in the altitude range 300–1,000 km. This phenomenon is known as equatorial spread F. A leading candidate to explain the generation of these irregularities is the generalized Rayleigh‐Taylor instability (GRTI). The phenomenon usually occurs after sunset but under certain conditions it can occur around midnight. In this paper, using the coupled ionosphere/thermosphere model SAMI3/WACCM‐X, we show that it can occur during a midnight temperature maximum where the neutral thermosphere temperature increases near the equator. This is associated with equatorward neutral wind flows that change the conductance of the ionosphere and leads to an increase in the growth rate of the GRTI and the development of a large equatorial plasma bubble. Key Points: An equatorial plasma bubble can develop during a midnight temperature maximum in the ionosphereEquatorward winds reduce the Pedersen conductance that enhances the growth rate of the generalized Rayleigh‐Taylor instabilityThe results are based on the coupled ionosphere/thermosphere model SAMI3/WACCM‐X [ABSTRACT FROM AUTHOR]

In this study we present AI Prediction of Equatorial Plasma Bubbles (APE), a machine learning model that can accurately predict the Ionospheric Bubble Index (IBI) on the Swarm spacecraft. IBI is a correlation (R2) between perturbations in plasma density and the magnetic field, whose source can be Equatorial Plasma Bubbles (EPBs). EPBs have been studied for a number of years, but their day‐to‐day variability has made predicting them a considerable challenge. We build an ensemble machine learning model to predict IBI. We use data from 2014 to 2022 at a resolution of 1s, and transform it from a time‐series into a 6‐dimensional space with a corresponding EPB R2 (0–1) acting as the label. APE performs well across all metrics, exhibiting a skill, association and root mean squared error score of 0.96, 0.98 and 0.08 respectively. The model performs best post‐sunset, in the American/Atlantic sector, around the equinoxes, and when solar activity is high. This is promising because EPBs are most likely to occur during these periods. Shapley values reveal that F10.7 is the most important feature in driving the predictions, whereas latitude is the least. The analysis also examines the relationship between the features, which reveals new insights into EPB climatology. Finally, the selection of the features means that APE could be expanded to forecasting EPBs following additional investigations into their onset. Key Points: AI Prediction of EPBs (APE) can accurately predict the Swarm Ionospheric Bubble IndexAPE is an XGBoost regressor that outperforms similarly trained linear and random forest modelsGame theory techniques reveals the influence of solar and geomagnetic activity as well as geo‐location, time, and season [ABSTRACT FROM AUTHOR]

Equatorial plasma bubbles are depleted plasma structures aligned to the geomagnetic field lines that are generated in the nighttime ionosphere bottomside and rise to the topside extending to increasingly higher latitudes. These structures induce fluctuations in the transionospheric radio signals causing the phenomenon known as ionospheric scintillation. Ionospheric scintillation is one of the main concerns for safety critical applications that rely on Global Navigation Satellite System information, therefore, the understanding about the occurrence and severity of scintillation is necessary. In this work, an analysis considering aspects of the orientation of the propagation of the radio signals through the plasma bubbles structures is performed to evaluate how this geometry can affect the scintillation profile. The dataset covers five months of records from three stations over the Brazilian region during the last solar cycle maximum. The initial results indicate that propagation paths fully aligned are consistently related to larger values of $S_{4}$ and more severe scintillation. The statistical evaluations with the $\alpha - \mu $ model show that during such cases stronger fading events are expected. According to the field measurements, fading events deeper than −15 dB are 73% more likely to occur under some aligned environments when compared to other propagation paths.

FORMOSAT‐7/COSMIC‐2 is the largest equatorial multi‐satellite constellation of six full‐size satellites to study the equatorial ionosphere. Each satellite is equipped by an ion velocity meter (IVM) instrument to provide high rate in situ plasma density observations along the low‐inclined satellite orbits at ∼530–550 km altitude. Six satellites provide an unprecedented dense coverage of the entire equatorial region around the globe and allow reliable detection of equatorial plasma bubbles (EPBs) and plasma density irregularities at different local times/longitudinal sectors simultaneously. We present a method for detection of EPBs in FORMOSAT‐7/COSMIC‐2 in situ plasma density data and construction of the global maps of EPB geolocations. The results in the form of time series and IVM‐based global Bubble Maps have a great potential for both near real‐time monitoring of space weather conditions and long‐term statistical analysis of EPB occurrence in regional or global scales. We present first FORMOSAT‐7/COSMIC‐2 derived climatological characteristics of the post‐sunset and post‐midnight EPBs occurrence probability and their apex altitudes during a period of low solar activity. Also, we demonstrate the good performance of the FORMOSAT‐7/COSMIC‐2 IVM‐based Bubble Maps when compared to optical images and ground‐based ionosonde observations. Plain Language Summary: Recently lunched satellite mission FORMOSAT‐7/COSMIC‐2 represents the largest equatorial mission of six spacecrafts that fly near the equator at 550 km altitude and measures various characteristics of the Earth's ionospheric plasma. Each satellite is equipped with an instrument for contact measurements of ionospheric plasma density and plasma motions—an ion velocity meter. Six satellites operate together around the globe, and they can probe the ionosphere at different longitudes and local times simultaneously. We use FORMOSAT‐7/COSMIC‐2 observations for detection of equatorial plasma bubbles (EPBs)‐huge plasma depletions developed after sunset over the equator. Detection of such structures is very important because of their impact on radio signals propagation. In this paper, we describe methodology on how we can detect EPBs and how to create global maps of their geolocation. The obtained results with time series of identified bubbles and global bubble maps can be used for near real‐time monitoring of ionospheric depletions and for statistical analysis of their occurrence around the globe. Key Points: Six satellites with in situ probe provide unprecedented coverage of equatorial region for reliable detection of equatorial plasma bubbles (EPBs)We developed new approaches for construction the global maps of EPBs geolocationsResults agreed well with independent observations and climatological characteristics of plasma bubbles occurrence [ABSTRACT FROM AUTHOR]

ESA's Swarm constellation entered in a "overfly" configuration in the period between September and October 2021, when the longitudinal distance between the lower pair and the upper satellite was at its minimum since the launch of the spacecrafts. In addition, the local time of the nighttime tracks was favorable to detect and study the morphology of post‐sunset equatorial plasma bubbles (EPBs). In this study, we focus on the Swarm overfly occurring between 00:41 UT and 00:59 UT on 30 September 2021, which covered one of the most densely instrumented regions for the study of the ionospheric irregularities embedded in the EPBs: the South American sector. By exploiting the use of ground‐based receivers of Global Navigation Satellite System (GNSS) signals in combination with the Swarm plasma density measurements, we study the irregularities in the EPB formed at ∼60°W and investigate the different scales of the irregularities and the cascading processes along the magnetic flux tubes. We also highlight how diffusion along the magnetic field lines occurs simultaneously with the plasma uplift, contributing then to the correct interpretation of the EPB evolution and decay process. The precious overfly conditions also allow the introduction of ionosphere‐related quantities, evaluated across the tracks at satellite altitudes enlarging the possibilities given by the same quantities already available along the tracks. Such opportunity envisages the possibility to proxy the impact of EPBs on GNSS signals with Low‐Earth Orbit satellite data provided by future missions specifically dedicated to the characterization of the near‐Earth environment and ionospheric studies. Plain Language Summary: Ionospheric Equatorial Plasma Bubbles (EPBs) are the most dangerous threat to radio‐wave propagation in the frequencies from HF, used for telecommunications, to L‐band, emitted by Global Navigation Satellite Systems (GNSS). Such bubbles form in the post‐sunset hours at equatorial latitudes and their occurrence is still a challenge for the scientific community, in terms of capability to model their triggering mechanisms, formation, growth and decay. On 30 September 2021, a precious occasion occurred. The 3 satellites constituting the ESA's Swarm constellation—a set of three satellites, two orbiting at 440 km and flying in pairs and one at 510 km—entered in a "overfly" configuration over Brazil. In other words, the lower pair and the upper satellite flew in a narrow longitudinal band and in one of the most densely instrumented regions in terms of ground‐based GNSS receiver. This translated into the possibility to step into a single EPB. We exploited this configuration and the ground‐based measurements to provide an unprecedented reconstruction of a plasma bubble and to introduce new measurements that can be used by future satellite missions to provide insights on the most dangerous natural hazard to radio waves. Key Points: Swarm overfly allowed offers an unprecedented possibility to investigate the features of a single plasma bubble at different altitudesScales of the irregularities and their cascading processes along the magnetic flux tubes are identifiedIonosphere‐related quantities, evaluated across the tracks at different satellite altitudes, are introduced [ABSTRACT FROM AUTHOR]

This study develops a new Bubble Index to quantify the intensity of 2‐D postsunset equatorial plasma bubbles (EPBs) in the American/Atlantic sector, using Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. A climatology and day‐to‐day variability analysis of EPBs is conducted based on the newly‐derived Bubble Index with the following results: (a) EPBs show considerable seasonal and solar activity dependence, with stronger (weaker) intensity around December (June) solstice and high (low) solar activity years. (b) EPBs exhibit opposite geomagnetic activity dependencies during different storm phases: EPBs are intensified concurrently with an increasing Kp, but are suppressed with high Kp occurring 3–6 hr earlier. (c) For the first time, we found that EPBs' day‐to‐day variation exhibited quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of Ionospheric Connection Explorer (ICON) winds and ionosonde data suggests that this multi‐day periodicity was related to the planetary wave modulation through the wind‐driven dynamo. Plain Language Summary: Equatorial plasma bubbles (EPBs) are plasma density depletions that frequently occur in the nighttime equatorial and low latitude ionosphere, which are well‐known for imposing significant space weather effects on navigation and communication systems. This study develops a novel Bubble Index to quantify EPBs' activity, using the Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. Based on this new Bubble Index, a climatology study and day‐to‐day variability analysis of GOLD‐captured EPBs were conducted. The EPBs' seasonal variation and solar and geomagnetic activity dependence were revealed and examined. Moreover, for the first time, we found the day‐to‐day variation of EPBs' intensity showed quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of neutral wind and ionosonde measurements suggests that this multi‐day periodicity was likely caused by planetary wave modulation via the wind‐driven dynamo. These new results shed new light on the day‐to‐day variability of the ionosphere and its possible connection with atmospheric waves. Key Points: A new Bubble Index is developed to quantify 2‐D EPBs' intensity over the American/Atlantic sector using GOLD nighttime disk image dataA first‐time statistical analysis of GOLD data shows that equatorial plasma bubbles (EPBs) have opposite geomagnetic activity dependencies during different storm phasesEPBs' intensity exhibits ∼3‐ and ∼6‐day periodicity, coinciding with planetary waves feature observed in atmospheric winds and hmF2 [ABSTRACT FROM AUTHOR]

Using the observations of the 630-nm all-sky imagers (ASIs) located in the geomagnetic conjugate points in the American sector from 2014 to 2017, this study statistically analyzed the features of conjugate equatorial plasma bubbles (EPBs), including their occurrence rate, zonal width, location and zonal drift velocity. The results show that the occurrence rate of the EPBs that occur simultaneously at geomagnetic conjugate points is ∼84%. The zonal widths of the EPBs are mainly ∼100 km, and the width differences of EPBs between the northern and southern hemispheres are mainly within ±30 km. The zonal displacements of the center locations of the northern and southern EPBs are within ±50 km. The zonal drift velocities of the northern and southern EPBs are nearly equal. However, it should be noted that the velocity of the EPBs in the northern hemisphere is 10% faster than that in the southern hemisphere. The results suggest that conjugate EPBs are common. Moreover, the non-conjugate EPBs in the northern and southern hemisphere can occur occasionally, which is probably associated with the different ionospheric backgrounds between the two hemispheres. The features of the conjugate EPBs as shown in this study provides support for the nowcasting of EPBs in the conjugate hemispheres. [ABSTRACT FROM AUTHOR]

This study investigates the Sub‐Auroral Polarization Streams (SAPS) and Sub‐Auroral Ion Drifts (SAID) along with the associated magnetospheric and ionospheric phenomena developed during 7–8 September 2017. Then, a series of substorms occurred before and during the geomagnetic storm (SYM‐Hmin = −146 nT). Results are obtained by utilizing multi‐point observations. These show that during the pre‐storm period, when the ElectroMagnetic Ion‐Cyclotron waves resulted in drop‐out events, a series of sunward (westward) SAID‐in‐SAPS flows developed in the dusk‐midnight sector due to the small‐scale flow bursts reaching the SAPS channel and the westward SAPS flows were streaming antisunward after midnight. During the storm period, dawnside sunward (eastward) SAPS also developed, Kelvin‐Helmholtz (K‐H) vortices rolled along the magnetopause and the hot zone became structured by reflected K‐H waves near the plasmapause leading to auroral undulations (AUs). New findings include the correlation of AUs and K‐H waves in the structured hot zone implying that the Near‐Earth Plasma Sheet acted as a resonator after activation by the K‐H vortices on the magnetopause. Further evidence is provided by the dayside Equatorward Boundary Intensification and Poleward Moving Auroral Forms of which development was supported by the K‐H vortices on the magnetopause by carrying the magnetosheath plasma into the magnetosphere. Adding to previous studies, new results also include the correlated observations of storm‐enhanced density (SED) plume and poleward drifts underlying both the equatorward plasma bubbles and the higher latitude plasma density depletions, and thus verifying their poleward movement within the SED base and SED plume. Key Points: Sunward Sub‐Auroral Ion Drifts‐in‐Sub‐Auroral Polarization Streams (SAPS) flows developed at dusk‐midnight and antisunward SAPS flows after midnightCorrelated auroral undulations and Kelvin‐Helmholtz waves (structuring the hot zone) suggest Near‐Earth Plasma Sheet resonator activationRam drifts depict the poleward drifts of plasma bubbles/depletions within the storm‐enhanced density base/plume [ABSTRACT FROM AUTHOR]

The nighttime ionospheric response to a geomagnetic storm that occurred on 23–29 September 2020 is investigated over the South American, Atlantic, and West African longitude sectors using NASA's Global‐scale Observations of the Limb and Disk measurements. On 27 September the solar wind conditions were favorable for prompt penetration electric fields to influence the equatorial ionosphere over extended longitudes. The equatorial ionization anomaly (EIA) crests were shifted 8°–10° poleward compared to the quiet time monthly mean across ∼65°–35°W during the main phase. Ionosonde hmF2 (peak electron density height) measurements from Fortaleza (GG: 3.9°S and 38.4°W) indicated a stronger prereversal enhancement this evening than other nights. As a result, equatorial plasma bubbles (EPB) occurred at these longitudes on this evening. This is the first simultaneous investigation of EIA morphology and EPB occurrence rate over an extended longitude range from geostationary orbit during a geomagnetic storm. Plain Language Summary: The effects of a geomagnetic storm that occurred during 23–29 September 2020 on the nighttime equatorial ionospheric behavior is investigated using NASA's Global‐scale Observations of the Limb and Disk (GOLD) measurements. On each evening, the equatorial ionization anomaly (EIA) crests locations and brightnesses and equatorial plasma bubbles (EPB) occurrence rates are obtained over the South American, Atlantic, and West African longitude sectors. On 27 September the solar wind conditions were favorable for the penetration of interplanetary electric fields to the equatorial ionosphere over the dusk longitude sectors (∼35°W Lon). The PPEF strengthened the pre‐reversal enhancement and thereby enhanced the plasma fountain effect. Ionospheric F2 layer height increase is confirmed by digisonde measurements at Fortaleza (GG: 3.9°S and 38.4°W). On this day, the maximum poleward shifts compared to the quiet time monthly mean values are observed across ∼65°–35°W longitude during the storm's main phase. EPBs occurrence rate was maximum on this night. The present study reports the first simultaneous investigation of EIA morphology (crests locations and brightnesses) and EPB occurrence rates over an extended longitude range during a geomagnetic storm from a geostationary orbit. Key Points: First simultaneous observations of geomagnetic storm effects on equatorial ionization anomaly (EIA) morphology and equatorial plasma bubble (EPB) occurrence rate from the geostationary orbitMaximum poleward shift of the EIA crests and increase in EPB occurrence rate is observed during storm's main phase on 27 September 2020Concurrent increase of hmF2 from digisonde observation confirms the strengthening of the plasma fountain effect during postsunset hours [ABSTRACT FROM AUTHOR]

Abstract We report results from a self‐consistent global simulation model in which a large‐scale equatorial plasma bubble (EPB) forms during a midnight temperature maximum (MTM). The global model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM‐X. We consider solar minimum conditions for the month of August. We show that an EPB forms during an MTM in the Pacific sector and is caused by equatorward neutral wind flows. Although this is consistent with the theoretical result that a meridional neutral wind (V) with a negative gradient (∂V/∂θ

Equatorial plasma bubbles (EPBs) can lead to signal degradation, affecting the measurement accuracy. Studying EPBs and their characteristics has gained increasing importance. The characteristics of EPBs were investigated using the rate of total electron content (TEC) index (ROTI) maps under different solar and magnetic activity conditions during two periods: July 2014–July 2015 (solar maximum activity with F10.7: 145.9 × 10−22 W⋅m−2⋅Hz−1) and July 2019–July 2020 (solar minimum activity with F10.7: 69.7 × 10−22 W⋅m−2⋅Hz−1). We also divided this analysis according to the magnetic activity levels based on Kp and Dst (disturbance storm time) indices, classified as follows: quiet+ (Kp ≤3 and Dst >−30 nT), quiet− (Kp ≤3 and Dst

Abstract This study develops a new Bubble Index to quantify the intensity of 2‐D postsunset equatorial plasma bubbles (EPBs) in the American/Atlantic sector, using Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. A climatology and day‐to‐day variability analysis of EPBs is conducted based on the newly‐derived Bubble Index with the following results: (a) EPBs show considerable seasonal and solar activity dependence, with stronger (weaker) intensity around December (June) solstice and high (low) solar activity years. (b) EPBs exhibit opposite geomagnetic activity dependencies during different storm phases: EPBs are intensified concurrently with an increasing Kp, but are suppressed with high Kp occurring 3–6 hr earlier. (c) For the first time, we found that EPBs' day‐to‐day variation exhibited quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of Ionospheric Connection Explorer (ICON) winds and ionosonde data suggests that this multi‐day periodicity was related to the planetary wave modulation through the wind‐driven dynamo.

This paper presents a multi-instrument observational analysis of the equatorial plasma bubbles (EPBs) variation over the American sector during a geomagnetically quiet time period of 07–10 December 2019. The day-to-day variability of EPBs and their underlying drivers are investigated through coordinately utilizing the Global-scale Observations of Limb and Disk (GOLD) ultraviolet images, the Ionospheric Connection Explorer (ICON) in-situ and remote sensing data, the global navigation satellite system (GNSS) total electron content (TEC) observations, as well as ionosonde measurements. The main results are as follows: 1) The postsunset EPBs’ intensity exhibited a large day-to-day variation in the same UT intervals, which was fairly noticeable in the evening of December 07, yet considerably suppressed on December 08 and 09, and then dramatically revived and enhanced on December 10. 2) The postsunset linear Rayleigh-Taylor instability growth rate exhibited a different variation pattern. It had a relatively modest peak value on December 07 and 08, yet a larger peak value on December 09 and 10. There was a 2-h time lag of the growth rate peak time in the evening of December 09 from other nights. This analysis did not show an exact one-to-one relationship between the peak growth rate and the observed EPBs intensity. 3) The EPBs’ day-to-day variation has a better agreement with that of traveling ionospheric disturbances and atmospheric gravity waves signatures, which exhibited relatively strong wavelike perturbations preceding/accompanying the observed EPBs on December 07 and 10 yet relatively weak fluctuations on December 08 and 09. These coordinate observations indicate that the initial wavelike seeding perturbations associated with AGWs, together with the catalyzing factor of the instability growth rate, collectively played important roles to modulate the day-to-day variation of EPBs. A strong seeding perturbation could effectively compensate for a moderate strength of Rayleigh-Taylor instability growth rate and therefore their combined effect could facilitate EPB development. Lacking proper seeding perturbations would make it a more inefficient process for the development of EPBs, especially with a delayed peak value of Rayleigh-Taylor instability growth rate.

Equatorial plasma bubbles (EPBs) have long been studied and are becoming increasingly important because they cause severe scintillations in radio waves from Global Navigation Satellite System (GNSS) satellites. In this review paper EPBs and their characteristics like generation mechanism, initial perturbation, occurrence variability, zonal drift velocity, vertical rise velocity, coupling with zonal neutral winds and secondary instabilities are thoroughly reviewed, and future aspects are discussed. [ABSTRACT FROM AUTHOR]

Hainan COherent scatter Phased Array Radar has recorded four patches of daytime F‐region field‐aligned irregularities on 31 May 2021, and it is the first time to investigate the daytime irregularities by the radar imaging technique. In‐beam radar images are plotted on the curved plane transverse to the geomagnetic field lines. The displayed daytime irregularities present the obliquely elongated band‐like structures and the corresponding scatter regions extend to hundreds of kilometers in range and narrow to a few kilometers in the zonal direction. The diffuse and isolated structures were also observed in the radar images. The radar image sequences indicate that the irregularities were continuous in height as they drifted westward. The daytime irregularities maintained the oblique elongated structures that resemble the morphological features of the nighttime equatorial plasma bubbles. Combined with the in situ satellite measurements, these daytime F‐region irregularities are interpreted to be the remnants of the previous night's bubbles and come from the higher altitudes near the magnetic equator. Key Points: Daytime F‐region irregularities in low latitudes were detected by applying in‐beam radar imaging techniqueRadar images of the daytime irregularities exhibit narrow band‐like structures extending hundreds of kilometers verticallyThe band‐like structures are interpreted as remnants of bubbles developed on previous night [ABSTRACT FROM AUTHOR]

We present high‐resolution simulation results of the response of the ionosphere/plasmasphere system to the 15 January 2022 Tonga volcanic eruption. We use the coupled Sami3 is Also a Model of the Ionosphere ionosphere/plasmasphere model and the HIgh Altitude Mechanistic general Circulation Model whole atmosphere model with primary atmospheric gravity wave effects from the Model for gravity wavE SOurces, Ray trAcing and reConstruction model. We find that the Tonga eruption produced a "super" equatorial plasma bubble (EPB) extending ∼30° in longitude and up to 500 km in altitude with a density depletion of 3 orders of magnitude. We also found a "train" of EPBs developed and extended over the longitude range 150°–200° and that two EPBs reached altitudes over 4,000 km. The primary cause of this behavior is the significant modification of the zonal neutral wind caused by the atmospheric disturbance associated with the eruption, and the subsequent modification of the dynamo electric field. Plain Language Summary: The Hunga Tonga‐Hunga Ha'apai volcanic eruption occurred on 15 January 2022 at 04:14 UT and generated a massive atmospheric disturbance that caused major effects in the ionosphere worldwide. Using a high‐resolution coupled ionosphere/thermosphere model we show that the changes in the thermospheric winds strongly modified the electrodynamics of the ionosphere. This led to the development of a "train" of equatorial plasma bubbles (EPBs), regions of very low electron density, in the western Pacific sector. Moreover, two EPBs reached unusually high altitudes, over 4,000 km. Key Points: Modeling of the Tonga volcanic eruption show equatorial plasma bubbles (EPBs) develop in the Pacific sectorA large equatorial bubble formed below 500 km roughly 30° in longitudeEPBs rose to very high altitudes (>4,000 km) [ABSTRACT FROM AUTHOR]

The use of Global Navigation Satellite System (GNSS) for air and terrestrial navigation and for many applications is increasing in the last decades. However, the Earth's ionosphere causes GNSS signal delay due to the total electron content (TEC) and scintillation in the signal phase and amplitude. This scintillation can give rise to deleterious effects in the GNSS positioning. So, it is important to assess the effects of the ionosphere over the GNSS signal. To achieve this goal, it is necessary to have a large spatial and temporal coverage of data from many different sounders, being the GNSS receivers of great importance due to their global coverage and availability. In this work, we present a retrospective of the scintillation monitoring networks in Brazil and their characteristics. As the RBMC network managed by the IBGE provides TEC and as rate of TEC index (ROTI) is well correlated with ionospheric irregularities, we present also the RBMC network description. These RBMC GNSS receivers provide data in regions with scarcity of scintillation monitors. The description of the Ionospheric Scintillation Monitoring Receivers (ISMR) Query Tool, that is a web software that has been supporting research on the ISMR data, is also presented. [ABSTRACT FROM AUTHOR]

The lengthiest GPS observations from a reference station during 2002–2018 and a colocated scintillation monitoring receiver during 2013–2018 at Hyderabad, India, are used to analyze the degradation in kinematic precise point positioning (KPPP) due to ionospheric irregularities. The GipsyX software is employed to estimate the KPPP of the reference station and the residuals for each coordinate are computed against the monthly mean values of the daytime observations. The results establish the long-term variability of the residuals of KPPP under the climatological and event-specific effect of the post-sunset equatorial plasma bubbles (EPBs). The extreme post-sunset fluctuations in the residuals during a few quiet and disturbed nights obtained from the 18 years of continuous observations are emphasized. The outstanding and persistent patterns in the residuals are majorly found during extended post-sunset hours with striking seasonal contrast. The large and rapid fluctuations in the residuals are found to exhibit long-term day-to-day, seasonal, and solar cycle variations, which correspond to the occurrence of the amplitude and phase scintillations. Thus, uniquely, the colocated observations of scintillations corroborate the impact of the simultaneously affected number of satellites (S4 index > 0.17) and indicate the possibility of cycle slips eventually affecting the KPPP solutions. The seasonal climatology of the scintillations is shown to translate over the 3-D position error, which modulates with change in the solar F10.7 cm flux. This study, thus probably for the first time, reports the long-term effect of solar cycle variability on the amplitude of perturbations in KPPP for the solar cycles 23 and 24. [ABSTRACT FROM AUTHOR]

• Plasma depletions are seen in simultaneity in OI 630 nm nightglow from two different low-latitudinal locations. • Strength of PRE decides the latitudinal extent of plasma bubbles over the low-latitudes. • Gravity waves with scale sizes in the range of 50–250 km seem to be responsible for the generation of EPBs. Equatorial plasma bubbles (EPBs), generated over the dip-equator during the sunset time, get mapped over to the low-latitudes through the geomagnetic field lines and contribute to structures of different scales in the nighttime thermosphere-ionosphere system. In the present study, the EPBs are observed in the OI 630 nm nightglow emissions over Mt. Abu and Kolhapur using a High Throughput Imaging Echelle spectrograph (HiTIES) and All-Sky Imager (ASI), respectively. Similar periodicities obtained in these measurements consistently corroborate the presence of the EPBs at both these locations. The strength of the Pre-Reversal Enhancement (PRE) in the zonal electric field has been investigated using the ionosonde data over Trivandrum, a dip-equatorial location. The strength of PRE decides the latitudinal extent of the EPBs. The eastward movement of the EPBs has been estimated to be in the range of 190–90 ms−1, decreasing in magnitude from post-sunset to midnight. The wave number analysis carried out using the observed OI 630 nm emissions in the zonal direction has resulted in a contrasting behaviour during the presence/absence of the EPBs. Based on the analysis of around 1300 images of data, it is revealed that the scale sizes in the range of 250–300 km are omnipresent, whereas, shorter scale sizes (50–250 km) are present only during the presence of EPBs. It is inferred that these shorter scale size gravity waves played a significant role in the seeding the perturbation of the EPBs. [ABSTRACT FROM AUTHOR]

The use of Global Navigation Satellite System (GNSS) for air and terrestrial navigation and for many applications is increasing in the last decades. However, the Earth’s ionosphere causes GNSS signal delay due to the total electron content (TEC) and scintillation in the signal phase and amplitude. This scintillation can give rise to deleterious effects in the GNSS positioning. So, it is important to assess the effects of the ionosphere over the GNSS signal. To achieve this goal, it is necessary to have a large spatial and temporal coverage of data from many different sounders, being the GNSS receivers of great importance due to their global coverage and availability. In this work, we present a retrospective of the scintillation monitoring networks in Brazil and their characteristics. As the RBMC network managed by the IBGE provides TEC and as rate of TEC index (ROTI) is well correlated with ionospheric irregularities, we present also the RBMC network description. These RBMC GNSS receivers provide data in regions with scarcity of scintillation monitors. The description of the Ionospheric Scintillation Monitoring Receivers (ISMR) Query Tool, that is a web software that has been supporting research on the ISMR data, is also presented.

Abstract We present high‐resolution simulation results of the response of the ionosphere/plasmasphere system to the 15 January 2022 Tonga volcanic eruption. We use the coupled Sami3 is Also a Model of the Ionosphere ionosphere/plasmasphere model and the HIgh Altitude Mechanistic general Circulation Model whole atmosphere model with primary atmospheric gravity wave effects from the Model for gravity wavE SOurces, Ray trAcing and reConstruction model. We find that the Tonga eruption produced a “super” equatorial plasma bubble (EPB) extending ∼30° in longitude and up to 500 km in altitude with a density depletion of 3 orders of magnitude. We also found a “train” of EPBs developed and extended over the longitude range 150°–200° and that two EPBs reached altitudes over 4,000 km. The primary cause of this behavior is the significant modification of the zonal neutral wind caused by the atmospheric disturbance associated with the eruption, and the subsequent modification of the dynamo electric field.

An overview of recent advances made in understanding the phenomenon of equatorial spread F (ESF) is presented and a discussion of unresolved issues that need to be addressed. The focus is on research that has occurred in the last decade. The topics include satellite observations, theory, and modeling. The suggested areas that require further exploration are a unified theory of turbulence extending from 100 s m to 10 s cm, the impact of geomagnetic storms on the development of equatorial spread F, the need for accurate thermospheric wind measurements and models, and identifying the underlying physics of ESF in the post-midnight sector during solar minimum.

The fluctuations in the electron density of the F-region in the ionosphere as compared to the background electron density are the sign of plasma irregularities which is a well-known phenomenon. They occur during post-sunset hours and degrade ionospheric radio wave communication majorly the navigation signals. To explore the features of equatorial plasma bubbles (EPBs) over the Indian region throughout the solar maximum year 2014 and the solar moderate year 2017, we have examined day and night hours plasma irregularities from Defense Meteorological Satellite Program (DMSP) with spacecraft as F15 with total crossings as 9698. From the total crossings, 1115 crossings were obtained with EPBs degrading the communication and navigation signals. The distribution of EPBs was studied monthly, seasonally, and longitudinally for the solar maximum and moderate years which has given significant results. We have also analyzed the occurrence of EPBs during the geomagnetic storms with Disturbed storm time index (Dst) ≤ -100 nT occurred in the solar maximum and moderate years. We have observed that the regular occurrence of EPBs was there during the initial and main phase but smoothens up in course of the recovery phases of the storm. These results are discussed with those of earlier results and are in good agreement too. The observed inferences will help to improve the telecommunication sector in the Indian region. [ABSTRACT FROM AUTHOR]

OI 630.0 nm airglow observations, from a new observatory at Bom Jesus de Lapa, were used to study the interaction between EPBs (Equatorial Plasma Bubbles) and the MSTID (Medium-Scale Traveling Ionospheric Disturbance) over the Northeast region in Brazil. On the night of September 16 to 17, 2020, an EPB was observed propagating eastward, in an apparent fossil stage, until it interacted with a dark band electrified MSTID (eMSTID). After the interaction, four EPBs merged, followed by an abrupt southward development and bifurcations. Analysis of the data suggests that an eastward polarization electric field, induced by the dark band eMSTID, forced the EPB into an upward drift, growing latitudinally along the magnetic field lines and then bifurcating.