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New compilations of records of ancient and medieval eclipses in the period 720 BC to AD 1600, and of lunar occultations of stars in AD 1600–2015, are analysed to investigate variations in the Earth's rate of rotation. It is found that the rate of rotation departs from uniformity, such that the change in the length of the mean solar day (lod) increases at an average rate of +1.8 ms per century. This is significantly less than the rate predicted on the basis of tidal friction, which is +2.3 ms per century. Besides this linear change in the lod, there are fluctuations about this trend on time scales of decades to centuries. A power spectral density analysis of fluctuations in the range 2–30 years follows a power law with exponent −1.3, and there is evidence of increased power at a period of 6 years. There is some indication of an oscillation in the lod with a period of roughly 1500 years. Our measurements of the Earth's rotation for the period 720 BC to AD 2015 set firm boundaries for future work on post-glacial rebound and core–mantle coupling which are invoked to explain the departures from tidal friction.

Global positioning system (GPS) radio occultation (RO) observations, first made of Earth’s atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of ROobservations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer.

This study assesses the potential influence of global navigation satellite system (GNSS) radio occultation (RO) data assimilation on the forecast skill of tropical cyclone formation over the western North Pacific in September–October 2019 through a regional model. Data from the Constellation Observing System for Meteorology, Ionosphere, and Climate mission II are applied. The forecast skill considers the hits and misses for nine developing cases and the false alarms and correct negatives for 23 non‐developing cases. Forecasts assimilating GNSS RO data reduce the false alarm ratio by 20% and increase the accuracy rate by 19%, compared to forecasts without GNSS RO data. Assimilation of GNSS RO data increases mid‐level moisture around the disturbance centers at the initial time of the forecasts. It also increases low‐level vorticity for developing cases but decreases vorticity throughout most of the troposphere for non‐developing cases. These lead to improved forecast performance for tropical cyclone formation. Plain Language Summary Tropical cyclone formation is very sensitive to local atmospheric conditions such as moisture, temperature, and winds. Getting these conditions correct is therefore critical for accurate forecasts of tropical cyclone formation. Satellite data are commonly used to characterize atmospheric conditions over the remote oceans where tropical cyclones tend to form. New satellite data, specifically from the radio occultation of global navigation satellite systems, provide fine scale information on moisture, temperature, and pressure. This new information suffers less from contamination by clouds and precipitation than the traditional microwave and infrared satellite observations, and may further improve the representation of atmospheric conditions. In addition to cases of tropical cyclone formation, this study pays particular attention to the quality of forecasts of disturbances that did not develop into tropical cyclones—something that is often overlooked when assessing the quality of tropical cyclone formation forecasts. We find that incorporating radio occultation data into the regional model can improve the local environmental conditions for tropical cyclone formation forecasts. This leads to fewer false alarms and higher hit rates. These improvements may provide critical guidance for tropical cyclone formation forecasters. Key Points Potential forecast skill of tropical cyclone formation is analyzed for nine developing and 23 non‐developing cases in September–October 2019 Assimilation of radio occultation data reduces the false alarm ratio by 20% and increases the accuracy rate by 19% The vorticity improved by radio occultation data is likely a key contributing factor to the improved tropical cyclone formation forecasts

The ionosphere is a part of the Earth’s atmosphere with complex temporal and spatial distribution and variation. Radio occultation can obtain the vertical distribution of ionosphere, effectively making up for the deficiencies of ground-based Global Navigation Satellite System (GNSS) and other ionospheric sounding technologies; however, the inversion results are subject to the hypothesis model and observation accuracy, making it important to conduct quality assessment. The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) project started in 2006 and has been renewed since 2019. So far, a large amount of observation data has been accumulated. In this paper, we used the electron density profile data obtained by COSMIC-1 and COSMIC-2 to evaluate the quality. Four evaluation parameters, which only consider the profile’s self-characteristics, were used, and the spatiotemporal distribution characteristics of the data quality were analyzed. The experimental results show that the number of occultation electron density profiles of COSMIC-2 is significantly greater, which is about seven times greater than COSMIC-1. In terms of spatial distribution, the unqualified ratio of COSMIC-1 profiles is less than 25% in most mid-latitude areas and about 15% near the equator, while the unqualified ratio rises to 25–50% in high-latitude areas and reaches 65% or even higher in part of the polar region. The unqualified ratio of COSMIC-2 profiles is about 25% near the equator and about 15% in middle and low-latitude areas. In terms of seasonal distribution, the quality of profiles is the worst in winter, followed by summer, and best in spring and autumn. In terms of day–night distribution, the unqualified ratio is higher at night than daytime and changes significantly at the turn of day and night. The nmF2 and hmF2 of the qualified profiles are significantly higher near the magnetic equator than in other regions, exhibiting a double-peaked phenomenon. The case of a large geomagnetic storm shows that the qualified ratio of inversion results can still maintain about 70% during extremely severe magnetic storms.

Juno performed close flybys of the innermost Galilean moon, Io, in December 2023 (I57) and February 2024 (I58). During these flybys, the radio link connecting the Juno spacecraft to Earth observing stations of NASA's Deep Space Network (DSN) propagated through the Alfvén wing, a magnetospheric feature in which plasma is produced between Io and Jupiter. The radio link is sensitive to the elevated electron densities in the Alfvén wing. A direct measurement of the total electron content was made by a linear combination of Juno's X‐band and Ka‐band downlink frequencies. Two different approaches were used in inverting the measurements into electron densities which assume different electron density distributions within the Alfvén wing. The maximum electron densities estimated in the Alfvén wing were 20,500–27,000 cm−3 on I57, in the northern Alfvén wing, and 15,300–31,000 cm−3 on I58, in the southern Alfvén wing.

The Global Navigation Satellite System Radio Occultation (GNSS-RO) technique has proven to be a powerful tool for studying E-region irregularities, i.e., Sporadic E (Es) which is primarily associated with the amplitude and phase scintillations. In the present study, an extensive 7-year GNSS-RO scintillation indices data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) observations was employed to investigate the global distribution and seasonal variation of the Es occurrences under solar activity near the magnetic dip equator. Our analysis from the Earth’s magnetic field parameters such as horizontal intensity and inclination estimated by the International Geomagnetic Reference Field model (IGRF) reveals that Earth’s magnetic field plays a crucial role in determining the global distribution of Es layers. Moreover, the abundance of Es shows a clear dependence on season/longitude, and the occurrence statistics of Es are closely aligned with the earlier reports. The solar activity dependence of the Es occurrence characteristics demonstrates its significant reduction with increased solar activity for most of the seasons in all longitude sectors. We address the Gradient Drift instability as a source mechanism of the Es layer’s appearance at the magnetic dip equator, where wind shear theory fails to operate because of the minimal inclination of the geomagnetic field.

On 29 September 2022 the Juno spacecraft flew within 354 km of Europa's surface while several instruments probed the moon's surroundings. During the close flyby, radio occultations were performed by collecting single‐frequency Doppler measurements. These investigations are essential to the study of Europa's ionosphere and represent the first repeat sampling of any set of conditions since the Galileo era. Ingress measurements resulted in a marginal detection with a peak ionospheric density of 4,000 ± 3,700 cm−3 (3σ) at 22 km altitude. A more significant detection emerged on egress, with a peak density of 6,000 ± 3,000 cm−3 (3σ) at 320 km altitude. Comparison with Galileo measurements reveals a consistent picture of Europa's ionosphere, and confirms its dependence on illumination conditions and position within Jupiter's magnetosphere. However, the overall lower densities measured by Juno suggest a dependence on time of observation, with implications for the structure of the neutral atmosphere. Plain Language Summary On 29 September 2022, NASA's Juno spacecraft flew very close to Jupiter's moon Europa. During the encounter, a radio occultation experiment was performed, where radio signals are exchanged between the spacecraft and ground stations as the former sets behind or rises from the moon as seen from the Earth. The scope of this experiment was studying the ionosphere of Europa, a layer of electrons and ions surrounding the moon. The Juno measurements confirmed the presence of the layer, with a structure similar to the one observed by the Galileo mission in the late 1990s. Key Points Europa's ionosphere was detected from Juno's X‐band Doppler data via NASA's Deep Space Network during a close encounter in 2022 Peak densities were 4,000 ± 3,700 cm−3 (3σ) at 22 km altitude during ingress and 6,000 ± 3,000 cm−3 (3σ) at 320 km during egress The Juno ionospheric profiles are consistent with Galileo measurements, and show a dependence on solar zenith and magnetospheric ram angles

The NOMAD (“Nadir and Occultation for MArs Discovery”) spectrometer suite on board the ExoMars Trace Gas Orbiter (TGO) has been designed to investigate the composition of Mars’ atmosphere, with a particular focus on trace gases, clouds and dust. The detection sensitivity for trace gases is considerably improved compared to previous Mars missions, compliant with the science objectives of the TGO mission. This will allow for a major leap in our knowledge and understanding of the Martian atmospheric composition and the related physical and chemical processes. The instrument is a combination of three spectrometers, covering a spectral range from the UV to the mid-IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and explain the technical principles of the three spectrometers. We also discuss the expected performance of the instrument in terms of spatial and temporal coverage and detection sensitivity.

The upper layers of Jupiter's atmosphere, offering critical insights into the planet's deeper structure, are accessible through radio occultation experiments. Since July 2023, NASA's Juno extended mission has provided the first high‐resolution radio occultation measurements since the Voyager era, probing the thermal structure and composition down to approximately 0.5 bar. We use these measurements to study Jupiter's latitudinally dependent vertical thermal structure. We observe cooler stratospheric and warmer tropospheric temperatures at the equatorial region compared to mid‐ and high‐latitudes, and temporal variations in the North Equatorial Belt's thermal structure on a time scale of a few months. These observations align with archival mid‐infrared data from Cassini's CIRS and current ground‐based Texas Echelon Cross Echelle Spectrograph observations, as well as previous studies based on Voyager radio occultations and the Galileo probe, offering an enhanced view of Jupiter's lower stratosphere and upper troposphere thermal structure.

This paper presents an analysis of Juno's first radio occultation experiments. Relying on two‐way radio links in the X‐ and Ka‐bands, we processed data from NASA's Deep Space Network antennas through a ray‐tracing inversion algorithm. By effectively isolating dispersive effects, we obtained measurements of the neutral atmosphere's characteristics. This enabled the derivation of pressure and temperature profiles from the recorded frequencies. These results complement prior data from Voyager occultations and CIRS observations, providing valuable contributions to our understanding of Jupiter's atmospheric dynamics.