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In the upper atmosphere (ionosphere), plasma with two electron populations is very common. The cold electrons ( T e ∼ 1 eV) usually originate in the ionosphere, while the hot electrons ( T e ∼ 100 eV) come from the magnetosphere. In addition to these two electron populations, there may be a beam of electrons streaming along the magnetic field lines. These electrons are responsible for exciting various electrostatic wave modes. In this article, we provide a systematic insight in the evolution of the system, based on the beam energy. With an increase in beam velocity, the beam temporarily recovers a portion of its initial energy as well as a higher saturation energy. The wave energy is lost significantly before saturation and after linear growth at a relatively modest beam velocity. In addition, the low beam velocity generates BGK electron holes in the phase space, which are missing at higher beam velocities. We also present the analysis of the condition of sustained electron holes in the phasespace. [ABSTRACT FROM AUTHOR]

High-resolution thermospheric mass density low Earth orbit (LEO)-based measurements are valuable for accurately estimating short-term atmospheric abrupt disturbances triggered by solar flux forcing. To investigate the enhancing status of solar cycle 25 between August 29 and December 31, 2020, we processed Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) 10-s accelerometer-derived thermospheric mass density (TMD) measurements normalized at 500 km altitude by the NRLMSIS-2.0 empirical model. These 4-month enhancing disturbance observations suggest a shift from relative quiescence to a much more active solar phase, revealing unexpected dependencies on temporal and spatial characteristics. The results indicated that the dominant driver is solar extreme ultraviolet radiation (EUV) during this ascending phase. Density enhancement was symmetric in both hemispheres around the autumn equinox. After the equinox, the neutral density enhancement intensity in the Southern Hemisphere surpasses that in the Northern Hemisphere. Density maxima occurred from high to low latitudes, accompanied by a 2–3 h time lag. The Wygant function was applied to model the response to solar wind geomagnetic field changes and quantify the impact of geomagnetic activities on upper atmosphere density, verifying the time lag of density disturbances. All these findings could potentially improve our understanding of the solar cycle and LEO orbital drag. [ABSTRACT FROM AUTHOR]

Atomic oxygen (AO) is the most abundant species in the terrestrial thermosphere at altitudes of 180 to 700 km and plays a crucial role in numerous upper atmospheric processes. The drag encountered by a spacecraft is directly influenced by the background atmospheric density, making AO a notable contributor in this regard. The objective of this study is to analyze the variability of the density of AO in the near-Earth space environment under different solar activity conditions, as well as during severe space weather events. The analysis is primarily focused on the altitude range of 300 to 500 km in the Earth's thermosphere across the globe, which is a crucial operating region for a large number of Low Earth Orbiting satellites. AO exerts significant drag on the Low Earth Orbit (LEO) satellites and reacts with the materials of the satellite body and in situ probes mounted therein, causing considerable degradation to both. Additionally, AO interferes with measurements made by optical instruments through phenomena such as 'shuttle glow'. Analysis based on NRLMSIS 2.0 model revealed that the concentration of AO displays significant seasonal variations primarily due to the asymmetry in solar insolation. The highest concentration of AO was observed during the spring equinoctial month of the solar maximum year, while the lowest concentration was noted during the winter solstitial month of the solar minimum year. The enhancement seen in AO density contours show an asymmetry in the zonal direction at approximately 50 ∘ N in June and 50 ∘ S in December. Similarly, during geomagnetically disturbed days, the AO density shows enhancements along various longitudes, as indicated by the elongated contours. During geomagnetic storms, the heightened Joule heating in the polar region modifies the meridional circulation, which in turn affects the global distribution of AO density. Similarly, an analysis of thermospheric neutral winds obtained from the HWM93 model reveals that the zonal winds exhibit a strong westward acceleration over the region where AO density is enhanced. Furthermore, such an increase in AO density during geomagnetic storms has been found to cause orbital degradation ranging from a few tens of meters to a few hundred meters, depending on the strength and geo-effectiveness of the storms. This study emphasizes the critical contribution of AO density to satellite drag force, necessitating in situ measurements for an accurate estimation of the altitude decay of the spacecraft. [ABSTRACT FROM AUTHOR]

• Filtering OH rotational temperature estimates perturbations having periods of 0.7–8.2 h. • Statistical approaches allow subtracting uncorrelated random noise. • Seasonal and interannual variations of residuals reflect climatology of correlated wave processes. Ground-based observations of the natural hydroxyl (OH) nightglow at altitudes of 85–90 km are used for deriving the rotational temperature of excited OH, which is close to the neutral atmospheric temperature. For filtering of mesoscale perturbations, we use differences between pairs of measured values of OH rotational temperature separated with fixed time intervals in the range of 0.5–2 h. The filtering is applied for studying mesoscale variations of temperature near the mesopause according to the data of spectral OH nightglow measurements at observatories of Zvenigorod (56°N, 37°E.) in the years 2004–2016, Tory (52°N, 103°E) in 2012–2017 and Maymaga (63°N, 130°E) in 2000–2015. Monthly-mean values and variances of temperature disturbances with periods 0.7–8.2 h are determined. Semiempirical and statistical approaches are used to estimate and subtract variances of the instrumental dark current noise and uncorrelated in time fluctuations. Seasonal and interannual variations in standard deviations of correlated in time mesoscale perturbations of the OH rotational temperature at the considered observational sites are studied. They can give information about multiyear changes in the activity of atmospheric acoustic-gravity waves propagating through the OH layer near the mesopause. [ABSTRACT FROM AUTHOR]

• Ionospheric F2 Layer on QTP is studied by multi-station collaborative method. • The altitude of the F2 layer on QTP has significant regional variations. • The causes of abnormal variations of the F2 layer on QTP are analyzed. The Qinghai-Tibet Plateau (QTP) region is the largest plateau in China and the highest altitude in the world, with a unique geographical environment. Based on the observation data of the 24th solar cycle from six sites on the QTP and its surrounding areas, the multi-station joint detection method is adopted to analyze the temporal variation and spatial distribution characteristics of the critical frequency of the F2 layer(f o F 2) and the bottom height of the F2 layer (h'F 2). The results indicate that the critical frequency of the ionospheric F layer in the QTP is controlled by solar illumination conditions, and conforms to the ionospheric variation law of middle and low latitudes. The height of the F layer has significant regional characteristics, and the F layer height variation in Kunming and Chongqing is particularly drastic, and the average height of the F layer in Chongqing is significantly higher than that in Kunming, which is at a lower latitude, and Lhasa, which is at the same latitude as Chongqing. This phenomenon may be attributed to the unique geographical environment of the QTP, which has an influential effect on the motion of the lower atmosphere, and through the coupling mechanism between the lower and upper atmosphere, has a substantial impact on the F2 layer. The research findings of this article are of great interest to the study of the coupling mechanism between the upper and lower atmosphere. [ABSTRACT FROM AUTHOR]

• We propose spaceborne snapshot GNSS receivers to estimate thermospheric density. • We show that these receivers (<0.1 mW) have a position accuracy 100 superior to TLEs. • We show (T_s ∼ 8 hrs) a relative density accuracy like from TLEs or geodetic receivers. • This improves the time update rate of density values from TLEs by a factor of 9. This paper proposes a novel concept of using highly efficient Snapshot Global Navigation Satellite Systems (GNSS) receivers to provide precise position fixes of single or multiple satellites in Low-Earth Orbit (LEO) to improve upper atmospheric modeling and thus contribute to superior space situational awareness (SSA). While tracking of LEO satellites and the use of onboard GNSS receivers for drag measurements and upper atmosphere modeling are well-established techniques, the expected advent of snapshot GNSS receivers for spaceborne scientific applications will allow massive improvements on the GNSS sensor's Size, Weight, Power and Cost (SWaP-C). With chip-size dimensions of 4x4 mm2, a mass of less than 5 gr, an average power level below 0.1 mW, snapshot receiver technology is expected to provide position fixes in space with an accuracy of ∼19 m (3D r.m.s.), which will surpass the accuracy of Two-Line Elements (TLE) provided by the US Joint Space Operations Center (JSpOC) by at least two orders of magnitude. Equally important to their SWaP-C benefits, Snapshot GNSS receivers will allow mission and spacecraft designers to trade onboard-processing requirements versus payload downlink requirements, leading to either minimum onboard processing or a minimum amount of downlinked data. In this research, we establish the concept and architectural overview of using snapshot GNSS receivers for SSA, including the role of using them in a Distributed Space System (DSS), and detail their characterization and performance in terms of the required GNSS hardware and the impact of these payload on the power budget, the link budget and the OnBoard Data Handling (OBDH) budget of a satellite. It will be shown that these receivers lend themselves especially to their use on femto-, pico- and nano-satellites, although integrated snapshot modules may be flown as auxiliary payloads on micro- or mini-satellites as well. While this work focuses on the implications of the use of snapshot GNSS receivers on spacecraft design for the use of upper atmosphere modeling and SSA, their use may open up other science applications which avoid the need for expensive high-grade GNSS receivers. [ABSTRACT FROM AUTHOR]

• We characterized the pressure wave produced by the Tonga volcano using cosmic rays. • We believe this is the first time that a Lamb wave is studied using cosmic rays. • Our high-altitude data provides additional information about this unique event. High-energy cosmic rays that hit the Earth can be used to study large-scale atmospheric perturbations. After a first interaction in the upper parts of the atmosphere, cosmic rays produce a shower of particles that sample it down to the detector level. The HAWC (High-Altitude Water Cherenkov) gamma-ray observatory in Central Mexico at 4,100 m elevation detects air shower particles continuously with 300 water Cherenkov detectors with an active area of 12,500 m2. On January 15th, 2022, HAWC detected the passage of the pressure wave created by the explosion of the Hunga volcano in the Tonga islands, 9,000 km away, as an anomaly in the measured rate of shower particles. The HAWC measurements are used to determine the propagation speed of four pressure wave passages, and correlate the variations of the shower particle rates with the barometric pressure changes. The profile of the shower particle rate and atmospheric pressure variations for the first transit of the pressure wave at HAWC is compared to the pressure measurements at the Tonga island, near the volcanic explosion. By using the cosmic-ray propagation in the atmosphere as a probe for the pressure, it is possible to achieve very high time-resolution measurements. Moreover, the high-altitude data from HAWC allows to observe the shape of the pressure anomaly with less perturbations compared to sea level detectors. Given the particular location and the detection method of HAWC, our high-altitude data provides valuable information that contributes to fully characterize this once-in-a-century phenomenon. [ABSTRACT FROM AUTHOR]

We analyze periodic features in the F10.7 cm solar radio flux and the Ap geomagnetic index to examine solar-terrestrial coupling in Earth's atmosphere above 100 km. The coupling is indicated by the same periodic features observed in the global infrared power radiated by Nitric Oxide (NO) and Carbon Dioxide (CO 2). The infrared power data have been measured by the SABER instrument on the NASA TIMED satellite since January 2002. A strong dominant feature for the length of the solar cycle (≈11 years) can be observed in all the datasets. The length of this periodicity can vary from 9.95 years to 12.25 years for different solar cycles. Periodic features larger than the length of the solar cycle are also observed in the F10.7 and Ap index (≈22 years and ≈44 years). A strong 180-day periodic feature can be observed in the Ap index which corresponds to the well-known Russell-McPherron effect. Additionally, the NO power and the Ap index exhibit statistically significant periodic features for shorter periods (e.g., 27, 13.5 and 9 days). This similarity indicates a strong link between the thermosphere's infrared energy budget and the geomagnetic conditions of the upper atmosphere. • LNP analysis has been applied to detect periodic features of solar activities. • F10.7 index, Ap index and SABER infrared measurements have been investigated. • A strong periodic feature for the solar cycle (≈11 years) exists in the datasets. • Larger (≥22 years) and shorter (≤365 days) periodic features are also present. • Earth's infrared radiated energy and geomagnetic behavior have a strong coupling. [ABSTRACT FROM AUTHOR]

Tropical cyclones are one of the potential sources of gravity waves, which can transport energy and momentum to higher heights through their vertical propagation and interaction with the background flow and thereby influence the dynamics of the upper atmosphere. High-resolution wind data acquired by the Medium Frequency (MF) radar at Kolhapur (16.69°N, 74.24°E) are utilized to study the high frequency gravity waves (20–60 min) associated with the tropical cyclone named "Vayu" formed in the Indian Arabian Sea in June 2019. An enhancement of the gravity wave (GW) activities in the meridional wind is observed during 13–15 June 2019. The source of the gravity wave is the tropical Vayu cyclone of category 2 storm. We employed the gravity wave variance data from NASA's Atmospheric Infrared Sounder (AIRS) satellite for our analysis over stratospheric heights. The enhancement of gravity wave variance in both the stratosphere mesosphere heights and the low values of OLR indicate the strong convection associated with cyclone, the source of observed gravity waves. In the present study we analyzed the horizontal propagation direction of the cyclone generated gravity waves using perturbation ellipse, it is found to be in the north–south plane. The temperature profiles obtained from the SABER instrument on board TIMED satellite indicate the presence of mesospheric inversion layer with an amplitude of nearly 40 K on the day of large gravity wave variance (June 13, 2019). These results indicate the cyclone can generate gravity waves which can propagate to higher heights and modify the MLT thermal structure, providing evidence for the vertical coupling between lower and upper atmosphere. • High-frequency GW activity in meridional wind associated with Tropical Cyclone "Vayu". • Studied cyclone generated high-frequency gravity waves (GWs) over Kolhapur. • Anticorrelation of GW activity with OLR reveals tropospheric convection associated with cyclone as a source. [ABSTRACT FROM AUTHOR]

The atmosphere of Titan is a unique natural laboratory for the study of atmospheric evolution and photochemistry akin to that of the primitive Earth, with a wide array of complex molecules discovered through infrared and sub-mm spectroscopy. Here, we explore high resolution visible spectra of Titan (obtained with VLT-UVES) and retrieve an empirical high resolution list of methane absorption features at high resolution, (R = 100. 000) between 5250 Å and 6180 Å, for which no linelists are yet available. Furthermore, we search for the predicted, but previously undetected carbon trimer, C 3 , on the atmosphere of Titan, at its 4051 Å band. Our results are consistent with the presence of C 3 at the upper atmosphere of Titan, with a column density of 1 0 13 cm−2. This study of Titan's atmosphere with high-resolution visible spectroscopy presents a unique opportunity to observe a planetary atmosphere where CH 4 is the main visible molecular absorber, from which CH 4 optical proprieties can be studied. It also showcases the use of a close planetary target to test new methods for chemical retrieval of minor atmospheric compounds, in preparation for upcoming studies of cold terrestrial exoplanet atmospheres. • Study of Titan's visible spectrum with VLT-UVES in very high spectral resolution. • Absorption lines Doppler Shift allow distinction between Solar and Planetary origin. • CH 4 visible absorption bands in Titan are studied in very high resolution. • 97 absorption lines are detected and characterised in CH 4 visible bands below 620 nm. • Absorption features are consistent with the existence of C 3 in Titan's atmosphere. [ABSTRACT FROM AUTHOR]