We have detected and characterized traveling ionospheric disturbances (TIDs) in the mid-latitude ionosphere over the European region using Kharkiv incoherent scatter (IS) radar data acquired near solstices and equinoxes during solar cycle 24 and under magnetically quiet conditions. We analyzed the diurnal, seasonal and solar activity dependence of both large-scale (LS) and medium-scale (MS) TIDs. We studied 140 TID events and estimated their energetic (relative amplitudes), temporal (dominant periods, frequency of occurrence), and spatial (heights of maximum relative amplitudes, vertical and horizontal phase velocities and wavelengths) characteristics. We suggest that the most probable generation mechanisms of magnetically quiet LS and MS TIDs are solar terminators passages, enhanced gravity wave activity and dissipation, severe tropospheric convection, coupling processes between Es layer and F region, polarization electric fields, and Perkins’s instability. We have shown a positive correlation between the heights of the maximum relative TID amplitudes and the solar activity for LSTIDs. The TID characteristics obtained during extremely quiet conditions provide a useful context for analysing the possible TID response to localized energy releases including those of anthropogenic origin, and enable improvements in global and regional ionospheric models by clarifying the contribution of wave processes to the overall energy budget of the atmosphere and ionosphere.

The continental United States is well instrumented with facilities for mid-latitude upper atmosphere research that operate on a continuous basis. In addition, citizen scientists provide a wealth of information when unusual events occur. We combine ionospheric total electron content (TEC) data from distributed arrays of GNSS receivers, magnetometer chains, and auroral observations obtained by citizen scientists, to provide a detailed view of the intense auroral breakup and westward surge occurring at the peak of the 10-11 May 2024 extreme geomagnetic storm. Over a 20-min interval, vertical TEC (vTEC) increased at unusually low latitude (~45°) and rapidly expanded azimuthally across the continent. Individual receiver/satellite data sets indicate sharp bursts of greatly elevated of vTEC (~50 TECu). Intense red aurora was co-located with the leading edge of the equatorward and westward TEC enhancements, indicating that the large TEC enhancement was created by extremely intense low-energy precipitation during the rapid substorm breakup.

Volcanic eruptions provide broad spectral forcing to the atmosphere and many previous studies have examined the IT disturbances caused by volcanic eruptions through both observations and modeling. Understanding the primary mechanisms that are relevant to explain the variety in waveform characteristics is still an important open question for the community. In this study, Global Navigation Satellite System (GNSS) Total Electron Content (TEC) data are analyzed and compared to simulations performed by the Global Ionosphere-Thermosphere Model with Local Mesh Refinement (GITM-R) for the first phase of the 2015 Calbuco eruption that occurred on 22 April. A simplified source representation and spectral acoustic-gravity wave (AGW) propagation model are used to specify the perturbation at the lower boundary of GITM-R at 100 km altitude. This modeling specification shows a good agreement with GNSS observations for some waveform characteristics such as travel/onset times and relative magnitudes. Most notably, GITM-R is able to reproduce the significance of AGWs as a function of radial distance from the vent, showing acoustic dominant forcing in the near field (<500 km) and gravity dominant forcing in the far-field (>500 km). The estimated apparent phase speeds from GITM-R simulations are consistent with observations with ~10% difference from observation for both acoustic wave packets and a trailing gravity mode. Relevance of the simplifications made in the lower atmosphere are then discussed and test changes to the assumed propagation structure, from direct propagation to ground-coupled propagation, show some improvement to the data-model comparison, especially the second acoustic wave-packet

Specification and forecast ionospheric parameters, such as ionospheric electron density (Ne), have been an important topic in space weather and ionosphere research. Neural networks (NNs) emerge as a powerful modeling tool for Ne prediction. However, heavy manual attention costs time to determine the optimal NN structures. In this work, we propose to use neural architecture search (NAS), an automatic machine learning method, to address this problem of NN models. NAS aims to find the optimal network structure through the alternated optimization of the hyperparameters and the corresponding network parameters. A total of 16-year data from Millstone Hill incoherent scatter radar (ISR) are used for NN models. One single-layer NN (SLNN) model and one deep NN (DNN) model are trained with NAS, namely SLNN-NAS and DNN-NAS, for Ne prediction and compared with their counterparts without NAS from previous studies, denoted as SLNN and DNN. Our results show that SLNN-NAS and DNN-NAS outperformed SLNN and DNN, respectively. NN models can reveal more finer details than the empirical ionospheric model developed using traditional data fitting approaches. DNN-NAS yields the best prediction accuracy measured by quantitative metrics and rankings of daily pattern prediction. The limited improvement of NAS is likely due to the network complexity and the limitation of fully connected NN without a memory mechanism.

This study reports a peculiar nighttime ionospheric enhancement in the total electron content (TEC) from the geostationary (GEO) satellites in response to the geomagnetic superstorm of May 2024. The enhancements occurred at low to mid-latitudes during the storm’s recovery phase on 11-12 May, with the TEC values almost twice as high as the quiet ones. Surprisingly, the nighttime ionospheric enhancements sub-rotated westward with a speed of ~130.5 m/s. The nighttime TEC enhancements lasted ~5-7 hours for a given location; however, they persisted over half a day over wide longitude ranges. Meanwhile, ionosonde data from two equatorial ionization anomaly (EIA) stations showed a higher peak height of the F2 layer, and a significant double-crest EIA structure was observed by both GEO TECs and in situ electron densities from China Seismo-Electromagnetic Satellite during this period of interest, indicating a possible contribution from disturbance electric fields producing nighttime eastward equatorial electric fields. Nevertheless, it remains a mystery whether the nighttime TEC enhancement and its movement were associated with the wind disturbance dynamo.

The high latitude ionospheric evolution of the May 10-11, 2024, geomagnetic storm is investigated in terms of Total Electron Content and contextualized with Incoherent Scatter Radar and ionosonde observations. Substantial plasma lifting is observed within the initial Storm Enhanced Density plume with ionospheric peak heights increasing by 150km to 300km, reaching levels of up to 630km. Scintillation is observed within the cusp during the initial expansion phase of the storm, spreading across the auroral oval thereafter. Patch transport into the polar cap produces broad regions of scintillation that are rapidly cleared from the region after a strong Interplanetary Magnetic Field reversal at 2230UT. Strong heating and composition changes result in the complete absence of the F2-layer on the 11th, suffocating high latitude convection from dense plasma necessary for Tongue of Ionization and patch formation, ultimately resulting in a suppression of polar cap scintillation on the 11th.

Upper atmospheric long-term trends could be examined in the ion temperatures ($T_i$) at the ionospheric F-region altitudes by the close coupling between neutrals and ions. We have analyzed the $T_i$ data sets of Arecibo Observatory (AO) incoherent scatter radar (18\textdegree20’N, 66\textdegree45’W) from 1985 to 2019, to examine the long-term trends of the ion temperature as a function of height from $\sim$140 km to $\sim$677 km. For this, the responses of $T_i$ to solar and geomagnetic activities have been taken into account as forcings of the $T_i$ behavior as well the annual and semi-annual oscillations. By removing the known forcing that govern the Ti behavior by the difference between the $T_i$ data and a climatological model, our results indicate that the upper atmosphere/ionosphere over Arecibo is cooling over the 35 years studied. Around 350 km, our findings also show that the rate of cooling over Arecibo is lower than previously reported for high latitudes, suggesting a latitudinal dependency. These cooling trends are believed to be the result of increasing green house gases, but the observed cooling trends exceed the magnitude of the cooling expected from green house gases. We have made an attempt to find the additional driver for observed cooling trends by linking the these upper atmospheric trends to lower atmospheric weather phenomena. We found that gravity waves in the lower atmosphere associated with terrestrial weather phenomena might be contributing to the observed cooling trends in the upper atmosphere.

The Tonga volcano eruption at 04:14:45 UT on 2022-01-15 released enormous amounts of energy into the atmosphere, triggering very significant geophysical variations not only in the immediate proximity of the epicenter but also globally across the whole atmosphere. This study provides a global picture of ionospheric disturbances over an extended period for at least four days. We find traveling ionospheric disturbances (TIDs) radially outbound and inbound along entire Great-Circle loci at primary speeds of ~300-350 m/s (depending on the propagation direction) and 500-1000 km horizontal wavelength for front shocks, going around the globe for three times, passing six times over the continental US in 100 hours since the eruption. TIDs following the shock fronts developed for ~8 hours with 10-30 min predominant periods in near- and far- fields. TID global propagation is consistent with the effect of Lamb waves which travel at the speed of sound. Although these oscillations are often confined to the troposphere, Lamb wave energy is known to leak into the thermosphere through channels of atmospheric resonance at acoustic and gravity wave frequencies, carrying substantial wave amplitudes at high altitudes. Prevailing Lamb waves have been reported in the literature as atmospheric responses to the gigantic Krakatoa eruption in 1883 and other geohazards. This study provides substantial first evidence of their long-duration imprints up in the global ionosphere. This study was enabled by ionospheric measurements from 5,000+ world-wide Global Navigation Satellite System (GNSS) ground receivers, demonstrating the broad implication of the ionosphere measurement as a sensitive detector for atmospheric waves and geophysical disturbances.

This paper investigates the local and global ionospheric responses to the 2022 Tonga volcano eruption, using ground-based Global Navigation Satellite System (GNSS) total electron content (TEC), Swarm in-situ plasma density measurements, the Ionospheric Connection Explorer (ICON) Ion Velocity Meter (IVM) data, and ionosonde measurements. The main results are as follows: (1) A significant local ionospheric hole of more than 10 TECU depletion was observed near the epicenter ~45~min after the eruption, comprising of several cascading TEC decreases and quasi-periodic oscillations. Such a deep local plasma hole was also observed by space-borne in-situ measurements, with an estimated horizontal radius of 10-15 deg and persisted for more than 10 hours in ICON-IVM ion density profiles until local sunrise. (2) Pronounced post-volcanic evening equatorial plasma bubbles (EPBs) were continuously observed across the wide Asia-Oceania area after the arrival of volcano-induced waves; these caused a Ne decrease of 2-3 orders of magnitude at Swarm/ICON altitude between 450-575~km, covered wide longitudinal ranges of more than 140 deg and lasted around 12 hours. (3) Various acoustic-gravity wave modes due to volcano eruption were observed by accurate Beidou geostationary orbit (GEO) TEC, and the huge ionospheric hole was mainly caused by intense shock-acoustic impulses. TEC rate of change index revealed globally propagating ionospheric disturbances at a prevailing Lamb-wave mode of ~315 m/s; the large-scale EPBs could be seeded by acoustic-gravity resonance and coupling to less-damped Lamb waves, under a favorable condition of volcano-induced enhancement of dusktime plasma upward ExB drift and postsunset rise of the equatorial ionospheric F-layer.

Travelling ionospheric disturbances (TIDs) and their neutral counterparts known as travelling atmospheric disturbances (TADs) are believed to play a central role in redistributing energy and momentum in the upper atmosphere and communicating inputs to other locations in the fluid. While these two phenomena are believed to be connected, they may not have a one-to-one correspondence as the geomagnetic field influences the TID but has no direct impact on the TAD. The relative amplitudes of the perturbations seen in the ionosphere and atmosphere have been observed but rarely together. This study reports results from a three-day campaign to observe TIDs and TADs simultaneously over a broad latitudinal region over the eastern United States using a combination of GOLD and a distributed network of ground based Global Navigation Satellite System (GNSS) receivers. These results demonstrate that GOLD and the ground-based total electron content (TEC) observations can see the atmospheric and ionospheric portions of a large-scale travelling disturbance. The phase difference in the perturbations to the GOLD airglow brightness, O/N2 and thermospheric disk temperature are consistent with an atmospheric gravity wave moving through this region. The ionospheric signatures move at the same rate as those in the atmosphere, but their amplitudes do not have a simple correspondence to the amplitude of the signal seen in the atmosphere. This campaign demonstrates a proof-of-concept that this combination of observations is able to provide information on TIDs and TADs, including quantifying their impact on the temperature and chemical composition of the upper atmosphere.

This work conducts a statistical study of the subauroral polarization stream (SAPS) feature in the North American sector using Millstone Hill incoherent scatter radar measurements from 1979 to 2019, which provides a comprehensive SAPS climatology using a significantly larger database of radar observations than was used in seminal earlier works. Key features of SAPS and associated Ne/Ti/Te are investigated using a superposed epoch analysis method. The characteristics of these parameters are investigated with respect to magnetic local time, season, geomagnetic activity, solar activity, and interplanetary magnetic field orientation, respectively. The main results are as follows: (1) Conditions for SAPS are more favorable for dusk than near midnight, for winter compared to summer, for active geomagnetic periods compared to quiet time, for solar minimum compared to solar maximum, and for IMF conditions with negative By and negative Bz. (2) SAPS is usually associated with a midlatitude trough of 15–20\% depletion in the background density. The SAPS-related trough is more pronounced in the postmidnight sector and near the equinoxes. (3) Subauroral ion and electron temperatures exhibit a 3–8\% (50–120 K) enhancement in SAPS regions, which tend to have higher percentage enhancement during geomagnetically active periods and at midnight. Ion temperature enhancements are more favored during low solar activity periods, while the electron temperature enhancement remains almost constant as a function of the solar cycle. (4) The electron thermal content, Te \times Ne, in the SAPS associated region is strongly dependent on 1/Ne, with Te exhibiting a negative correlation with respect to $Ne$.

We present a new high resolution empirical model for the ionospheric total electron content (TEC). TEC data are obtained from the global navigation satellite system (GNSS) receivers with a 1 x 1 spatial resolution and 5 minute temporal resolution. The linear regression model is developed at 45N, 0E for the years 2000 - 2019 with 30 minute temporal resolution, unprecedented for typical empirical ionospheric models. The model describes dependency of TEC on solar flux, season, geomagnetic activity, and local time. Parameters describing solar and geomagnetic activity are evaluated. In particular, several options for solar flux input to the model are compared, including the traditionally used 10.7cm solar radio flux (F10.7), the Mg II core-to-wing ratio, and formulations of the solar extreme ultraviolet flux (EUV). Ultimately, the extreme ultraviolet flux presented by the Flare Irradiance Spectral Model, integrated from 0.05 to 105.05 nm, best represents the solar flux input to the model. TEC time delays to this solar parameter on the order of several days as well as seasonal modulation of the solar flux terms are included. The Ap_3 index and its history are used to reflect the influence of geomagnetic activity. The root mean squared error of the model (relative to the mean TEC observed in the 30-min window) is 1.9539 TECu. A validation of this model for the first three months of 2020 shows excellent agreement with data. The new model shows significant improvement over the International Reference Ionosphere 2016 (IRI-2016) when the two are compared during 2008 and 2012.