## 4 Local Variations of Ne in the Topside Ionosphere during Seismic Activity
### Swarm Satellites and Data  
The three identical satellites of the ESA's Swarm mission [53] are in specific low-quasi-polar orbits. They are not sun-synchronous, and as such, they allow the satellites to move rapidly through local time (LT). Two satellites (SwA and SwC) fly in tandem at an altitude of \(\sim\)460 km (in the upper F-layer) close to each other at a distance of about 150 km, with a differential delay in orbit of several seconds. The third satellite (SwB) flies in another meridional plane at an altitude of \(\sim\)530 km. Satellites orbit the Earth 14-15 times a day, flying in certain LT sectors. Gradually drifting in longitude, the orbits cover all LTs in a few months. Each satellite is equipped with scalar and vector magnetometers, sampling at 1 Hz (scalar) and 50 Hz (vectorial). A Langmuir probe (LP) monitors the ionospheric plasma density with a sampling rate of 2 Hz. The Level 2 data products are freely available on the ESA server (swarm-diss.e.oe.esa.int) (last access: 20 January 2023). The details are available in the Swarm Level 2 Processing handbook: ([[https://earth.esa.int/eogateway/missions/swarm/product-data-handbook/level-2-product-definitions](https://earth.esa.int/eogateway/missions/swarm/product-data-handbook/level-2-product-definitions)]([https://earth.esa.int/eogateway/missions/swarm/product-data-handbook/level-2-product-definitions](https://earth.esa.int/eogateway/missions/swarm/product-data-handbook/level-2-product-definitions)); accessed on 2 March 2024).  
Figure 6 illustrates the global distribution of Ne as measured by an individual Swarm satellite. In late January 2024, SwA and SwC orbits were centered at about 17 LT (the ascending branch corresponding to the day-side half-orbit) and 05 LT (the descending branch corresponding to the night-side half-orbit).  
Figure 6 depicts the latitudinal profiles of Ne along the SwC passages for a full day on 22 January. At 17 LT, Ne reaches 2 \(\times\) 10\({}^{6}\) el/cm\({}^{3}\), while at 05 LT, almost on the night-side of the Earth, Ne is by an order of magnitude smaller. The transition to the mid-latitude ionosphere (20-30\({}^{\circ}\)) is characterized by a rapid change in plasma density. In both hemispheres, as the satellite moves from the middle latitudes to the auroral zone, Ne increases and becomes more variable. At mid/high latitudes, seasonal interhemispheric asymmetry is manifested in the predominance of the southern hemisphere. In Figure 6, the first and last passages of the day are shown by colored lines. The first orbit passed along the meridians at approximately 120\({}^{\circ}\)E and 50\({}^{\circ}\)W. The last orbit passed along 90\({}^{\circ}\)E (ascending) and 80\({}^{\circ}\)W (descending), i.e., the last descending passage was almost along the meridian of  
Figure 6: Latitudinal profiles of the SwC Ne (in 10\({}^{6}\) cm\({}^{-3}\)) measured along (**a**) the ascending (17 LT) and (**b**) the descending (05 LT) trajectories on 22 January. The first (last) passage during the day is shown in blue (red).  
the EQ epicenter. At the end of 22 January, the Ne measured along both branches of the orbit was considerably larger than during the previous hours. This is consistent with the increase in TEC observed at 23 UT on 22 January (cf. Figures 4 and 5).