In this study, two ionospheric nighttime enhancement (INE) events at low latitudes are selected to investigate their spatial features through the observations from Global Navigation Satellite System (GNSS) receivers and ionosondes. For the first time, we present the detailed spatial pictures of premidnight and postmidnight INEs under geomagnetically quiet conditions. The two INE events have the maximum extents of about 11° × 34° and 17° × 25° (longitude × latitude), respectively. Dramatic latitudinal and longitudinal features are revealed in the two INEs. We perform a comparison between the products of Global Ionospheric Maps (GIMs) and total electron content (TEC) measurement from GNSS receivers. However, GIMs fail to capture the TEC distribution during INEs owing to their limited spatial and temporal resolution. Considering the extent of INEs from the observations, the spherical harmonic (SH) expansion adopted by the GIM models needs to upgrade the degree and order to 36. The pixel-based methods developed from two GIM models are required to reduce their grid size for higher spatial resolution. The recommended time interval is shorter than 30 min. Among seven GIMs, CODG and JPLG maps generally have the best performance in reproducing the latitudinal structure of the ionosphere.
Atmospheric waves play an important role in the vertical coupling of atmosphere and ionosphere. Recent studies revealed that the mesospheric wave activities associated with sudden stratospheric warming events (SSWs) are more complicated than previously thought. This study investigates the activities of the dominant solar tide‐like harmonics in the ionosphere during SSWs. Significant tide‐like harmonics are present in the residuals of total electron content (TEC) detected by five receivers at different latitudes using signals from BeiDou geostationary (BD‐GEO) C3 satellite during the 2018 SSW. The results reveal that after the SSW onset, the third harmonic enhances and the fourth harmonic weakens at all selected receivers. However, the second harmonic exhibits a response varying with latitude. We then compare the ionospheric tide‐like signatures with those in mesospheric winds observed by four meteor radars. A good agreement is found after the SSW onset. The measurements at Mohe (53.5°N, 122.3°E) show that the response of each specific ionospheric tide‐like signature is similar to the tidal result in the mesospheric winds after the onset of three SSW events in 2017–2019. In addition, we suggest that besides the direct or indirect effects from the interactions of various waves in the lower atmosphere, the non‐sinusoidal waveform of the ionospheric diurnal variation may also contribute to solar harmonic signatures in the ionosphere.
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