For the purpose of building a regional (bound 20–60°N in latitude and 110–160°E in longitude) ionospheric nowcast model, we investigated the performance of IDA4D (Ionospheric Data Assimilation Four‐Dimension) technique considering International Reference Ionosphere model as the background. The data utilized in assimilation were slant total electron content (STEC) from 27 ground GPS (Global Positioning System) receiver stations and NmF2 (ionospheric F2 peak density) from five ionosondes and COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) Data Analysis Archive Center. The period analyzed covered both geomagnetic quiet and disturbed days (15–18 March 2015). Assimilations were run under the following data combinations (cases): (1) GPS‐STEC's only; (2) GPS‐STEC's and NmF2's from five ionosondes; (3) only NmF2's from five ionosondes; and (4) GPS‐STEC's and NmF2's from both five ionosondes and COSMIC. Results showed that under case 1 the root‐mean‐square error (RMSE) in STEC reduced by 44% over the background International Reference Ionosphere values and on averaged over all ionosonde stations in the analysis RMSE values of foF2 (F2 layer critical frequency) reduced by 21%. Furthermore, foF2 RMSE values under Case 2 were 36% smaller than those under Case 1. Under Case 4, IDA4D performance improved even further in areas not covered by GPS and ionosonde measurements. Therefore, IDA4D is a potential candidate for regional ionosphere modeling that exhibits improved performance with assimilation of different data types.

A statistical study on the relationship between the perturbation component (ΔTEC (total electron content)) and the F2 layer peak height (hmF2) during nighttime medium‐scale traveling ionospheric disturbances is presented. The results are obtained by using a time‐dependent computerized ionospheric tomography (CIT) technique. This was realized by using slant total electron content observations from a dense Global Positioning System receiver network over Japan (with more than 1000 receivers), together with a multiplicative algebraic reconstruction technique. Reconstructions from CIT were validated by using ionosonde and occultation measurements. A total of 36 different time snapshots of the ionosphere when medium‐scale traveling ionospheric disturbances (MSTIDs) were eminent were analyzed. These were obtained from a data set covering years from 2011 to 2014. The reconstructed surface wavefronts of ΔTEC and hmF2 structure were found to be aligned along the northwest‐southeast direction. These results confirm that nighttime MSTIDs are driven by electrodynamic forces related to Perkins instability which explains the northwest‐southeast wavefront alignment based on the F region electrodynamics. Furthermore, from the statistical analysis hmF2 varied quasiperiodically in altitude with dominant peak‐to‐peak amplitudes between 10 and 40 km. In addition, ΔTEC and hmF2 were 60% anticorrelated.

A sudden stratospheric warming (SSW) is an extremely rare event in the Southern Hemisphere (SH), but occurred in early September 2019. From the Antarctic meteor radar (MR) stations, Davis (68.6˚S, 77.9˚E) and King Sejong Station (62.2˚S, 58.8˚W), quasi 10-day oscillations were clearly observed in the zonal mesospheric winds before the central date (DOY 253) of the SSW. From the northern low-latitude Tirupati (13.6˚N, 79.4˚E) MR, a strong wave activity with a period of ∼6 days was detected in the zonal winds right after the central date. This oscillation is also seen in the geopotential height measurements from the Microwave Limb Sounder (MLS) on board the Aura satellite near the Tirupati region. To elucidate the possible source of the quasi 6-day wave (Q6DW), we use a specified dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM) constrained by the reanalysis data from the surface to 50 km. The simulation results show that the amplitude of the westward and equatorward propagating Q6DW was enhanced after the SSW central date in the MLT region, and the Q6DW can be attributed to the baroclinic/barotropic instability in the SH high-latitude mesosphere where the divergence of Eliassen-Palm flux occurred. Thus, we suggest that the Q6DW activity observed by the Tirupati MR and MLS originated from the SH high-latitude mesospheric region. Both the observation and the simulation results clearly demonstrate that the 2019 SH SSW affected not only the high-latitude MLT region but also the low-latitude MLT region.Plain Language Summary A sudden stratospheric warming (SSW) is an extremely rare event in the Southern Hemisphere (SH) but occurred in early September 2019. We report the planetary wave activity (∼10-day and ∼6-day periods) in the mesospheric winds measured by Antarctica and tropical meteor radars during the SSW. The 6-day wave has also been reported in the equatorial electrojet measurements by satellites. The observed planetary wave activity was successfully simulated by a theoretical global circulation model constrained by reanalysis data only up to the stratospheric altitude. The simulation identifies the source of the waves at the high latitude of the Southern Hemisphere and the propagation to the tropical region during the SSW. LEE ET AL.

This paper presents a study on the possibility of predicting the regional ionosphere at midlatitude by assimilating the predicted ionospheric parameters from a neural network (NN) model into the Sami2 is Another Model of the Ionosphere (SAMI2). The NN model was constructed from the data set of Jeju ionosonde (33.43°N, 126.30°E) for the period of 1 January 2011 to 31 December 2015 by using the long-short term memory (LSTM) algorithm. The NN model provides 24-hr prediction of the peak density (NmF2) and peak height (hmF2) of the F2 layer over Jeju. The predicted NmF2 and hmF2 were used to compute two ionospheric drivers (total ion density and effective neutral meridional wind), which were assimilated into the SAMI2 model. The SAMI2-LSTM model estimates the ionospheric conditions over the midlatitude region around Jeju on the same geomagnetic meridional plane. We evaluate the performance of the SAMI2-LSTM by comparing predicted NmF2 and hmF2 values with measured values during the geomagnetic quiet and storm periods. The root-mean-square error values of NmF2 (hmF2) from Jeju ionosonde measurements are lower by 45% and 45% (30% and 11%) than those of the SAMI2 and IRI-2016 models during the geomagnetic quiet periods. However, during the geomagnetic storm periods, the performance of the SAMI2-LSTM model does not predict positive geomagnetic storms well. Comparing the quiet and storm periods for the SAMI2-LSTM model, the root-mean-square error (RMSE) of the storm period was calculated to be 2.76 (3.2) times higher at Jeju (Icheon) than in the quiet period. From these results, we demonstrated that in this study, the combination of the NN-LSTM model and physics-based model could improve the ionosphere prediction of existing theoretical and empirical models for midlatitude regions, at least in geomagnetically quiet conditions. We strongly suggest that this attempt, which has not been reported before, could be used as one of the keys to advance the physics-based model further.