We performed a comprehensive comparison between GPS global ionosphere map (GIM) and TOPEX/Jason (T‐J) total electron content (TEC) data for the periods of 1998–2009 in order to assess the performance of GIM over the global ocean where the GPS ground stations are very sparse. Using the GIM model constructed by the Center for Orbit Determination in Europe at the University of Bern, the GIM TEC values were obtained along the T‐J satellite orbit at specific locations and times of measurements and then binned into various geophysical conditions for direct comparison with the T‐J TEC. On the whole, the GIM model was able to reproduce the spatial and temporal variations of the global ionosphere as well as the seasonal variations. However, the GIM model was not accurate enough to represent the well‐known ionospheric structures such as the equatorial anomaly, the Weddell Sea Anomaly, and the longitudinal wave structure. Furthermore, a fundamental limitation of the model seems to be evident in the unexpected negative differences (i.e., GPS < T‐J) in the northern high‐latitude and the southern middle‐ and high‐latitude regions in comparison with the T‐J TECS. The positive relative differences (i.e., GIM > T‐J) at night represent the plasmaspheric contribution to GPS TEC, which is maximized, reaching up to 100% of the corresponding T‐J TEC values in the early morning sector. In particular, the relative differences decreased with increasing solar activity, and this may indicate that the plasmaspheric contribution to the maintenance of the nighttime ionosphere does not increase with solar activity, which is different from what we normally anticipate.

We compared the global plasmaspheric total electron content (pTEC) with the ionospheric TEC (iTEC) simultaneously measured by Jason‐1 satellite during the declining phase of solar cycle 23 (2002–2009) to investigate the global morphology of the plasmaspheric density in relation to the ionosphere. Our study showed that the plasmaspheric density structures fundamentally follow the ionosphere, but there are also significant differences between them. Although the diurnal variations are very similar to each region, the plasmasphere shows much weaker variations, only approximately 1 TECU day‐night difference. By analyzing the day‐night differences in the plasmasphere, we found that the plasmaspheric contribution to the nighttime ionosphere does not increase with solar activity and the largest contribution occurs during June solstice. The plasmasphere shows similar seasonal variations to the ionosphere, except for the semiannual variation, which is essentially absent in the plasmasphere. There is also an important difference in the annual variation: although the annual variation in the ionosphere exists regardless of longitude, it occurs only at American sector in the plasmasphere. As solar activity increases to moderate level, the pTEC substantially enhances from approximately 2 to 4 TECU at the initial increase of solar activity below F10.7p = 100 and then quickly slows down while the iTEC almost linearly enhances. Although it is well known that magnetic storms are the major source of plasmaspheric density depletion, pTEC does not show this aspect of the plasmasphere probably due to the relatively small K_p values for high magnetic activity (K_p > 2.5) in the current study.

On November 1st and 2nd, 2021, four Halo coronal mass ejections were ejected from the Sun, releasing billions of tons of high-energy particles into interplanetary space. These were directed towards the Earth and reached our planet on November 3rd and 4th, 2021, generating the first G3-level extreme geomagnetic storm since the beginning of the 25th solar cycle. In this study, we investigate the thermospheric and ionospheric responses in the European sector to a G3-level storm using various observational data from Fabry-Perot interferometer, Ionospheric Connection Explorer/Michelson Interferometer for Global High-resolution Thermospheric Imaging (ICON/MIGHTI), and Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI). The results show positive ionospheric storms in the middle and low latitudes of Europe which may be associated with the equatorward and westward neutral winds induced by heating in the polar region. In contrast, negative storms were detected at high latitudes in association with the increase in thermospheric density (upwelling). These two antithetical responses were confirmed by using European ionosonde and total electron contents (TEC) observation chains distributed over a wide range of latitudes. Finally, we, for the first time, attempt to identify the imaginary boundary line between the two responses.

Plasma bubbles in the equatorial F region are thought of as nighttime phenomena because they develop at night and are assumed to vanish after sunrise. However, bubbles occasionally persist throughout the night and into the day. This study investigates the origin of daytime irregularities and their evolution using data from the first Republic of China satellite. Our results show that daytime irregularities occur in the longitudes where bubbles have developed on previous nights. A newly reported feature is the observation of the temporal variation of the locations of daytime irregularities; daytime irregularities are concentrated near the magnetic equator early in the morning, but the location gradually shifts to higher latitudes with time. This phenomenon is explained in terms of the latitudinal redistribution of fossil bubbles by the ionospheric fountain effect.

A total solar eclipse occurred in the Southern Hemisphere on 2 July 2019 from approximately 17 to 22 UT. Its effect in the thermosphere over South America was imaged from geostationary orbit by NASA's Global‐scale Observation of Limb and Disk (GOLD) instrument. GOLD observed a large brightness reduction (>80% around totality) in OI 135.6 nm and N₂ LBH band emissions compared to baseline measurements made 2 days prior. In addition, a significant enhancement (with respect to the baseline) in the ΣO/N₂ column density ratio (~80%) was observed within the eclipse's totality. This enhancement suggests that the eclipse induced compositional changes in the thermosphere. After the eclipse passed, a slight enhancement in ΣO/N₂ column density ratio (~7%) was also seen around the totality path when compared to measurements before the eclipse. These observations are the first synoptic imaging measurements of an eclipse's thermospheric effects with the potential to drastically improve and test our understanding of how the thermosphere responds to rapid, localized changes in solar short wavelength radiation.