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The receiver operating characteristic (ROC) and the area under the curve (AUC), initially developed for signal processing and psychology, are a test for assessing the performance of a binary classification problem at varying threshold values. The ion density (Ni) observed by FORMOSAT‐7/COSMIC‐2 is used to study traveling ionospheric disturbances (TIDs) triggered by the 15 January 2022 Tonga volcanic eruption. We examine parameters of Ni, differential Ni, and standard deviation Ni (STD_Ni) in January 2022, simulate TID wavefronts traveling with various speeds from 10 to 1,000 m/s, and apply the ROC curve to globally identify the significance of TIDs in STD_Ni triggered by the volcanic eruption. ROC and AUC results show that in addition to TIDs related to tsunami/tropospheric Lamb waves and a series of fast‐moving waves with propagation speeds of 180–350 and 450–600 m/s, respectively, those long‐lasting low‐speeds less than 70 m/s and high‐speeds about 690, 860, and 990 m/s meet 95% statistical significance, which confirms TIDs being detected. These show that the FORMOSAT‐7/COSMIC‐2 ion density can be used to globally detect various TIDs triggered by the Tonga volcanic eruption. The ROC test results also show a potential use case for detecting other geophysical signals in future applications. Plain Language Summary: Large‐scale natural phenomena such as volcanoes and tsunami induce prominent traveling atmospheric disturbances (TADs) which also simultaneously and locally influence the above ionosphere activating prominent traveling ionospheric disturbances (TIDs). To assess the significance of such TIDs, we treat the TIDs as a series of signals, and the receiver operating characteristic (ROC) and the area under the curve (AUC), developed to address binary classification problems, are used to evaluate these signals at varying threshold values. The 15 January 2022 Tonga volcanic eruption triggers a series of TADs, TIDs, and tsunami waves propagating around the globe. Measurements of the ion density (Ni) by IVM (ion velocity meter) onboard FORMOSAT‐7/COSMIC‐2 (F7C2) are used to identify TIDs triggered by the volcanic eruption. We examine TIDs of F7C2 Ni, differential Ni, and standard deviation (STD) of Ni in January 2022, and find that TIDs associated with the tsunami waves, tropospheric Lamb waves, and high‐speed waves within 450–600 m/s triggered by the Tonga volcanic eruption on the globe are very prominent on the eruption day. In addition, TIDs with long‐lasting low‐speeds less than 70 m/s and high‐speeds about 690, 860, and 990 m/s can be identified in the ROC curves and AUC indices. Key Points: The FORMOSAT‐7/COSMIC‐2 ion density globally observes various traveling ionospheric disturbances triggered by the Tonga volcanic eruptionThe receiver operating characteristic (ROC) test can detect traveling ionospheric disturbances and potentially other geophysical signalsDisturbances with low‐speeds of 10–70 m/s, tsunami, tropospheric Lamb wave, and high‐speeds of 490/690/860/990 m/s are identified [ABSTRACT FROM AUTHOR]

The Iterative Driver Estimation and Assimilation (IDEA) data assimilation technique was used with the Whole Atmosphere Model (WAM) to improve neutral density specification in the upper thermosphere. Two different neutral density data sources were examined to enhance the capability of simulating the global thermospheric state. The first were accelerometer estimates of neutral density from the Challenging Mini‐Satellite Payload (CHAMP) satellite. The second were neutral density estimates from the Global Ultraviolet Imager (GUVI) limb‐scan airglow observations aboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. Due to the intensity of the November 2003 storm, two changes were necessary in WAM. The first was allowing the Kp geomagnetic index to exceed 9 and the second was changing the relationship between Kp and the solar wind parameters used to drive the model. With these changes, results show that IDEA effectively captures the thermospheric neutral density at the CHAMP satellite altitude and follows the time‐dependence through the November 2003 storm period. Furthermore, a cross‐comparison was conducted with the GUVI dayside limb scan measurements. GUVI neutral densities within 270–320 km show the closest agreement with WAM when CHAMP data was assimilated by IDEA. We speculate on the potential for observations from GUVI at 300 km to be used as a data source in the IDEA‐WAM simulations. These simulations demonstrate the utility of the IDEA data assimilation technique with physical models and that using either accelerometer observations or ultraviolet airglow limb measurement during extreme storm periods could be used. Plain Language Summary: Strong variations in the neutral density of the thermosphere heavily impact the position of low Earth orbit objects as they respond to radiative inputs from the Sun in the extreme ultraviolet (UV) wavelength range, energetic particle precipitation in the auroral regions, and global‐scale electrical currents generated during geomagnetic storms. In this study, the Iterative Driver Estimation and Assimilation (IDEA) data assimilation technique was used with the Whole Atmosphere Model (WAM) to improve neutral density specification in the upper thermosphere. The Challenging Mini‐Satellite Payload (CHAMP) satellite and the Global Ultraviolet Imager (GUVI) limb scan UV airglow observations aboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite were examined to enhance the capability of simulating the global thermospheric state. Results show that IDEA effectively captures the thermospheric neutral density at the CHAMP satellite altitude throughout the November 2003 storm period. Furthermore, GUVI limb scan measurements on the dayside were cross‐compared with IDEA densities. We speculate on the potential for observations from GUVI during the daytime to be used as a data source in the IDEA‐WAM simulations. This study demonstrates data assimilative physical models are capable of improving neutral density specification for better orbit determination and prediction of low Earth orbiting satellites. Key Points: Iterative Driver Estimation and Assimilation (IDEA) data assimilation system greatly improves the Whole Atmosphere Model (WAM) thermospheric neutral density nowcasting capabilityDifferent locations of observed and modeled high‐latitude density hole structures lead to discrepancy between Challenging Mini‐Satellite Payload and the model outputIDEA output is validated against Global Ultraviolet Imager data, indicating a potential use case for limb‐scan measurements in the WAM neutral density forecast [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]

This letter reports first simultaneous detection of F‐region plasma‐depleted structure in O(1D) 630.0 and O(1S) 557.7 nm airglow images on a geomagnetically quiet‐night (Ap = 3) of 26 June 2021 from mid‐latitude station (Hanle, India) due to enhanced thermospheric 557.7 nm emission. Since nighttime thermospheric 557.7 nm emission over mid‐latitudes is predominantly masked by significantly larger mesospheric component, F‐region plasma structures are rarely observed in 557.7 nm images. Interestingly, thermospheric 557.7 nm emission was not significant on the following geomagnetically quiet‐night as bands of medium‐scale traveling ionospheric disturbance were only observed in 630.0 nm images. Poleward wind generated by Equatorial Temperature and Wind Anomaly transported plasma from the boundary of equatorial ionization anomaly, causing significant electron density enhancement around 250 km and descent of F‐layer peak over Hanle on 26 June 2021. This amplified the dissociative recombination enabling the simultaneous detection of plasma‐depleted structure in 557.7 and 630.0 nm images. Plain Language Summary: The thermospheric O(1S) 557.7 generated through dissociative recombination of O2+ in the F‐region is significantly lower than its mesospheric counterpart which is generated via the Barth mechanism in the Mesosphere‐Lower‐Thermosphere region. This causes difficulties in the simultaneous detection of mid‐latitude F‐region plasma structures in 630.0 and 557.7 nm airglow images during geomagnetically quiet nights of low solar active years. We report, for the first time, such simultaneous detection of plasma‐depleted structure from mid‐latitude station (Hanle, India) on a geomagnetically quiet night of 26 June 2021. Interestingly, the thermospheric 557.7 nm emission was not significant on the following geomagnetically quiet night as bands of medium‐scale traveling ionospheric disturbance were detected only in the 630.0 nm images. Results from multi‐instrument data sets showed the descent of F‐layer peak and significant electron density enhancement over Hanle. The local poleward wind generated by stronger Equatorial Temperature and Wind Anomaly on 26 June 2021 transported the plasma from the boundary of the equatorial ionization anomaly region to Hanle through geomagnetic field lines. This caused the enhancement of electron density around 250 km and descent of F‐layer peak over Hanle that created suitable condition for the amplification of dissociative recombination reaction. Key Points: Simultaneous observation of mid‐latitude F‐region plasma‐depleted structure in O(1D) 630.0 and O(1S) 557.7 nm airglow imagesSignificantly higher electron density is observed over the region on 26 June 2021 than on the following night at airglow emission altitudeThermospheric O(1S) 557.7 nm emission contributed significantly due to the enhancement in the dissociative recombination reaction [ABSTRACT FROM AUTHOR]

The NASA global‐scale observations of the limb and disk (GOLD) mission is a measurement opportunity to scan the far ultraviolet airglow at ∼134–162 nm over the American Hemisphere since October 2018. The FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission has provided thousands of daily radio occultation soundings in the low‐ and mid‐latitude regions since July 2019. The nighttime OI–135.6 nm emission is mainly through radiative recombination, and the radiance is used to derive the peak electron density. Comparison with corresponding F7/C2 observations demonstrates good correlation in low‐latitudes, while is overestimated near mid‐latitudes in winter, induced by the photoelectrons emanating from magnetically conjugate Hemisphere. The machine learning technique Bagged Trees is implemented to develop an intensity to peak density model training from GOLD and F7/C2 observations. The validation demonstrates that Bagged Trees peak‐density has less influence from conjugate photoelectrons and indicates the power of machine learning techniques for geophysics data processing. Plain Language Summary: The global‐scale observations of the limb and disk (GOLD) mission can scan wide ranges of far‐ultraviolet airglow at ∼134–162 nm over the American Hemisphere, providing the intensity of the airglow radiance and related maximum electron density of the ionosphere. Meanwhile, the FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission receives GNSS radio signal to do radio occultation soundings and provides more than 4,000 of vertical electron density profiles daily. This study examines the two observations from different sounding techniques and algorithms, which generally yield good agreements, except that in the winter Hemisphere, the GOLD images experience the twilight airglow excited by conjugate photoelectrons emanating during May, June, and July. Moreover, the machine learning technique Bagged Trees is implemented to develop an intensity to peak density model training from two satellite observations, and the validation shows that the machine learning technique can detect and reduce the influence of the conjugate photoelectrons. The results indicate that the model can improve and increase the accuracy of the application of GOLD intensity for retrieving the peak electron density of the ionosphere. Key Points: The F7/C2 NmF2 and global‐scale observations of the limb and disk (GOLD) 135.6 nm airglow peak density are consistent except in the mid‐latitude region in the winter HemisphereThe machine learning technique bagged tree is used to develop an intensity to NmF2 model based on GOLD and F7/C2 observationsThe machine learning model can reduce the overestimation of NmF2 influenced by the photoelectrons emanating from the opposite Hemisphere [ABSTRACT FROM AUTHOR]

Intense eruptions of the Tonga volcano activated prominent traveling atmospheric disturbances (TADs) at 04:05UT on 15 January 2022. Himawari‐8 satellite images depict that TADs of the tropospheric Lamb wavefront propagate with a speed of 315 m/s and arrive in Taiwan at 11:30UT. Networks of 98 barometers, 28 tide gauges, an ionosonde, and 10 magnetometers are used to study the responses of magnetic fields to TADs in Taiwan. The horizontal components in magnetic field changes of the Taiwan magnetometers all point toward and away from the Tonga volcano at 11:00–12:00UT upon the tropospheric Lamb wavefront arrival and at 22:00–23:00UT when the thermospheric Lamb wavefront with speeds of 487 m/s coming, respectively. Analyses of the raytracing and beamforming techniques on the horizontal components in magnetic field changes of 69 INTERMAGNET magnetometers show that both tropospheric and thermospheric Lamb waves efficiently activate traveling ionospheric disturbances and modify ionospheric currents of the globe. Plain Language Summary: At 04:05UT on 15 January 2022, intense Tonga volcanic eruptions induce prominent atmospheric disturbances and tsunami waves. Himawari‐8 meteorological satellite images depict the induced upper‐level tropospheric disturbances with horizontal speeds of about 315 m/s at 8.2 km altitude in the Lamb wave mode travel worldwide. Upon the traveling atmospheric disturbances (TADs) of the tropospheric Lamb wavefront arriving in Taiwan at 11:30UT, 98 ground‐based barometers register increases and reach peaks at about 11:50UT in the atmospheric pressure; 28 tide gauges record enhancements and maximums of sea level fluctuations at about 14:30–17:30UT; and a local ionosonde observes that the ionosphere reaches the highest altitude at 14:30UT. The changes of the horizontal component of the Earth's magnetic fields measured by 10 Taiwan magnetometers almost all point exactly toward the Tonga volcano upon the tropospheric Lamb wavefront arrival at 11:00–12:00UT, and away from the volcano at 22:00–23:00UT, which suggests a 487 m/s TAD (or thermospheric Lamb wavefront) at about 130 km altitude also being activated. The horizontal components in magnetic field changes of 69 INTERMAGNET magnetometers show that both tropospheric and thermospheric Lamb waves triggered by Tonga volcanic eruptions are very powerful, and can induce intense dynamo currents and electric fields on the globe. Key Points: Tropospheric and thermospheric Lamb waves of the Tonga volcanic eruption activate dynamo currents and electric fieldsTraveling atmospheric disturbances of the Tonga volcanic eruption significantly uplift the ionosphereTropospheric Lamb waves of the Tonga volcanic eruption modulate ground‐based air pressures and sea levels [ABSTRACT FROM AUTHOR]

Solar eclipse is a daytime phenomenon that significantly disturbs the ionosphere, but whether the eclipse induces ionospheric irregularities in the nighttime remains unknown. In this study, we analyzed the dense total electron content (TEC) observations from the ground‐based Global Navigation Satellite System receivers over East and South Asia to examine the development of the irregularities in the nighttime on the day of the 21 June 2020 annular solar eclipse. The rate of TEC index (period <5 min) indicates the occurrence of the irregularities that evolve from the large or coarse structures with a period ranging from hours to dozens of minutes in the nighttime due to the eclipse. We take advantage of the data‐adaptive analysis method, Hilbert‐Huang transform, to derive the instantaneous amplitude and frequency of the TEC time series, which exposes the temporal and spatial evolutions of the irregularities from larger structures continuously. Plain Language Summary: The moon obscuration of a solar eclipse suddenly reduces solar irradiation in the daytime, which generates large atmospheric and ionospheric perturbations. The massive total electron content observations from the dense ground‐based Global Navigation Satellite System network recorded the occurrence of the large‐scale ionospheric perturbations and plasma irregularities in the nighttime on 21 June 2020. The occurrence of nighttime irregularities due to a solar eclipse was never known. This study further shows that the Hilbert‐Huang transform is an efficient technique to sift out the structures with large, coarse, or fine scales (Figure S1 in Supporting Information S1) from the total electron content time series. This data‐adaptive method benefits capturing the transient evolution of the ionospheric weather. Key Points: Development of ionospheric plasma irregularities after the solar eclipse in postsunset hoursEvolution of the irregularities from the convergence of large enhancementsHilbert‐Huang transform analysis sifts out the density structures with scales from coarse to fine [ABSTRACT FROM AUTHOR]

The two‐dimensional concentric traveling ionospheric disturbances (TIDs) over Japan on the landfall day of Typhoon Faxai in 2019 were detected in total electron content by the ground‐based Global Navigation Satellite System observations. They propagated in the radial direction away from the typhoon eye with a horizontal velocity of ∼190 m/s, a period of ∼24 min, and a horizontal wavelength of ∼180 km. Then, we systematically investigated the corresponding three‐dimensional ionospheric responses for the first time. Remarkable wavelike structures of ionospheric electron density (Ne) and inverted conic‐like structures concerning the disturbed Ne (dNe) were visualized in the F region. Furthermore, the descending phase progression of dNe with increasing time was detected at 220–550 km altitudes, which powerfully revealed that the typhoon‐related TIDs were dominated by the upward atmospheric gravity waves (AGWs) propagating from the troposphere into the ionospheric F2 layer. By analyzing the spatial and temporal evolutions of Ne and dNe, we postulated that the transport process of neutral wind fluctuations associated with the upward AGWs were likely responsible for generating TIDs during Typhoon Faxai. In the end, the background wind velocity was estimated based on the recovered vertical wavelength of ∼460 km and the dispersion equation of AGWs. Our results also demonstrated that the weak background wind condition is essential for gravity waves propagating in the ionosphere. Key Points: The wavelike structures of Ne in concentric traveling ionospheric disturbances (TIDs) caused by Typhoon Faxai are detected in the F regionThe distributions of dNe exhibit oblique alignments and downward phase progression in the vertical directionThe neutral wind fluctuations due to the upward atmospheric gravity waves are likely responsible for generating typhoon‐related TIDs [ABSTRACT FROM AUTHOR]

Latitudinal distances between the two equatorial ionization anomaly (EIA) crests in total electron content (TEC) of global ionosphere map (GIM) are used to derive the daily dynamo eastward electric field by comparing the eastward electric field calculated by ROCSAT‐1 vertical ion velocity in four seasons. The electric fields derived by the EIA crests of GIM TEC, ROCSAT‐1 ion velocity, and the incoherent scatter radar (ISR) observations at Jicamarca are generally in good agreement in longitudes and seasons. Meanwhile, the electric fields derived by the EIA crests agree well with those by ROCSAT‐1, FORMOSAT‐7/COSMIC‐2, and Jicamarca ISR on individual days. A long‐term study of the GIM‐TEC crest distance reveals obvious solar activity dependency of the daily dynamo electric field during 1999–2020. These agreements confirm that the EIA crest can be used to derive the daily eastward dynamo electric field and the associated vertical ion velocity which is responsible to the strength of equatorial plasma fountain and the latitudinal extend of the EIA. Moreover, the vertical ion velocity derived by the GIM‐TEC crest distance is further used to improve the SAMI2 model for space weather applications. Plain Language Summary: Equatorial ionization anomaly (EIA) is the most prominent feature in the ionosphere. During the morning, the eastward electric field induced by the tidal wind in the off‐equator E‐region activates the E × B plasma fountain and results in the prominent EIA crests on both sides of the magnetic equator. The global ionosphere map (GIM) routinely and globally displays the EIA crest of total electron content (TEC). Here, we show that the latitudinal distance of the GIM TEC crests between northern and southern hemispheres can be used to derive the daily eastward dynamo electric field and the associated vertical ion velocity in the equatorial ionosphere. Moreover, the derived vertical ion velocity is further used to improve ionosphere models for space weather applications. Key Points: Daily eastward dynamo electric fields in the equatorial ionosphere are derived by the distance of equatorial ionization anomaly (EIA) crests in global ionosphere mapsThe derived dynamo electric fields are used as input to ionosphere models to have a better understanding on the space weatherThe distance of EIA crests minimizes the trans‐equatorial wind effects in the estimation of daily dynamo electric fields [ABSTRACT FROM AUTHOR]

The sudden eruption of the Tonga underwater volcano (20.53°S, 175.38°W) on 15 January 2022 generated explosions that triggered blast waves traveling away from the eruption. In this study, the analysis of the geomagnetic field observations on the ground shows that the eruption perturbed the E‐region current density by 22–55 mA/m within a radius of 8,000 km away from the eruption. The perturbation evolved into large scales of ∼5 hr and thousands of kilometers as it traveled away. The traveling speed of the leading front is ∼740 m/s that is near acoustic in the ionosphere. The magnetic fields and total electron content observations suggest that the dynamics changes further induced significant ionospheric disturbances that lasted ∼10 hr after the eruption. The examination of the Tonga volcanic eruption inspires us that a near‐surface perturbation can change the dynamics of the upper atmosphere. Plain Language Summary: As we drop a stone into the still water, the water piles up around the rock and induces ripples in the form of concentric circles that expand and travel away from the center point. A similar phenomenon happened in the atmosphere after the eruption of the Tonga underwater volcano (20.53°S, 175.38°W) at ∼04:15 UT on 15 January 2022. The Earth's atmosphere is a pond of air. The explosive eruption triggered prosperous waves, which is similar to dropping a stone into water. The waves, ripples, and disturbances in different spheres are fantastic due to the eruption (Figure S8 in Supporting Information S1). Besides the tsunamis, atmospheric concentric ripples, and ionospheric traveling disturbances, this study for the first time showed that the blast wave due to the eruption changed the dynamics of the ionosphere or thermosphere (upper atmosphere). It is well known that a significant volcanic eruption can release tremendous ashes and gases into the troposphere and stratosphere (lower atmosphere), which change the chemical and dynamic process there. The observations in this study surprisingly show that the eruption changed the dynamics of the upper atmosphere, where the volcanic cloud cannot reach. Key Points: The eruption of the Tonga underwater volcano perturbed the E‐region current and, in turn, changed the geomagnetic field by ∼20–50 nTA large near‐surface perturbation can modulate the ionospheric dynamics and further induce ionospheric disturbancesThe significant ionospheric disturbances with periods ranging from several minutes to hours lasted more than 10 hr after the eruption [ABSTRACT FROM AUTHOR]

The explosive eruption of the Hunga‐Tonga volcano in the southwest Pacific at 0415UT on 15 January 2022 triggered gigantic atmospheric disturbances with surface air pressure waves propagating around the globe in Lamb mode. In space, concentric traveling ionosphere disturbances (CTIDs) are also observed as a manifestation of air pressure waves in New Zealand ∼0500UT and Australia ∼0630UT. As soon as the air pressure waves reached central Australia ∼0800UT, conjugate CTIDs appeared almost simultaneously in the northern hemispheres through interhemispheric coupling, much earlier than the arrival of the surface air pressure waves to Japan after 1100UT. Combining observations over Australia and Japan between 0800 and 1000UT, both direct and conjugate CTIDs show similar horizontal phase velocities of 320–390 m/s, matching with the dispersion relation of Lamb mode. The arrival of atmospheric Lamb wave to Japan later created in situ CTIDs showing the same Lamb mode characteristics as the earlier conjugate CTIDs. Plain Language Summary: The Hunga‐Tonga volcano eruption on 15 January 2022 has created the impulsive Lamb wave propagation on the surface air pressure that has been observed globally. The Lamb wave, typically moving near the ground at the sound speed of ∼340 m/s, traveled 6 hr to reach Japan. However, the Lamb wave signature in the Earth's ionosphere, the ionized component of the atmosphere, arrived 3‐hr earlier than expected, thanks to the property of the ionospheric plasma being controlled by the Earth's magnetic field. As soon as the surface Lamb wave reached Australia, the ionosphere above showed concentric wave shape of the traveling ionospheric disturbances (CTIDs), and the effect was mapped to Japan in the northern hemisphere along the conductive magnetic field lines. The much faster appearance of the ionosphere disturbances to the northern hemisphere through the magnetic field lines is beyond expectations. This is the first time such concentric waves from a volcanic eruption have been observed simultaneously in both hemispheres. Monitoring the ionospheric disturbances could be a robust early warning tool for diagnosing such explosive events on the planet Earth. Key Points: Concentric traveling ionospheric disturbances (CTIDs) driven by volcano excited Lamb wave are observed simultaneously in Australia and JapanDue to magnetic conjugate effect, the northern hemisphere CTIDs appear 3‐hr prior to the arrival of the surface Lamb waveBoth direct and conjugate CTIDs match with the theoretical dispersion relation of the atmospheric Lamb and gravity modes [ABSTRACT FROM AUTHOR]

Geostationary BeiDou satellites monitor the total electron content (TEC) in the ionosphere over certain locations 24 hr per day without interruption and act as ionosphere‐based seismometers. The system detected perturbations in TEC before both the M6.1 Dali and M7.3 Qinghai earthquakes that occurred during the night of 21–22 May 2021. The TEC perturbations reside mainly over an area within a distance of ∼700 km from the epicenters of the earthquakes. The standing waves revealed the persistence of a subsurface wave source before the occurrences of the earthquakes, which differs from the co‐seismic ionospheric distributions propagating away from the epicenters. The resident waves in TEC and ground vibrations share a frequency of ∼0.004 Hz, which can be attributed to the resonant coupling between the lithosphere and ionosphere. Plain Language Summary: Large earthquakes seriously threat human life and induce economic loss. However, predicting an earthquake remains as a huge challenge due to the lack of reliable precursor signals. Something must be happened before a large earthquake. Therefore, we are looking for the reliable geophysical parameters for identifying the precursor that human beings are desired. In this study, the ionospheric total electron content (TEC) from the geostationary satellites of the Beidou global navigation satellite system system recorded an unusual phenomenon of TEC perturbations resides in a particular area that appeared after mid‐May in the post‐sunset hours over China. The unusual phenomenon is significantly different from the morphologies of the traveling ionospheric disturbances and plasma bubbles over Asia in summer. The TEC perturbations reside over both two areas within a distance of ∼700 km where cover the epicenters of the M6.1 Dali and M7.3 Qinghai earthquakes occurred during the night of 21–22 May 2021. The perturbations in TEC and ground vibrations share a frequency of ∼0.004 Hz, which can be attributed to the resonant coupling between the lithosphere and ionosphere during the earthquakes. Key Points: Ground vibrations and total electron content (TEC) perturbations share a frequency of ∼4 mHzGeostationary BeiDou satellites detected TEC wave prior to large earthquakesTEC perturbations resided persistently around the epicenters [ABSTRACT FROM AUTHOR]

We investigate ionospheric disturbances using the total electron content (TEC) data obtained from the radio signals emitted from three satellites after the foreshock of the 2011 Tohoku Earthquake on March 9, 2011. Co‐seismic ionospheric disturbances (CIDs) appeared to extend from an onset point concentrically in all of the satellite data. We have found, however, that the geographic coordinates of the onset points did not coincide if the observed CIDs were assumed to occur at one altitude. Admitting that the altitudes of the onset points are different, we searched for coinciding geographic coordinates of the onset points by changing the altitudes and identified the altitude of the three onset points to be 107.8, 131.8, and 133.3 km and the onset time at these altitudes. As a result, the vertical velocity of an acoustic‐gravity wave is estimated to be 448 m/s from the travel time between the altitudes of 107.8 and 131.8 km and 370 m/s between the altitude of 107.8 and 133.3 km. This is comparable to the speed of sound calculated using the empirical model NRLMSISE‐00. The present study provides a method of determining the altitude and geographic coordinates of the onset point from the ionospheric TEC analysis independently of the seismic data. Furthermore, the source location of the acoustic‐gravity wave is simultaneously determined because its wavefront propagating vertically arrives earliest at an altitude that TEC disturbance is detectable. Key Points: We propose a new method of determining the location of ionospheric disturbance detected in total electron content (TEC) independent of the seismic dataThe ionospheric disturbances in TEC were found to be below the F2‐peak at the foreshock of the 2011 Tohoku Earthquake on March 9, 2011Source location of an acoustic‐gravity wave can be determined by analyzing TEC disturbances [ABSTRACT FROM AUTHOR]

Geomagnetic pulsations with an enhanced amplitude of 0.1–1 nT in a frequency band of 0.1–0.2 Hz were observed by three three‐component magnetometers. The geomagnetic pulsations exhibit slight time difference that is unusually observed within a limited area of ∼5 × 5 km2 at the northern end of Taiwan. The time difference suggests the geomagnetic pulsations are not dominated by global effects but local ones. We compare the seismic signals recorded by the cosited broadband seismometers for investigating potential mechanisms of these geomagnetic pulsations. Results from analysis of both seismic and geomagnetic data exhibit consistent source azimuths from the Pacific Ocean to the northeast of the observation sites with a wave propagation speed of ∼2.9 km/s estimated based on P‐SV‐type microseisms. Polarization and particle motion analyses demonstrate that the geomagnetic pulsations move with an elliptical rotation form, showing a close association with the P‐SV‐type microseisms. The 90° phase difference between the horizontal and vertical components of the geomagnetic signals suggests that the time‐lagged geomagnetic pulsations are possibly caused by the motional induction effect after the arrivals of P‐SV‐type microseisms. Key Points: Time‐lagged magnetic pulsations ranged at 0.1–0.2 Hz are observed in a limited areaElliptical particle motion suggests dominant P‐SV‐type microseismsTime‐lagged magnetic pulsations can be generated by the motional induction effect [ABSTRACT FROM AUTHOR]

The ionospheric total electron content (TEC) derived from dense ground‐based Global Navigation Satellite System receivers over the continental United States and those from global ionosphere maps are utilized to find the ionosphere response to the August 21, 2017 total solar eclipse. Maximum obscurations and their associated TEC major depressions appear simultaneously at midlatitudes, while major depressions elongate toward the magnetic equator with some delays in the equatorial ionization anomaly (EIA) region. The former is due to the photochemical loss process, while the latter is caused by the plasma transport of E×B drifts and lunar gravitation forces. TECs of predawn reductions, morning enhancements, afternoon reductions, and nighttime enhancements reveal that the semidiurnal lunar tide are essential. Since a solar eclipse always occurs on a new moon day, the lunar tide results in the early EIA appearance and major depressions being underestimated/diminished before and overestimated/enhanced after about 14:00 local time. Plain Language Summary: Observations during an eclipse offer a special opportunity for studying the Earth's ionospheric response to changes in solar ionizing radiation. Although ionospheric solar eclipse signatures in many events have been studied, we report the lunar tide effect on the signatures for the first time. A total solar eclipse swept across the continental United States (CONUS) from the west to east coast on August 21, 2017. The total electron content (TEC) along seven longitudes of the US continent derived from more than 2,200 ground‐based Global Navigation Satellite System receivers in the CONUS and extracted from global ionosphere maps (GIMs) is employed to study ionospheric solar eclipse signatures. The most prominent solar eclipse signature is major depressions (MDs) in both the CONUS and GIM TECs, when the maximum obscuration occurs. TEC extrema of predawn reductions, morning enhancements, afternoon reductions, and nighttime enhancements reveal that the semidiurnal lunar tide of about 12.42‐h period is prominent. The lunar tide causes the early appearance of equatorial ionization anomalies, which further results in the MDs being weakened in the morning and enhanced in the afternoon on the solar eclipse day. The lunar tide has to be taken into consideration for studying ionospheric solar eclipse effects. Key Points: The lunar tide results in the early appearance of the equatorial ionization anomaly on solar eclipse daysTotal electron contents tend to enhance before and decrease after 14:00 local time (LT) on a solar eclipse dayThe lunar tide causes major depressions being weakened before and enhanced after 14:00 LT [ABSTRACT FROM AUTHOR]

The FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission was launched on 25 June 2019 with six low‐Earth‐orbit satellites and can provide thousands of daily radio occultation (RO) soundings in the low‐latitude and midlatitude regions. This study shows the preliminary results of space weather data products based on F7/C2 RO sounding: global ionospheric specification (GIS) electron density and Ne‐aided Abel and Abel electron density profiles. GIS is the ionospheric data assimilation product based on the Gauss‐Markov Kalman filter, assimilating the ground‐based Global Positioning System and space‐based F7/C2 RO slant total electron content, providing continuous global three‐dimensional electron density distribution. The Ne‐aided Abel inversion implements four‐dimensional climatological electron density constructed from previous RO observations, which has the advantage of providing altitudinal information on the horizontal gradient to reduce the retrieval error due to the spherical symmetry assumption of the Abel inversion. The comparisons show that climatological structures are consistent with each other above 300 km altitude. Both the Abel electron density profiles and GIS detect electron density variations during a minor geomagnetic storm that occurred within the study period. Moreover, GIS is further capable of reconstructing the variation of equatorial ionization anomaly crests. Detailed validations of all the three products are carried out using manually scaled digisonde NmF2 (hmF2), yielding correlation coefficients of 0.885 (0.885) for both Abel inversions and 0.903 (0.862) for GIS. The results show that both GIS and Ne‐aided Abel are reliable products in studying ionosphere climatology, with the additional advantage of GIS for space weather research and day‐to‐day variations. Plain Language Summary: This study presents two ionosphere products from the innovative satellite constellation mission launched recently. Global ionospheric specification is an ionospheric data product that assimilates ground‐based Global Positioning System and FORMOSAT‐7/COSMIC‐2 radio occultation observation of total electron content, to generate hourly global three‐dimensional electron density for monitoring space weather condition. Ne‐aided Abel electron density profile is an improved retrieval product of FORMOSAT‐7/COSMIC‐2 radio occultation observations by imposing asymmetry information of ionosphere to mitigate the error introduced by the assumption of spherical symmetry in the Abel inversion. The comparisons and validations confirm that these two data products are reliable for the study of ionosphere climatology and weather. They are operationally produced and released at Taiwan Analysis Center for COSMIC. Key Points: All the three F7/C2 products capture similar climatological structure of ionosphere in longitudes (Wave 4) and latitudes (EIA crests)Abel electron density profiles detect responses to geomagnetic storm, but GIS performs better in reconstructing the EIA crests variationsDigisonde validations demonstrate that the GIS NmF2 has excellent performance when there are RO observations available for assimilation [ABSTRACT FROM AUTHOR]

The strength, appearance time, and latitudinal location of the equatorial ionization anomaly (EIA) crest response to the lunar phase are examined using the total electron content of global ionosphere maps during the 18‐year period of 2000–2017. The total electron content of the EIA crest reveals semidiurnal tides with 12.42‐hr period, while the appearance time and latitude of the EIA crests present prominent semimonthly lunar tides of 14.77‐day period. The EIA crests on new/full moon (first/third quarter) lead (lag) those of the overall 18‐year average by about 20–40 min, while the EIA crests move the furthest poleward and equatorward 2–5 days after new/full moon and first/third quarter, respectively. Amplitudes of the semimonthly lunar tide in the EIA crest appearance times and latitudes yield the greatest value around perihelion and the smallest value around aphelion, which shows that the solar declination and seasonal effects are important. Plain Language Summary: The most prominent feature in the ionosphere is the equatorial ionization anomaly (EIA), which is characterized by two enhanced plasma crests at low latitudes straddling the magnetic equator. Global ionosphere maps of total electron content are analyzed to see how the ionospheric EIA crests respond to changes in lunar phases and solar declinations. The results show that the EIA crests exhibit prominent semimonthly lunar tides with 14.77‐day period. Appearance times of the EIA crests on new/full moon (first/third quarter) lead (lag) those on the associated overall 18‐year average by about 20–40 min, while the EIA crests move the furthest poleward during new/full moon and equatorward during first/third quarter with the delays of about 2–5 days. These indicate that the lunar phase can significantly modulate the ionospheric EIA. The semimonthly lunar tide in the crest appearance times and latitudes become most prominent around perihelion but the least around aphelion, which shows that the solar declination and seasonal effects are important. Key Points: Appearance times of the EIA crests on new/full moon lead those on first/third quarterThe EIA crests move most poleward (equatorward) 2–5 days after new/full moon (first/third quarter)The semimonthly lunar tide of the EIA crest becomes most prominent around perihelion [ABSTRACT FROM AUTHOR]

Abstract: Global Navigation Satellite System radio occultation signals often show extremely strong levels of scintillation when passing through the ionospheric E region. This is related to sporadic E(Es)—dense layers of metallic ions that can form in the E region, influencing terrestrial and satellite radio propagation. In our report on the 2007–2014 variation of E region S4 amplitude fluctuation indices measured by the FORMOSAT‐3/Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) satellite constellation, we find that the spatial and temporal variation of the maximum S4 index in the E region is proportionate to the occurrence rate of extreme scintillation and by extension, sporadic E. We also find that the monthly median extreme S4 amplitude fluctuation index in the E region midlatitudes shows a dependence on variation of the El Niño–Southern Oscillation (ENSO) in the troposphere that has not been previously reported. The ENSO‐related variation of the E region median extreme S4 indices varies closely with the tropopause height, with both parameters lagging the Oceanic Niño Index by roughly 1 to 2 months, while also displaying a similar spectrum of periodicities. This similarity is especially strong in the southern midlatitudes. These results indicate that ENSO signatures can be transmitted to Es formation mechanisms, potentially through modulation of vertically propagating atmospheric tides that alter lower thermospheric wind shears. The end result is the modulation of the interannual variation of extreme Es values by ENSO. [ABSTRACT FROM AUTHOR]

Abstract: The launch of SpaceX Falcon 9 rocket delivered Taiwan's FORMOSAT‐5 satellite to orbit from Vandenberg Air Force Base in California at 18:51:00 UT on 24 August 2017. To facilitate the delivery of FORMOSAT‐5 to its mission orbit altitude of ~720 km, the Falcon 9 made a steep initial ascent. During the launch, the supersonic rocket induced gigantic circular shock acoustic waves (SAWs) in total electron content (TEC) over the western United States beginning approximately 5 min after the liftoff. The circular SAWs emanated outward with ~20 min duration, horizontal phase velocities of ~629–726 m/s, horizontal wavelengths of ~390–450 km, and period of ~10.28 ± 1 min. This is the largest rocket‐induced circular SAWs on record, extending approximately 114–128°W in longitude and 26–39°N in latitude (~1,500 km in diameter), and was due to the unique, nearly vertical attitude of the rocket during orbit insertion. The rocket‐exhaust plume subsequently created a large‐scale ionospheric plasma hole (~900 km in diameter) with 10–70% TEC depletions in comparison with the reference days. While the circular SAWs, with a relatively small amplitude of TEC fluctuations, likely did not introduce range errors into the Global Navigation Satellite Systems navigation and positioning system, the subsequent ionospheric plasma hole, on the other hand, could have caused spatial gradients in the ionospheric plasma potentially leading to a range error of ~1 m. [ABSTRACT FROM AUTHOR]

Abstract: A moon shadow of the total solar eclipse swept through the continent of United States (CONUS) from west to east on 21 August 2017. Massive total electron content (integration of electron density from 0 km to 20,200 km altitude) observations from 2,255 ground‐based Global Navigation Satellite System receivers show that the moon shadow ship generates a great ionospheric bow wave front which extends ~1,500 km away from the totality path covering the entire CONUS. The bow wave front consists of the acoustic shock wave due to the supersonic/near‐supersonic moon shadow ship and the significant plasma recombination due to the reduction in solar irradiation within the shadow area. The deep bow wave trough (−0.02 total electron content unit (1 TECU = 1016 el m−2) area) nearly coincides with the 100% obscuration moving along the totality path over the CONUS through the entire eclipse period. The supersonic moon shadow ship induces a bow wave crest in front of the ship (~80% obscuration). It is the first time to find the acoustic shock wave‐formed bow wave trough and crest near the totality. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we present a comprehensive study of occurrence of L‐band scintillation in association with the appearance of sporadic E (Es) along the magnetic dip equator during daytime in 2013. The presence of L‐band scintillation was determined from signals collected with GNSS (Global Navigation Satellite Systems) ground‐based Scintillation Network Decision Aid receivers from five stations situated at the magnetic dip equator. The detection and analysis of Es layers were obtained from GNSS FORMOSAT‐3/Constellation Observing System for Meteorology, Ionosphere, and Climate (F3/C) radio occultation (RO) data. Combining ground‐based data with the limb‐viewing geometry from space provides a unique opportunity to retrieve complementary information about scintillation and association with equatorial E region irregularities (i.e., Es) during daytime. Results for the first time show that daytime scintillation does occur at the magnetic dip equator and the occurrence is associated with the appearance of Es observed using GNSS F3/C RO data. [ABSTRACT FROM AUTHOR]

An obvious magnetic disturbance has been repeatedly observed by magnetometers located outside of urban areas in Yangmingshan National Park and has affected the monitoring of volcanic activity in the northern region of Taiwan. The disturbance has amplitudes of approximately 3-5 nanoteslas, and regularly appears for approximately 21 hours per day, except during the New Year period, when it is observed continuously for 45 hours. This change coincides with the annual extended operation of the Mass Rapid Transit ( MRT) system. A 750-volt direct current drives the MRT trains and leaks to the ground, with a maximum value of approximately 1000 amperes measured along the routes. The disturbance is dominated by the unbalanced supply and return currents resulting from the leakage. The daily changes in the magnetic noise levels are correlated with the electricity consumption of the MRT system because the leakage is proportional to the use of electric power. [ABSTRACT FROM AUTHOR]

This study examined the FORMOSAT‐3/COSMIC radio occultation soundings of total electron content in the ionosphere (from 150 to 550 km) and refractivity index in the lower atmosphere (from 0 to 60 km) after/during the 2011 Mw9.0 Tohoku earthquake/tsunami. The refractivity index observations show the first evidence of the Tohoku earthquake/tsunami‐induced vertical oscillations in the lower atmosphere. The atmospheric oscillations with vertical wavelength ranging from 0.5 to 8 km and from 10 to 40 km, respectively, appear in the stratosphere and ionosphere after the earthquake onset. The short vertical wavelengths suggest that the oscillations are the atmospheric oscillatory tail due to the wavefront of the earthquake/tsunami. The radio occultation technique is a tool for detecting the atmospheric waves induced by earthquake or some other sources. Plain Language Summary: It has been well known that the sudden uplift of the Earth's surface due to earthquake/tsunami significantly disturbs the upper atmosphere and ionosphere. However, the evidence of earthquake/tsunami‐induced waves in the lower atmosphere is insufficient due to the lack of observation supporting. Here the FORMOSAT‐3/COSMIC (F3/C) radio occultation refractivity index is for the first time to capture the Tohoku earthquake‐induced vertical oscillations at the stratosphere altitudes from 30 to 40 km. The results in this study not only shed lights on understanding of earthquake/tsunami waves perturbing the whole atmosphere but also suggest that the radio occultation technique can benefit to the earthquake/tsunami warning system. Key Points: The study shows the first evidence of the Tohoku earthquake/tsunami‐induced vertical oscillations in the stratosphereAtmospheric oscillations with vertical scale range from 10 to 40 km and from 0.5 to 8 km, respectively, in the ionosphere and stratosphereThe oscillations are mainly the atmospheric oscillatory tail due to the tsunami [ABSTRACT FROM AUTHOR]