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Objects in Low-Earth Orbit (LEO) experience a range of environmental conditions that influence their trajectories. Aside from gravitational effects and solar radiation pressure, drag due to the residual background atmosphere is conventionally viewed as the primary perturbing factor. The ionosphere is an additional background environment composed of charged particles that is well-known to result in spacecraft charging, and in the worst case, cause arcing or unwanted electrostatic discharges. While the ionospheric plasma density is typically an order of magnitude (or more) lower than the atmospheric neutral gas density, electrostatic charging can lead to the formation of plasma sheath and wake structures around an object that artificially increase its effective collecting area. Direct charged particle collection and indirect charged particle deflection gives rise to a drag force that can exceed that due to atmospheric neutral gas alone at some altitudes and charging potentials. Here, we present a model accounting for charged particle flow effects (i.e. charged aerodynamics) around objects in LEO, and validate this model with previous particle-in-cell simulations and experiments. Using atmospheric properties from the Mass Spectrometer and Incoherent Scatter radar (MSIS) model and ionospheric properties from the International Reference Ionosphere (IRI), we show that plasma-induced drag in LEO can be significant and may have important orbit prediction implications for space domain awareness and space traffic management. Through differential spacecraft biasing, ionospheric plasma drag can also be used as a propellantless and "solid-state" mechanism to achieve in-orbit mobility for precision maneuvers, formation flying, deorbiting, and even partial attitude control. [Display omitted] • Charging in the ionosphere generates electrostatic fields around spacecraft. • Collection/deflection of charged particles produces a drag force. • Sheath expansion increases the effective collecting area, amplifying plasma drag. • Plasma drag is an additional environmental disturbance affecting spacecraft motion. • Plasma drag can be exploited as a propellantless propulsion mechanism. [ABSTRACT FROM AUTHOR]

Ionospheric equatorial plasma bubbles (EPBs) are usually generated around sunset over equatorial to low latitudes. In this study, super EPBs extending to middle latitudes were observed, which were freshly generated at both post‐sunset and near‐sunrise periods over East/Southeast Asia during the geomagnetic storm on 26–28 February 2023. The post‐sunset (near‐sunrise) EPB persisted ∼4 (8) hours, drifting eastward (westward) and extending up to ∼35°N. Strong L‐band scintillations, deep total electron content (TEC) depletions and significant positioning errors were caused by the post‐sunset EPB. However, the scintillations and TEC depletions linked with the near‐sunrise EPB were relatively weaker, and no apparent positioning error was caused. Before the onsets of the post‐sunset EPBs, rapid upward vertical plasma drifts of ∼60 m/s, which could be linked with storm‐time prompt penetration electric fields, were observed at magnetic equator. The upward vertical drifts could amplify the F‐region bottomside perturbation via increasing the growth rate of Rayleigh‐Taylor (R‐T) instability and lead to the generation of post‐sunset EPBs. On the other hand, before the onset of near‐sunrise EPBs, the uplift of F layer was more significant at higher latitudes than that at magnetic equator, probably indicating the presence of equatorward neutral wind. Both the equatorial F layer uplift and equatorward neutral wind could contribute to the growth of R‐T instability and favor the generation of near‐sunrise EPBs. Plain Language Summary: Equatorial plasma bubble (EPB) is a kind of ionospheric irregularity which could cover a large area up to thousands of kilometers and cause strong radio signal scintillation and positioning error. It is usually generated at post‐sunset hours when the pre‐reversal enhancement of the eastward electric field could increase the growth rate of the Rayleigh‐Taylor instability, and observed at equatorial and low latitudes. Here we report unusual cases of super EPBs extending to middle latitudes, which were freshly generated at both post‐sunset and near‐sunrise periods during the geomagnetic storm on 26–28 February 2023. Based on the observations by multiple instruments from equatorial to middle latitudes, a comparison study is performed to analyze the characteristics of the EPBs during the two periods, that is, sunset and sunrise hours, including their evolution processes, scintillation characteristics and effects to GNSS positioning, and possible driving mechanisms. The results showed that relatively weaker scintillation, total electron content depletion and positioning error were associated with the near‐sunrise EPB than the post‐sunset EPB. Storm‐time electric field and/or equatorward neutral wind, together with the bottomside perturbations could contribute to the generation of the EPBs at different periods. Key Points: Ionospheric super bubbles extending to middle latitudes were freshly generated at post‐sunset and near‐sunrise periods during storm timeThe sunrise bubbles caused relatively weaker scintillation, total electron content depletion and positioning error than the post‐sunset bubblesRapid uplift of F layer driven by storm‐time electric fields, and equatorward neutral wind could contribute to the generation of the bubbles [ABSTRACT FROM AUTHOR]

The interplanetary magnetic field (IMF) significantly influences the global ionospheric electrodynamics, but it is still largely unknown the high‐ and low‐latitude ionospheric electrodynamical coupling under long‐duration northward IMF Bz (NBz) and By conditions. During the long‐duration NBz and duskward By conditions (Kp < 2, AE < 100 nT, SYM‐H > −25 nT) on 20 August 2014, six pairs of magnetometer data showed that daytime equatorial electrojet (EEJ) underwent strong decreases (reach up to 90%) at wide longitudes. Thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) captured well the decrease and the control simulations revealed that both effects of the NBz and By significantly change the plasma convection, Joule heating, and thermospheric winds at high latitudes; and penetration electric field (PEF) due to the NBz plays an important role in the decrease of the equatorial electric field. This study indicates the significant NBz effects on the equatorial ionospheric electrodynamics even during weak geomagnetic conditions, apart from the widely believed meteorological effects. Further study should be taken to disclose the relative contribution between geomagnetic and meteorological effects on the decrease of the equatorial electric field. Plain Language Summary: Interplanetary magnetic field significantly influences the global ionospheric electrodynamics, and the electric field plays essential roles in shaping the spatio‐temporal characteristics of ionospheric plasma, generating the ionospheric plasma instabilities, and influencing various applications related to the radio wave propagation. However, limited attention has been paid to the coupling of high‐ and low‐latitude ionospheric electrodynamics under the prolonged periods of northward IMF Bz (NBz) and By conditions. During the long‐duration NBz, duskward IMF By, and weak geomagnetic conditions (Kp < 2, AE < 100 nT, SYM‐H > −25 nT) on 20 August 2014, the daytime equatorial electrojet (EEJ) underwent strong decreases (reach up to 90%) at wide longitudes from 77°W to 39°E. Thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) effectively captured the decrease and the control simulations revealed that both effects of the NBz and By significantly change the plasma convection, Joule heating, and thermospheric winds at high latitudes; and penetration electric field due to the NBz plays an important role in the decrease of the equatorial electric field. This study highlights the significant NBz effects on the equatorial ionospheric electrodynamics even during weak geomagnetic conditions. This effect is different from the widely believed meteorological effects, which modulate the ionospheric dynamo by the lower atmospheric waves. Key Points: Thermosphere‐ionosphere‐electrodynamics general circulation model reproduces well the observed long‐duration decreases of equatorial electrojet at wide longitudes (∼120°) during the weak geomagnetic activityBoth effects of the interplanetary magnetic field (IMF) Bz and By significantly change the plasma convection, Joule heating, and thermospheric winds at high latitudesPenetration electric field due to IMF Bz is important to the decrease of the equatorial electric field [ABSTRACT FROM AUTHOR]

Electron density and Neutral Wind (ENWi) probe and/or Langmuir probe were flown on three different rocket flights from equatorial station Thumba (8.5°N, 77°E), during daytime, eclipse time, and post sunset time respectively, of low solar activity period, to study ionization irregularities in the E region of the ionosphere. The spectral characteristics observed using the payloads were analyzed during the three different rocket flights. The 15 January 2010 solar eclipse event and "SOUREX 1" wind and electron density measurements confirm the validity of the wind shear theory for equatorial regions during the eclipse and post sunset periods, when electrojet is weak, using wind and electron density irregularity measurements from the same instrument (ENWi) for the first time. The spectral indices in the range of −1.2 to −1.78 and the strong wind shear due to gravity wave winds during the eclipse time as well as the post sunset time, confirm the role of neutral turbulence in the generation of irregularities in the 95–112 km altitude region. The spectral index of −2.0 to −2.7 and the presence of irregularities in the negative gradient regions establishes the role of wind‐driven gradient drift instability (GDI) in generating irregularities during the SOUREX 1 post sunset flight for altitudes below 93 km. Further, the role of gradient drift instability in triggering ionization irregularities during the daytime on 14 January 2010 is unraveled by the spectral index of −2.03 and the presence of irregularities in the positive gradient regions alone. Plain Language Summary: The spectral characteristics of ionospheric plasma irregularities were studied using rocket‐borne in situ measurements of electron density and neutral winds. The Electron density and Neutral Wind probe and/or Langmuir probe were flown during different geophysical conditions from equatorial station, Thumba. The validity of the wind shear theory for equatorial region and the role of neutral turbulence in the generation of irregularities were verified by the electron density and neutral wind measurements during daytime for the 15 January 2010 solar eclipse event and post sunset time for the "SOUREX 1" experiment. The presence of irregularities in the positive electron density gradient regions alone and the spectral index value of −2.03 together confirm the role of gradient drift instability in the generation of daytime E region irregularities. The irregularities generated through wind‐driven GDI during post sunset time is characterized by the spectral index of −2.0 to −2.7 and the irregularities were present in the negative electron density gradient regions. Key Points: In situ measurements of electron density and neutral wind as well as spectral analysis of irregularitiesRole of neutral turbulence in irregularity generation above 95 km during eclipse time and post sunset time unraveledRole of wind‐driven gradient drift instability in post sunset irregularity generation below 93 km is deduced [ABSTRACT FROM AUTHOR]

The directions of Martian magnetic field, which depends on the interaction between the crustal magnetic field and the time-various interplanetary magnetic field (IMF), is in a state of disorder and plays a significant role in ions vertical transport, such as ions upwelling and precipitation. In general, vertical diffusion transport of ionospheric plasma tends to be promoted around vertical magnetic field regions but be suppressed around horizontal magnetic field area. However, the mechanism behind this phenomenon is not clear to date. Based on three-dimensional multi-fluid Hall magneto-hydrodynamic (MHD) equations in conjunction with an equivalent source dipole (ESD) model, we investigated mechanism behind the control of vertical transport by different magnetic field directions in the Martian ionosphere. Numerical results showed that the same direction of interplanetary magnetic field with ESD results in the formation of mini-magnetosphere to protect the ionosphere, which reflected in higher density distributions comparing to the no-ESD case due to joint effects of thermal pressure increasing and plasma vertical transport. At low altitude of ionosphere, O + ion inward transport (precipitation) tends to be inhibited particularly around horizontal magnetic field, with high contribution from the motional electric force. In addition, at high altitude, O + outward transport (upwelling) is likely to be facilitated over vertical-field areas in significant part controlled by the ambipolar electric force, whereas it is likely to be inhibited in horizontal-field region, contributed mainly from the Hall electric force. These results will enrich our understanding of mechanisms behind the control of ions vertical transport by magnetic field directions. [ABSTRACT FROM AUTHOR]

The ratio of O to N2 number densities (O/N2) at different altitudes is an important parameter in describing thermospheric neutral composition changes and their effects on the ionosphere during geomagnetic storms. However, storm‐induced vertical variations in O/N2 and its dependence on the O and N2 perturbations are still not fully understood. Here, the Thermosphere/Ionosphere Electrodynamics General Circulation Model simulations were used to investigate the responses of thermospheric composition at different pressure levels to the super geomagnetic storm occurred on November 20 and 21 in 2003. Our analysis shows that the behaviors of O/N2 perturbations on different pressure levels are similar above ∼180 km altitude. In the middle and low thermosphere of below ∼300 km, the storm‐time O/N2 decrease is mainly caused by a large reduction of O number density. However, N2 enhancement plays a vital role in O/N2 decreases in the upper thermosphere. The O/N2 enhancement is mainly attributed to the N2 decreases at all pressure levels. The changes of O and N2 number densities at a constant pressure level can be explained by the perturbations of their mass mixing ratio (mmr) and total mass density (ρ). The regions of the O/N2 decrease are characterized by the O mmr decrease and N2 mmr enhancement, whereas the regions of the O/N2 increase are characterized by the O mmr increase and N2 mmr decrease. The ρ value that shows the decrease globally at most pressure levels during the storm either enhance or reduce the O and N2 perturbations. Plain Language Summary: The column O/column N2 density ratio (∑O/N2) was usually used to describe thermospheric neutral composition responses to geomagnetic storms and the storm effects on ionospheric plasma density. However, thermospheric circulation changed considerably during the storm, resulting in discrepancies in composition at different altitudes. Additionally, the daytime electron density changes during geomagnetic storms are more related to those of local O/N2 at a given altitude, not the ∑O/N2. Therefore, it is important to fully understand the storm‐induced vertical variations in O, N2 and O/N2 perturbations. In this paper, the vertical variations in O/N2 and its dependence on the O and N2 perturbations during the 20–21 November 2003 storm are investigated by the numerical simulations. Our results shows that the behaviors of O and N2 perturbations depend much on the altitude, but those of O/N2 on different pressure levels are similar, especially above ∼180 km. This study helps us better understand the physical process of storm‐time ∑O/N2 variations based on the observations. Key Points: In middle and low thermosphere of below ∼300 km, storm‐time decreases of the ratio of O/N2 volume density are mainly caused by O reductionIn the upper thermosphere, N2 enhancement plays a vital role in the decreases of the ratio of O/N2 volume density during the stormAt all pressure levels, storm‐time increases of the ratio of O/N2 volume density depend more on the N2 decreases [ABSTRACT FROM AUTHOR]

Ionospheric scintillations, which are due to ionospheric plasma density anomalies, negatively impact trans-ionospheric signals and the positioning accuracy of the global navigation satellite system (GNSS). One of the crucial variables for comprehending space weather conditions is the total electron content (TEC) of the ionosphere. It is vital to predict the ionospheric TEC before making efforts to enhance the GNSS system. In this article, the long short-term memory machine learning approach for TEC prediction is presented, based on which the ionospheric phase scintillations are identified and classified using popular classifiers: support vector machines and decision trees. In this article, the comparative analysis of these classifiers is presented using the standard performance metrics: accuracy, recall, precision, and F1 score. [ABSTRACT FROM AUTHOR]

The study of ionospheric effects1 on the satellite-based communication signals has been a continued topic of interest both for academic interest and global positioning system aided navigation applications. These communication signals are affected by the fieldaligned irregularities, suffers from fading and range spreading when they pass through the ionosphere.2 To study the low latitude ionospheric plasma irregularities, dedicated radar with wide beam steering capabilities is required. Recently Gadanki Ionospheric Radar Interferometer, which operates at 30 MHz frequency, has been realized at National Atmospheric Research Laboratory, Gadanki, to scan a larger part of the sky up to -50°C in East-West direction. The radar system employs a 160 element phased antenna array, antenna system, state-of-the-art 8 kW high power solid-state transmit-receive modules, and direct digital receiver. In order to carryout round-the-clock scientific observations, highly reliable, robust designs are required to realize the high power transmit-receive modules. Detailed design philosophies are described with modular concept and better thermal designs in this paper to achieve the high power requirements. Performance results of the transmit-receive module and the sample scientific results obtained by employing these transmit-receive modules are presented in this paper. [ABSTRACT FROM AUTHOR]

Using a large data set of ground‐based GNSS scintillation observations coupled with in situ particle detector data, we perform a statistical analysis of both the input energy flux from precipitating particles, and the observed occurrence of density irregularities in the northern hemisphere cusp. By examining trends in the two data sets relating to geomagnetic activity, we conclude that observations of irregularities in the cusp grows increasingly likely during storm‐time, whereas the precipitating particle energy flux does not. We thus find a weak or nonexistent statistical link between geomagnetic activity and precipitating particle energy flux in the cusp. This is a result of a previously documented tendency for the cusp energy flux to maximize during northward IMF, when density irregularities tend not to be widespread, as we demonstrate. At any rate, even though ionization and subsequent density gradients directly caused by soft electron precipitation in the cusp are not to be ignored for the trigger of irregularities, our results point to the need to scrutinize additional physical processes for the creation of irregularities causing scintillations in and around the cusp. While numerous phenomena known to cause density irregularities have been identified and described, there is a need for a systematic evaluation of the conditions under which the various destabilizing mechanisms become important and how they sculpt the observed ionospheric "irregularity landscape." As such, we call for a quantitative assessment of the role of particle precipitation in the cusp, given that other factors contribute to the production of irregularities in a major way. Plain Language Summary: The cusp ionosphere is a particularly turbulent part of the interaction between Earth and the solar wind. Here, magnetic field lines connect the ionosphere directly to the solar wind, and a range of plasma physical mechanisms can studied. "Soft" (low‐energy) electrons raining into Earth's atmosphere inside the cusp has been suggested several times in the past as an important contributor to the various energetic phenomena found in the cusp, but little evidence exists that can tell us how important they are. We present a large statistical study of the cusp's input energy (the soft electrons) and how it affects the cusp's turbulent output (widespread plasma irregularities). We find a discrepancy in that soft electrons largely exhibit opposite long‐term trends compared to the cusp plasma turbulence. Put simplistically, the energy budget provided by soft electrons might not be sufficient to explain the enormous energy that are at times pent up in the ionospheric cusp plasma turbulence. These results point to the fact that the community needs to address the role of particle precipitation (electrons, but also ions) in producing the observed ionosphere "irregularity landscape." Key Points: Cusp scintillation occurrence increases significantly with rising geomagnetic activityThe energy flux of soft electron precipitation in the cusp has a statistical tendency to decrease as geomagnetic activity increasesThe increase in cusp scintillation with geomagnetic activity must be caused by drivers other that soft electron precipitation [ABSTRACT FROM AUTHOR]

Recently, it has been shown that the Spread Spectrum Interferometric Multistatic meteor radar Observing Network radar system located in northern Norway is capable of measuring ionospheric E‐region coherent scatter with spatial and temporal resolutions on the order of 1.5 km and 2 s, respectively. Four different events from June and July of 2022 are examined in the present study, where the coherent scatter measurements are used as a tracer for large‐scale ionospheric phenomena such as plasma density enhancements and ionospheric electric fields. By applying a two‐dimensional Fourier analysis to range‐time‐intensity data, we perform a multi‐scale spatial and temporal investigation to determine the change in range over time of large‐scale ionospheric structures (>3 km) which are compared with line‐of‐sight velocities of the small scale structures (∼5 m) determined from the Doppler shift of the coherent scatter. The spectral characteristics of the large‐scale structures are also investigated and logarithmic spectral slopes for scale sizes of 100–10 km were found to be between −3.0 and −1.5. This agrees with much of the previous work on the spatial spectra scaling for ionospheric electric fields. This analysis aids in characterizing the source of the plasma turbulence and provides crucial information about how energy is redistributed from large to small scales in the E‐region ionosphere. Plain Language Summary: Large electromagnetic structures and processes at scales greater than 1 km influence the ionosphere of the Earth. Coherent scatter ionospheric radars measure meter‐scale plasma turbulence that can be caused by these structures. This small‐scale turbulence in the E‐region (90–150 km altitude) is related to the electric fields and plasma density of the ionosphere. In this study, we use the coherent scatter measurements as a tracer for the large‐scale processes occurring. By examining the structure of the small‐scale turbulence within a large field of view, we can then look at the physics behind the large‐scale structures and how they drive the small‐scale turbulence. We can also investigate the cascade of energy between the different scales in turbulent regions. Key Points: E‐region coherent scatter is utilized as a tracer for larger‐scale ionospheric plasma turbulence generating structuresThe dominant velocity of ionospheric structuring at large scales is found to be slowly moving from a two‐dimensional Fourier analysis methodThe spectral logarithmic structuring cascade slope in the spatial domain at 100–10 km scales is between −1.5 and −3.0 [ABSTRACT FROM AUTHOR]

The ducting of whistler waves by systems of small‐scale field‐aligned plasma density depletions is studied. Similarly to our previous paper (Zudin et al., 2019, https://doi.org/10.1029/2019ja026637), we carry out analytical calculations and numerical simulations for the parameters of an active experiment in which very low frequency whistler waves emitted by a ground‐based transmitter at a frequency of 18 kHz were received onboard the DEMETER satellite at 700 km above the SURA heating facility. Random‐sized density depletions with a level around 10%–20% and perpendicular sizes ranging from 10 m up to about 300 m are considered. The properties of ducted waves are determined by the perpendicular size of individual depletions. Particularly, depletions with a width of more then d0 ∼ 100 m form separate ducting structures, that is, coupled waveguides capable of exchanging energy by means of mode overlap. Depletions with a width of less than d0 ∼ 100 m form a common waveguide structure, whose properties are equivalent to those of a wider irregularity with a smoothed density profile. Two important differences are revealed in ducting properties of density depletions compared to density enhancements considered in Zudin et al. (2019, https://doi.org/10.1029/2019ja026637). First, depletions support highly oblique Gendrin mode waves, rather than quasi‐longitudinal whistlers as in the case of density enhancements. Second, the characteristic perpendicular size d0 ∼ 100 m of density depletions separating the regimes of "coupled waveguides" and of "equivalent ducting structure" with smoothed density profile is by an order of magnitude smaller than for density enhancements of the same 10–20% relative level. Key Points: Small‐scale field‐aligned ionospheric plasma density depletions can serve as specific ducting structures for very low frequency whistlersThe ducting properties of systems from multiple small‐scale density depletions are studied both analytically and numericallyThe ducting properties of density depletions and enhancements are compared [ABSTRACT FROM AUTHOR]

An analytical model of the vertical acoustic resonance between the thermosphere and the earth's surface and its geomagnetic effect has been constructed. Resonance with a frequency of several mHz occurs when an atmospheric acoustic wave (AW) is excited by vibrations of the earth or ocean surface. The derived analytical relationships give a possibility to examine the dependence of the fundamental resonant frequency on the height of the reflecting atmospheric layer and horizontal AW number. Geomagnetic disturbances caused by the impact of AW on the ionosphere are calculated within the framework of a multi‐layered model of the ground‐atmosphere‐ionosphere system. The E‐layer of the ionosphere is treated in the approximation of a thin layer with an inclined geomagnetic field, while the topside ionosphere is assumed to consist of a cold collisionless plasma. The dependence of the spectral power of magnetic perturbations on the direction of the horizontal AW propagation is examined. The magnetic perturbation spectra are shown to have maxima at frequencies close to the acoustic resonance frequency. The spectral powers of magnetic perturbations and barometric variations measured during the Iwate‐Miyagi Nairik earthquake are in accordance with the model predictions for realistic media and AW parameters. Field‐aligned currents that arise when AWs enter the ionospheric E‐layer is shown to carry measurable electromagnetic disturbances into the geomagnetically conjugated region. Plain Language Summary: Geophysical phenomena with large energy yield, such as earthquakes and volcano eruptions, can excite specific oscillations of the geomagnetic field and ionospheric plasma with frequencies around a few mHz. These oscillations are spatially localized in the vicinity of earthquake/volcano epicenter and last up to 1–2 hr. This phenomenon was interpreted as the vertical resonance of the acoustic wave (AW) trapped between the ground/ocean surface and thermosphere (∼80 km). It is important to know geomagnetic response to the acoustic resonance because it will give possibility to use data from magnetometer array for in‐depth study of this phenomenon. Presented theoretical model of the acoustic resonance provides analytical relationships to examine the dependence of geomagnetic signature of the acoustic resonance on atmosphere/ionosphere parameters. These simple relationships anybody can use to estimate without sophisticated computer modeling an expected geomagnetic effect of the acoustic resonance for any atmosphere/ionosphere parameters. Recorded spectral powers of magnetic perturbations and variations of atmospheric pressure are found to be in accordance with the model predictions. Currents along the geomagnetic field line that arise when AW interacts with ionospheric plasma can carry electromagnetic disturbances into the opposite hemisphere. This theoretical prediction has been confirmed by observations by Pacific magnetometers. Key Points: Theory describes analytically magnetic pulsations observed after seismic/volcano events driven by vertical acoustic resonance in atmosphereA dominant magnetic component of pulsations excited on the ground is directed along the horizontal wave vector of the acoustic waveTheory predicts that ionospheric field‐aligned currents can cause detectable magnetic response to acoustic resonance in conjugate ionosphere [ABSTRACT FROM AUTHOR]

The Hunga Ha'apai volcano eruption (20.536°S, 175.382°W in Tonga) reached its maximum outbreak on 15 January 2022, at 04:15 UT, leading to huge oceanic fluctuations and atmospheric disturbances. This study focuses on the response of the ionosphere to the eruption of Tonga volcano, based on observations from a low-latitude station of the Meridian Project at Fuke, Hainan (19.310°N, 109.080°E). We identified the anomalies in the plasma drift caused by the volcanic eruption and discussed the possible mechanisms. The following results were obtained: (1) The anomalies of ionospheric plasma drift were observed at Fuke Station, during the main eruption; (2) A sudden increase and inversion of the plasma drift velocity occurred on January 15, and a large fluctuation of the drift velocity occurred afterwards; (3) By comparing the anomalous propagation velocity with the background drift, it was confirmed that the anomaly was the response of the low latitude ionosphere to the Tonga volcano eruption. Furthermore, we analyzed a possible mechanism for the effect of volcanic eruptions on ionospheric plasma drift. A large number of charged particles could be brought out by the explosion to generate an atmospheric electric field, which may cause the ionospheric plasma to change its original motion. [ABSTRACT FROM AUTHOR]

High-latitude ionospheric plasma constitutes a very complex environment, which is characterized by turbulent dynamics in the presence of different ion species. The turbulent plasma motion produces statistical features of both electromagnetic and velocity fields, which have been broadly studied over the years. In this work, we use electric field high-resolution observations provided by the China-Seismo Electromagnetic Satellite-01 in order to investigate the properties of plasma turbulence within the Earth's polar cap. We adopt a model of turbulence in which the fluctuations of the electric field are assimilated to a stochastic process evolving throughout the scales, and we show that such a process (i) satisfies the Markov condition (ii) can be modeled as a continuous diffusion process. These observations enable us to use a Fokker–Planck equation to model the changes in the statistics of turbulent fluctuations throughout the scales. In this context, we discuss the advantages and limitations of the proposed approach in modeling plasma electric field fluctuations. [ABSTRACT FROM AUTHOR]

The ionosphere can be perturbed by solar and geomagnetic activity, earthquakes, thunderstorms, etc. In particular, electromagnetic pulses produced by thunderstorms can generate wave structures in the ionospheric plasma, which are known as atmospheric gravity waves (AGWs), which can be detected by measuring the total electron content (TEC). We studied ionospheric variations resulting from thunderstorms on 10 November 2018, between 00:00 and 08:00 UTC, in the central region of Argentina, site of the RELAMPAGO–CACTI Project (Remote sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations; Clouds, Aerosols, and Complex Terrain Interactions). Atmospheric electrical activity data were provided by the Earth Networks Total Lightning Network (ENTLN) and the TEC was computed from Global Navigation Satellite System (GNSS) measurements provided by the Argentinian Continuous Satellite Monitoring Network (RAMSAC by its Spanish acronym). We found AGWs with periods less than or equal to 100 min and peak-to-peak Differential Vertical Total Electron Content (DVTEC) amplitude values up to 1.35 TECU (1 total electron content unit = 10 16 electrons/m 2 ). We observed that AGWs show the highest peak-to-peak amplitudes during intense thunderstorm periods. On a day without thunderstorms, the peak-to-peak amplitudes were approximately 2.91 times lower. [ABSTRACT FROM AUTHOR]

Based on the continuity and momentum equations, a self-consistent simulation model is developed for describing the localized electron density enhancement caused by high-pressure H2O gas release in the ionosphere. The chemical reaction and momentum exchange process between the neutral gas and ionospheric plasma species are considered in the theoretical simulation model. The finite element method is used to solve the simulation model for H2O gas release, and the expansion and ionospheric disturbance process at the early stage of high-pressure gas release are studied. It is shown that the space expansion of the released gas is mainly dominated by the pressure difference between the H2O gas and the ionospheric plasma at the early stage of release. Then the diffusion process becomes the dominant process of the space transport of H2O molecules. An electron depletion region forms near the center of the release region due to the chemical reaction and collision process with H2O molecules. Meanwhile, an electron density enhancement region forms on both sides along the magnetic field direction due to the electron snowplow effect. With the increase in the released mass of H2O gas, the intensity and duration of the electron density enhancement increase gradually. With the release position rising, it is found that the intensity of the electron density enhancement has a peak near 380 km and the duration increases slightly with the increasing release height. [ABSTRACT FROM AUTHOR]

The aim of this work is to investigate a three-dimensional second-order monotone finite-difference scheme for the transport equation. The investigation is conducted for model three-dimensional transport equations of an incompressible medium. The properties of the three-dimensional extension of the Z-scheme with nonlinear correction are studied in this work. This study is an extension of the author's previous works, where a nonlinear correction of one-dimensional transport equations was constructed. The proposed scheme uses "skew differences" containing values from different time layers instead of fluxes for the correction. The monotonicity of the obtained nonlinear scheme is numerically studied for a family of limiter functions for both smooth and non-smooth continuous solutions. The constructed scheme is absolutely stable but loses the monotonicity property when the Courant step is exceeded. The distinctive feature of the proposed finite-difference scheme is the minimalism of its template. The constructed numerical scheme is designed for models of plasma instabilities of various scales in the low-latitude ionospheric plasma of the Earth. One of the real problems that arise in solving such equations is the numerical modeling of strongly non-stationary medium- and small-scale processes in the low-latitude ionosphere of the Earth under conditions of the occurrence of the Rayleigh–Taylor instability and other types of instabilities, leading to the phenomenon of F‑scattering. Due to the fact that transport processes in the ionospheric plasma are controlled by the magnetic field, it is assumed that the plasma is incompressible in the direction perpendicular to the magnetic field. [ABSTRACT FROM AUTHOR]

This work examines the weak stratospheric polar vortex event in late February 2008 and the strong polar vortex event in late February 2021 to understand the potential influence of polar vortex strength on midlatitude thermospheric composition and ionospheric plasma density. The weak polar vortex event shows a reduction in thermospheric O/N2 at all longitudes, changes in the ionospheric total electron content (TEC) that vary with longitude, and an intensification of the semidiurnal tides. The strong polar vortex event shows O/N2 elevated by up to 25% in the Asian sector and a 25% decrease in semidiurnal tidal amplitude. The TEC response during the strong polar vortex event is complex and shows enhanced TEC in the Asian sector where O/N2 is elevated. These are the first observations of anomalies in the thermospheric composition and ionospheric plasma associated with a strong polar vortex event. Plain Language Summary: The stratospheric polar vortex is a belt of westerly winds in the midlatitudes that exists in the middle atmosphere during the winter months. The Arctic polar vortex is highly variable; sometimes it is weak and distorted, while other times it is strong. Vortex strength is known to impact weather in the troposphere and variability in the mesosphere, thermosphere, and ionosphere. The state of the polar vortex is predictable up to 2 weeks in advance, so knowledge of the vortex may increase predictability in other regions of the atmosphere. Here, we use two case studies to examine how a weak vortex and a strong vortex impact the composition of the thermosphere and the density of the ionosphere at midlatitudes. During the weak vortex case, the thermospheric composition is changed such that there is a decrease in the ratio of atomic oxygen to molecular nitrogen (O/N2). However, during the strong vortex case, there are longitude sectors with elevated values of O/N2. These regions of elevated O/N2 correspond to increased ionospheric plasma density. These are the first observations of anomalies in the thermosphere and ionosphere associated with a strong vortex event. Key Points: For the first time, a strong polar vortex event is shown to have an impact on both ionospheric electron density and O/N2 at midlatitudesBoth strong and weak vortex events have an influence on thermospheric composition, possibly through modified thermospheric circulationRegions of enhanced O/N2 coincide with elevated total electron content in the Asian sector during the strong vortex event [ABSTRACT FROM AUTHOR]

Low‐altitude observations of magnetospheric particles provide a unique opportunity for remote probing of the magnetospheric and plasma states during active times. We present the first statistical analysis of a specific pattern in such observations, energetic electron flux dropouts in the low‐altitude projection of the plasma sheet. Using 3.5 years of data from the ELFIN CubeSats we report the occurrence distribution of 145 energetic electron flux dropout events and identify characteristics, including their prevalence in the dusk and premidnight sectors, their association with substorms and enhanced auroral activities, and their correlation with the region‐1 (R1) field‐aligned current region. We also investigate three representative dropout events which benefit from satellite conjunctions between ELFIN, GOES, and THEMIS, to better understand the magnetospheric drivers and magnetic field conditions that lead to such dropouts as viewed by ELFIN. One class of dropouts may be associated with magnetic field mapping distortions due to local enhancements and thinning of cross‐tail current sheets and amplification of R1 field‐aligned currents. The other class may be associated with the increase in perpendicular anisotropy of magnetospheric electrons due to magnetic field dipolarizations near premidnight. These plasma sheet flux dropouts at ELFIN provide a valuable tool for refining magnetospheric models, thereby improving the accuracy of field‐line mapping during substorms. Plain Language Summary: Low‐Earth Orbit (LEO) observations of magnetospheric particles can sometimes provide critical information on complex high‐altitude processes that are challenging to access. Here we introduce a novel type of LEO particle measurement: energetic electron flux dropouts in the low‐altitude projection of the plasma sheet. These particle observations may prove valuable for remotely probing magnetospheric configurations during geomagnetically disturbed times. Utilizing 3.5 years of data from the ELFIN CubeSats, we conduct the first statistical analysis of these flux dropout events. Our findings reveal that they predominantly occur in the dusk and premidnight magnetic local time sectors during substorms, and exhibit a well‐defined relationship with the large‐scale field‐aligned current region. We also examine three representative dropout events to gain a deeper understanding of the magnetospheric drivers and magnetic field conditions that lead to such dropouts at low altitudes. These plasma sheet flux dropouts at ELFIN provide a valuable tool for refining magnetospheric models, consequently enhancing the accuracy of field‐line mapping during disturbed times. Key Points: We report statistical observations of energetic electron flux dropouts at the low‐altitude extent of the plasma sheet by ELFINFlux dropouts occur preferentially at dusk and premidnight and are collocated with R1 FAC poleward of the electron isotropy boundaryTwo types of magnetospheric drivers of ELFIN flux dropouts are discussed by analyzing three conjunction events with GOES and THEMIS [ABSTRACT FROM AUTHOR]

It has long been established that ionospheric electron density irregularities cause random rapid amplitude and phase fluctuations in trans‐ionospheric radio signals. Various types of ground‐based measurements, including radar and Global Navigation Satellite System (GNSS) receivers, have been utilized to gain insights into the plasma mechanisms that could lead to the development of these ionospheric irregularities. The spectral characteristics of radio wave signals emitted by GNSSs and detected by the Canadian High Arctic Ionospheric Network Global Positioning System (GPS) receivers are analyzed to try and identify the spectral signatures of the ionospheric plasma irregularities. In the polar region, we have confirmed that phase fluctuations in the GPS signal are not always accompanied by amplitude fluctuations. For the first time, a systematic comparison is conducted on the spectral properties of phase‐fluctuation events in the absence and presence of amplitude scintillations, respectively. More specifically, a spectral characterization is performed on 41 events recorded by three receivers positioned within the auroral oval. When fluctuations above the background level are solely observed in the signal's phase, the spectrum of phase variations exhibits a consistent steepness. Conversely, the presence of amplitude scintillation is accompanied by an increase in power at higher frequencies, resulting in shallower spectra. This analysis provides yet another physical element suggesting that the rapid fluctuations observed in the signal's amplitude may be caused by Fresnel‐scale irregularities arising from a gradient‐drift‐like instability while the phase variations seem to be consistent with a refractive mechanism caused by large‐scale ionospheric structures. Key Points: Forty one phase and amplitude scintillation events in the high‐latitude region were analyzed to study the properties of the spectraIn the presence of amplitude‐scintillation, all spectra exhibited a shallower slopeFresnel‐scale structures are found at the edge of large‐scale ionospheric density gradients [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument analysis and data assimilation study of midlatitude ionospheric disturbances over the European and North American longitude sectors during a strong geomagnetic storm on 26–28 February 2023. The study uses a set of ground‐based (GNSS receivers, ionosondes) observations, space‐borne (DMSP, GOLD) measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS). We observed a series of distinct storm‐time features with regard to storm‐enhanced density (SED) and subauroral polarization stream (SAPS) as follows: (a) Under multiple ring current intensifications, the storm‐time subauroral ionosphere produced long‐lasting duskside SAPS for ∼36 hr along with considerable dawnside SAPS for several hours. (b) Associated with long‐lived SAPS, strong SED occurred consecutively in the European longitude sector near local noon during a positive ionospheric storm and later in the North American longitude sector near local dusk during a negative ionospheric storm. (c) The 3‐D morphology of SED in multiple longitude sectors was reconstructed using TIDAS data assimilation technique with fine‐scale details, which revealed a narrow ionospheric plasma channel with electron density enhancement and layer uplift. Key Points: The storm‐time subauroral ionosphere produced long‐lasting duskside subauroral polarization stream (SAPS) for ∼36 hr and noticeable dawnside SAPS for several hoursAssociated SED occurred in European longitudes during a positive storm period and then in North America during a negative storm periodThe 3‐D SED morphology in multiple longitude sectors was reconstructed using a new total electron content (TEC)‐based ionospheric data assimilation system [ABSTRACT FROM AUTHOR]

The mid‐latitude upper atmosphere is directly affected by the inner magnetosphere through the interactions at the closed geomagnetic field line boundary. Several space weather phenomena associated with magnetospheric‐plasmaspheric exchanges can be mapped to mid‐latitudes. One such phenomena that occurs over mid‐latitudes during geomagnetic storms is the storm enhanced density (SED). So far, storm time ionospheric features have been studied primarily using radar, GPS and in situ measurements. We present high spectral resolution ground‐based measurements of OI 630.0 nm dayglow emissions obtained from Boston to investigate thermospheric behavior during both geomagnetic quiet and disturbed times, as these are very sensitive to changes in the ionospheric plasma temperatures, and densities. During a geomagnetic storm on 7–9 Nov 2004 (Kp = 9‐), an anomalous enhancement in brightness (>1 kR) was observed in the dayglow for about an hour duration near local sunset (dusk hours), which was not seen on other neighboring days. This enhancement in OI 630.0 nm dayglow observed on 9 Nov 2004 is confirmed to be a SED event as seen from the complimentary datasets, the Millstone Hill incoherent scatter radar (ISR), Defence Meteorological Satellite Program satellite and GPS total electron content. Further, model estimates of OI 630.0 nm dayglow intensities using ISR measured electron/ion temperature and electron densities as inputs show good agreement with our measurements. These results make this study possibly the first report on the daytime optical signatures of SED. Key Points: Anomalous enhancement in OI 630.0 nm dayglow emissions was observed during superstorm over mid‐latitude location, BostonThis enhanced dayglow intensity is in simultaneity with increased electron density possibly making it the first daytime storm enhanced density (SED) observationsModel estimates show the peak emission altitude of dayglow emission shifted to higher altitudes during the SED observation duration [ABSTRACT FROM AUTHOR]

A study of the behavior of the main characteristics of the ionosphere over Europe during the 26–28 February 2023 ionospheric storm was carried out in this present work. The additional influence of sudden stratospheric warming on the ionosphere was considered. The behavior of the critical frequency of the ionosphere foF2 (characterizing the maximum electron density), the peak height of the F2-layer (hmF2), and Total Electron Content (TEC) were investigated through their relative deviations from the quiet conditions. The behavior of the TEC over Europe showed the geographic latitudinal dependence of the response. The variability in the ionospheric critical frequency was represented by the data of 10 ionospheric stations for vertical sounding located in two groups: (i) near the prime meridian and (ii) near the 25° E meridian. Some differences were found in the response compared to the TEC response, which was explained by the different responses of the top maximum region and bottom maximum region. The peak height of the F2 layer varied strongly during the storm, which was due to the forced drift of ionospheric plasma induced by additional electric fields. The present detailed analysis of the ionospheric response shows that the considered storm exhibited characteristic features inherent in the winter season but with some manifestations of reactions in equinox conditions. [ABSTRACT FROM AUTHOR]

The data of simultaneous measurements of the polarization jet from the Yakutsk ground-based vertical radio sounding station and satellite observations of narrow electron density dips or fast westward drifts of the ionospheric plasma from satellites of the Defense Meteorological Satellite Program (DMSP) series. The events are based on ground-based ionospheric measurements and cover the time interval from March 1989 to December 2015, i.e. about 26 years. The simultaneity of observations is ensured by a time period of approximately ±1.5 h from the time of registration of signs of a polarization jet according to the data of a ground-based ionospheric sounding station or by the an orbital period of DMSP satellites around the Earth. Based on data of long-term simultaneous satellite and ground-based measurements (126 events), it was shown and confirmed that the presence of characteristic additional traces of reflections on ionograms indicates the presence of narrow and fast drifts of ionospheric plasma or a polarization jet near the zenith of the observation station. It is also shown that the quasi-instantaneous longitude extent of a polarization jet at subauroral latitudes can in some cases reach 8 h or 120° by longitude. [ABSTRACT FROM AUTHOR]

We examined evolution of Global Positioning System (GPS) scintillation during a substorm in the nightside high latitude ionosphere, using 1‐s phase and amplitude scintillation indices from the Canadian High Arctic Ionospheric Network (CHAIN) network. The traditional 1‐min scintillation indices showed that the phase scintillation was dominant, while the amplitude scintillation was weak. However, the 1‐s amplitude scintillation occurred more often in association with major auroral structures (polar cap arc, growth phase arc, onset arc, poleward expanding arc, poleward boundary intensification, and diffuse aurora) that were detected by the THEMIS all‐sky imagers (ASIs). The 1‐min index missed much of the amplitude fluctuations because they only lasted ∼10 s near a local peak or at the gradients of the auroral structures. The 1‐s phase scintillation was concurrent with the amplitude scintillation but was much weaker than the 1‐min phase scintillation. The frequency spectral analysis showed that the spectral power above ∼1 Hz was diffractive and below ∼1 Hz was refractive. We suggest that the amplitude scintillation in the high‐latitude ionosphere is much more common than previously considered, and that a short time window of the order of 1 s should be used to detect the scintillation. The 1‐min phase scintillation index is largely influenced by refractive effects due to total electron content (TEC) variations, and the spectral power below ∼1 Hz should be removed to identify diffractive scintillation. Plain Language Summary: The Global Positioning System (GPS) is widely used in the society for navigation. The GPS radio signal is disturbed by aurora due to modulation of ionospheric plasma density, and the signal disturbance is called scintillation. The scintillation indices are widely used to quantify the level of scintillation. We show that the conventional scintillation indices often miss the scintillation, because the indices assume that aurora is steady for a minute but aurora evolves rapidly in ∼10 s. We use a modified version of the indices and show that the scintillation of GPS signal amplitude occurs more commonly than previously considered. In addition, we show that the conventional phase scintillation index overestimates the scintillation of GPS signal phase, because it is sensitive to variation of the ionospheric plasma density. Our modified indices remove this effect and characterize the scintillation more accurately. We find that the scintillation is closely related to active auroral forms during an auroral substorm. Key Points: The 1‐s amplitude scintillation index shows much more frequent occurrence of diffractive scintillation than the 1‐min indexThe 1‐s phase scintillation index is smaller than the 1‐min index but is better correlated with the amplitude scintillationThe higher cutoff frequency and shorter time window are important for removing refractive effects and identifying diffractive scintillation [ABSTRACT FROM AUTHOR]

A thermospheric wind data set from a Fabry‐Perot interferometer (630 nm) and the ion velocity from a Dynasonde in Tromsø, Norway, was analyzed for nine winter seasons to study the dynamics of the thermosphere and F‐region ionosphere at an auroral latitude. This study focused on bifurcation in the zonal component of the neutral wind and ion velocity at midnight and its dependence on the Y component of the interplanetary magnetic field (IMF). Ionospheric plasma convection patterns are evidently imprinted on the thermospheric wind variations as aspects of the westward and eastward accelerations at dusk and late morning, respectively. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may result in the dependence because of anti‐sunward plasma flow distorted in the polar cap. Plain Language Summary: The ionosphere is partially ionized plasma, but the particle minority of ions plays an important role in controlling dynamics of the thermosphere. Particle collision is the fundamental process for momentum transfer from ionospheric ions to thermospheric neutral particles. The ionospheric plasma flow pattern at high latitudes depends on the direction of the interplanetary magnetic field (IMF), and the pattern may be projected on the thermospheric wind. However, the dependence is not yet well understood. This study derived statistical experimental features regarding the dependence of the thermospheric wind, analyzing data from an optical interferometer (Fabry‐Perot interferometer) and a radio wave technique (Dynasonde) in Tromsø, Norway. The wind pattern around midnight is different from the ionospheric plasma convection, in accordance with the IMF direction. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may cause the dependence because of anti‐sunward plasma flow distortion in the polar cap. In summary, this study concludes that the zonal wind bifurcation at auroral latitudes is caused by the ion velocity bifurcation, and that advection from the polar cap region affects the wind response time to the ion velocity bifurcation. Key Points: The thermospheric wind from a Fabry‐Perot interferometer (630 nm) and the ionospheric plasma velocity from a Dynasonde were comparedThe zonal wind bifurcates immediately before midnight for interplanetary magnetic field By < 0, but for By > 0, it inverts gradually into the postmidnight sectorThe wind bifurcation signature is different from the ion velocity bifurcation, probably due to advection from the polar cap region [ABSTRACT FROM AUTHOR]

During the storm on 3 and 4 November 2021, the Global‐scale Observations of the Limb and Disk (GOLD) mission observed well separated equatorial ionization anomaly (EIA) crests post sunset on 3 November, but merged EIA on 4 November. We used the Whole Atmosphere Community Climate Model‐eXtended to simulate the EIA structures during the two nights. The simulations show two separated post sunset EIA crests on 3 November but merged post sunset EIA crests on 4 November, which are qualitatively consistent with the GOLD observations. Numerical simulations and Ionospheric Connection Explorer neutral wind observations illustrate that the formation of merged EIA crests was due to several hours of downward E × ${\times} $ B drifts before and after sunset. Further diagnostic analysis revealed that it was mainly driven by westward electric fields caused by the disturbance dynamo electric field during the recovery phase of the storm. Plain Language Summary: The Ionospheric equatorial ionization anomaly (EIA) occurs almost every day and has been studied extensively. A large number of studies have shown that EIA has usually two separated electron density crests around ± $\pm $15° off the magnetic equator in the ionosphere. A structure of merged EIA with just one density crest located near the magnetic equator was observed by a satellite on 4 November 2021. We successfully simulate the merged EIA structure using a first‐principles model. Analysis of the model results reveals that thermospheric neutral winds were disturbed by a geomagnetic storm on 3 and 4 November 2021; the disturbance dynamo electric field due to the disturbed winds changed the usual drift pattern of the plasma, which eventually drive the ionospheric plasma to move toward the magnetic equator causing the merged EIA structure. Key Points: Merged equatorial ionization anomaly (EIA) structure was observed by Global‐scale Observations of the Limb and Disk during the storm of 3 and 4 November 2021This EIA structure was successfully reproduced by Whole Atmosphere Community Climate Model‐eXtended simulationsModel diagnostic analysis reveals that the merged EIA was mainly driven by disturbance dynamo electric fields during the storm [ABSTRACT FROM AUTHOR]

In this paper, (pure, slow, and fast) Alfvèn waves for the accepted conditions in the Northern-hemisphere at the EF-region of ionospheric plasma were calculated with low latitudes using Eqs. (20, 25, 26) and the real geometry of the Earth's magnetic field, at hours 12.00 LT for the 1990 year when sunspot activity was at its peak. One of the most important findings of this research is that the phase velocities of magnitudes of "MHD modes = (pure, slow, and fast) Alfvèn waves" are analytically dependent not only on the angle between the wave propagation vector (k) and the magnetic field (B), but also on the declination (D = It is the angle value between the direction of the sun's rays and the equatorial plane) and magnetic dip angle (I = It is the angle between real north and magnetic north). According to the results, the behavior of the magnitudes of the squares of the phase velocities of all MHD modes is consistent with the behavior of the distribution of electron density with low geographic latitude, even when the magnetic field vector is perpendicular and parallel to the wave's propagation vector. In addition, the wave phase velocities are greater in the summer than in the winter. In this point, MHD wave behavior is critical in magnetosphere-ionosphere coupling. On the other hand The propagation velocities of the fast and slow MHD modes in the magnetic equatorial trough region have been determined to be very small at (= I), with almost no energy, but if = 90 + I, the energy increases with latitude and is approximately maximum at the low latitude limit. The minimum points are between 0 and 10 degrees latitude, where the wave energies are the smallest, and the maximum points are between 20 and 30 degrees latitude, where the wave energies are the greatest. [ABSTRACT FROM AUTHOR]

Forecasting of the effects of thermospheric drag on satellites will be improved significantly with better modeling of space weather effects on the high‐latitude ionosphere, in particular the Joule heating arising from electric field variability. We use a regression analysis to build a model of the ionospheric convection drift velocity which is driven by relatively few solar and solar wind variables. The model is developed using a solar cycle's worth (1997–2008 inclusive) of 5‐min resolution Empirical Orthogonal Function (EOF) patterns derived from Super Dual Auroral Radar Network (SuperDARN) line‐of‐sight observations of the convection velocity across the high‐latitude northern hemisphere ionosphere. At key stages of development of the model, we use the percentage of explained variance P to see how well the model reproduces the EOF data. The final model is driven by four variables: (a) the interplanetary magnetic field component By, (b) the solar wind coupling parameter epsilon ε, (c) a trigonometric function of day‐of‐year, and (d) the monthly F10.7 index. The model can reproduce the EOF velocities with a characteristic P = 0.7. The model and EOF data compare best around the solar maximum of 2001. P $P$ is lower around solar minimum, due to occasional limitations in the geographical and temporal coverage of the SuperDARN measurements. This may indicate the need to modify our model around the minimum of the solar cycle. Our model has the potential to be used to forecast the ionospheric electric field using the real‐time solar wind data available from spacecraft located upstream of the Earth. Plain Language Summary: Variations in space weather in the ionized region of the Earth's atmosphere (the ionosphere) can result in expansion of the atmosphere, increasing the atmospheric drag on objects, such as satellites, in the thermosphere. We aim to significantly improve the forecasting of the effects of atmospheric drag on satellites by more accurate modeling of space weather effects on the motion of ionized particles (plasma) in the ionosphere. We have developed a model of the variation in plasma motion using a few solar wind variables which are all now available in real time from satellites upstream of the Earth. The model was built using 5‐min resolution observations of the ionospheric plasma motion from a 12‐year interval, to capture effects on the solar cycle timescale. Our model is good at reproducing the original data set—if 0 indicates that there is no reproduction and 1 indicates exact reproduction, then our model scores 0.7. Data set reproduction is best around the maximum in the solar cycle and worst at solar minimum. This is mainly due to differences in the spatiotemporal data coverage between these times but possibly also the model's specification of the physical processes coupling the Sun to the Earth's ionosphere. Key Points: We present a regression model of high‐latitude ionospheric plasma convection driven by a small number of space weather variablesThe model could be a valuable tool for operational forecasting due to its simplicityThe model reproduces its parent data set best around solar maximum, with a mean percentage of explained variance of 0.7 over 12 years [ABSTRACT FROM AUTHOR]

Data measured by the Digisonde at the low‐latitude station Hainan from 2003 to 2016 are statistically analyzed to specify the diurnal average variations of the bottom‐side F region ionospheric plasma velocity vector V. This is the first comprehensive analysis of Digisonde measurements of low latitude F region plasma velocities in the East Asian sector that use a database covering more than one solar cycle. The velocity components VN (Northward), VE (Eastward), and VZ (Upward) are analyzed for two levels of solar flux and two levels of geomagnetic activity, respectively. The diurnal variations of the average VZ show three positive peaks near the prereversal enhancement (PRE) period, pre‐midnight, and before sunrise, respectively, and a prominent valley in the early morning. The averaged VZ significantly increased with solar flux in the period of PRE during equinoxes, but it was only slightly affected by Kp. The VE component was westward in daytime and eastward in nighttime. The average eastward VE increased significantly with solar flux but decreased with Kp, whereas the average westward VE exhibited only a small variation with solar flux and Kp. The average VN was almost southward independent of solar flux and Kp. The plasma velocities over the Hainan station were mainly caused by the electric field and neutral wind. Our results show that the features of the vertical and meridional velocities over the Hainan station in the morning are associated with the formation of the equatorial ionization anomaly (EIA). Plain Language Summary: The ionospheric plasma drift has been studied with data from ground‐based observations for decades. However, there is no corresponding study for the east Asian sector. In 2002, a Digisonde named DPS‐4 was installed at the Hainan station which can directly measure the plasma velocities with Drift mode. Using the data from the DPS‐4 at Hainan, we perform a statistical study on the bottom‐side F region plasma velocities at two levels of F10.7 and two levels of Kp, respectively. The results show that the velocities are mainly caused by the electric field and neutral wind, and the VN and VZ components are associated with the EIA formation. We also compared our results with that measured by incoherent scatter radar at the Jicamarca station. Some statistical results are similar, but some are different in the Hainan station and Jicamarca station, though the locations, the observed altitudes, and the observation periods differ between the two stations. Key Points: Bottom‐side ionospheric plasma velocities in the East Asian sector are comprehensively analyzed using Drift mode data of the DigisondeStatistical features of ionospheric plasma velocities over the low latitude station Hainan for two levels of F10.7 and two levels of KpVelocities are mainly caused by the electric field and neutral wind. The VN and VZ components may be associated with the formation of EIA [ABSTRACT FROM AUTHOR]

The oblique radio wave incidence on a turbulent equatorial conductive collision plasma layer is considered. The "Compensation Effect" has been discovered by us. A complex refractive index of the equatorial terrestrial ionosphere has been derived for the first time. Second-order statistical moments of the spatial power spectrum (SPS) of scattered radio waves are obtained for the first time using the WKB method, taking into account the asymmetry of the problem: the inclined incidence of the wave on a plasma boundary and the asymmetry of the magneto-ionic parameters. It was established for the first time that a certain direction exists along which the inclined incidence radio wave on a plasma layer and the anisotropy parameters of a magnetoplasma compensate each other. This result will have great practical application in communication. In this case, the SPS of scattered radio waves neither widens nor is its maximum displaced. The behavior of this spectrum versus distance propagated by radio waves in the conductive equatorial ionosphere is analyzed numerically for different penetration angles and anisotropy factors of asymmetric anisotropy electron density irregularities. It was shown that the anisotropy factor of elongated plasmonic structures has a substantial influence on the "Compensation Effect" of scattered ordinary and extraordinary waves penetrating in the conductive collision ionospheric plasma the slab. Numerical calculations are carried out for the anisotropic Gaussian correlation function applying IRI experimental data. [ABSTRACT FROM AUTHOR]

In our exploration, we aimed at identifying seismic anomalies using limited ionospheric data for earthquake forecasting and we meticulously compiled datasets under conditions of minimal geomagnetic disturbance. Our systematic evaluation affirmed the ITransformer as a potent tool for the feature extraction of ionospheric data, standing out within the domain of transformer-based time series prediction models. We integrated the maximum entropy principle to fully leverage the available information, while minimizing the influence of presuppositions on our predictions. This led to the creation of the MaxEnt SeismoSense Model, a novel composite model that combines the strengths of the transformer architecture with the maximum entropy principle to improve prediction accuracy. The application of this model demonstrated a proficient capability to detect seismic disturbances in the ionosphere, showcasing an improvement in both recall rate and accuracy to 71% and 69%, respectively, when compared to conventional baseline models. This indicates that the combined use of transformer technology and the maximum entropy principle could allow pre-seismic anomalies in the ionosphere to be sensed more efficiently and could offer a more reliable and precise approach to earthquake prediction.

• Low frequencies prevailed in the longitudinal wave spectrum. • Two trends in spatial distribution of surface wave peak frequencies are revealed. • Long-lived non-travelling ionospheric disturbances follow TID over UNT site. We investigated lithospheric and ionospheric disturbances caused by the 3 September 2017 underground nuclear test (UNT) in North Korea based on data from networks of seismic stations and GPS/GLONASS receivers. We analyzed frequency composition of longitudinal and surface waves detected at more than 40 seismic stations. Low frequencies (∼0.45 ± 0.21 Hz) were found to prevail in the spectrum of longitudinal waves. For both types of waves, frequencies decreased with distance according to the power law. Analysis revealed two trends in the spatial distribution of peak frequencies. First, high and mid-frequencies (0.14–0.50 Hz) are typical for the continental landmass regions, and low frequencies of surface waves (0.13 Hz and below) are more common for the regions adjacent to fringe seas. Second, uneven frequency variations of seismic waves for different azimuths relative to the UNT epicenter (frequencies subside rapidly in the east, southeast, and southwest directions from the epicenter; towards the continental landmass inner parts, frequency variation is much slower). Records of longitudinal waves made at different distances from the epicenter allowed estimating the size of the focal area. Analysis of data from GPS/GLONASS receiving stations near the Korean Peninsula revealed ionospheric disturbances that were most likely caused by UNT. The ionospheric disturbances started to be detected ∼ 8 min after UNT and were observed for about 5 hrs. During the first 1.5–2 hrs after UNT, travelling ionospheric disturbances (TID) were recorded; they propagated from the epicenter at ∼ 600, 250 and 133 m/s. These TIDs had periods of 1–10.5 min and could be associated with acoustic waves induced in the Earth's atmosphere by the underground nuclear test. After TID passage over the UNT site, there was a long-lived (more than 3.5 hrs) region of non-travelling perturbations of ionospheric plasma with the velocity about 7 m/s. This velocity describes not so much the disturbance travel, but the time when the "receiver – satellite" line of sight (LOS) crosses the disturbance. This region can possibly occur due to the formation of standing waves in the atmosphere, development of plasma instabilities or penetration into the ionosphere of an anomalous electric field generated by radioactive substances leaked out onto the surface. [ABSTRACT FROM AUTHOR]

Ionospheric irregularities are plasma density variations that occur at various altitudes and latitudes and whose size ranges from a few meters to a few hundred kilometers. They can have a negative impact on the Global Navigation Satellite Systems (GNSS), on their positioning accuracy and even cause a signal loss of lock (LoL), a phenomenon for which GNSS receivers can no longer track the satellites' signal. Nowadays, the study of plasma density irregularities is important because many of the crucial infrastructures of our society rely on the efficient operation of these positioning systems. It was recently discovered that, of all possible ionospheric plasma density fluctuations, those in a turbulent state and characterized by extremely high values of the Rate Of change of the electron Density Index appear to be associated with the occurrence of LoL events. The spatial distributions of this class of fluctuations at mid and high latitudes are reconstructed for the first time using data collected on Swarm satellites between July 15th, 2014 and December 31st, 2021, emphasizing their dependence on solar activity, geomagnetic conditions, and season. The results unequivocally show that the identified class of plasma fluctuations exhibits spatio-temporal behaviours similar to those of LoL events. [ABSTRACT FROM AUTHOR]

We investigate the prospects for the direct detection of dark matter (DM) particles, incident on the upper atmosphere. A recent work relating the burst-like temperature excursions in the stratosphere at heights of ≈38–47 km with low speed incident invisible streaming matter is the motivation behind this proposal. As an example, dark photons could match the reasoning presented in that work provided they constitute part of the local DM density. Dark photons emerge as a U(1) symmetry within extensions of the standard model. Dark photons mix with real photons with the same total energy without the need for an external field, as would be required, for instance, for axions. Furthermore, the ionospheric plasma column above the stratosphere can resonantly enhance the dark photon-to-photon conversion. Noticeably, the stratosphere is easily accessible with balloon flights. Balloon missions with up to a few tons of payload can be readily assembled to operate for months at such atmospheric heights. This proposal is not limited to streaming dark photons, as other DM constituents could be involved in the observed seasonal heating of the upper stratosphere. Therefore, we advocate a combination of different types of measurements within a multi-purpose parallel detector system, in order to increase the direct detection potential for invisible streaming constituents that affect, annually and around January, the upper stratosphere. [ABSTRACT FROM AUTHOR]

The nonlinear system of equations for Rossby-Khantadze waves in a weakly ionospheric plasma by incooperating sheared zonal flow is given. It is shown that in the presence of multiple-scale analysis, our obtained set of equations can be decomposed into one21 dimensional equation which we call nonlinear modified KdV (MKdV) equation illustrating the propagation of solitary Rossby-Khantadze waves. [ABSTRACT FROM AUTHOR]

Measuring the plasma motion vector field accurately is beneficial to understanding the physical processes in the ionosphere. As a newly built advanced modular phase array Incoherent Scatter Radar in Sanya (SYISR), SYISR can measure the line‐of‐sight (LOS) velocity of ion drift in multiple directions, potentially yielding the spatial distribution of ionospheric plasma drift at low latitudes in East Asia. Compared to the traditional ISR, it can operate continuously for a long‐time and perform with fast and flexible scanning. This paper evaluates different beam configurations and inversion methods for vector ion velocity detection by comparing the vector ion velocities input in the forward modeling and derived by inversion. For the mono‐static SYISR, the all‐sky scan mode is suitable for observing the distribution of vector ion velocity with height, and the fine scan mode is suitable for observing the spatial distribution of two components of vector ion drift perpendicular to the magnetic field in the northern region of SYISR's non‐grating lobe. For the future tri‐static SYISR, it could simultaneously observe the spatial distribution of all three components of the vector ion velocity. Besides, the inversion method based on Bayesian estimation shows good accuracy and stability in deriving the vector ion velocity, especially when the LOS ion velocity has a large error. These results will benefit the design and optimization of the actual experiments for the vector ion velocity measurement and inversion using the SYISR system. Key Points: A procedure is constructed to guide and evaluate the detection of the vector ion velocity by the Incoherent Scatter Radar in Sanya (SYISR) systemThe ion velocity inversion method based on Bayesian estimation shows good accuracy and stability in deriving the vector ion velocityMono‐static and tri‐static SYISR can be used to detect the height profile of vector ion velocity and spatial distribution, respectively [ABSTRACT FROM AUTHOR]

We report on the development of plasma density irregularities in the subauroral ionosphere over the North American sector during the 17 March 2015 Saint Patrick's Day geomagnetic storm. Data from network of ground‐based observation instruments including the National Oceanic and Atmospheric Administration Continuously Operating Reference Station global navigation satellite system receivers and Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imagers, as well as in situ measurements from the Defense Meteorological Satellite Program (DMSP) and the Van Allen Radiation Belt Storm Probes (RBSP) spacecrafts, were examined to characterize the spatial and temporal development of subauroral irregularities. The auroral electrojet (AE) index was used as primary indicator of substorm occurrences, aided by some cross‐referencing with the SYM‐H index and SYM‐H time derivative. Special attention was given to substorms that happened at the beginning of this geomagnetic storm. Analysis of Global Positioning System (GPS) rate‐of‐total electron content index (ROTI) data along an east‐west cut line near the US‐Canada border indicates that subauroral ionospheric irregularities may start to form as early as a few minutes after a substantial increase in the AE index. Contemporaneous DMSP and RBSP observations confirmed the presence of subauroral polarization streams wave structures when the aforementioned enhancement in AE index and GPS ROTI occurred. An equatorward expansion of bright auroral arc and subsequent auroral breakups were also seen in the THEMIS all‐sky imager observation data. The relatively short time scales for irregularities to develop suggest possible roles played by the penetration of magnetotail plasma flow bursts into the plasmasphere and the substorm current wedge development in their formation in the subauroral ionosphere. Key Points: Development of subauroral ionospheric density irregularities during the 17 March 2015 Saint Patrick's Day geomagnetic storm was examinedThe empirical analysis comprised of a combination of Global Positioning System rate‐of‐total electron content index, Defense Meteorological Satellite Program, Van Allen Radiation Belt Storm Probes, and Time History of Events and Macroscale Interactions during Substorms all‐sky imager data over the North American sectorSubauroral ionospheric density irregularities started to appear a few minutes after the initial increase in the auroral electrojet index [ABSTRACT FROM AUTHOR]

Ion drag associated with the ionospheric plasma convection plays an important role in the high-latitude thermospheric dynamics, yet changes in the thermospheric wind with geomagnetic activity are not fully understood. We performed a statistical analysis of the thermospheric wind measurements with a Fabry–Perot interferometer (FPI; 630 nm wavelength) in Tromsø, Norway, in the winter months for 9 years. The measurements were sorted by a SuperMAG (SME) index, and a quiet-time wind pattern was defined as an hourly mean under SME ≤ 40 nT. The quiet-time wind pattern can be expected to be represented by a pressure gradient associated with the solar radiation and a geostrophic force balance. With an increase in the geomagnetic activity level, the thermospheric wind turned over from eastward to westward at dusk and increased the equatorward magnitude from midnight to dawn. Deviations from the quiet-time wind presented similar patterns in the direction with the ionospheric plasma convection but were larger in magnitude at dusk than at dawn. This is the first study to report a dawn-dusk asymmetry of the thermospheric wind acceleration feature and signatures of the eastward wind acceleration at dawn by ion drag. [ABSTRACT FROM AUTHOR]

The results of temperature measurements in the lower thermosphere at altitudes of 90–130 km by the method of resonant scattering of radio waves on artificial periodic inhomogeneities (APIs) of the ionospheric plasma are presented. These inhomogeneities are created when the ionosphere is exposed to powerful HF radio emission. The temperature profile was obtained from measurements of the relaxation time of the API scattered signal. The data processes and the method of the temperature determination are given in detail. The height and temporal resolutions of the API technique are of the order of 1 km and 15 s, respectively, making it possible to study both fast and slow processes in the lower thermosphere. Large temperature variability at altitudes of 90–130 km during the day and from day to day, due to the propagation of atmospheric waves, has been confirmed. The temporal variations of the atmospheric parameters take place with periods from 15 min to some hours. There are often height profiles of the temperature with the wave-like variations and with the vertical scale of about 4–10 km. The irregular temperature profiles were observed above 100 km. [ABSTRACT FROM AUTHOR]

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]

In Trung et al. (2019b, https://doi.org/10.1029/2019ja026636), we compared the geopause, a surface where the ionospheric plasma and solar wind plasma has equal parameters (by mass density, number density, or pressure), to the magnetopause to show that there was a transition in physics governing the motion of the plasmas between the different boundaries. In this study, we use multifluid magnetohydrodynamics simulation data to examine the relation of the source terms in the momentum equation and to the geopauses. We find that the geopauses do represent a transition in the physics governing the motion of the plasmas. While friction does not play a dominant role in the motion of the heliospheric and ionospheric plasmas, we find that friction shows indirectly that the ionospheric plasma can travel faster than the heliospheric plasma. Further satellite data is needed to verify the possibility of ionospheric plasma reaching speeds exceeding the solar wind. Key Points: We used numerical simulations to compare the structure of source terms in the multifluid momentum equation to the geopauseGeopauses represent a transition in how source terms govern the motion of individual plasma componentsFriction shows that ionospheric plasma can travel faster than the solar wind [ABSTRACT FROM AUTHOR]

• Results of ionosphere observation during geospace storm on 25 September 2016 over Kharkiv (Ukraine) are presented. • The Kharkiv incoherent scatter radar, digital ionosonde and magnetometer-fluxmeter were used for this. • Geospace storm led to changes in the spatial structure of the ionosphere. • Geospace storm led to significant restructuring of thermal and dynamic state of the ionosphere. Instrumental observations of variations in the level of the geomagnetic field and ionospheric parameters were conducted during a weak geospace storm on September 25, 2016. It was found that a moderate magnetic storm was accompanied by variations in the level of the H component of the geomagnetic field up to 20–30 nT, as well as variations in the level of fluctuations of the horizontal components of the geomagnetic field up to ±(2–3) nT. It was revealed that the ionospheric storm was three-phase, mostly moderate. The decrease in the electron density reached a factor of 1.5, and its increase reached a factor of 2.2. The increase in the electron temperature during the negative phase of ionospheric storm did not exceed 200–300 K, while its increase during the positive phase of ionospheric storm reached 400–500 K. The ion temperature variations were within ±100 K. During the negative phases of the ionospheric storm, individual deviations in the vertical plasma drift velocity from those on a geomagnetically quiet day were observed both in the direction of decreasing the downward drift velocity (increasing the upward drift velocity at high altitudes) and vice versa. Significant (17–53 m/s) deviations in velocity variations towards positive values (decrease of downward and increase of upward drift velocity), accompanied by the rise of the F2 layer, after significant changing in the zonal component of the magnetospheric electric field (from –2 to 3.5 mV/m), contributed to the increase in the electron density and to the appearance of the positive phase of the ionospheric storm. During the positive phase, the increase in the velocity of downward plasma drift reached 9–20 m/s at altitudes above 450 km. The negative phases of the ionospheric storm were formed due to an increase in the electron and ion temperatures and a decrease in the O/N2 ratio during the storm. The generation of ionospheric storm positive phase at mid-latitudes apparently was caused by several mechanisms at once: a penetrating electric field, a meridional wind directed toward the equator, and downward plasma flows in the protonosphere. The phases of the ionospheric storm were accompanied by significant variations in the dynamic and thermal processes in the ionosphere. [ABSTRACT FROM AUTHOR]

A newly developed three-dimensional electrostatic fluid model solving continuity and current closure equations aims to study phenomena that generate ionospheric turbulence. The model is spatially discretized using a pseudo-spectral method with full Fourier basis functions and evolved in time using a four-stage, fourth-order Runge Kutta method. The 3D numerical model is used here to investigate the behavior and evolution of ionospheric plasma clouds. This problem has historically been used to study the processes governing the evolution of the irregularities in the F region of the ionosphere. It has been shown that these artificial clouds can become unstable and structure rapidly (i.e., cascade to smaller scales transverse to the ambient magnetic field). The primary mechanism which causes this structuring of ionospheric clouds is the E × B , or the gradient drift instability (GDI). The persistence and scale sizes of the resulting structures cannot be fully explained by a two-dimensional model. Therefore, we suggest here that the inclusion of three-dimensional effects is key to a successful interpretation of mid-latitude irregularities, as well as a prerequisite for a credible simulation of these processes. We investigate the results of 2D and 3D nonlinear simulations of the GDI and secondary Kelvin–Helmholtz instability (KHI) in plasma clouds for three different regimes: highly collisional (≈200 km), collisional (≈300 km), and inertial (≈450 km). The inclusion of inertial effects permits the growth of the secondary KHI. For the three different regimes, the overall evolution of structuring of plasma cloud occurs on longer timescales in 3D simulations. The inclusion of three-dimensional effects, in particular, the ambipolar potential in the current closure equation, introduces an azimuthal "twist" about the axis of the cloud (i.e., the magnetic field B). This azimuthal "twist" is observed in the purely collisional regime, and it causes the perturbations to have a non-flute-like character ( k ‖ ≠ 0 ). However, for the 3D inertial simulations, the cloud rapidly diffuses to a state in which the sheared azimuthal flow is substantially reduced; subsequently, the cloud becomes unstable and structures, by retaining the flute-like character of the perturbations ( k ‖ = 0 ). [ABSTRACT FROM AUTHOR]

Generation of ionospheric-like plasma is important for laboratory investigations of ionospheric physics. In this work, the design and fabrication of a magnetic filter source for the ground simulation of ionospheric-like low density plasma are presented. Four groups of permanent magnets were placed at different regions to form a magnetic filter configuration, and filaments were used to produce the low-density plasmas. Operating with adjustable plasma source conditions can generate plasmas with variable density and energy similar to those of the ionosphere, which were measured using tailor-made plasma diagnostic tools. The results indicate that homogeneous distributed low-density plasmas on the order of 105 cm−3 were produced using the plasma source. In addition, ion and electron energies that are similar to those of the actual ionosphere were also achieved. Based on the plasma source, ionospheric plasma physics can be investigated in a controlled manner in the laboratory. In addition, it can also be extended to the calibration and testing of payloads for ionospheric plasma measurement before launching. [ABSTRACT FROM AUTHOR]

Multiyear researches show that weak and moderate magnetic storms may induce considerable and unpredictable changes in the ionosphere state. The problems of predicting the ionosphere response in a certain region to space weather changes currently remain topical since the physical processes occurring in the ionospheric plasma are variable and complicated. Particular interest is attracted by ionospheric disturbances with variable phases at middle latitudes and their propagation to low latitudes and the occurrence of strong ionospheric storms as a result of moderate or weak magnetic storms. The objective of this study is to perform the experimental studies of variations in the ionospheric plasma parameters over Ukraine during the very moderate magnetic storm on December 18, 2019. The study was carried out by using the incoherent scatter of radio waves as providing the most complete diagnostic capabilities and the vertical sounding method. Observations were performed in the Ionospheric Observatory of the Institute of Ionosphere (National Academy of Sciences of Ukraine, Ministry of Education and Science of Ukraine, Kharkiv) with an incoherent scatter radar. The critical frequencies were measured with a portable ionosonde. In addition, the geophysical information about the space weather and magnetosphere parameters was used. The ionosphere response to the geospace storm on December 18, 2019, over Kharkiv was analyzed. The very moderate magnetic storm (Kp = 4) was established to induce positive ionospheric disturbance. An increase in the critical frequency (up to 1.6 times) and a corresponding increase in the ionospheric F2 peak electron density (up to 2.6 times) was accompanied by a sequence of changes in the variations of principal ionospheric plasma parameters, such as the F2 layer peak height (a decrease by 30 km), the electron density throughout the entire range of studied altitudes (200–450 km), the electron and ion temperatures, and the vertical ionospheric plasma velocity component (with a decrease in the downward plasma drift velocity Vz at the noon after the magnetic storm began with further velocity recovery, the occurrence of fluctuations in the variations Vz with a quasi-period of 1 h 50 min at 15:40 UT (Universal Time) at altitudes of 360–420 km, and weakening of the evening extremum effect in the Vz variations and a maximum decrease in the velocity to 40–70 m/s at these altitudes). A substantiation was given for the following mechanism of the formation of a positive ionospheric storm: the downward plasma drift is weakened in the mid-latitude ionosphere during the winter daylight due to the fact that normal circulation is weakened by reverse storm-induced circulation. The very moderate magnetic storm on December 18, 2019, induced appreciable changes in the ionospheric plasma parameters throughout the entire range of studied altitudes. The measured data provided additional information for solar-terrestrial relationships study and the ionosphere state prediction. [ABSTRACT FROM AUTHOR]

The occurrence of plasma irregularities and ionospheric scintillation over the Caribbean region have been reported in previous studies, but a better understanding of the source and conditions leading to these events is still needed. In December 2021, three ground-based ionospheric scintillation and Total Electron Content monitors were installed at different locations over Puerto Rico to better understand the occurrence of ionospheric irregularities in the region and to quantify their impact on transionospheric signals. Here, the findings for an event that occurred on March 13–14, 2022 are reported. The measurements made by the ground-based instrumentation indicated that ionospheric irregularities and scintillation originated at low latitudes and propagated, subsequently, to mid-latitudes. Imaging of the ionospheric F-region over a wide range of latitudes provided by the GOLD mission confirmed, unequivocally, that the observed irregularities and the scintillation were indeed caused by extreme equatorial plasma bubbles, that is, bubbles that reach abnormally high apex heights. The joint ground- and space-based observations show that plasma bubbles reached apex heights exceeding 2600 km and magnetic dip latitudes beyond 28°. In addition to the identification of extreme plasma bubbles as the source of the ionospheric perturbations over low-to-mid latitudes, GOLD observations also provided experimental evidence of the background ionospheric conditions leading to the abnormally high rise of the plasma bubbles and to severe L-band scintillation. These conditions are in good agreement with the theoretical hypothesis previously proposed. [ABSTRACT FROM AUTHOR]

It is well known that there exists a thin layer in the lower boundary of the ionosphere between altitudes of 80 and 140 km in which collisional ions and collisionless electrons mix. Local breakdown of charge neutrality may be initiated in this layer by electric fields from the magnetosphere as well as by electric fields generated there by the local neutral winds. The breakdown may be momentarily canceled by Pedersen currents, but complete neutralization is prevented because some ionospheric plasmas are released as outflows by parallel electric fields. Those parallel electric fields are produced by inherent plasma processes in the polar ionosphere and act as auroral drivers. A new scenario creating parallel potential gradients is proposed. [ABSTRACT FROM AUTHOR]

Plasma blobs are night-time ionospheric irregularities whose generation mechanism is still under investigation. A large number of observations highlighted several aspects of their morphology and dynamics. However, the plasma blobs have not been attributed convincingly to a known mechanism. We analyzed the OI 630.0 nm emission images during March and October of 2019 and 2020 (minimum solar activity) using the ground-based all-sky imager at ZF-2 (2.58° S, 60.22° W) in the Amazon region of Brazil. The novelties of the present study are the rarely reported observation of both plasma blob merging and bifurcation. We studied the evolutional dynamics of plasma blobs and observed that blobs are distinct phenomena with unique properties. We attribute the merging of plasma blobs to the "wind reversion effect" (WRE) mechanism caused by a change in the direction of the zonal thermospheric wind from east to west. In some cases, the slower-drifting plasma blobs may merge with the faster ones. Moreover, blobs were observed initially bifurcating at the topside and later divided into two. The activity of the polarized electric field inside the plasma bubble mapping along the magnetic field lines is possibly responsible for the blob's bifurcation. Subjecting the two features of ionospheric plasma blobs to simulation may reveal further the physics of blobs' merging and bifurcation. [ABSTRACT FROM AUTHOR]