No abstract

The vacuum-calibration apparatus is mainly composed of the extreme high-vacuum ͑XHV͒ system, ultrahigh vacuum system, and separated-flow system. The ultimate pressure of 7.9ϫ 10 −10 Pa was obtained in the XHV calibration chamber by combining the magnetically levitated turbomolecular pump and nonevaporable getter pump ͑NEGP͒. The separated-flow method was used to extend the lower limit of vacuum-gauge calibration to 10 −10 Pa. The uncertainty at lower limit was reduced when taking inert gases as test gases because NEGP has no pumping speed for inert gases. For this apparatus, the combined standard uncertainties were estimated to range from 1.5% at 10 −4 Pa to 3.5% at 10 −10 Pa.

The devastating Hunga Tonga-Hunga Ha’apai underwater volcano erupted at ~04:15 UT on 15 January 2022. We captured the waves that erupted from the volcano propagating in the ionosphere by monitoring total electron content (TEC) perturbations utilizing ground-based global navigation satellite system (GNSS) receivers that receive electromagnetic signals transmitted from the geostationary satellites operated by the BeiDou Navigation Satellite System (BDS). Meanwhile, ground barometers detected unusual enhancements of air pressure traveling in the troposphere. A novel phenomenon shows that the waves can individually propagate with a speed of ~335 m/s in the ionosphere, which is faster than its’ ~305 m/s in the troposphere. We further examined multiple geophysical data at the particular site of the novel instrumental array. Analytical results show that the pressure enhancements traveling in the troposphere not only downward trigger ground vibrations mainly in the horizontal components without obvious time difference, but also upward, leading the secondary TEC perturbations with a ~12-min delay.

Geostationary BeiDou satellites monitor the total electron content (TEC) in the ionosphere over certain locations 24 hr per day without interruption and act as ionosphere‐based seismometers. The system detected perturbations in TEC before both the M6.1 Dali and M7.3 Qinghai earthquakes that occurred during the night of 21–22 May 2021. The TEC perturbations reside mainly over an area within a distance of ∼700 km from the epicenters of the earthquakes. The standing waves revealed the persistence of a subsurface wave source before the occurrences of the earthquakes, which differs from the co‐seismic ionospheric distributions propagating away from the epicenters. The resident waves in TEC and ground vibrations share a frequency of ∼0.004 Hz, which can be attributed to the resonant coupling between the lithosphere and ionosphere.