ALBERT

Description: Here we report an experiment of measuring the geopotential difference between two stations separated with a 1240 m height difference by comparing two hydrogen atomic clocks’ frequencies via a satellite common view time-frequency transfer approach. After comparing the two clocks at one station for calibration, one of the two clocks was transported to another station and continued the clock comparison. By this way, we determined the gravity frequency shift and consequently, the geopotential difference between the two remote sites. The experiment result deviates from that determined by the Earth gravity model EIGEN-6C4 by 8.9 (24.6) m〈sup〉2〈/sup〉/s〈sup〉2〈/sup〉. This study provides a prospective way to directly measure the geopotential difference between two remote sites using precise atomic clocks based on satellite links. This study is supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 42030105, 41721003, 42274011), Space Station Project (2020)228, and Fundamental Research Funds for the Central Universities (2022).

Description: Throat auroras frequently observed near local noon have been confirmed to correspond to magnetopause indentations, but the generation mechanisms for these indentations and the detailed properties of throat aurora are both not fully understood. Using all‐sky camera and magnetometer observations, we reported some new observational features of throat aurora as follows. (1) Throat auroras can occur under stable solar wind conditions and cause clear geomagnetic responses. (2) These geomagnetic responses can be simultaneously observed at conjugate geomagnetic meridian chains in the Northern and Southern Hemispheres. (3) The initial geomagnetic responses of throat aurora show concurrent onsets that were observed at all stations along the meridians. (4) Immediately after the concurrent onsets, poleward moving signatures and micropositive bays were observed in the X components at higher‐ and lower‐latitude stations, respectively. We argue that these observations provide evidence for throat aurora being generated by low‐latitude magnetopause reconnection. We suggest that the concurrent onsets reflect the instantaneous responses of the reconnection signal arriving at the ionosphere, the followed poleward moving signatures reflect the antisunward dragging of the footprint of newly opened field lines, and the micropositive bays may result from a pair of field‐aligned currents generated during the reconnection. This study may shed new light on the geomagnetic transients observed at cusp latitude near magnetic local noon.