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Thermal electron depression and energetic electron depletion are two well-known features of the nightside Martian ionosphere. While the latter has been extensively studied in the past, there is no report of systematic and firm identification of the former and hence the correlation between them is unknown. Applying an automatic procedure to the Mars Express total electron content (TEC) measurements, we identify a large number of TEC depression features that amount to one-third of the available nightside measurements. Similar to the observation of energetic electron depletion, large amplitude TEC depression is more frequently observed near strong magnetic fields, as an expected result of the established solar-wind-driven scenario that the shielding of precipitating electrons by closed magnetic loops is responsible for the observed reduction in thermal electron density. However, small amplitude depression appears to be uniformly distributed across the Martian surface, implying an alternative origin yet to be determined. Contrary to our expectation, TEC depression and energetic electron depletion are not usually collocated, but this dilemma may simply reflect the complexity of the magnetic field topology with respect to the spacecraft trajectory.

We investigated the variations of equatorial plasma bubbles (EPBs) in the East-Asian sector during a strong geomagnetic storm in October 2016, based on observations from the Beidou geostationary (GEO) satellites, Swarm satellite and ground-based ionosonde. Significant nighttime depletions of F region in situ electron density from Swarm and obvious nighttime EPBs in the Beidou GEO observations were observed on 13 October 2016 during the main phase. Moreover, one interesting feature is that the rare and unique sunrise EPBs were triggered on 14 October 2016 in the main phase rather than during the recovery phase as reported by previous studies. In addition, the nighttime EPBs were suppressed during the whole recovery phase, and absent from 14 to 19 October 2016. Meanwhile, the minimum virtual height of F trace (h’F) at Sanya (18.3°N, 109.6°E, MLAT 11.1°N) displayed obvious changes during these intervals. The h’F was enhanced in the main phase and declined during the recovery phase, compared with the values at pre- and post-storm. These results indicate that the enhanced nighttime EPBs and sunrise EPBs during the main phase and the absence nighttime EPBs for many days during the recovery phase could be associated with storm-time electric field changes.

The photoelectron peak at 22–27 eV, a distinctive feature of the energetic electron distribution in the dayside Martian ionosphere, is a useful diagnostic of solar extreme ultraviolet (EUV) and X-ray ionization as well as of large-scale transport along magnetic field lines. In this work, we analyze the pitch angle distribution (PAD) of energetic electrons at 22–27 eV measured during several representative Mars Atmosphere and Volatile Evolution (MAVEN) orbits, based on the electron spectra gathered by MAVEN’s Solar Wind Electron Analyzer (SWEA) instrument. On the dayside, most photoelectron spectra show an isotropic PAD as is expected from production via solar EUV/X-ray ionization. The photoelectron spectra occasionally observed on the nightside show instead a strongly anisotropic PAD, indicative of cross-terminator transport along ambient magnetic field lines. This would in turn predict the presence of dayside photoelectrons, also with a strongly anisotropic PAD, which was indeed revealed in SWEA data. Comparison with magnetic field measurements made by the MAVEN Magnetometer suggests that on average the photoelectrons with anisotropic PAD stream away from Mars on the dayside and towards Mars on the nightside, further supporting the scenario of day-to-night transport. On both sides, anisotropic photoelectrons tend to be observed above the photoelectron exobase at ~160 km where photoelectron transport dominates over local production and energy degradation.

O++ is an interesting species in the ionospheres of both the Earth and Venus. Recent measurements made by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft provide the first firm detection of O++ in the Martian ionosphere. This study is devoted to an evaluation of the dominant O++ production and destruction channels in the dayside Martian ionosphere, by virtue of NGIMS data accumulated over a large number of MAVEN orbits. Our analysis reveals the dominant production channels to be double photoionization of O at low altitudes and photoionization of O+ at high altitudes, respectively, in response to the varying degree of O ionization. O++ destruction is shown to occur mainly via charge exchange with CO2 at low altitudes and with O at high altitudes. In the dayside median sense, an exact balance between O++ production and destruction is suggested by the data below 200 km. The apparent discrepancy from local photochemical equilibrium at higher altitudes is interpreted as a signature of strong O++ escape on Mars, characterized by an escape rate of 6×1022 s–1.

The \begin{document}$ {\rm{CO}}_2^+$\end{document} ultraviolet doublet (UVD) emission near 289 nm is an important feature of dayside airglow emission from planetary upper atmospheres. In this study, we analyzed the brightness profiles of \begin{document}$ {\rm{CO}}_2^+$\end{document} UVD emission on Mars by using the extensive observations made by the Imaging Ultraviolet Spectrograph on board the recent Mars Atmosphere and Volatile Evolution spacecraft. Strong solar cycle and solar zenith angle variations in peak emission intensity and altitude were revealed by the data: (1) Both the peak intensity and altitude increase with increasing solar activity, and (2) the peak intensity decreases, whereas the peak altitude increases, with increasing solar zenith angle. These observations can be favorably interpreted by the solar-driven scenario combined with the fact that photoionization and photoelectron impact ionization are the two most important processes responsible for the production of excited-state \begin{document}$ {\rm{CO}}_2^+$\end{document} and consequently the intensity of \begin{document}$ {\rm{CO}}_2^+$\end{document} UVD emission. Despite this, we propose that an extra driver, presumably related to the complicated variation in the background atmosphere, such as the occurrence of global dust storms, is required to fully interpret the observations. In general, our analysis suggests that the \begin{document}$ {\rm{CO}}_2^+$\end{document} UVD emission is a useful diagnostic of the variability of the dayside Martian atmosphere under the influences of both internal and external drivers.

Electron density ( N e ) and effective reflection height ( H ' , where the electron density reaches its peak value) in the D region observed by medium frequency (MF) radar at Kunming (25.6°N, 103.8°E) were utilized to quantitatively study the ionospheric response to solar flares. It was found that the H ' and N e had profound responses to solar flares. The results showed a sharp decrease in the H ' from 76 to 80 km during the non-flare period to nearly 58 km with flares, and a sudden increase in the N e from below 4 × 10 8 m3 in non-flare period to 1.8 × 10 9 m3 during 13 M-level flares. The behavior of N e was strongly and positively correlated with the variation of the X-ray flux, with the largest coefficient being 0.93, while the changes of H ' exhibited a strongly negative correlation of −0.96 with that of the X-ray flux. Moreover, variations of H ' and Δ N e depended on not only the flare level, but also upon the onset time and duration of flare.

In this study, we utilized both ground‐based and space‐borne observations including total electron content (TEC) from Beidou geostationary satellites, two‐dimensional TEC maps from the worldwide dense Global Navigation Satellite System receivers, ionosondes, and in situ electron density (Ne) and electron temperature (Te) from both Swarm and China Seismo‐Electromagnetic Satellite satellites, to investigate the low‐latitude ionospheric responses to the annular solar eclipse on 21 June 2020. The decrease in TEC during the eclipse at low latitudes showed a local time dependence with the largest depletions in the noon and afternoon sectors. It was also found that the TEC depletions at different latitudes in the equatorial ionization anomaly (EIA) region over the East Asian sector cannot solely be explained by the solar flux changes associated with the obscuration rate. The differences in TEC reduction between stations can be more than a factor of 2 at latitudes with the same obscuration rate of over 90%. Compared with TEC variations in the Northern Hemisphere, the TEC also underwent a considerable decrease in the EIA region in the conjugate hemisphere without eclipse shadow. Meanwhile, the hmF2 near the magnetic equator increased around the onset of the eclipse, indicating an enhancement of the eastward equatorial electric field. Furthermore, the TEC decrease during the eclipse in the EIA region in both hemispheres lasted for a long period of more than 7 hr after the eclipse, with a TEC depletion of 2–6 TEC units. The Ne from Swarm and China Seismo‐Electromagnetic Satellite satellites showed a complicated variation after the eclipse, whereas no visible change was observed in Te. The enhanced equatorial electric field, neutral wind changes, and the associated plasma transport act together to generate the observed ionospheric effects at low latitudes during the eclipse. Our results also suggest that the eclipse‐induced perturbations of dynamic processes can continue to impact the ionosphere after the eclipse.

In this study, the variations of topside ionospheric irregularities during 24 geomagnetic storms with Dst ≤ −50 nT in 2015 were examined through an algorithm specifically designed to detect a significant level of ionospheric irregularities. The algorithm was developed through the use of several parameters derived from the topside total electron content (TEC) observations from GRACE, Swarm-C, and Swarm-B. The local time characteristics of the observed equatorial plasma irregularities (EPIs) were analyzed during different phases of the storms, within 30° S–30° N magnetic latitudes. By comparing its results with corresponding in-situ electron density data and the results of previous studies, the algorithm was found to be efficient. It was observed that the detected EPIs at different stages of the storm showed local time dependence. For instance, EPIs were observed during nighttimes, but took place in the daytime occasionally during the storm main phase. Furthermore, the percentage occurrence rates were most prominent during the main phase at the post-sunset sector within less than 6 h of the storm onset. On the other hand, the occurrence rates became prominent in the post-midnight/morning sector during the recovery phase and even higher than observed in the post-sunset sector. Based on these findings it was concluded that the dominant driver of the enhanced EPIs during the post-midnight/daytime sector could be associated with disturbance dynamo electric fields.

Statistical analyses were conducted to investigate the nighttime medium-scale traveling ionospheric disturbances (MSTIDs) for the first time by using airglow images and Global Positioning System (GPS) data over central China during 2013–2015. Our results show that the phase fronts of perturbations are aligned from northwest to southeast direction and propagate toward the southwest direction. The characteristics of the nighttime MSTIDs observed by OI 630.0 nm images are consistent with those of the nighttime MSTIDs obtained from the GPS data. The phase velocity, period, wavelength, and amplitude of nighttime MSTIDs are 50–150 m/s, 0.5–1.5 h, 150–400 km, and 2%–15%, respectively, as measured from 630.0 nm images and GPS data. In addition, we utilized the simultaneous observations from OI 630.0 nm and OI 557.7 nm images to explore the relationship between nighttime MSTIDs and gravity waves (

Topside ionospheric total electron content (TEC) observations from multiple low-Earth orbit (LEO) satellites have been used to investigate the local time, altitudinal, and longitudinal dependence of the topside ionospheric storm effect during both the main and recovery phases of the March 2015 geomagnetic storm. The results of this study show, for the first time, that there was a persistent topside TEC depletion that lasted for more than 3 days after the storm main phase at most longitudes, except in the Pacific Ocean region, where the topside TECs during the storm recovery phase were comparable to the quiet time ones. The observed depletion in the topside ionospheric TEC was relatively larger at higher altitudes in the evening sector and greater at local times closer to midnight. Moreover, the topside TEC patterns observed by MetOp-A (832 km) were different from those seen by other LEO satellites with lower orbital altitudes during the storm main phase and at the beginning of the recovery phase, especially in the evening sector. This suggests that the physical processes that control the storm time behavior of topside ionospheric response to storms are altitude-dependent.

Ionospheric observations from the ground-based GPS receiver network, CHAMP and GRACE satellites and ionosondes were used to examine topside and bottomside ionospheric variations at low and middle latitudes over the Pacific and American sectors during the October 2003 superstorms. The latitudinal variation and the storm time response of the ground-based GPS total electron content (TEC) were generally consistent with those of the CHAMP and GRACE up-looking TEC. The TECs at heights below the satellite altitudes during the main phases were comparable to, or even less than, the quiet time values. However, the storm time CHAMP and GRACE up-looking TECs showed profound increases at low and middle latitudes. The ground-based TEC and ionosonde data were also combined to study the TEC variations below and above the F2 peak height (hmF2). The topside TECs above hmF2 at low and middle latitudes showed significant increases during storm time; however, the bottomside TEC below hmF2 did not show so obvious changes. Consequently, the bottomside ionosphere made only a minor contribution to the ionospheric positive phase seen in the total TEC at low and middle latitudes. Moreover, at middle latitudes F2 peak electron densities during storm time did not have the obvious enhancements that were seen in both the ground-based and topside TECs, although they were accompanied by increases of hmF2. Therefore, storm time TEC changes are not necessarily related to changes in ionospheric peak densities. Our results suggest that TEC increases at low and middle latitudes are also associated with effective plasma scale height variations during storms.

In this study, the ionospheric observations from ionosondes, ground-based GPS receivers, Gravity Recovery and Climate Experiment (GRACE) and MetOp-A satellites, and Fabry-Perot interferometer over the Asian-Australian sector have been used to investigate the responses of the F2 peak and the topside ionosphere to the 2 October 2013 geomagnetic storm, particularly during the recovery phase. The comparison between the multiple simultaneous observations revealed a contrasting behavior of the topside ionosphere and the F2 peak in East Asia during the recovery phase. The upward looking total electron content from low-Earth orbit (LEO) satellites did not undergo such depletions as seen in the region near the F2 peak, and they even showed increases. Furthermore, the simulation results of the Thermosphere Ionosphere Electrodynamics General Circulation Model are used to explore the possible mechanisms responsible for the observed features. The model results and observations suggested that the contrasting behavior of the F2 peak and the topside ionosphere is mainly associated with the enhancement of the equatorward winds, albeit the disturbed electric fields could play an important role in producing it.