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For the past few decades, it has been demonstrated that gravity waves (GWs) and neutral winds can drive ionospheric irregularities on Earth. Still, as far as we know, the formation of ionospheric irregularities on Mars due to GWs has not been well studied. In this study, we use data from the NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission to show evidence of an irregularity event in the Martian ionosphere, potentially seeded by the GWs break (GWB). Statistical findings indicate that the observed ratio of GWB‐related irregularity events varies from ∼0.25 to ∼0.47 each year, and the average ratio in 2015–2020 is ∼0.37. We perform a numerical simulation to provide further insight into the processes behind irregularity formation, which employs neutral wind shear as a source of perturbation in the context of the GWB. The simulations yield results fundamentally aligned with the observed characteristics of ionospheric irregularities in the 2018 event by considering the wind shear as the disturbance source. This study provides supplementary insights into the perturbation sources involved in shaping irregularities within the Martian ionosphere and presents valuable information about the coupling between the Martian ionosphere and the lower atmosphere. Plain Language Summary: Many remote and in situ observations of the Martian ionosphere have been made by landers, orbiting or flyby spacecrafts, for example, Viking, Mars Global Surveyor (MGS), Mars Express (MEX), and Mars Atmosphere and Volatile EvolutioN (MAVEN) missions. It gives us a great opportunity to know the basic ionospheric structure and composition of Mars. However, there are still many scientific questions about the Martian ionosphere that have yet to be studied, for example, the structure of the lower ionosphere, small‐scale ionospheric irregularities, and the role of lower atmospheric effects on the ionosphere. Normally, the ionosphere is a stable stratified structure. However, in some situations, the ionosphere becomes unstable, generating perturbations in the electron density, leading to these so‐called "irregularities." In this study, we report that the break of gravity waves potentially accounts for the formation of ionospheric irregularities observed in the Martian ionosphere by NASA's MAVEN spacecraft. The results of our statistical study support the notion that gravity waves are a crucial seeding source in the formation of irregularities. This investigation adds to the current understanding of perturbation sources responsible for irregularity formation in the Martian ionosphere, while also contributing to our knowledge of the coupling between the Martian ionosphere and the lower atmosphere. Key Points: An example of a Martian ionospheric irregularity event potentially seeded by GWB (the break of gravity waves) is presentedStatistical results show that the proportion ratio of GWB‐related irregularities peaked at ∼0.47 in 2018, reached a minimum of ∼0.25 in 2017, and averaged ∼0.37 over the six yearsThe simulation shows that the formation of the irregularities can be fundamentally reproduced using the GWB‐related wind shear as a perturbation source [ABSTRACT FROM AUTHOR]

The intensity of the equatorial electrojet (EEJ) derived from the magnetic field measurements by the China Seismo-Electromagnetic Satellite (CSES) is analyzed for the low solar activity period of July 2018–April 2022. The CSES spacecraft flies in a Sun-synchronous orbit, providing the first continuous satellite observations of the afternoon EEJ at a fixed local time at 2 p.m. The EEJ intensities from CSES and concurrent observations from the Swarm satellite mission show a good correlation, supporting the reliability of the CSES EEJ data. Spectral analysis of the CSES data reveals the presence of three distinct oscillatory components in the day-to-day variation of the afternoon EEJ: (1) an eastward-propagating 2–3-day oscillation with zonal wavenumber 1, (2) a westward-propagating 5–6-day oscillation with zonal wavenumber 1, and (3) a zonally-symmetric 14–15-day oscillation. These oscillations result from upward-propagating waves in the atmosphere. That is, the first two can be attributed to the ultra-fast Kelvin wave and quasi-6-day wave, respectively, while the latter is likely due to the atmospheric lunar tide. The CSES EEJ data are also compared with lower thermospheric wind measurements by the Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) onboard the Ionospheric Connection Explorer (ICON). The results suggest that the EEJ intensity correlates negatively with the equatorial eastward wind at 100–115 km, consistent with earlier studies. Contributions of different tidal wind components to longitudinal structures of the EEJ are evaluated. A four-peak structure during July–September can be largely explained by the eastward-propagating diurnal tide with zonal wavenumber 3 (DE3), while a two- or three-peak structure during December–January is mainly due to the combined effect of DE3 and the eastward-propagating diurnal tide with zonal wavenumber 2 (DE2). Furthermore, the CSES EEJ data are compared with the electron density measurements from the Langmuir probe onboard CSES. There is a positive correlation between the EEJ intensity and in-situ electron densities at ∼510 km from the same orbit, reflecting the plasma fountain effect. The correlation tends to be higher in the summer hemisphere, which may be due to the meridional wind in the thermosphere.

Quasi 16‐day waves (Q16DWs) are a prominent and recurrent phenomenon in the middle atmosphere, typically observed over winter mid and high latitudes. This study investigates the intense Q16DW event during the 2018–2019 Northern Hemisphere (NH) winter, and explores its propagation in the middle atmosphere and its notable influence on the E‐region ionosphere. Long‐term geopotential height estimates of Aura Microwave Limb Sounder (MLS) reveal that the wave activity under consideration exhibited the largest amplitudes in the mesosphere for past 16 years. An analysis of wind data obtained from medium frequency (MF) and meteor radars, as well as from Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) reanalysis, reveals the presence of a westward‐propagating Q16DW with zonal wavenumber 1 exhibiting notable asymmetry about the equator, with the majority of the wave activity being confined to the NH. The prominently large amplitudes and vertical wavelengths of the wave suggest potential for the wave propagation to extend deep into the E‐region ionosphere. Swarm satellite observations reveal concurrent ∼16‐day oscillations in the eastward component of the geomagnetic field at low latitudes. These oscillations can be attributed to the periodic variations in interhemispheric field‐aligned currents (IHFACs). The ∼16‐day oscillations in the IHFACs are likely a consequence of asymmetric wind‐dynamo action, which is directly or indirectly associated with the Q16DW. These findings suggest that planetary waves originating in the middle atmosphere can cause interhemispheric coupling in the ionosphere. Plain Language Summary: This study examines the strong quasi‐16‐day wave activity that occurred during the winter of 2018–2019 over mid‐to‐high latitudes. A data set consisting of medium frequency (MF) and meteor radar winds, reanalysis data, Aura Microwave Limb Sounder (MLS) and Swarm geomagnetic field measurements is utilized to characterize the wave. The simultaneous occurrence of wave signatures in the middle atmosphere and the ionosphere suggests some atmosphere‐ionosphere coupling. The wave signatures observed in the eastward component of the geomagnetic field are attributed to interhemispheric field‐aligned currents. Key Points: Strong quasi‐16‐day wave signatures in middle atmospheric temperatures and winds during the northern hemispheric winter of 2018–2019Simultaneous occurrence of the wave in the eastward component of the geomagnetic fieldPlanetary wave oscillations of ∼16‐day period observed in the interhemispheric field aligned currents [ABSTRACT FROM AUTHOR]

An exceptionally strong westward propagating quasi‐6‐day wave (Q6DW) with zonal wavenumber 1 in connection with the rare 2019 Southern Hemispheric Sudden Stratospheric Warming (SSW) is observed by two meteor radars at 30°S and is found to modulate and interact with the diurnal tide and gravity waves (GWs). The diurnal tide is amplified every 6 days and a prominent 21 hr child wave attributed to Q6DW‐diurnal tide nonlinear interaction occurs. Q6DW modulation on GWs is confirmed as the 4–5 day periodicity in GW variances. Simultaneously, the Q6DW appears to shift its period toward the periodicity of the modulated GW variances. Enhancement is also observed in the first results of meteor radar observed Q6DW Eliassen‐Palm flux, which may facilitate the global perturbation and persistence of this Q6DW. We conclude that the observed SSW triggered Q6DW‐tide and Q6DW‐GW interactions play an important role in coupling the lower atmospheric forcings to ionospheric variabilities. Plain Language Summary: Our work provides observational evidence for the 6‐day planetary wave‐tide and 6‐day planetary wave‐gravity wave interactions at the Earth's mesosphere and lower thermosphere. The results strongly support the theory that wave‐wave interactions are the primary mechanism coupling planetary waves to ionospheric variability and provide an additional mechanism as the 6‐day wave modulation on the gravity waves. We utilize measurements from two meteor radars to diagnose planetary wave characteristics and identify wave‐wave interactions, and compute the first‐time meteor radar observed Eliassen‐Palm flux. Enhancement is observed in the Eliassen‐Palm flux of 6‐day wave following the SSW maximum phase, which demonstrates that energy of the 6‐day wave is enhanced and therefore, facilitates the global perturbation and persistence of the 6‐day wave for an extended time period. While meteor radar observations are widely used to investigate planetary waves and tides, high meteor detection rate is required for further studying temperature perturbations and small scale waves (e.g., gravity waves). Thus, this work also highlights the capability of a modern multi‐static meteor radar system, Chilean Observation Network De meteOr Radars, in resolving oscillations of small spatial scales over a broad range of periods, and for calculating Eliassen‐Palm flux of planetary waves. Key Points: A dominating W1 Q6DW is observed at 30°S and its Eliassen‐Palm flux is enhanced during the 2019 SH SSWQ6DW amplifies the diurnal tide every 6 days and a strong 21 hr child wave is observedQ6DW modulates the gravity wave variances and its frequency appears to shift accordingly [ABSTRACT FROM AUTHOR]

In this paper, we attempt to compare the Antarctic SSW of 2019 with the Arctic SSW of 2013. Despite being a minor SSW, the rare Antarctic event of September 2019 caused a significant enhancement of stratospheric temperature at high latitudes with more temperature deviation than the Arctic major SSW of 2013. On the basis of the duration and extent of stratospheric zonal mean zonal wind, the 2019 SSW is comparable to major SSWs. An exceptionally strong wave 1 planetary wave was observed during the 2019 SSW. We have investigated the response of the ionospheric total electron content (TEC) during both the SSWs over two low mid-latitude locations Okinawa and Darwin, which form magnetic conjugate pair. The extent of maximum ionospheric deviations during the 2019 SSW is almost comparable to the 2013 SSW in their respective hemispheres. We have observed mostly night time enhancement in TEC during the Arctic SSW, while daytime enhancement is noted during the Antarctic SSW. Hemispheric asymmetry is observed during both the SSWs. Minor geomagnetic storm has been noticed coinciding with each SSW during different phases of the warming. The SSW, solar flux, and storm contribution on the TEC have been calculated during both the SSWs, which shows clear contribution of SSW on the total TEC despite the presence of storm and during moderately high solar activity. We have also investigated the foF2 variations from ionosonde and IRI-16 at the same two locations. The peak electron density from ionosonde responds to the warming in a manner similar to TEC. The foF2 of IRI-16 exhibits overestimation during daytime and underestimation during nighttime on quiet days during low solar activity (2019), while during moderate solar activity (2013), it overestimates during morning hours. However, it can not predict the enhancements of foF2 during SSWs. [ABSTRACT FROM AUTHOR]

Abstract An exceptionally strong westward propagating quasi‐6‐day wave (Q6DW) with zonal wavenumber 1 in connection with the rare 2019 Southern Hemispheric Sudden Stratospheric Warming (SSW) is observed by two meteor radars at 30°S and is found to modulate and interact with the diurnal tide and gravity waves (GWs). The diurnal tide is amplified every 6 days and a prominent 21 hr child wave attributed to Q6DW‐diurnal tide nonlinear interaction occurs. Q6DW modulation on GWs is confirmed as the 4–5 day periodicity in GW variances. Simultaneously, the Q6DW appears to shift its period toward the periodicity of the modulated GW variances. Enhancement is also observed in the first results of meteor radar observed Q6DW Eliassen‐Palm flux, which may facilitate the global perturbation and persistence of this Q6DW. We conclude that the observed SSW triggered Q6DW‐tide and Q6DW‐GW interactions play an important role in coupling the lower atmospheric forcings to ionospheric variabilities.

Using observations from the Global‐scale Observations of the Limb and Disk (GOLD) mission, we investigate post‐sunset ionospheric responses to the September 2019 Antarctic sudden stratospheric warming –first ever from a synoptic perspective. Observations reveal a prevalent quasi‐6‐day periodicity in the equatorial ionization anomaly region over South America and the Atlantic, coincident with enhanced quasi‐6‐day wave (Q6DW) activity in the mesosphere (Liu et al., 2021, https://doi.org/10.1029/2020JA028909). The atmosphere‐ionosphere coupling via large‐scale waves is rarely studied over the ocean due to the lack of observations. More importantly, further analyses suggest that multiple pathways are involved in transmitting the quasi‐6‐day periodicity from the middle atmosphere into the post‐sunset F‐region ionosphere, including modulation of F‐region field aligned winds and pre‐reversal enhancements by the tides and or Q6DW. A remarkable depletion in electron density, attributable to the overall change in thermosphere composition driven by the dissipative tides and or Q6DWs, is also seen during the period of enhanced Q6DW activity. Plain Language Summary: The equatorial ionization anomaly (EIA) owes an appreciable amount of variability to the lower/middle atmosphere wave forcing of various scales. The 2019 September Antarctic sudden stratospheric warming (SSW)—rarely occurs in the southern hemisphere—provides a great opportunity to explore the effects atmospheric waves have on the EIA, as a bunch of waves gets significantly reinforced following the polar vortex disruption. Combining the observations made by the NASA GOLD and TIMED missions reveals a coincident quasi‐6‐day modulation of the post‐sunset EIA crests and mesospheric temperatures during the course of this SSW event, demonstrating a solid case that the ionosphere couples with the atmosphere via planetary‐scale waves. Key Points: A prominent quasi‐6‐day periodicity in the post‐sunset equatorial ionization anomaly (EIA) is observed during the 2019 Antarctic sudden stratospheric warming, coincident with mesospheric quasi‐6‐day waves (Q6DWs)The EIA displays an overall depletion over South America and the Atlantic when the Q6DWs are quite activeMultiple mechanisms play roles in coupling of the Q6DW with the post‐sunset EIA [ABSTRACT FROM AUTHOR]

• Atmospheric variability leads to noise in TEC response to proton precipitation. • Atmosphere driven noise of TEC disturbances has UT variation. • Such noise strongly change during the solar proton event. • Atmospheric variability mask ionospheric response to space weather events. • Likelihood of ionospheric afterstorm effects decreases due to atmospheric variability. The purpose of this study is to investigate the role of internal atmospheric variability in the detection of ionospheric response to precipitating protons of solar and magnetospheric origin during 17–23 January 2005. For this purpose, we analyzed the results of ensemble runs of the whole atmosphere model EAGLE. We confirmed that the proton precipitation produces (directly and indirectly) significant TEC (Total Electron Content) enhancements at different latitudes. However, we also showed that internal atmospheric variability can strongly mask the forced signal, especially its after-effects, which can lead to false interpretation if only one model realization is considered. The ensemble standard deviation, or an atmospheric noise level, in TEC has a clear UT variation and spatial distribution, which defines the derivability of the signal depending on the time and the region of the forced effects. The noise itself is also affected by the increased ionization from protons, but its behavior is nonlinear and depends on the latitude, spectrum of the event and its time of occurrence. We conclude that studies of the ionospheric response to various space weather phenomena require the application of whole atmosphere models in ensemble mode to ensure statistical significance of the obtained results and a reliable interpretation of the observational data. [ABSTRACT FROM AUTHOR]

We present a study of the major and minor SSWs that have occurred in the Northern Hemisphere (NH) over the past four decades (1981–2020) from a statistical perspective. This study investigates the frequency and timing of occurrence of major and minor SSWs during the specified period. A correlation is observed between the timing of the earliest minor SSWs and final events, having different correlation coefficients in all four decades, with minimum correlation during the decade 2001–2010, including the extended solar minima. In the second part of our work, we examined SSW events of the last 18 winters based on ERA5 and MERRA2 reanalysis data. Surprisingly, the stratospheric parameters have been found to show different behaviors depending on the number of SSW events in a winter. The temperature and zonal mean zonal wind variations are stronger during winters with one major SSW, rather than winters having major SSW accompanied by minor SSWs. Finally, we conducted four case studies to investigate the atmospheric and ionospheric response to major and minor SSWs over three stations, namely Okinawa (26.21 ° N , 127.68 ° E), Yamagawa (35.45 ° N , 133.62 ° E), and Wakkanai (45.16 ° N , 141.75 ° E). Like the stratospheric parameters, ionospheric parameters are not found to be much depended on the number of SSWs in a winter, but they are found to vary from event to event. Significant latitudinal variations in ionospheric parameters were observed in all the events, with more pronounced effects toward lower latitudinal station Okinawa. Although ionospheric disturbances are more pronounced during major SSWs, some severe minor events also have notable ionospheric impacts just as the major SSW. [ABSTRACT FROM AUTHOR]

Abstract Using observations from the Global‐scale Observations of the Limb and Disk (GOLD) mission, we investigate post‐sunset ionospheric responses to the September 2019 Antarctic sudden stratospheric warming –first ever from a synoptic perspective. Observations reveal a prevalent quasi‐6‐day periodicity in the equatorial ionization anomaly region over South America and the Atlantic, coincident with enhanced quasi‐6‐day wave (Q6DW) activity in the mesosphere (Liu et al., 2021, https://doi.org/10.1029/2020JA028909). The atmosphere‐ionosphere coupling via large‐scale waves is rarely studied over the ocean due to the lack of observations. More importantly, further analyses suggest that multiple pathways are involved in transmitting the quasi‐6‐day periodicity from the middle atmosphere into the post‐sunset F‐region ionosphere, including modulation of F‐region field aligned winds and pre‐reversal enhancements by the tides and or Q6DW. A remarkable depletion in electron density, attributable to the overall change in thermosphere composition driven by the dissipative tides and or Q6DWs, is also seen during the period of enhanced Q6DW activity.

This brief report examines the ground-based total electron content (TEC) in Asian sector during August–October 2019, covering the period of a stratosphere sudden warming (SSW) occurred in Antarctica. The analysis reveals pronounced ionospheric day-to-day variability with distinct periodicities. The most dominant and long-lasting periodicities are quasi-10 days and quasi 14-day during September and October, while a quasi 6-day also present in September. The 10-day and 6-day TEC oscillations were attributed by previous studies solely to the Antarctic SSW while assuming negligible geomagnetic effects. By comparing co-located ground mesospheric wind observations, along with the interplanetary electric field (IEF) and geomagnetic activity (Kp index), we demonstrate that the quasi 14-day oscillation is mainly driven by low-level geomagnetic activities, while quasi-6 days oscillation is driven by mesospheric wind changes during the SSW. The 10-day oscillation, on the other hand, is driven by both IEF and mesospheric wind in September, but by IEF in October. These results demonstrate that low-level geomagnetic activities traditionally classified as “quiet conditions” can induce significant day-to-day oscillations in TEC, and their impacts should not be ignored when studying meteorological (e.g., SSWs) impacts on the ionosphere.

The strong quasi 2‐day wave (Q2DW) event that occurred during January 2015 was studied by employing medium frequency and meteor radar observations of winds from three different locations at low latitudes with considerable longitudinal separation. The large‐amplitude wave activity that occurred in mid‐January is the focus of this study. Using the advantage offered by the longitudinal separation among the stations, the zonal structure of the Q2DW has been examined. The presence of a dominant westward propagating zonal wavenumber 3 (W3) mode was revealed in the analysis. The availability of geomagnetic field data from a number of stations at low latitudes enabled us to observe the wave event at ionospheric dynamo region heights. The phase‐longitude cross‐section of the quasi 2‐day oscillation in the Y component of the geomagnetic field reiterates the presence of the dominant W3 mode observed at lower altitudes. Similar zonal characteristics and periods of the wave signatures found at mesospheric altitudes and in the manifestation of quiet‐time ionospheric currents in geomagnetic field are in conformity with the notion of the wave coupling of the atmosphere‐ionosphere system. Plain Language Summary: This study examines the strong quasi 2‐day wave event that occurred during January 2015 at low latitudes using medium frequency and meteor radar observations. The analysis reveals the presence of a dominant westward propagating zonal wavenumber 3 mode. Geomagnetic field data from multiple stations at low latitudes show similar zonal characteristics and periods of the wave signatures, which suggests wave coupling of the atmosphere‐ionosphere system. Key Points: Strong quasi 2‐day wave in the mesosphere during January 2015Simultaneous presence of quasi 2‐day oscillation in magnetic field with similar zonal structureEvidence for vertical coupling by quasi 2‐day wave [ABSTRACT FROM AUTHOR]

The mesospheric polar vortex (MPV) plays a critical role in coupling the atmosphere-ionosphere system, so its accurate simulation is imperative for robust predictions of the thermosphere and ionosphere. While the stratospheric polar vortex is widely understood and characterized, the mesospheric polar vortex is much less well-known and observed, a short-coming that must be addressed to improve predictability of the ionosphere. The winter MPV facilitates top-down coupling via the communication of high energy particle precipitation effects from the thermosphere down to the stratosphere, though the details of this mechanism are poorly understood. Coupling from the bottom-up involves gravity waves (GWs), planetary waves (PWs), and tidal interactions that are distinctly different and important during weak vs. strong vortex states, and yet remain poorly understood as well. Moreover, generation and modulation of GWs by the large wind shears at the vortex edge contribute to the generation of traveling atmospheric disturbances and traveling ionospheric disturbances. Unfortunately, representation of the MPV is generally not accurate in state-of-the-art general circulation models, even when compared to the limited observational data available. Models substantially underestimate eastward momentum at the top of the MPV, which limits the ability to predict upward effects in the thermosphere. The zonal wind bias responsible for this missing momentum in models has been attributed to deficiencies in the treatment of GWs and to an inaccurate representation of the high-latitude dynamics. In the coming decade, simulations of the MPV must be improved.

Modeling studies have predicted that the acoustic resonance of the atmosphere during geophysical events such as earthquakes and volcanos can lead to an oscillation of the geomagnetic field with a frequency of about 4 mHz. However, observational evidence is still limited due to scarcity of suitable events. On 15 January 2022, the submarine volcano Hunga Tonga‐Hunga Ha'apai (20.5°S, 175.4°W, Tonga) erupted in the Pacific Ocean and caused severe atmospheric disturbance, providing an opportunity to investigate geomagnetic effects associated with acoustic resonance. Following the eruption, geomagnetic oscillation is observed at Apia, approximately 835 km from Hunga Tonga, mainly in the Pc 5 band (150–600 s, or 1.7–6.7 mHz) lasting for about 2 hr. The dominant frequency of the oscillation is 3.8 mHz, which is consistent with the frequency of the atmospheric oscillation due to acoustic resonance. The oscillation is most prominent in the eastward (Y) component, with an amplitude of ∼3 nT, which is much larger than those previously reported for other events (<1 nT). Comparably large oscillation is not found at other stations located further away (>2700 km). However, geomagnetic oscillation with a much smaller amplitude (∼0.3 nT) is observed at Honolulu, which is located near the magnetic conjugate point of Hunga Tonga, in a similar wave form as at Apia, indicating interhemispheric coupling. This is the first time that geomagnetic oscillations due to the atmospheric acoustic resonance are simultaneously detected at magnetic conjugate points. Key Points: The effect of the January 2022 Hunga Tonga‐Hunga Ha'apai volcano eruption on the geomagnetic field is examinedGeomagnetic oscillation with a frequency of ∼3.8 mHz is observed simultaneously near the volcano and its magnetic conjugate pointThe oscillation is attributed to the acoustic resonance of the atmosphere [ABSTRACT FROM AUTHOR]

The flight of rocket‐propelled vehicles triggers acoustic waves that propagate through the atmosphere and induce electron density changes at ionospheric heights. These perturbations can be observed using multi‐frequency Global Navigation Satellite System (GNSS) receivers, sounding the ionosphere by taking advantage of its dispersive property. We extend an existing lightweight modeling approach to reproduce the shape, amplitude and timing of the observed perturbations using a semi‐empirical source model based on flight conditions and engine characteristics to express the acoustic perturbations generated by the exhaust gases and a three‐dimensional ray tracing method accounting for dissipative and dispersive effects to propagate the waveform through the atmosphere. Perturbations of the ionospheric plasma are estimated using a simplified model of collisions between neutral and charged particles. The developed simulation framework is applied to the launch of SpaceX's Crew Dragon spacecraft on May 30, 2020 from Kennedy Space Center. The amplitude, shape and timing of simulated perturbations are in good agreement with measurements made by GNSS ground‐based receivers located in the region. This approach is a new step toward rapid characterization of anthropogenic activity in the atmosphere as well as natural events releasing energy near the Earth's surface (earthquake, volcanic activity, etc.) based on GNSS technology. Plain Language Summary: Perturbations of the ionosphere triggered by the ascent flight of a Falcon 9 rocket are simulated using a semi‐empirical source model describing the disturbances caused by the energy released in the environment by the rocket engines. The results are successfully compared to measurements made by ground‐based receivers sounding the ionosphere. This approach is a new step toward rapid characterization of anthropogenic activity in the atmosphere as well as natural events releasing energy near the Earth's surface (earthquake, volcanic activity, etc.). Key Points: The ascent flight of the Crew Dragon spacecraft induced perturbations of the ionospheric plasma observed using ground‐based Global Navigation Satellite System (GNSS) receiversPerturbations caused by exhaust gases are simulated using linear acoustic propagation and simplified atmosphere‐ionosphere couplingProposed approach opens the possibility of characterization of rocket‐propelled vehicles engines and trajectory based on GNSS soundings [ABSTRACT FROM AUTHOR]

The paper presents the results of observations of the sporadic Es layer during the period of meteorological disturbances in Kaliningrad in October 2017 and 2018 under quiet geomagnetic conditions. During the meteorological storms (October 29–30, 2017 and October 23–24, 2018), significant changes occurred in the dynamics of the Es-layer critical frequency. Observations of atmospheric and ionospheric disturbances in the Kaliningrad region show that the delay between the ionospheric response and the moment of maximum disturbances in atmospheric parameters is about 3 hours. These phenomena at the heights of the E-region might have been caused by propagation of acoustic-gravity waves generated by convective processes in the lower atmosphere during periods of a meteorological storm. Intensification of turbulent processes in the lower thermosphere leads to an increase in the atmospheric density and, accordingly, to higher recombination rates. This leads to a rapid decrease in the concentration of ions and, consequently, to a decrease in the critical frequency of the sporadic layer below the sensitivity threshold of ionosondes.

This study examines the day-to-day variability of low-latitude ionosphere using global ionospheric specification (GIS) electron density profiles derived from FORMOSAT-7/COSMIC-2 radio occultation measurements during a deep solar minimum period of August 2019 to July 2020. The measurements reveal significant daily variations over dayside low latitudes, yielding about 10 - 20% standard deviation in equinoxes, 20 - 30% in solstices, reaching 40 - 50% in winter. The nighttime deviations could be 30 - 60%, being largest in solstices. Day-to-day variations are also observed in the longitudinal wave-4 structures. The period mostly remained geomagnetically quiet except for some moderate disturbances on a few days. Tidal decomposition of the GIS electron density shows that in-situ forced migrating diurnal (DW1) terdiurnal (TW3) oscillations and the background zonal mean yield only ~25% of the daily variations despite accounting for almost 75 - 90% of the observed electron density. Thus, forcing from lower atmosphere dominates the contribution (~75%) to the observed daily variations. Only about one third of this lower atmospheric forcing comes from the migrating semidiurnal SW2 and the usually investigated nonmigrating diurnal eastward DE2, DE3, stationary planetary wave SPW3, SPW4, and semidiurnal eastward SE1, and SE2 components. The residual tides other than those mentioned above, including secondary waves through non-linear interactions and other planetary waves, thus significantly influence the day-to-day variations in electron density and modify the longitudinal wave structures. [ABSTRACT FROM AUTHOR]

Mesospheric winds from two longitudinal sectors at 53°N latitude are combined to investigate quasi‐two‐day waves (Q2DWs) and their nonlinear interactions with tides. In a summer 2019 case study, we diagnose the zonal wavenumber m of spectral peaks at expected frequencies through two dual‐station approaches, a phase differencing technique (PDT) on individual spectral peaks and a least squares procedure on family batched peaks. Consistent results from the approaches verify the occurrences of Rossby‐gravity modes (m = 3 and 4 at periods T = 2.1 and 1.7 days), and their secondary waves (SWs) generated from interactions with diurnal, semi‐diurnal, ter‐diurnal, and quatra‐diurnal migrating tides. We further extend the PDT to 2012–2019, illustrating that Q2DWs exhibit significant interannual variability. Composite analysis reveals seasonal and altitude variations of the Rossby‐gravity modes and their SWs. The Rossby‐gravity modes maximize in local summer, whereas their 16‐ and 9.6‐h SWs appear more in winter. Plain Language Summary: The quasi‐two‐day wave is the strongest and most widely studied planetary wave occurring in the mesosphere. Existing observational analyses are based on either single‐satellite or ‐station approaches, which suffer from temporal and spatial aliasing, respectively. The current study implements and develops dual‐station approaches to investigate the mesospheric quasi‐two‐day wave at 53°N latitude, in a case and a statistical study. Our approaches allow diagnosing both the frequency and zonal wavenumber. In the case study, we diagnosed two Rossby‐gravity modes and the secondary waves (SWs) of the nonlinear interactions between the Rossby‐gravity modes and the migrating tides at periods of 24, 12, 8, and 6 h. While the interactions with the 24‐ and 12‐h tides are expected, those with the 8‐ and 6‐h tides are reported for the first time. In the statistical study, we report the seasonality and altitude variation of the Rossby‐gravity modes and their most dominant SWs. Key Points: Multi‐station approaches are developed and applied to diagnose zonal wavenumber m of near‐2‐day, ‐16‐h, ‐9.6‐h, and ‐6.9‐h spectral peaksDiagnosed are Rossby‐gravity modes with m = 3 and 4, and their secondary waves from nonlinear interactions with 24‐, 12‐, 8‐, and 6‐h migrating tidesSeasonally, the most dominant near‐2‐day, ‐16‐h, ‐9.6‐h waves occur in summer, winter, and winter, respectively [ABSTRACT FROM AUTHOR]

We present the results of complex obser-vations of various parameters of the middle and upper atmosphere over Siberia in December 2012 – January 2013, during a major sudden stratospheric warming (SSW) event. We analyze variations in ozone concentration from microwave measurements, in stratosphere and lower mesosphere temperatures from lidar and satellite measurements, in the F2-layer critical frequency (foF2), in the total electron content (TEC), as well as in the ratio of concentrations of atomic oxygen to molecular nitrogen (O/N2) in the thermosphere. To interpret the observed disturbances in the upper atmosphere, the experimental measurements are compared with the results of model calculations obtained with the Global Self-consistent Model of Thermosphere—Ionosphere—Protonosphere (GSM TIP). The response of the upper atmosphere to the SSW event is shown to be a decrease in foF2 and TEC during the evolution of the warming event and a prolonged increase in O/N2, foF2, and TEC after the SSW maximum. For the first time, we observe the relation between the increase in stratospheric ozone, thermospheric O/N2, and ionospheric electron density for a fairly long time (up to 20 days) after the SSW maximum at midlatitudes.

Abstract The relationship between day-to-day variability of equatorial plasma bubbles (EPBs) and the neutral atmosphere is studied. This study is based on the previous study in which the GPS scintillation index and the tropospheric cloud-top temperature are used as proxies for EPB activity and atmospheric perturbations, respectively, and a correlation was found between their day-to-day variations. In this paper, we maintained the same GPS scintillation data but substituted the atmospheric data via an assimilation run of the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). Cross-correlation between the EPB activity and the atmospheric temperature is similar to the results in Ogawa et al. (Earth Planets Space 61:397–410, 2009). The new findings from our study include (1) an enhanced correlation between the EPB activity and the neutral atmosphere is found in horizontally and vertically large areas, (2) the longitudinal disturbance of atmospheric temperature and wind velocity during the EPB-active days is enhanced, and (3) the enhancement of atmospheric disturbance during the EPB-active days shows a similarity to the characteristics of large-scale wave structures in the ionosphere. These results more clearly support couplings between EPBs and the neutral atmosphere.

Determination of the physical mechanisms of energy transfer of tropospheric disturbances to the ionosphere is one of the fundamental problems of atmospheric physics. This article presents the results of observations carried out using two-wavelength lidar sensing at tropospheric altitudes and satellite GPS measurements during a meteorological storm in Kaliningrad (Russia, 54.7° N, 20.5° E) on 1 April 2016. During lidar sensing, it was found that the amplitudes of variations in atmospheric parameters with periods of acoustic (AWs) and internal gravity (IGWs) waves significantly increased. As a result of numerical modeling using the AtmoSym software package, it was shown that there is a noticeable increase in the period of temperature disturbances from 6–12 min to 10–17 min at altitudes from 150 km up to 230 km during the vertical propagation of acoustic waves and internal gravity waves from the troposphere. Nonlinear and dissipative processes in this layer lead to the formation of sources of secondary waves in the thermosphere with periods longer than those of primary ones. In this case, the unsteady nature of the wave source and the short duration of its operation does not lead to significant heating of the thermosphere. Simultaneous satellite observations demonstrate the response of the ionosphere (total electron content (TEC) disturbance) to tropospheric disturbances. Analysis of the time series of the amplitudes of the reflected lidar signal and TEC made it possible to determine that the response time of the ionosphere to tropospheric disturbances is 30–40 min.

The feasibility of detection of electromagnetic response in the upper ionosphere to ground large‐scale extremely low frequency (ELF) transmitters (e.g., submarine communication systems) by low‐orbiting satellites is discussed. Several times when the DEMETER satellite (660 km) was in the vicinity of the ELF transmitter on the Kola Peninsula, the electric and magnetic sensors operating in a burst mode detected a narrowband 82‐Hz emission. The same emission associated with the ELF transmitter was observed by a ground‐based magnetometer. We modeled the rate of the ELF wave energy leakage into the upper ionosphere from an oscillating 82‐Hz linear current with an infinite length suspended above a high‐resistive ground. A realistic altitudinal profile of the plasma parameters has been reconstructed with the use of the IRI ionospheric model. The modeled amplitudes and polarization of electromagnetic response of the upper ionosphere are in reasonable agreement with the properties of emission recorded by the satellite. Key Points: An 82‐Hz emission from ground ELF transmitter was recorded in the upper ionosphere by satellite DEMETERSpectral power of electric and magnetic components in the nightside ionosphere is about several nT2/Hz and tens of (μV/m)2/HzModeling the electromagnetic response to oscillating line current above a high‐resistive crust matches the observational result [ABSTRACT FROM AUTHOR]

This paper presents the observations of ionospheric parameters in Kaliningrad (54° N, 20° E) during a meteorological storm in the Baltic Sea during October 2017 and 2018. Analysis of the total electronic content (TEC) during the storm showed that perturbations of the TEC values from the median can reach two standard deviations of the value. For the critical frequency of the F2 layer, it was 1.5–1.6 times the standard deviations. On days of a meteorological storm, significant changes were noted in the dynamics of the E-layer’s critical frequency. The reasons for the occurrence of the observed phenomena were due to the propagation of acoustic-gravity waves generated by convective processes in the lower atmosphere during periods of a meteorological storm. Spectral analysis of TEC variations revealed an increase in the amplitudes of ionospheric variations 10–16 min over the area of a meteorological storm. The analysis allowed us to conclude that ionospheric perturbations during the meteorological perturbation were caused by increased acoustic-gravity wave (AGW) generation processes in the lower atmosphere. The most likely cause of negative ionospheric disturbances were processes associated with the dissipation of AGW propagating from the area of a meteorological storm and increased turbulence in the lower thermosphere.

Abstract We present the results of studying the behavior of parameters of the mesosphere and lower thermosphere (MLT) and the ionosphere over the Eastern Siberia region during a minor winter sudden stratospheric warming (SSW) in early February 2016. We used the data from spectrometric measurements of OH ((6-2), 834.0 nm, ∼87 km) and O 2 At ((0–1), 864.5 nm, ∼94 km), emissions from the Geophysical Observatory at the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (51.8°N, 103.1°E, Tory). These emissions originate at the MLT heights. We also used the vertical sounding data on the peak electron density (NmF2), and on the peak height (hmF2). These data were obtained with the DPS-4 Irkutsk ionosonde (52.3°N, 104.3°E). For the analysis, we also involved the MLS Aura satellite data of measuring vertical temperature profiles and the MERRA reanalysis data. We found the MLT and ionospheric signatures for the 2016 February SSW. At the MLT heights, the OH and O 2 emissions intensities increased by a factor of ∼2 and ∼3, respectively, and the temperature fell by ∼20 K. Analyzing the mesopause temperature variability and comparing the mean seasonal values showed an essential increase in the wave activity at the MLT. At the F2-region ionospheric heights, we revealed significant (up to ∼80%) NmF2 positive disturbances in the postmidnight hours and an essential increase (up to ∼ 2 times relative to root-mean-square values) in the amplitudes of the tidal component of the NmF2 disturbance. The results of the study show that a minor SSW may significantly impact the state of the MLT and the ionosphere at midlatitudes. Highlights • Minor SSW in February 2016 affected the upper neutral atmosphere and ionosphere. • MLT temperature variability, caused by wave activity, increased significantly. • Positive NmF2 disturbances in the postmidnight and early morning hours occurred. • The results confirm couplings between the lower atmosphere, MLT, and ionosphere. [ABSTRACT FROM AUTHOR]

Abstract We study the effects of the middle atmosphere dynamics on the mid-latitude and subauroral ionosphere during the winter jet stream that occurred at the stratosphere/lower mesosphere heights in the Northern Hemisphere over 2008–2013. The used ionospheric data were obtained from measurements taken at the vertical sounding ionosonde chain covering the entire Eurasian continent. We reveal regional features of the F2 -layer ionospheric response to dynamic processes during the winter circumpolar vortex evolution in the strato-mesosphere. The variations in the ionospheric parameters observed at different longitudes in the Northern Hemisphere significantly depended on the ionosonde position relative to the jet stream zone in the strato-mesosphere. The difference in critical frequency values for the ionosondes spaced longitudinally by only 15–20° could reach about 1.5–2 MHz, depending on the observation point location under the jet stream or off the latter. We consider the molecular gas upwelling/downwelling to/from the lower thermosphere in active regions of circulation as a mechanism for the underlying atmosphere impact on the ionosphere. Irregular variations in the ionospheric parameters could also be caused by wave disturbances of various spatial and temporal scales (including internal gravity waves) propagating into the thermosphere from the underlying atmosphere. The mesoscale wave generation source at the strato-mesospheric heights was associated with baroclinic instabilities in the spatially non-uniform, high-velocity winter jet stream. Highlights • Ionosphere response on winter stratosphere/mesosphere jet stream. • Eurasian ionosonde chain data are used for ionosphere study. • Specific longitudinal ionosphere dynamics over jet stream was found. • Different mechanisms of thermosphere [O]/[N 2 ] ratio change are discussed. [ABSTRACT FROM AUTHOR]

We examine excitation of ultralow frequency (ULF) electromagnetic waves by an atmospheric lightning stroke in the upper ionosphere and the role of the ionospheric Alfvén resonator (IAR) in this process. We have theoretically calculated with the developed numerical model the spatial and spectral structures of electromagnetic disturbance in the ULF frequency range 0.1–6.0 Hz excited by an atmospheric lightning stroke on the ground and at ionospheric altitudes. The frequency band under consideration comprises typical frequencies of the IAR and the ionospheric waveguide. The spectra of horizontal magnetic and electric components reveal a spectral multiband structure in the upper ionosphere. The form of spectra depends significantly on the horizontal distance ρ from the source: spectral peaks associated with the IAR are evident at ρ≤400 km, whereas at ρ≥1,000 km the spectral peaks (>4 Hz) corresponding to the ionospheric waveguide modes can be seen. The model predicts that a vertical electric discharge with the charge moment MQ=106 C·m produces at altitude 500 km and ρ = 40 km a pulse with electric and magnetic amplitudes of about 4 mV/m and 4 nT, correspondingly, and duration ∼0.2 s. The pulse amplitude decays rather slowly with distance ∝ρ−1. Detection of ULF response in the upper ionosphere to isolated intense lightning stroke by low‐orbiting satellites with magnetic or electric sensors onboard is quite feasible. Key Points: Spectra of ULF electromagnetic field excited by atmospheric lightning stroke reveal spectral multiband structure in the upper ionosphereSpectral peaks associated with the ionospheric Alfven resonator are evident at ≤400 kmAn electric discharge with the charge moment 106 C m produces at 500 km a pulse with amplitudes about 4 mV/m and 4 nT [ABSTRACT FROM AUTHOR]

Abstract: Using a global atmosphere‐ionosphere coupled model, the characteristics and excitation source of traveling ionospheric disturbances (TIDs) during geomagnetically quiet periods are studied. This is the first paper concerning the simulation of TIDs generated by upward propagating gravity waves (GWs) that are spontaneously generated in the model. The dominant horizontal wavelengths of the simulated TIDs range from 700 to 1,500 km. The dominant periods and horizontal phase velocities of TIDs are 45–90 min and 250–300 m s−1, respectively. These features are the same as those of GWs in the 250–300 km height region. The phase of the electron density variations due to TIDs descends with increasing time, which is characteristic of the upward propagation of GWs. These electron density variations that are caused due to TIDs are explained by the transport processes of a neutral wind along a geomagnetic field line. These results indicate that the electron density variations respond locally to the passage of neutral wind fluctuations associated with upward propagating GWs. The GWs that excite TIDs are secondary GWs, which are generated in the mesosphere and lower thermosphere via the dissipation/breaking of tropospheric GWs. The magnitudes of TIDs at middle latitudes are larger in winter than in summer. The mechanisms of seasonal and day‐to‐day variations in TIDs that are caused due to GWs are discussed in this study. [ABSTRACT FROM AUTHOR]

Quantitative information about the effect of atmospheric influences on the variability of upper mesosphere–lower ionosphere (UMLI) region is not clearly known yet. To investigate the relationship between the lower atmosphere and UMLI region, we compared the amplitude of very low frequency (VLF) signals with the atmospheric parameters such as total column Ozone (TCO) density and stratospheric temperature at various heights for the first time for three different latitudinal regions. We show that the VLF amplitude is strongly correlated with the TCO density, stratospheric temperatures for mid-latitude propagation paths throughout the years. For high and low latitude regions, this correlation between the VLF amplitude and atmospheric parameters is poor and not significant. This study indicates the experimental observation of latitudinal dependence of atmospheric influence on the upper mesosphere. [ABSTRACT FROM AUTHOR]

We studied ionospheric disturbances caused by the wave-like processes in the middle atmosphere. The ionospheric data were obtained from continuous measurements with the DPS-4 vertical sounding ionosondes in Irkutsk and Norilsk over 2008–2010. The ionospheric disturbances were considered as deviations of the F 2 layer peak density variations from the daily average values. We also used ECMWF ERA-Interim reanalysis data for the middle atmosphere dynamics analysis, and the Aura MLS data on the atmospheric temperature at the stratosphere and upper mesosphere heights obtained within the above period. The analysis allowed us to reveal periods of middle-scale wave motions in the stratosphere and lower mesosphere during winter seasons in the Northern Hemisphere. The wave motions observed in the study were associated with the jet streams at the stratosphere/lower mesosphere heights localized mainly between 50 and 80°N. The middle-scale waves in the stratosphere were compared with ionospheric disturbances over two ionosonde stations in Irkutsk and Norilsk. A noticeable increase in the wave activity was found at the F 2 layer heights during the periods of stratospheric wave activity. For both stations, the maximal variability in the ionosphere F 2 layer parameters was observed in winter with the minimal variability observed in summer. The summer–winter difference was more pronounced in the case of the high-latitude ionosonde in Norilsk. [ABSTRACT FROM AUTHOR]

This paper presents a simple linear regression model that enables to quantify the contribution of high-latitude stratospheric temperature and solar radiation (describes by its proxy F10.7) to the variability of the low-latitude TEC during winter. The model is based on cross-correlation analysis performed on the Aura MLS temperature measurements and the global CODE TEC data for the period of time 2005–2010, i.e. at low to moderate solar activity (F10.7 changes between ~65 and ~140 solar flux units). It revealed that the temperature at altitude of ~40 km and latitude of ~60°N describes the most typical winter conditions and shows the largest negative correlation with the low-latitude TEC. This temperature namely is included in the regression model. The model results have been compared with the TEC data by calculating the standard deviation (STD). The comparison indicated that the regression model describes almost half of the real variability of the global TEC and that the contribution of the temperature (that is only a part of forcing from below) is almost half of the solar variability (i.e. external forcing related to the photo-ionization). A possible mechanism for explaining the relationship between the high-latitude stratospheric increase of the temperature and low-latitude decrease of the TEC is suggested. [ABSTRACT FROM AUTHOR]

We investigated variations in the neutral atmosphere and ionosphere parameters within a large range of heights in the Eastern Siberia region during the 2013 January sudden stratospheric warming (SSW). The analysis is based on: the data from spectrometric measurements of OH (∼87 km, 834.0 nm, (6–2)) and At O 2 (∼94 km, 864.5 nm, (0–1)) upper atmospheric emissions, the data from the Irkutsk DPS-4 Digisonde, the data on electron and ion temperatures and the meridional component of the neutral wind from the Irkutsk Incoherent Scatter Radar, the satellite data on the vertical temperature distribution in the atmosphere from Aura MLS v3.3, and the MERRA reanalysis data. We detected the disturbances of the neutral atmosphere temperature from the stratosphere to the mesosphere and lower thermosphere (MLT). The temperature at 10 hPa (∼32 km) increased by ∼70 K up to ∼270 K, the temperature at 0.01 hPa (∼80 km) decreased by 50 K, and reached ∼170 K. At the MLT heights, an increase in the intensities of the OH and O 2 emissions by a factor of 2–2.5 relative to the undisturbed conditions was revealed. At the F2-layer height, the plasma parameter disturbances were found. After 2013 January 10, interruption of the correlation between NmF2 and hmF2 occurred. Ion temperature cooling reaching 50 K was observed on January 1–10, changing to a quick increase by 50 K for several days after January 10. The neutral wind meridional component and the electron temperature decreased over January 1–21. The observed effects can be probably caused by atmospheric circulation disturbances and amplification in the vertical transfer. The disturbances in the upper atmospheric and in ionospheric parameters during SSW can evidence the coupling between the lower and upper atmosphere. [ABSTRACT FROM AUTHOR]

The Canadian Ionosphere and Atmosphere Model (Canadian IAM or C-IAM) is a new project aimed at the development of whole atmosphere modeling capabilities in Canada. The first version of the C-IAM is comprised of the extended Canadian Middle Atmosphere (CMAM) and the Murmansk's Upper Atmosphere Model (UAM), currently coupled in a one-way manner. The model extends from the surface to the inner magnetosphere and is able to describe the impact on the upper atmosphere and ionosphere of self-consistently generated lower atmosphere dynamical variability. In the current study, the C-IAM has been used to investigate the physical mechanisms responsible for forming the 4-peak longitudinal structure of the 135.6 nm ionospheric emission observed by the IMAGE satellite over the tropics at 20:00 local time from March 20 to April 20, 2002. To perform this study, the C-IAM has been run for this whole observation period taking into account a realistic day-to-day variation in solar and geomagnetic activity. The 4-peak longitudinal structure produced by the model and averaged over the observation period is in a good agreement with the observations. Analysis of the model results suggests that the main mechanism for generating the longitudinal structure in the ionospheric emission after sunset is a modification of the ionospheric electric field in the E region caused in the course of the daytime by differences in the diurnal evolution of the zonal wind in different longitudinal sectors due to waves penetrating from the lower atmosphere. In agreement with the earlier findings, our study showed that this mechanism is driven mainly by the diurnal eastward propagating tide with zonal number 3. A small contribution to the formation of the 4-peak structure is also provided by longitudinal variations in meridional wind and neutral composition at the F2 layer altitudes. The 4 peaks being clearly visible in the monthly mean result are not, however, present every day. Day-to-day variability of both the diurnal wind evolution and geomagnetic activity can significantly modify the longitudinal structure of the ionospheric emission. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the global spatial (latitude and altitude) structure and temporal variability of the ∼23-day ionospheric zonally symmetric (s =0) planetary wave (PW) seen in the Northern winter of 2008/2009 (October 2008–March 2009). It is shown that these ∼23-day ionospheric oscillations are forced from PWs propagating from below. The COSMIC ionospheric parameters foF2 and hmF2 and electron density at fixed altitudes and the SABER temperatures were utilized in order to define the waves which are present simultaneously in the atmosphere and ionosphere. The long-period PWs from the two data sets have been extracted through the same data analysis method. The similarity between the lower thermospheric ∼23-day (s =0) temperature PW and its ionospheric electron density response provides valuable and strong experimental evidence for confirming the paradigm of atmosphere–ionosphere coupling. [Copyright &y& Elsevier]

This paper provides a comprehensive update on sporadic E layers that is placed in the context of atmosphere-ionosphere coupling, exemplified here by the fundamental windshear theory processes that govern sporadic E layer formation and variability. Some basics of windshear theory are provided first, followed by a summary of key experimental results, their interpretation and physical understanding. The emphasis is placed on the wind shear control of the diurnal and sub-diurnal variability and altitude descent of sporadic E layers and the key role behind these properties of the diurnal and semidiurnal tides. Furthermore, the paper summarizes recent observations that establish a role also for the planetary waves in sporadic E layer occurrence and long-term variability. The possible mechanisms behind this interaction are examined and evidence is presented which shows that planetary waves affect sporadic E layers indirectly though the amplitude modulation of tides at lower altitudes in the MLT region. Only a brief mention is made about gravity wave effects on sporadic E, which apparently exist but cannot be as crucial in layer forming as thought in the past. There is now enough evidence to suggest that mid- and low-latitude sporadic E is not as 'sporadic' as the name implies but a regularly occurring ionospheric phenomenon. This may suggest that the sporadic E layer physics can be incorporated in large-scale atmosphere-ionosphere coupling models. [ABSTRACT FROM AUTHOR]

There are several external sources of ionospheric forcing, including these are solar wind-magnetospheric processes and lower atmospheric winds and waves. In this work we review the observed ion-neutral coupling effects at equatorial and low latitudes during large meteorological events called sudden stratospheric warming (SSW). Research in this direction has been accelerated in recent years mainly due to: (1) extensive observing campaigns, and (2) solar minimum conditions. The former has been instrumental to capture the events before, during, and after the peak SSW temperatures and wind perturbations. The latter has permitted a reduced forcing contribution from solar wind-magnetospheric processes. The main ionospheric effects are clearly observed in the zonal electric fields (or vertical E× B drifts), total electron content, and electron and neutral densities. We include results from different ground- and satellite-based observations, covering different longitudes and years. We also present and discuss the modeling efforts that support most of the observations. Given that SSW can be forecasted with a few days in advance, there is potential for using the connection with the ionosphere for forecasting the occurrence and evolution of electrodynamic perturbations at low latitudes, and sometimes also mid latitudes, during arctic winter warmings. [ABSTRACT FROM AUTHOR]

In this paper, we focus on ionospheric absorption in the East Asia sector, and look for manifestations of atmospheric influences in this area. First, a 4-year historical record of absorption measurement at Beijing is presented. This record was obtained by a sweep frequency technique, in which 27-days periodic variation of the absorption level was found to be dominant, appearing in most seasons except winters. Instead, unusual enhancements of the absorption level appeared in winters (winter anomaly), at the meantime the level varied with periods mainly in the range of 8-12 days. Comparing to 27-days period from the Sun, the shorter period oscillations should be related to planetary wave activities in lower atmosphere. Second, f data from 5 mid-latitude ionosondes in Japan were used as an indirect but long-term measurement. With the f data covering two solar cycles, disturbances with various periods were found to be active around solar maximum years, but the 8-12 days oscillations always existed in winter, showing seasonal dependence instead of connection to solar activity. These results given in this paper demonstrate seasonal and solar cycle-dependent features of the ionospheric absorption in East Asia sector, and confirm the existence of influence from atmosphere-ionosphere coupling in this area, as well as the relationship between ionospheric winter anomaly and planetary wave activity. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents for the first time the global spatial (latitude and altitude) structure of the mean ionospheric response to sudden stratospheric warming (SSW) events in winters of 2007/2008 and 2008/2009. To elucidate the effect of the SSWs on the ionosphere the COSMIC foF2, hmF2, and electron density data at fixed altitudes are analyzed. Both the mean foF2 and hmF2 parameters and the mean electron density at fixed heights indicate regular negative responses to the SSW temperature pulses at high latitudes. Similar response is found for the diurnal variability of the COSMIC electron density. The response is confined mainly to low and middle latitudes. A possible mechanism causing the observed negative ionospheric response is suggested. [Copyright &y& Elsevier]

It is investigated theoretically the nonlinear down-conversion of powerful extremely low frequency (ELF) modulated acoustic wave beams at the carrier frequencies 1–20 Hz into ultra-low frequency (ULF) acoustic waves at the frequencies 0.01–0.1 Hz under the propagation upwards within the atmosphere and ionosphere. The approximation of the slowly varying profile is used. It is shown that under a fixed value of the integral power of the initial acoustic ELF beam there exists the optimal value of the initial transverse width where the maximum down-conversion is realized. The different types of the modulation are considered, namely the simplest two-frequency modulation and multifrequency noise-like one. The using of the complex multifrequency modulation results in the increase of the efficiency of the down-conversion within the ionosphere E- and F- layers. • The frequency down-conversion of modulated extremely low frequencies acoustic waves takes place in the atmosphere. • The frequency down-conversion of acoustic waves moving upwards is important for the modification of the ionosphere. • This down-conversion can be optimized. • Under a fixed initial acoustic power there is an optimal initial width of the acoustic beam. • The additional increase of the down-conversion efficiency can be reached with the complex noise-like modulation. [ABSTRACT FROM AUTHOR]

Determination of the physical mechanisms of energy transfer of tropospheric disturbances to the ionosphere is one of the fundamental problems of atmospheric physics. This article presents the results of observations carried out using two-wavelength lidar sensing at tropospheric altitudes and satellite GPS measurements during a meteorological storm in Kaliningrad (Russia, 54.7° N, 20.5° E) on 1 April 2016. During lidar sensing, it was found that the amplitudes of variations in atmospheric parameters with periods of acoustic (AWs) and internal gravity (IGWs) waves significantly increased. As a result of numerical modeling using the AtmoSym software package, it was shown that there is a noticeable increase in the period of temperature disturbances from 6–12 min to 10–17 min at altitudes from 150 km up to 230 km during the vertical propagation of acoustic waves and internal gravity waves from the troposphere. Nonlinear and dissipative processes in this layer lead to the formation of sources of secondary waves in the thermosphere with periods longer than those of primary ones. In this case, the unsteady nature of the wave source and the short duration of its operation does not lead to significant heating of the thermosphere. Simultaneous satellite observations demonstrate the response of the ionosphere (total electron content (TEC) disturbance) to tropospheric disturbances. Analysis of the time series of the amplitudes of the reflected lidar signal and TEC made it possible to determine that the response time of the ionosphere to tropospheric disturbances is 30–40 min. [ABSTRACT FROM AUTHOR]

This paper presents the observations of ionospheric parameters in Kaliningrad (54° N, 20° E) during a meteorological storm in the Baltic Sea during October 2017 and 2018. Analysis of the total electronic content (TEC) during the storm showed that perturbations of the TEC values from the median can reach two standard deviations of the value. For the critical frequency of the F2 layer, it was 1.5–1.6 times the standard deviations. On days of a meteorological storm, significant changes were noted in the dynamics of the E-layer's critical frequency. The reasons for the occurrence of the observed phenomena were due to the propagation of acoustic-gravity waves generated by convective processes in the lower atmosphere during periods of a meteorological storm. Spectral analysis of TEC variations revealed an increase in the amplitudes of ionospheric variations 10–16 min over the area of a meteorological storm. The analysis allowed us to conclude that ionospheric perturbations during the meteorological perturbation were caused by increased acoustic-gravity wave (AGW) generation processes in the lower atmosphere. The most likely cause of negative ionospheric disturbances were processes associated with the dissipation of AGW propagating from the area of a meteorological storm and increased turbulence in the lower thermosphere. [ABSTRACT FROM AUTHOR]