Longitudinal differences of low‐latitudinal ionospheric responses during five major, four minor, and a none‐stratospheric sudden warming (SSW) winters from 2009 to 2018 between East Asian and American sectors are studied with total electron content (TEC). The time‐shifted semidiurnal (TS) pattern and the amplitude (AM2), phase angle (PAM2), and relative strength (RSM2) of the lunar semidiurnal tide (M2) harmonic in TEC are compared between the two sectors. Main results are as follows: (1) TS patterns, AM2,and RSM2 tend to be more discernable or larger in the American sector than in the East Asian sector. (2) TS patterns and PAM2 correspond well with the moon phase, and the occurrence of TS patterns coincides well with the enhancement of AM2 and RSM2. (3) Such patterns sometimes occur before the polar peak warming and experience several cycles during one event, but the most significant one tends to follow the peak warming. These characteristics are most distinct during major events with low solar activities. Our results support the mechanism that TS patterns in low‐latitudinal ionosphere parameters are due to enhanced lunitidal effects on the E region dynamo. Besides, changes in temperature and wind during SSWs can also contribute to the generation of TS signatures. Longitudinal differences in TEC suggest that the M2 influence on the low‐latitudinal ionosphere tends to be more prominent in the American sector than in the East Asian sector during SSWs. These differences probably result from a combined effect of the longitudinal variety in atmospheric (especially tidal) and electrodynamic processes.

Using the ionospheric total electron content (TEC) and the location of the equatorial ionization anomaly (EIA) crest derived from GPS data observed at Nanning (22.84°N, 108.33°E, dip latitude ~ 12.14°N), China, a GPS station situated near the daytime EIA crest region, the local time, seasonal, and solar cycle variations of EIA crests from 2006 to 2015 was investigated. The TEC and the magnetic latitude (MLAT) location of EIA crest experience an ascendant and descendant process with the daytime evolution of EIA crest. Also, this diurnal variation of EIA crest depends on season and solar cycle. The TEC and MLAT location of EIA crest is stronger and higher in equinoctial months than that in solistial months showing a semiannual variation. The TEC and MLAT location of EIA crest increase with the solar activity. The local time, seasonal, and solar cycle variations of EIA crest is consistent with that of the strength of equatorial electrojet (EEJ). Comparatively, the correlation of the EEJ strength with the EIA location is better than that with the TEC of EIA crest.

The ionospheric perturbations during sudden stratospheric warming events (SSWs) have attracted broad attention recently, including depressions and enhancements (

This study presents a comprehensive observation of the large-scale traveling ionospheric disturbances (LSTIDs) in the eastern Asian sector during the 2015 St. Patrick 's Day (17 March 2015) geomagnetic storm. For the first time, three dense networks of GPS receivers in China and Japan are combined together to obtain the twodimensional (2-D) vertical total electron content (VTEC) perturbation maps in a wider longitudinal range than previous studies in this region. Results show that an LSTID spanning at least 60 • in longitude (80-140 • E) occurs as a result of possible atmospheric gravity waves (AGWs) propagating from high to lower latitudes at around 09:40-11:40 UT (universal time), and the crest of this LSTID shows a tendency of dissipation starting from the eastern side. The manifestation of the 2-D VTEC perturbation maps is in good agreement with the recordings from two high-frequency Doppler sounders and the iso-frequency lines from eight ionosondes. Then, the propagation parameters of the LSTIDs are estimated by applying least-square fitting methods to the distinct structures in the 2-D VTEC perturbation plots. In general, the propagation parameters are observably longitudinally dependent. For example, the propagation direction is almost due southward between 105 and 115 • E, while it is slightly southwest and southeast in the western and eastern sides of this region. This feature is probably related to the regional geomagnetic declination. The mean values of the period, trough velocity (V t ), crest velocity (V c ), and wavelength of the LSTIDs in the studied longitudinal bands are 74.8 ± 1.4 min, 578 ± 16 m s −1 , 617 ± 23 m s −1 , and 2691 ± 80 km, respectively. Finally, using the VTEC map data from the Madrigal database of the MIT Haystack Observatory, the characteristics of the ionospheric disturbances over the European sector (30-70 • N, 10-20 • E) are also studied. The results are very different from those in the eastern Asian sector in parameters like the occurrence time, oscillation period, and propagation velocities.

The unexpected relationship between the ionosphere and the sudden stratospheric warming (SSW) event has been a research hot spot for over a decade. SSW event is a polar stratospheric phenomenon caused by the breakdown of the polar vortex due to its interaction with the enhanced planetary wave (PW) (

The equatorial ionization anomaly (EIA) crest derived from GPS observations, F2‐layer peak height (hmF2), critical frequency (foF2), and equatorial electrojet (EEJ) in China low latitude are used to study the lunar tidal effect on EIA region for the years from 2006 to 2015. A predominant 14.76‐day periodic component in EIA crest, hmF2, and EEJ concurrently appears in some seasonal intervals, which coincides with the half of the lunar revolution period (29.53 days) and the lunar phase, indicating that the 14.76‐day periodic oscillation in EIA region is modulated by the lunar tide. This 14.76‐day periodic oscillation occurs in all Northern Hemisphere (NH) winter and corresponds to their respective sudden stratospheric warming (SSW) period from 2006 to 2015. In addition, this 14.76‐day periodic oscillation also occurs in other seasons for some years, for example, May and August, in which wave power is sometimes comparable to that in SSW period. The occurrence of the 14.76‐day periodic oscillation and its wave power during non‐SSW seasons in the solar maximum years (2012–2015) is more frequent and higher than that in the solar minimum years (2008–2010). Our results suggest that, besides the NH SSW condition, the solar activity and some other unclear factors are also responsible for the lunar tide enhancement in EIA region.

No abstract

<p>During Sudden Stratospheric Warming events, the ionosphere exhibits phase-shifted semi-diurnal perturbations, which are typically attributed to vertical coupling associated with the semi-diurnal lunar tide (M2). Our understanding of ionospheric responses to M2 is limited. This study focuses on fundamental vertical coupling processes associated with the latitudinal extent and hemispheric asymmetry of ionospheric M2 signatures, using total electron content data from the eastern Asian and American sectors. Our results illustrate that the asymmetry maximizes at around 15°N and 20°S magnetic latitudes. In the southern hemisphere, the M2-like signatures extend deep into midlatitude and, in the American sector, encounter the Weddell Sea Anomaly. The M2 amplitude is larger in the northern hemisphere and such asymmetry is more distinct in the eastern Asian sector. The hemispheric asymmetry of M2 signatures in the low latitude can be primarily explained by the trans-equatorial wind modulation of the equatorial plasma fountain. Other physical processes could also be relevant, including hemispheric asymmetry of the M2 below the F region, the ambient thermospheric composition and ionospheric plasma distribution, and the geomagnetic field configuration.</p>