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We report thermospheric exospheric temperature and composition responses on the 15 January 2022 Tonga volcanic eruption. The temperature and composition profiles are inversed from three ionosonde (MHJ45, EG931, FF051) observed electron density profiles (∼150–200 km) using our new method (Li, Ren, et al., 2023, https://doi.org/10.1029/2022ja030988). The retrieved exospheric temperatures all showed obvious eruption‐induced perturbations, with maximum disturbance magnitude of ∼200 K at MHJ45 and ∼100 K at EG931 and FF051. The temperature variations were related to eruption‐excited thermospheric waves and their propagation with different speeds. While column ∑O/N2 had no evident changes similar to temperatures, which were basically consistent with GOLD observations. In comparison, higher thermospheric O/N2 has larger eruption‐related changes, maybe due to the exponential increase of thermospheric wave amplitudes with height. The application of our inversion method, combined with continuous observations and global coverage of ionosonde data, provide a possibility to further investigate thermospheric responses to different geophysical conditions. Plain Language Summary Extreme volcanic eruptions and resulted tsunami at 04:14:45 UT on 15 January 2022 generated a series of atmospheric waves, which can propagate out globally and up into the thermosphere. The ionosphere responses on this eruption, relative to thermosphere, have been reported a lot due to the large amounts of ionospheric observations. Here, we used the new method proposed by Li, Ren, et al. (2023), https://doi.org/10.1029/2022ja030988 to inverse daytime thermospheric parameters (neutral temperature and composition) from ionospheric electron density profiles (∼150–200 km). We selected ionosonde data at three stations (MHJ45, EG931, FF051) to verify the thermospheric responses during this eruption. The retrieved temperature at three stations showed the obvious eruption‐induced perturbations, but ∑O/N2 not, which were basically consistent with GOLD observations. However, O/N2 in higher thermosphere had larger eruption‐related changes. The comparison with GOLD observations and observed F2 layer peak electron densities verified the credibility of our inversion method again. Thus, the application of the method to the continuous and high‐covering ionosonde data provides a possibility to further investigate thermospheric responses to different geophysical conditions. Key Points Inversed exospheric temperatures showed obvious eruption‐induced perturbations on the 15 January 2022 Tonga eruption ∑O/N2 had no evident eruption‐induced changes similar to the temperature, neither in our inversion data nor in GOLD observations Ionosonde can expand the understanding of thermospheric responses to different geophysical conditions by our inversion method

Direct electric field measurements during certain ionosphere‐magnetosheath transitions on the dayside of Mars reveal a presence of localized (<${< } $ 20 km thickness along vertical direction) strong (>${ >} $ 40 mV/m) electric field located at the solar wind stagnation point. This electric field is nearly collocated with the ion composition boundary where ionospheric oxygen ions are observed to be accelerated up to ∼${\\sim} $ 1 keV, forming a layer of higher temperature plasma around the stagnation point. Simulations demonstrate that the observed localized electric field enhancement can create this hotter plasma layer population on either side of the boundary. This plasma layer can have an impact on the solar wind coupling with the planet and forms a reservoir for heavy ion escape. Plain Language Summary Solar wind, being heated and decelerated at the bow shock of Mars, begins to flow around the planet, and its ions generally reach altitudes as low as ∼${\\sim} $ 500–700 km on the dayside of the planet. Below this altitude, in the ionosphere, the plasma environment is dominated by heavier ions of planetary origin, and the region in between, where number densities of both population are equal, is often referred to as the ion composition boundary (ICB). Observations show that sometimes particles of the upper part of the ionosphere are accelerated to energies as high as ∼${\\sim} $ 1 keV and are diverted back to the planet or leak to the shocked solar wind region located above. Interestingly, these instances coincide with the presence of a localized large amplitude electric field enhancement at the ICB. Current study employs particle tracing simulations to determine if the observed feature in the electric field may be responsible for backward acceleration of ionospheric ions and their upstream leakage. The results provide an insight on a new feature of solar wind interaction with planetary plasma envelope which helps to better understand the processes of ion energization and escape on Mars. Key Points A localized electric field enhancement is observed at the solar wind stagnation point Observations of the electric field enhancement coincide with accelerated ionospheric oxygen ions up to ∼${\\sim} $ 1 keV Simulation suggests that the observed ion acceleration can be explained by the measured electric field enhancement

Observation‐based simulations of the ionosphere were performed with the NRLMSISE‐00 model for six locations around the globe during 1–9 February 2022, which includes the so‐called Starlink Storm. Unlike other studies, we focused on the magnetically quiet days around the storm. Unexpectedly, the observed values of the F2‐layer peak density were ∼50% larger than the simulated values. We show that this implies that the daytime O density in the thermosphere was systematically ∼30% larger than the NRLMSISE‐00 predicts. Further investigation shows that this discrepancy is not an exclusive feature of the period around the Starlink Storm and a similar problem happens for some periods for different years. It is unclear if the reason is an actual increase of the O density or its underestimation by the model. Resolving this problem is critical for providing accurate predictions of the atmosphere to avoid the degradation of normal operation or even loss of space assets. Plain Language Summary In early February 2022, dozens of Starlink satellites were lost after a moderate magnetic storm which was not expected to be able to cause significant perturbations in the Earth's atmosphere but did that. Numerous investigations showed the Starlink loss happened because of a remarkable increase of density of the upper atmosphere (thermosphere) caused by the storm. Teams of researchers were focused on the storm days 3 and 4 February. We got curious about the conditions in the thermosphere around the storm days. We employed observational data collected by ionospheric radars around the globe and conducted simulations with a physical model of the ionosphere‐thermosphere system, which allowed us to obtain the thermosphere composition. Our results indicate that the density of atomic oxygen in the thermosphere was one third larger than predictions of the standard model of the atmosphere during the periods before and after the storm. This means that the thermosphere was already poorly predictable prior to the storm occurrence. It is possible that this affected the thermosphere behavior during the storm. Also, we found that a similar problem with the prediction of the thermosphere by the standard model happens from time to time posing threat to the safety of spacecraft. Key Points For magnetically quiet days, the observed daytime NmF2 values were ∼50% larger than those simulated using the NRLMSISE‐00 model This implies ∼30% larger neutral O density comparing with the NRLMSISE‐00 model prediction or the density underestimation by the model Similar problem with the O density prediction happens for some periods for different years

Ionospheric chemistry plays an unexpectedly important role in the evolution of planetary habitability. This study is dedicated to a detailed modeling of the nightside Martian ionospheric structure and composition, a topic that has been poorly explored due to the absence of relevant measurements, but now becomes tractable owing to the unprecedented measurements made by the Mars Atmosphere and Volatile Evolution. Two-stream kinetic calculations and time-dependent fluid calculations are coupled to derive the nightside density profiles at 100–300 km for a large number of ion species, assuming solar wind electron precipitation as the only viable ionizing source in the ideal nonmagnetized atmosphere. Our calculations indicate the presence of a well-defined ionospheric peak at 146 km with a peak density of 8500 cm−3, as driven by the strong atmospheric “absorption” of precipitating electrons at low altitudes. The distribution of nonterminal species is roughly under chemical equilibrium below 170 km, whereas for terminal species such as NO+ and HCO+, diffusion is effective at essentially all altitudes, in direct contrast to the dayside behavior. In the more realistic magnetized atmosphere, the ionospheric peak seldom exists due to the patchiness of electron precipitation. In particular, our model results agree fairly well with the MAVEN measurements, especially in view of the coincidence between electron depletion and thermal plasma void seen along many MAVEN orbits. Compared to the dayside, the nightside ionospheric composition has a much higher proportion of NO+ and lower proportion of CO2 +, likely indicative of nightside enhancement of atmospheric O and N.

Long-lived metallic ions in the Earth's atmosphere (ionosphere) have been investigated for many decades. Although the seasonal variation in ionospheric “sporadic E” layers was first observed in the 1960s, the mechanism driving the variation remains a long-standing mystery. Here, we report a study of ionospheric irregularities using scintillation data from COSMIC satellites and identify a large-scale horizontal transport of long-lived metallic ions, combining the simulations of the Whole Atmosphere Community Climate Model with the chemistry of metals and ground-based observations from two meridional chains of stations from 1975–2016. We find that the lower thermospheric meridional circulation influences the meridional transport and seasonal variations of metallic ions within sporadic E layers. The winter-to-summer meridional velocity of ions is estimated to vary between −1.08 and 7.45 m/s at altitudes of 107–118 km between 10–60∘ N. Our results not only provide strong support for the lower thermospheric meridional circulation predicted by a whole atmosphere chemistry–climate model, but also emphasize the influences of this winter-to-summer circulation on the large-scale interhemispheric transport of composition in the thermosphere–ionosphere.

Current methods for estimating ion density on Swarm rely on the assumption of 100% O + and no along-track ion velocity flows. These assumptions are routinely violated, particularly on the nightside and during high-latitude and polar cap traversals, compromising the accuracy of the measurements. The use of faceplate current data along with the Langmuir probe ion admittance measurements, and orbital-motion limited (OML) theory, make it possible to relax some of the assumptions inherent in current ESA Swarm density estimates. This further yields along-track ion drift and effective ion mass estimates. This paper describes the theoretical basis for estimating revised ion density, providing a new estimate for effective ion mass, as well as an alternative way of estimating along-track ion drift. The complete Swarm historical data set has been generated and validated using empirical models (International Reference Ionosphere, and an empirical electric field model), as well as ground-spacecraft conjunctions. Case studies and statistical results reveal clear geophysical signatures in the new product of light ions at low- and mid-latitudes and along-track ion drift at high latitudes, and their response to space weather.Key pointsA new data product for Swarm along-track ion drift velocity, density and effective mass has been derivedThe addition of faceplate current to ion admittance enables a refinement to Swarm ion densityThe estimations have been validated against a variety of independent measurements and empirical models

The Ionospheric CONnections (ICON) mission has been continuously operating during the period from January 2020 to December 2021 providing simultaneous measurements of the thermal plasma properties near 600 km altitude and the neutral atmosphere and ionosphere in the altitude range 100 km to 500 km at low and middle latitudes. During this period of extremely low to moderately low solar activity, the evolving properties of the topside ionospheric density, composition, temperature and drift velocity at the satellite location are described using measurements from the Ion Velocity Meter (IVM). In the early months of 2020, the very low solar activity and relatively high abundance of H + in the total plasma density present a challenge to a robust description of the full local time distribution of the topside ion drifts. However, the quality of measurements of the ionospheric composition and temperature are not impacted by low solar activity conditions and changes in the O + and H + concentrations and their effects on the energy balance in the topside can be investigated as solar activity changes. As the relative abundance of O + increases, the susceptibility of the ion drift determination to the local plasma environment around the spacecraft is reduced and a more robust determination of the ion drift at all local times is possible. From October 2020 onward, the relationships between the topside ionospheric dynamics and the ionospheric density and temperature can be investigated and the relationships between the plasma drifts and the underlying neutral wind drivers can be established.

Leveraging the unique perspective enabled by Global‐scale Observations of the Limb and Disk, we examined the characteristics of equinox transitions in the thermospheric column integrated ratio of atomic oxygen to molecular nitrogen (O/N2) in the Northern Hemisphere. We found that the timing of the O/N2 equinox transition from winter to summer or vice versa exhibits a progression with latitude, particularly, near spring equinox. The O/N2 equinox transition is far slower during spring compared to fall, leading to a remarkable seasonal asymmetry. Ionospheric Connection Explorer observed a prominent asymmetry in the summer‐to‐winter circulation in the middle to upper thermosphere, implying that the inter‐hemispheric circulation plays a crucial role in the O/N2 equinox transition. Additionally, since the wave‐driven meridional circulation in the lower thermosphere displays a seasonal asymmetry between the northward‐to‐southward and southward‐to‐northward transitions, we would anticipate that the O/N2 equinox transition is also influenced by the lower atmospheric forcing. Plain Language Summary Thermospheric equinox transition refers to the most dynamic portion of the seasonal variation, which occurs near equinox and is featured by switching from either winter state to summer state or vice versa. Despite considerable advance being made in characterizing and understanding thermospheric seasonal variations over the past few decades, the rapid transitional process around equinox remains poorly understood. Very recently, NASA's Global‐scale Observations of the Limb and Disk and Ionospheric Connection Explorer missions provided a brief chance of simultaneous observations of composition and neutral winds in the thermosphere. Leveraging two missions offers an unparallel opportunity to delineate the equinox transition and to unveil the governing physical processes. Our data analysis reveals the complex nature of the equinox transitions, which is a combined effect of both the summer‐to‐winter interhemispheric circulation and the lower thermospheric wave‐driven circulation. Key Points We examine the thermospheric equinox transition in the Northern Hemisphere with Global‐scale Observations of the Limb and Disk O/N2 and Ionospheric Connection Explorer winds Equinox transition exhibits an evident progression in latitude and a seasonal asymmetry between spring and fall Thermospheric summer‐to‐winter circulation and wave‐driven circulation regulate the O/N2 equinox transitions

Storm-time topside ionosphere plasma composition, especially the light ion fraction, is an important parameter which controls magnetosphere–ionosphere coupling, plays a part in the growth of local instabilities, and provides information about the ring current, ion upflow, movement of ionization and other important physical processes and parameters. Ion composition is difficult to estimate on fine scales as empirical models tend to be parametrized by fixed inputs, ignoring the role of memory in plasma, and to preferentially capture large scales, while ground radars have limited coverage. In particular, ionospheric composition measurements at mid-latitude are lacking. Here we show, using the new Swarm SLIDEM effective ion mass measurement, a superposed epoch analysis of storm-time dayside and nightside effective ion mass changes, demonstrating the extent and timescales of motion of the [O+]/[H+] transition height with the main phase of geomagnetic storms, as well as directly observing evidence for the latitude dependence of these dynamics.

We discuss robust estimations for the variance of normally distributed random variables in the presence of interference. The robust estimators are based on either ranking or the geometric mean. For the interference models used, estimators based on the geometric mean outperform the rank-based ones in both mitigating the effect of interference and reducing the statistical error when there is no interference. One reason for this is that estimators using the geometric mean do not suffer from the “heavy tail” phenomenon like the rank-based estimators do. The ratio of the standard deviation over the mean of the power random variable is sensitive to interference. It can thus be used as a criterion to combine the sample mean with a robust estimator to form a hybrid estimator. We apply the estimators to the Arecibo incoherent scatter radar signals to determine the total power and Doppler velocities in the ionospheric E-region altitudes. Although all the robust estimators selected deal with light contamination well, the hybrid estimator is most effective in all circumstances. It performs well in suppressing heavy contamination and is as efficient as the sample mean in reducing the statistical error. Accurate incoherent scatter radar measurements, especially at nighttime and at E-region altitudes, can improve studies of ionospheric dynamics and composition.