Explore the vast range of titles available.

The October effect is known as a rapid and strong decrease in the signal amplitude of radio waves with very low frequency (VLF), reflected at the lowest edge of the ionosphere. This strong decrease can be observed only during the daytime. Although the October effect is long known, it is hardly investigated and its mechanism is still unknown. To get closer to a mechanism, we answer why the October effect does not occur during nighttime. Therefore, average characteristics of the October effect are obtained from different VLF transmitter‐receiver combinations. The occurrence of the October effect is then compared with characteristics of the neutral atmosphere temperature at VLF reflection heights as it seems to act as a proxy for the unknown mechanism. The temperature shows an asymmetric seasonal behavior at daytime VLF reflection heights poleward of 50°N but not during the nighttime, resulting in the October effect. Plain Language Summary The October effect is known as a rapid and strong decrease in the signal amplitude of radio waves with very low frequency, reflected at the lowest edge of the ionosphere (60–90 km). This strong decrease can be observed only during the daytime. Although the October effect has been long known, it is hardly investigated and its mechanism is still unknown. To get one step closer to a mechanism, we want to answer the question of why the October effect does not occur during nighttime. There are two main reasons why the October effect does not occur during nighttime. First, the radio wave reflection height is at around 70 km during daytime and at 85 km during nighttime. The second is the different behavior of the temperature at these two altitudes. While the temperature follows the seasonal cycle of the sun at 85 km, it shows an asymmetric behavior between spring and autumn at 70 km. This unexpected behavior of the temperature at 70 km leads to the October effect during the daytime only. Key Points Strong and rapid decrease in VLF amplitude in October Spring‐fall asymmetry in VLF amplitude and lower mesospheric temperature Asymmetry only at daytime VLF reflection height

High-Frequency Doppler (HFD) sounders at low-latitudes often detect characteristic oblique spreading Doppler traces in the spectrogram, known as Oblique Spread Structure (OSS). OSS has been expected to be generated by the dispersion of radio wave reflection due to equatorial plasma bubbles (EPBs). However, it has not yet been confirmed whether OSS is surely a manifestation of EPB by conducting simultaneous observations of EPB and OSS with different observational techniques. Additionally, it remains unclear what kinds of properties of EPB are reflected in the fine structure of OSS. In this study, we investigated three cases of OSSs and EPBs simultaneously observed by a HFD sounding system and an all-sky airglow imager in Taiwan. For the three cases presented here, the timing of OSS occurrence in the HFD data well coincided with that of the EPB appearance in the airglow data. The frequency shift of OSS is quantitatively explained assuming a radio wave reflection at 250–300 km altitudes. These results strongly indicate that OSS is formed by electron density variations at F-region altitudes accompanying EPB; thus, OSS is a manifestation of EPB in the HFD observations. Furthermore, it was suggested that the fine structure of OSS reflected the branching structure of EPB when the multiple branches of EPB reached the intermediate reflection point of the HFD observation. The detection of EPB occurrence and its fine structure using HFD observation enables monitoring of EPB regardless of weather conditions, which will contribute to monitoring the space weather impact of EPBs, for example, on GNSS navigation, in a wide area.

The dynamic structure of the ionosphere and its changes play an important role in comprehending the natural cycle by linking earth sciences and space sciences. Ionosphere research includes a variety of fields like meteorology, radio wave reflection from the atmosphere, atmospheric anomaly detection, the impact on GNSS (Global Navigation Satellite Systems) signals, the exploration of earthquake precursors, and the formation of the northern lights. To gain further insight into this layer and to monitor variations in the total electron content (TEC), ionospheric maps are created using a variety of data sources, including satellite sensors, GNSS data, and ionosonde data. In these maps, data deficiencies are addressed by using interpolation methods. The objective of this study was to obtain high-accuracy VTEC (Vertical Total Electron Content) information to analyze TEC anomalies as precursors to earthquakes. We propose an innovative approach: employing alternative mathematical surfaces for VTEC calculations, leading to enhanced change analytical interpretation for anomaly detections. Within the scope of the application, the second-degree polynomial method, kriging (point and block model), the radial basis multiquadric, and the thin plate spline (TPS) methods were implemented as interpolation methods. During a 49-day period, the TEC values were computed at three different IGS stations, generating 1176 hourly grids for each interpolation model. As reference data, the ionospheric maps produced by the CODE (Center for Orbit Determination in Europe) Analysis Center were used. This study’s findings showed that, based on statistical values, the TPS model offered more accurate results than other methods. Additionally, it has been observed that the peak values in TEC calculations based on polynomial surfaces are eliminated in TPSs.

An analytical solution of direct and inverse problems arising in the study of the internal gravity waves (IGWs) dynamic via recording of the Doppler frequency shift, is presented. The direct problem is to determine the response of the Doppler shift to IGWs in the region of the radio wave reflection point; the inverse problem is the determination of IGW parameters from data on the Doppler frequency shift. Solutions were obtained in an approximation of the isothermal ionosphere for the heights of the F-region. They are presented in a form convenient for their practical use and can have a wide range of applications, including the detection of soliton-like wave structures in the F-region of the ionosphere.

The seasonal variation of the 162 kHz radio wave reflection height was measured indirectly at Panská Ves using the IPHA method described in Fišer and Matys (1992). The fact that these waves are reflected at the height where the electron concentration is about 3.5 × 108m−3was used to model this height using a 1-D model of the lower ionosphere electron concentration described by Ondrášková (1993). The comparison of the measurements and model results indicates that: 1. The annual mean of the reflection height derived from measurements is several km higher than that derived from the model; 2. the character of the modelled seasonal variation is different from the real variation shown in Fišer and Laštovička (1992). The differences are discussed.

Saturn Kilometric Radiation (SKR), being the dominant radio emission at Saturn, has been extensively investigated. The low‐frequency extension of SKR is of particular interest due to its strong association with Saturn's magnetospheric dynamics. However, the highly anisotropic beaming of SKR poses challenges for observations. In most cases, the propagation of SKR is assumed to follow straight‐line paths. We explore the propagation characteristics of SKR across different frequencies in this study. An extended equatorial shadow region for low‐frequency SKR is identified, resulting from the merging of the Enceladus plasma torus and the previously known equatorial shadow zone. Ray‐tracing simulations reveal that low‐frequency (≲$\\lesssim $ 100 kHz) SKR is unable to enter the shadow region and is instead reflected toward high latitudes. In contrast, high‐frequency SKR (≳$\\gtrsim $ 100 kHz) generally propagates without hindrance. Observations suggest that some low‐frequency SKR can enter the shadow region through reflection by the magnetosheath or leakage from the plasma torus. Plain Language Summary Saturn Kilometric Radiation (SKR) is a natural electromagnetic wave generated in Saturn's high‐latitude region along its magnetic field lines. Variations in SKR frequency could offer insights into Saturn's magnetic conditions, especially its interaction with the solar wind. However, the observed frequency characteristics of SKR depend on viewing geometry due to its directional nature. While past studies assumed SKR travels in straight lines, this may not hold true for low‐frequency SKR. These emissions can change direction when they encounter dense plasma, similar to light reflecting off a mirror or bending when entering water. At Saturn's equatorial region, the plasma torus created by Enceladus, one of Saturn's moons, contains dense plasma and significantly affects radio wave propagation. Our study investigates the distribution of SKR at different frequencies and identifies a shadow region where low‐frequency SKR emissions are rarely seen. Using numerical simulations of ray propagation paths, we discover that low‐frequency SKR emissions cannot reach these shadow regions because they are reflected by the dense plasma torus. However, occasionally, we observe low‐frequency SKR in the shadow region, suggesting the possibility of reflection by Saturn's magnetosheath or leakage through the plasma torus. Key Points The propagation characteristics of Saturn Kilometric Radiation (SKR) are established statistically and by ray‐tracing A shadow region of the low‐frequency SKR near the equatorial region at large radial distances is discovered and discussed Low‐frequency SKR may enter the shadow region due to torus leakage or reflection at the magnetosheath

The concept that geospace storms are comprised of synergistically coupled magnetic storms, ionospheric storms, atmospheric storms, and storms in the electric field originating in the magnetosphere, the ionosphere, and the atmosphere (i.e., electrical storms) was validated a few decades ago. Geospace storm studies require the employment of multiple-method approaches to the Sun–interplanetary medium–magnetosphere–ionosphere–atmosphere–Earth system. This study provides general analysis of the 30 August–2 September 2019 geospace storm, the analysis of disturbances in the geomagnetic field and in the ionosphere, as well as the influence of the ionospheric storm on the characteristics of high frequency (HF) radio waves over the People's Republic of China. The main results of the study are as follows. The energy and power of the geospace storm have been estimated to be 1.5×1015 J and 1.5×1010 W, and thus, this storm is weak. The energy and power of the magnetic storm have been estimated to be 1.5×1015 J and 9×109 W, i.e., this storm is moderate, and a characteristic feature of this storm is the duration of the main phase of up to 2 d. The recovery phase also was lengthy and was no less than 2 d. On 31 August and 1 September 2019, the variations in the H and D components attained 60–70 nT, while the Z-component variations did not exceed 20 nT. On 31 August and 1 September 2019, the level of fluctuations in the geomagnetic field in the 100–1000 s period range increased from 0.2–0.3 to 2–4 nT, while the energy of the oscillations showed a maximum in the 300–400 to 700–900 s period range. During the geospace storm, a moderately to strongly negative ionospheric storm manifested itself by the reduction in the ionospheric F-region electron density by a factor of 1.4 to 2.4 times on 31 August and 1 September 2019, compared to the its values on the reference day. Appreciable disturbances were also observed to occur in the ionospheric E region and possibly in the Es layer. In the course of the ionospheric storm, the altitude of reflection of radio waves could sharply increase from ∼150 to ∼300–310 km. The atmospheric gravity waves generated within the geospace storm modulated the ionospheric electron density; for the ∼30 min period oscillation, the amplitude of the electron density disturbances could attain ∼40 %, while it did not exceed 6 % for the ∼15 min period. At the same time, the height of reflection of the radio waves varied quasi-periodically with a 20–30 km amplitude. The results obtained have made a contribution to the understanding of the geospace storm physics, to developing theoretical and empirical models of geospace storms, to the acquisition of detailed understanding of the adverse effects that geospace storms have on radio wave propagation, and to applying that knowledge to effective forecasting of these adverse influences.

What are the main findings? * Ambient noise seismic interferometry successfully retrieves empirical Green’s functions containing the vertical component of Rayleigh waves and P-wave reflection energy, enabling passive seismic imaging of Antarctic drilling sites. * The method provides stable and repeatable constraints on firn structure, ice thickness, and the ice–bedrock interface. Ambient noise seismic interferometry successfully retrieves empirical Green’s functions containing the vertical component of Rayleigh waves and P-wave reflection energy, enabling passive seismic imaging of Antarctic drilling sites. The method provides stable and repeatable constraints on firn structure, ice thickness, and the ice–bedrock interface. What are the implications of the main findings? * Passive seismic imaging provides a fast and cost-effective tool for Antarctic drilling site selection and subsurface characterization. * More generally, it can serve as a complement to active-source seismic methods, helping extend subsurface investigation to areas where conventional exploration is limited by logistical, environmental, or operational constraints. Passive seismic imaging provides a fast and cost-effective tool for Antarctic drilling site selection and subsurface characterization. More generally, it can serve as a complement to active-source seismic methods, helping extend subsurface investigation to areas where conventional exploration is limited by logistical, environmental, or operational constraints. Antarctic drilling projects provide critical information for investigating ice-sheet stability, reconstructing paleoclimate evolution, and characterizing subglacial geological structures through ice-core and bedrock recovery. Drilling site selection currently relies on high-resolution geophysical methods such as radio echo sounding and active-source seismic methods; however, radar imaging near the ice–bedrock interface is limited by electromagnetic attenuation, while active-source seismic methods in polar regions are constrained by logistical complexity and high cost. To address these limitations, this study proposes a passive integrated imaging approach that integrates P-wave responses and vertical-component Rayleigh-wave information retrieved from continuous ambient noise recordings near drilling sites using seismic interferometry. Based on their distinct propagation characteristics, signal selection and processing workflows are developed to jointly image near-surface firn structure, ice-sheet thickness, and subglacial bedrock structure. Application to the Princess Elizabeth Land drilling project in East Antarctica demonstrates that high- signal-to-noise-ratio P-wave responses and vertical-component Rayleigh-wave signals can be retrieved from as little as 24 h of ambient noise data, while stacking the full 20-day record further suppresses incoherent noise and yields more reliable imaging of the ice–bedrock interface. These results indicate that passive seismic imaging provides a rapid, cost-effective, and environmentally friendly complement for drilling site selection and operational support.

Energetic electron precipitation (EEP) from the Earth's radiation belts can ionize neutral molecules in the D-region ionosphere (60–90 km altitude), significantly influencing the conductivity and chemical species therein. However, due to the limited resolution of space-borne instruments, the energy and fluxes of electrons that truly precipitate into the atmosphere still remain poorly investigated. To resolve this problem, in this study, we have utilized the wave and particle data measured by the Electric Field Detector (EFD) and High-Energy Particle Detector (HEPP) on board the China Seismo-Electromagnetic Satellite (CSES-01) during nighttime conditions between 2019 and 2021. Using the measurements of extremely low frequency (ELF) waves, we have derived the reflection height of the D-region ionosphere, which turn out to be highly consistent with the electron and X-ray measurements of the CSES. Our results show that the influence of EEP on the two hemispheres is asymmetric: the reflection height in the Northern Hemisphere is in general lowered by 2.5 km, while that in the Southern Hemisphere is lowered by 1.5 km, both of which are consistent with first-principles chemical simulations. We have also found that the decrease in reflection height exhibits strong seasonal variation, which appears to be stronger during wintertime and relatively weaker during summertime. This seasonal difference is likely related to the variation of the background ionospheric electron density. Our findings provide a quantitative understanding of how EEP influences the lower ionosphere during solar minimum periods, which is critical for understanding the magnetosphere–ionosphere coupling and assessing the impact on radio wave propagation.

The Very Low Frequency (VLF) radio wave propagation characteristics play a very important role in understanding the behaviour of the D-region. The earth-ionosphere wave guide theory has been used to evaluate the reflection height of VLF radio waves using the electron density profiles obtained from the International Reference Ionosphere (IRI) 2012 and 2016 models. For calculating the conductivity parameter, two different collision frequency models have been used. The diurnal shift in reflection height of 16-kHz VLF waves is evaluated for the midpoint of Visakhapatnam-Rugby path using the two IRI models and the results are compared with those values derived from VLF phase measurements made at Visakhapatnam. The theoretically evaluated values using the FT-2001 option for the D-region electron density profile in the IRI-2012 and IRI–2016 models are in good agreement with those obtained from phase measurements, especially in summer. The day to night shift in reflection height obtained using exponential collision critical frequency model are in good agreement with those derived from VLF phase measurements. The diurnal shift in reflection height of VLF radio waves during winter months derived from IRI models are much lower than those obtained from measurements.