EPP

Through respectively adding June tide and December tide at the low boundary of the GCITEM-IGGCAS model (Global Coupled Ionosphere–Thermosphere–Electrodynamics Model, Institute of Geology and Geophysics, Chinese Academy of Sciences), we simulate the influence of atmospheric tide on the annual anomalies of the zonal mean state of the ionospheric electron density, and report that the tidal influence varies with latitude, altitude, and solar activity level. Compared with the density driven by the December tide, the June tide mainly increases lower ionospheric electron densities (below roughly the height of 200 km), and decreases electron densities in the higher ionosphere (above the height of 200 km). In the low-latitude ionosphere, tides affect the equatorial ionization anomaly structure (EIA) in the relative difference of electron density, which suggests that tides affect the equatorial vertical E×B plasma drifts. Although the tide-driven annual anomalies do not vary significantly with the solar flux level in the lower ionosphere, in the higher ionosphere the annual anomalies generally decrease with solar activity.

The wavenumber spectral components WN₄ at the mesosphere and low thermosphere (MLT) altitudes (70–10 km) and in the latitude range between ±45° are obtained from temperature data (T) observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments on board the National Aeronautics and Space Administration (NASA)’s Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) spacecraft during the 11-year solar period from 2002 to 2012. We analyze in detail these spectral components WN_k and obtain the main properties of their vertical profiles and global structures. We report that all of the wavenumber spectral components WN_k occur mainly around 100 km altitude, and that the most prominent component is the wavenumber spectral component WN₄ structure. Comparing these long duration temperature data with results of previous investigations, we have found that the yearly variation of spectral component WN₄ is similar to that of the eastward propagating non-migrating diurnal tide with zonal wavenumber 3 (DE3) at the low latitudes, and to that of the semi-diurnal tide with zonal wavenumber 2 (SE2) at the mid-latitudes: the amplitudes of the A₄ are larger during boreal summer and autumn at the low-latitudes; at the mid-latitudes the amplitudes have a weak peak in March. In addition, the amplitudes of component WN₄ undergo a remarkable short period variation: significant day-to-day variation of the spectral amplitudes A₄ occurs primarily in July and September at the low-latitudes. In summary, we conclude that the non-migrating tides DE3 and SE2 are likely to be the origins, at the low-latitudes and the mid-latitudes in the MLT region, respectively, of the observed wavenumber spectral component WN₄.

With the method of Hough mode decomposition (HMD), the tidal sources of the three main tidal components, namely, the migrating components DW1 (diurnal westward propagating wavenumber 1) and SW2 (semidiurnal westward propagating wavenumber 2) and the non-migrating component DE3 (diurnal eastward propagating wavenumber 3), at the tropospheric altitudes (1–12 km) and in the latitude range of ±60°, were obtained from National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data during the interval from 1988 to 2011. We analyzed these sources in detail at 6 km and obtained the main properties of their yearly variations. The DW1 source was found to present a weak seasonal variation in the lower latitudes (about ±10°–15°). That is, the amplitudes of the DW1 sources were larger in the summer months than in the winter months, and DW1 presented semi-annual variation near the equator (±10°) such that the DW1 source was larger at the equinoxes than at the solstices. In addition, the SW2 source was symmetric and was stronger in the southern hemisphere than in the northern hemisphere. The SW2 source presented remarkable annual and semi-annual variation such that the amplitudes were largest during the March equinox months and larger during the June solstice months. In contrast, DE3 appeared mainly around the equatorial latitudes within about ±30°. The DE3 source presented remarkable semi-annual variation that was larger around the solstices than the equinoxes in the southern hemisphere, and it was opposite in the northern hemisphere. By HMD, we found that the tropospheric tides were primarily dominated by some leading propagating Hough modes, specifically, the (1, 1), (2, 3), and (3, 3) modes; the influences of the other Hough modes were negligible. The consequences of an El Niño–Southern Oscillation modulation of tidal amplitudes for the energy and momentum budgets of the troposphere may now be expected to attract attention. In summary, the above yearly variations of the main tidal sources and the Hough coefficients demonstrate that an HMD analysis can be used to investigate the tropospheric tides.

On 21 May 2021 (UTC), an M_W 7.4 earthquake jolted the east Bayan Har block in the Tibetan Plateau. The earthquake received widespread attention as it is the largest event in the Tibetan Plateau and its surroundings since the 2008 Wenchuan earthquake, and especially in proximity to the seismic gaps on the east Kunlun fault. Here we use satellite interferometric synthetic aperture radar data and subpixel offset observations along the range directions to characterize the coseismic deformation of the earthquake. Range offset displacements depict clear surface ruptures with a total length of ~170 km involving two possible activated fault segments in the earthquake. Coseismic modeling results indicate that the earthquake was dominated by left-lateral strike-slip motions of up to 7 m within the top 12 km of the crust. The well-resolved slip variations are characterized by five major slip patches along strike and 64% of shallow slip deficit, suggesting a young seismogenic structure. Spatial–temporal changes of the postseismic deformation are mapped from early 6-day and 24-day InSAR observations, and are well explained by time-dependent afterslip models. Analysis of Global Navigation Satellite System (GNSS) velocity profiles and strain rates suggests that the eastward extrusion of plateau is diffusely distributed across the east Bayan Har block, but exhibits significant lateral heterogeneities, as evidenced by magnetotelluric observations. The block-wide distributed deformation of the east Bayan Har block along with the significant co- and post-seismic stress loadings from the Madoi earthquake imply high seismic risks along regional faults, especially the Tuosuo Lake and Maqên–Maqu segments of the Kunlun fault that are known as seismic gaps.