This study provides wide coverage for the Pc5 ultra-low frequency discrete spectrum observed in the magnetosphere and high-to-lower latitude ionosphere during noon hours ( 10–14 MLT) on November 4, 2021. During the recovery phase, magnetospheric toroidal mode oscillations (GOES-16 Bn) appeared in response to dayside solar wind dynamic pressure enhancements during high solar wind speeds. At the same time, the super dual auroral radar network at Saskatoon recorded oscillations in both ionospheric line-of-sight velocity and echo power at discrete frequencies of 1.7 and 2.2 mHz (9.8 and 7.5 min). The geomagnetic field components exhibited the same frequency pulsations from high to lower latitudes. The oscillation frequency of the H-component persisted consistently at 1.7 mHz to the lower latitudes. Similar wave oscillations were noticed in the total electron content (TEC) of the Global Positioning System (GPS). The complex demodulation analysis suggests that the observed Pc5 oscillations are consistent with toroidal mode Alfvén waves and field line resonances (FLRs) during the storm recovery phase. The observations indicate a possible association with solar wind dynamic pressure enhancements under high-speed solar wind conditions, while cavity/waveguide modes and magnetopause Kelvin–Helmholtz instability (KHI) may have contributed to the excitation of the observed waves.

Seasonal variation in westward-propagating quasi-10 d waves (Q10DWs) in the mesosphere and lower thermosphere of the Southern Hemisphere (SH) high-latitude regions is investigated using meteor radar (MR) observations for the period of 2012–2016 and using the Specified Dynamics (SD) version of the Whole Atmosphere Community Climate Model (WACCM). The phase difference in meridional winds measured by two MRs located in Antarctica gives observational estimates of the amplitude and phase of the Q10DW with zonal wavenumber 1 (W1). The amplitude of the observed Q10DW-W1 is large around equinoxes. In order to elucidate the variations in the observed Q10DW-W1 and its possible amplification mechanism, we carry out two SD-WACCM experiments nudged towards the MERRA-2 reanalysis from the surface up to ∼ 60 km (EXP60) and ∼ 75 km (EXP75). Results of the EXP75 indicate that the observed Q10DW-W1 can be amplified around regions of barotropic and/or baroclinic instability in the middle mesosphere around 60–70° S. In the EXP60 experiment, it was also found that the Q10DW-W1 is amplified around the regions of instability, but the amplitude is too large compared to MR observations. The large-scale instability in the EXP60 in the SH summer mesosphere is stronger than that in the EXP75 and Microwave Limb Sounder observations. The larger instability in the EXP60 is related to the large meridional and vertical variations in polar mesospheric zonal winds in association with gravity wave parameterization (GWP). Given uncertainties inherent in GWP, these results can suggest that it is possible for models to spuriously generate traveling planetary waves such as the Q10DW, especially in summer, due to excessively strong large-scale instability in the SH high-latitude mesosphere.

We present oscillating features from long-term neutral wind (meteor radar) and pressure (microwave limb sounder) measurements at Esrange (67°N, 20°E, 2007–2018) in the northern hemisphere and King Sejong Station (KSS; 62°S, 58°W, 2007–2017) in the Southern Hemisphere using the Lomb-Scargle periodogram and wavelet analysis. In zonal winds and pressure, we estimated the height profiles for the amplitude ratio between annual oscillation (AO) and semi-annual oscillation (SAO) at both sites. Over KSS, the ratio indicates that SAO increases with altitude, whereas AO decreases, with SAO becoming the dominant oscillating component at around 90 km. However, the ratio mostly remains constant with an altitude due to the steady formation of a westward wind field throughout the height in summer. More intensive gravity wave activity over KSS drives meridional residual circulation in summer mesopause, creating a more powerful SAO signature above 90 km.

We conducted a statistical study of polar mesospheric summer echoes (PMSEs) in relation to magnetic local time (MLT), considering the geomagnetic conditions using the K-index (or K). Additionally, we performed a case study to examine the velocity profile, specifically for high velocities (≥ ~100 m/s) varying with high temporal resolution at high K-index values. This study utilized the PMSE data obtained from the mesosphere–stratosphere–troposphere radar located in Esrange, Sweden (63.7°N, 21°E). The change in K-index in terms of MLT was high (K ≥ 4) from 23 to 04 MLT, estimated for the time PMSE was present. During the near-midnight period (0–4 MLT), both PMSE occurrence and signal-to-noise ratio (SNR) displayed an asymmetric structure with upper curves for K ≥ 3 and lower curves for K < 3. Furthermore, the occurrence of high velocities peaked at 3–4 MLT for K ≥ 3. From case studies focusing on the 0–3 MLT period, we observed persistent eastward-biased high velocities (≥ 200 m/s) prevailing for ~18 min. These high velocities were accompanied with the systematic motion of profiles at 85–88 km, including large shear formation. Importantly, the rapid variations observed in velocity could not be attributed to neutral wind effects. The present findings suggest a strong substorm influence on PMSE, especially in the midnight and early dawn sectors. The large zonal drift observed in PMSE were potentially energized by local electromagnetic fields or the global convection field induced by the electron precipitation during substorms.

This study presents ultra-low frequency (ULF) Pc5 discrete spectrum simultaneously observed in the magnetosphere and high- to low-latitude ionospheres near noon hours (~10-14 MLT) during the recovery phase of geomagnetic storm on November 4, 2021. During the recovery phase, magnetospheric toroidal mode oscillations (GOES-16 Bn) appeared according to solar wind dynamic pressure enhancements after GOES Bp and Be (poloidal mode) oscillations precede during high solar wind speeds. When Bn oscillates, the ionospheric line-of-sight (LOS) velocity and echo power oscillate at the same discrete frequencies of 1.7 and 2.2 mHz (9.7 and 7.5 min), observed by Super Dual Auroral Radar Network (SuperDARN) at Saskatoon (eastward LOS). The period of negative LOS velocity (away from the radar) for 7.5 min or 9.7 min corresponds to echo power increase. This signifies that both the ionospheric density and poleward convection velocity increase are driven by the periodic forcing of the convection electric field and energetic electron precipitation. The same frequency pulsations have also been observed in the geomagnetic field (H-component) and Global Positioning System (GPS) total electron content (TEC) from high- to low-latitude ionosphere. The oscillation frequency of the H-component is consistently preserved at 1.7 mHz (9.7 min) down to low latitudes. The Pc5 oscillations at high to low latitudes can be attributed to toroidal mode Alfven waves and the compressional mode propagating across magnetic field lines as well as the fast magnetosonic waveguide mode at work by the solar wind dynamic pressure enhancements at high solar wind speeds.

Using a network of meteor radar observations, observational evidence of polar-to-tropical mesospheric coupling during the 2018 major sudden stratosphere warming (SSW) event in the northern hemisphere is presented. In the tropical lower mesosphere, a maximum zonal wind reversal (−24 m/s) is noted and compared with that identified in the extra-tropical regions. Moreover, a time delay in the wind reversal between the tropical/polar stations and the mid-latitudes is detected. A wide spectrum of waves with periods of 2 to 16 days and 30–60 days were observed. The wind reversal in the mesosphere is due to the propagation of dominant intra-seasonal oscillations (ISOs) of 30–60 days and the presence and superposition of 8-day period planetary waves (PWs). The ISO phase propagation is observed from high to low latitudes (60° N to 20° N) in contrast to the 8-day PW phase propagation, indicating the change in the meridional propagation of winds during SSW, hence the change in the meridional circulation. The superposition of dominant ISOs and weak 8-day PWs could be responsible for the delay of the wind reversal in the tropical mesosphere. Therefore, this study has strong implications for understanding the reversed (polar to tropical) mesospheric meridional circulation by considering the ISOs during SSW.

In this study, the first high-resolution regional ionospheric model over Africa and adjacent areas (-40-40 degrees N latitude, 30 degrees W-60 degrees E longitude, and 80-1,400 km in altitude) is constructed by assimilating ground-based slant total electron content (STEC) from 40 GPS (Global Positioning System) receiver stations and space-based NmF2 (ionospheric F2 peak density) data from C2 (Constellation Observing System for Meteorology, Ionosphere, and Climate-2) into the International Reference Ionosphere (IRI-2016) model. An Ionospheric Data Assimilation Four-Dimensional (IDA4D) technique was used to estimate electron densities as high as 1.5 degrees x 3 degrees in latitude and longitude, 10 km in altitude in the E and F regions, and 15 min in universal time. Two experiments were run for the following data sets: (a) GPS-STECs only and (b) GPS-STECs and NmF2s from C2 during geomagnetically quiet (6-11 May 2021) and storm periods (12-14 May 2021). The IDA4D assimilation results are validated using independent C2 control group, ionosonde, and JASON-3 observations. Results for the storm period show that experiment 2 reduces the average root-mean-square error (RMSE) of NmF2, foF2, and VTEC by 34%, 31%, and 34%, respectively, and increases the associated correlations by 10%, 14%, and 2% over IRI, respectively. Using IDA4D, we observed enhancement of the northern crest equatorial ionization anomaly in the late evening that was caused by upward and northward plasma transport.

Using meteor radar observations over King Sejong Station (62.22 degrees S, 58.78 degrees W) in the Antarctic Peninsula, we investigated the wave-wave interaction in the mesosphere and lower thermosphere (MLT) region. We analyzed hourly horizontal wind measurements at altitudes of 84-96 km from March 2016 through February 2017 and found that the MLT region is dominated by a semidiurnal tide (SDT) and the signature of planetary waves (PWs) with a period of similar to 8-27 days. The PW activity was substantially intensified during the winter. The day-to-day variability of SDT amplitudes are estimated and the higher-order spectral analysis of SDT amplitudes exhibits intermittent modulation of SDT at PW periods similar to 8 and 27 days in the zonal winds and periods of 8 and 16 days in the meridional winds. We observed an intense episode of tidal modulation from June through September 2016, during which a set of subsidiary spectral components around the SDT were found as a result of wave-wave interaction. Further, the bi-spectral analysis reveals that quadratic coupling (nonlinear interaction) occurred between SDT and PWs at a band period of 8-27 days, at above 90 km. We compared the spectral powers between the secondary waves created by the nonlinear interaction between the SDT and 16-day PWs. Further, we found that sum-secondary waves generated by the nonlinear interaction are stronger than the difference-secondary waves during austral winter 2016. We thus conclude that the non-linear interaction between the tides and PWs may play a major role in the short-term tidal variability in the MLT region during austral winter 2016.

Abstract. Seasonal variation of westward-propagating quasi-10-day wave (Q10DW) in the mesosphere and lower thermosphere of the Southern Hemisphere (SH) high-latitude regions is investigated using meteor radar (MR) observations for the period of 2012–2016 and Specified Dynamics (SD) version of the Whole Atmosphere Community Climate Model (WACCM). The phase difference of meridional winds measured by two MRs located in Antarctica gives observational estimates of the amplitude and phase of Q10DW with zonal wavenumber 1 (W1). The amplitude of the observed Q10DW-W1 is large around equinoxes. In order to elucidate the variations of the observed Q10DW-W1 and its possible amplification mechanism, we carry out two SD-WACCM experiments nudged towards the MERRA-2 reanalysis from the surface up to ~60 km (EXP60) and ~75 km (EXP75). Results of the EXP75 indicate that the observed Q10DW-W1 can be amplified around the barotropic/baroclinic instability regions in the middle mesosphere around 60° S–70° S. In the EXP60, it is also found that Q10DW-W1 is amplified around the instability regions, but the amplitude is too large compared with MR observations. The large-scale instability in the EXP60 in the SH summer mesosphere is stronger than that in the EXP75 and Microwave Limb Sounder observation. The larger instability in the EXP60 is related to the large meridional and vertical variations of polar mesospheric zonal winds in associated with gravity wave parameterization (GWP). Given uncertainties inherent in GWP, these results can suggest that it is possible for models to spuriously generate traveling planetary waves such as Q10DW, especially in summer, due to the excessively strong large-scale instability in the SH high-latitude mesosphere.

On November 1st and 2nd, 2021, four Halo coronal mass ejections were ejected from the Sun, releasing billions of tons of high-energy particles into interplanetary space. These were directed towards the Earth and reached our planet on November 3rd and 4th, 2021, generating the first G3-level extreme geomagnetic storm since the beginning of the 25th solar cycle. In this study, we investigate the thermospheric and ionospheric responses in the European sector to a G3-level storm using various observational data from Fabry-Perot interferometer, Ionospheric Connection Explorer/Michelson Interferometer for Global High-resolution Thermospheric Imaging (ICON/MIGHTI), and Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI). The results show positive ionospheric storms in the middle and low latitudes of Europe which may be associated with the equatorward and westward neutral winds induced by heating in the polar region. In contrast, negative storms were detected at high latitudes in association with the increase in thermospheric density (upwelling). These two antithetical responses were confirmed by using European ionosonde and total electron contents (TEC) observation chains distributed over a wide range of latitudes. Finally, we, for the first time, attempt to identify the imaginary boundary line between the two responses.

The ionospheric density displays hemispheric asymmetries in the polar region due to various hemispheric differences, for example, in the offset between geographic and geomagnetic poles and in the geomagnetic field strength. Using ground‐based ionospheric measurements from Vertical Incidence Pulsed Ionospheric Radar with Dynasonde analysis at Jang Bogo Station (JBS), Antarctica and from EISCAT Svalbard Radar (ESR) where both sites are located mostly in the polar cap, we investigate the hemispheric differences in the ionospheric density between the northern and southern hemispheres for geomagnetically quiet and solar minimum condition. The results are also compared with Thermosphere Ionosphere Electrodynamic Global Circulation Model (TIEGCM) simulations. The observations show larger density and stronger diurnal and seasonal variations at JBS in the southern hemisphere than at Svalbard in the northern hemisphere. The diurnal variations of the density peak height are also observed to be much larger at JBS. In both hemispheres, the ionospheric density is significantly reduced in winter due to the limited solar production at high geographic latitudes, but TIEGCM considerably overestimates winter density, which is even larger than summer density, especially in the northern hemisphere. Also existed are the differences in the equinoctial asymmetry between the observations and the simulations: the daytime F‐region density is observed to be larger in fall than in spring in both hemispheres, but TIEGCM shows the opposite. In general, most of the observed asymmetrical density are much weaker in the model simulation, which may result from lack of proper magnetospheric forcings and neutral dynamics in the model.

From meteor radar observations typically measuring neutral winds and temperatures in the upper atmosphere, we showed that under-dense meteor plasma trails respond to geomagnetic activity. Depending on the geomagnetic activity conditions, the parameters of the under-dense trail echoes show substantial anomalies, and their responses are immediate. As geomagnetic activity increases, the decay time decreases up to about 10% at Kp = 8, while both the returned signal power and radial velocity error increase. In addition, the types of diffuse evolution vary with geomagnetic activity, resulting in strong geomagnetic activity that immediately affects the trail. The evolution of the trail is affected not only by the ambient mesospheric neutral atmospheric properties but also by electromagnetic effects, such as strong electric fields inside meteor trails. Furthermore, our findings suggest that the geomagnetic effect should be considered when estimating atmospheric parameters from MR during a geomagnetically active period.

Five floor-fractured craters (FFCs), Lavoisier crater and four craters surrounding it (Lavoisier C, Lavoisier E, Lavoisier F, and Lavoisier H), are distributed along the boundary between the northwestern part of the Oceanus Procellarum and the highlands. This study examines the uplifted or exposed materials on the fractured floors of these five impact craters using petrological, mineralogical, and morphological analyses. We inferred the processes that uplifted or exposed the materials from the subsurface to the crater floor using the Chandrayaan-1 Moon Mineralogy Mapper (M3) level 1b (thermally and topographically corrected spectral radiance) data and level 2 (spectral reflectance) data. The elemental abundances, petrological, and mineralogical characteristics of the study regions were mapped. We confirm that mare basalts and dark mantle deposits exist on the floors of these five craters. These two materials (mare basalts and dark mantle deposits) were used to identify minerals exposed on the floor surface of craters using spectral reflectance spectra. Two mineral groups were identified: pigeonite (or orthopyroxene in norite (low-Ca pyroxene) occurred in the craters Lavoisier, Lavoisier F, and Lavoisier H, and subcalcic augite (high-Ca pyroxene) occurs in the craters Lavoisier C and E. Our approach demonstrates that the characterization of uplifted or exposed surface minerals using elemental maps, spectral parameter composite maps, and reflectance spectra can provide information critical for prospective studies involving lunar geology and in situ resource utilization.

We have recently developed a Korean ionosphere physics-based prediction model (KIPM) that can predict the state of the ionosphere over and around the Korean peninsula as a function of latitude, longitude, altitude, and time. The KIPM model is an extended version of the SAMI2-CNU model (Kim et al., 2016), which extended the SAMI2 model into a three-dimensional space. The KIPM model has a spatial resolution of 1 degrees for longitude (120-140 degrees E) and latitude (20-40 degrees N) near the Korean Peninsula, with a height resolution of 10 km between 85 km and 1000 km. We upgraded the solar irradiance model and the thermospheric wind field and density models used in the SAMI2-CNU model to the latest versions. To evaluate the performance of the KIPM model, we compared its predictions with various ionospheric measurements (Ionosonde, GPS TEC) observed in Korea, along with the SAMI2, SAMI3, and IRI-2016 models. Through this comparison, we found the KIPM model showed 37% (10.2%), 8% (-4.6%), and 14.7% (1.1%) better predictive performance in terms of NmF2 (hmf2) at the Jeju location than the SAMI2, SAMI3, and IRI-2016 models, respectively. For vertical TEC, our model showed improvements of 14.2%, 20.7%, and 28.7% over the SAMI3, IRI-2016, and SAMI2 models, respectively, on average overall seasons. The SAMI3 model, a three-dimensional global model, can calculate the effect of plasma transport in the longitudinal direction, but it takes too long (several hours) to predict 24 h. On the other hand, the KIPM model only takes about 5-7 min, though it does not include the effect of longitudinal plasma transport. The fast calculation and performance of the KIPM should be significantly advantageous when estimating the 3-D regional ionosphere with data assimilation in near real time. (C) 2021 COSPAR. Published by Elsevier B.V.

This study reconstructs total electron content (TEC) maps in the vicinity of the Korean Peninsula by employing a deep convolutional generative adversarial network and Poisson blending (DCGAN‐PB). Our interest is to rebuild small‐scale ionosphere structures on the TEC map in a local region where pronounced ionospheric structures, such as the equatorial ionization anomaly, are absent. The reconstructed regional TEC maps have a domain of 120°–135.5°E longitude and 25.5°–41°N latitude with 0.5° resolution. To achieve this, we first train a DCGAN model by using the International Reference Ionosphere‐based TEC maps from 2002 to 2019 (except for 2010 and 2014) as a training data set. Next, the trained DCGAN model generates synthetic complete TEC maps from observation‐based incomplete TEC maps. Final TEC maps are produced by blending of synthetic TEC maps with observed TEC data by PB. The performance of the DCGAN‐PB model is evaluated by testing the regeneration of the masked TEC observations in 2010 (solar minimum) and 2014 (solar maximum). Our results show that a good correlation between the masked and model‐generated TEC values is maintained even with a large percentage (∼80%) of masking. The performance of the DCGAN‐PB model is not sensitive to local time, solar activity, and magnetic activity. Thus, the DCGAN‐PB model can reconstruct fine ionospheric structures in regions where observations are sparse and distinguishing ionospheric structures are absent. This model can contribute to near real‐time monitoring of the ionosphere by immediately providing complete TEC maps.

The Ionospheric Data Assimilation Four-Dimension (IDA4D) is, developed by Bust et al. (2007), a continuous time and three-dimension variational (3D-Var) algorithm that can optimally estimate the ionosphere from a model and measured data. In this study, we utilized three different data types into IDA4D for a regional ionosphere estimation: slant total electron contents (STEC) obtained from a Global Positioning System (GPS) network, NmF2 (peak electron density of the F2 layer) and STEC from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellite. These multiple type data were assimilated into a background model, the International Reference Ionosphere (IRI – 2016). To evaluate the effect of each data type, the assimilation was performed on the following data combinations (cases): (1) GPS-STEC’s only; (2) GPS-STEC’s and NmF2′s from COSMIC; (3) GPS-STEC’s and COSMIC-STEC’s; and (4) all three data types. For each case computed foF2′s (F2 layer critical frequency) from IDA4D were compared with measured values from five ionosondes (I-Cheon, Jeju, Okinawa, Kokubunji, and Wakkanai) in the region of Korea and vicinities for the test periods of March, June, September and December in 2015. The comparison shows that computed foF2′s for all cases have higher correlation coefficients (CC) and less root mean square errors (RMSE) from measured values than IRI estimated values. In Cases 2, 3 and 4, IDA4D performance progressively improved in areas where GPS measurements were not covered but COSMIC STEC and NmF2 data were available. Furthermore, the IDA4D assimilation yields better results for electron density profiles and TEC values in comparison with observed values than the IRI model does. Thus, our study suggests that the IDA4D model with multiple data types can provide a reliable estimation of the regional ionosphere over the Korean Peninsula and vicinities.

A new method of estimating mesospheric temperature has recently been proposed by utilizing an apparent linear relation between atmospheric temperatures and full widths at half maximum (FWHMs) of meteor height distributions measured by a meteor radar (MR). However, the new method assumes that the meteor height distribution is dominantly dependent on the atmospheric conditions, rather than on meteoroid characteristics (mass and velocity). In order to verify this assumption, we have developed a meteor ablation model and applied it to the observed parameters by a MR at King Sejong Station (62.2 degrees S, 58.8 degrees W). The simulation results show that the FWHM of meteor height distribution increases linearly with the mesospheric temperature and its linear relation matches well with the observed relation. We found that the seasonal variation of meteor velocity distributions is significant but has only little effect on the variation of the height distribution. We also found that the observed characteristics of meteors are consistent with a Gaussian distribution of logarithmic masses, and this distribution is nearly invariable throughout the year with the average peak value of 10-6.2kg ${10}<^>{-6.2}\hspace*{.5em}\text{kg}$. Thus, we conclude that observed meteor height distributions are mainly dependent on the mesospheric temperature, and can be used as a mesospheric temperature indicator.