This letter proposes a novel keypoint detection paradigm for the direct parameter regression of Maximum Usable Frequency (MUF) for the F and E layers from oblique ionograms. Unlike traditional approaches that rely on susceptible multi-stage image processing pipelines, the proposed method conceptualizes ionogram echo traces as structured objects. By employing a You Only Look Once Pose (YOLOPose)-based architecture, the physical characteristics of traces are modeled as regressable keypoints, enabling direct parameter extraction. Validated on a self-collected dataset of 1,804 annotated ionograms, the method achieves detection precisions of 98.90% and 95.69% for the F and E layers, respectively, with regression errors satisfying practical communication standards. By obviating the complex post-processing steps typical of rule-based workflows, this framework offers a robust and efficient technical pathway for real-time ionospheric monitoring and frequency management.

While the bottomside electron density profile (EDP) of the ionosphere can be widely estimated by ionosonde observations, accurate estimation of the topside EDP from ground-based ionospheric observations remains challenging. Furthermore, the impact of initialization parameters on EDP reconstruction accuracy has not been systematically investigated. This study proposes a method to reconstruct the electron density profile from Global Navigation Satellite System (GNSS) vertical total electron content (vTEC), in which the QPS model and the semi-Epstein model are employed to construct the bottomside and topside electron density profiles, respectively. The required parameters for the E and F1 layers are specified by the IRI-2020 model, whereas the optimal foF2 and hmF2 are determined through an iterative adjustment process designed to minimize the absolute difference between the modeled and measured TEC. To evaluate the impact of the initial parameters on reconstruction accuracy, initial parameters from different sources are used to reconstruct the electron density profile: (1) foF2 and hmF2 from the IRI-2020 model, (2) foF2 and hmF2 from a locally built EOF model, and (3) foF2 estimated directly from vTEC with hmF2 from the IRI-2020 model (hereafter referred to as the TEC-foF2 method). Results show that all three methods were able to reproduce the variations in foF2 and hmF2. The root mean square errors (RMSE) of foF2 obtained by three methods are below ∼0.62 MHz, which are lower than those of the initial foF2. However, the accuracy of hmF2 is not improved compared with the initial value. The RMSE values of hmF2 from the three methods are about 20 km. Furthermore, an analysis of the diurnal variations in the reconstruction errors reveals that the TEC-foF2 method minimizes the foF2 error during the daytime, whereas the local EOF model achieves the highest accuracy at night. Motivated by these distinct diurnal characteristics, a time-segmented hybrid initialization strategy was developed. By optimally integrating these two approaches, the overall RMSE of the reconstructed foF2 is further constrained to approximately 0.517 MHz. Moreover, case studies reveal that the TEC-foF2 method demonstrates robustness in foF2 estimation during geomagnetic storms compared to the IRI-2020 model. However, to improve the accuracy of hmF2 estimated from vTEC, the future studies should focus on the initial hmF2 from models.

Ionospheric scintillation caused by ionospheric plasma bubbles/irregularities is a common ionospheric phenomenon in the global navigation satellite system (GNSS), which can lead to a decline in the performance of GNSS systems in severe cases. Therefore, the study of the characteristics of ionospheric scintillation is of great significance. The characteristics of ionospheric scintillation are closely related to the characteristics of ionospheric plasma bubbles/irregularities (including disturbance intensity and spectral indices). In this study, the characteristics of ionospheric scintillation caused by equatorial plasma bubbles (EPBs) are studied by using a new 2-D model of plasma bubble and spectral analysis method. First, the spatial and temporal distributions of electron density in the equatorial ionosphere were simulated by the new model. Then, the intensity and phase power spectra of ionospheric scintillation signals are constructed based on the parameter estimation results. Finally, the temporal variations of intensity and phase of GNSS signal are simulated. Results show that the strength and phase of GNSS signals can be well reproduced by a new 2-D model of plasma bubbles. Due to the low-performance computing facilities compared with 3-D models, a 2-D model of a plasma bubble might be an acceptable method for practical usage in assessing the severity of ionospheric threats.

The air inside the lightning channel can be almost completely ionized into plasma because of the strong current and extremely high temperature. The electrons in the channel can persist for some time after the current disappears. This article proposes a numerical model to study the spatiotemporal variation of electron density during the no-current phase in the lightning channel after a single return stroke. The model consists of a convection-diffusion equation and an electron-particle reaction equation to calculate the distribution and variation of electron density in the radial direction over the cross section of the lightning channel. On the results of the numerical simulation of the electron density, the reflection durations of the lightning channel for electromagnetic waves of different frequencies and the channel's radar scattering cross section (RCS) are calculated. This article provides a new and valuable perspective for active radar detection of lightning and the study of the atmospheric effects of lightning based on numerical simulations.

In this work, we investigate the generation of E-region and F-region irregularities at equatorial ionization anomaly crest in the East Asian longitude sector, focusing on whether the daytime F-region irregularities are exclusively linked with E-region irregularities. Coordinated observations were conducted on a geomagnetically quiet day, 22 June 2020, using Qujing and Wenshan very high frequency (VHF) radars. Results reveal that daytime E-region echoes are closely related to local sporadic E (ES) layer with quasi-periodic (QP) echoes occurring during periods of strong ES activity. Zonal wind observations from MIGHTI/ICON measurement displayed that strong negative wind shear appeared during this period of observed E-region echoes. Results from current case study suggest that strong negative zonal wind shear can induce both Kelvin-Helmholtz (K-H) instability and gradient drift instability (GDI), leading to daytime E-region irregularities. Moreover, the results also indicated that daytime F-region irregularities were accompanied by strong QP echoes in the E region. The enhanced eastward polarized electric field excited by Hall polarized process in the E-region can be mapped along geomagnetic field lines to the F-region and further excite the generation of daytime F-region irregularity through E-F coupling system during the period of a significant increase in ΣHEs/ΣPF.

The ionosphere, as an essential component of the Earth’s space environment, contains free electrons that influence the amplitude and phase of radio signals. In the equatorial and low-latitude regions, plasma bubbles in the F region, manifested as electron density depletion, are observed frequently during the postsunset period. This phenomenon severely impacts the performance of modern electronic systems, such as short-wave communications and satellite navigation systems. In short-wave communication and target localization systems, the propagation mode of high-frequency electromagnetic waves in the ionosphere is typically oblique propagation. Therefore, studying the effects of equatorial plasma bubbles (EPB) on the oblique propagation characteristics of short waves is of significant engineering application importance. In this study, first, a model of plasma bubbles was used to produce equatorial plasma bubbles in the ionosphere. Then, a ray-tracing method was utilized to simulate the oblique propagation of radio waves in the ionosphere and synthesize oblique ionograms. Finally, the influence of EPB on the morphology of oblique ionograms was investigated, and the synthesized ionograms were compared with the measured oblique ionogram. Results showed that the features of the echoes such as satellite traces in the low working frequency band and diffuse echoes in the high working frequency band could be well reproduced in the simulated oblique ionograms, and that EPB could cause severe spread in both the group path and frequency of the high working frequency band of oblique ionograms, which was highly similar to the characteristics of spread F in the measured ionograms.

The ionosonde is one of the most widely used ionospheric detection device. It has significant implications for the study of space physics and wireless communication technology. To solve the problem of low resolution with traditional iososonde imaging method, this letter proposes a high-resolution two-dimensional spatial-frequency spectral estimation imaging method. By reconstructing the echo signal matrix and constructing the guidance vector that simultaneously reflects the range and motion characteristics of the target, high-precision estimation of the range-Doppler spectrum is achieved in twodimensional Capon estimation. The analysis of experimental Sporadic-E sounding data of August 13, 2021 in Wuhan (114 degrees 22 ' E, 30 degrees 30 ' N), China, demonstrates its ability of the precise measurement for single-layer echoes and the identification ability for multi-layer fine structures. This method further enhances the potential of traditional ionosonde for precise observation and analysis of the ionosphere.

In the contemporary era, there has been a surge in the development of innovative radio communication systems. However, HF communication, a conventional mode with a long history, continues to play a pivotal role and is undergoing rapid advancements across the globe. The maximum usable frequency (MUF) in HF communication is a crucial parameter influenced by the condition of the ionosphere. Accurate predictions of MUF are essential for ensuring the efficacy of communication. This paper assesses the precision of the methodology of employing the IRTAM combining with the INGV model to forecast MUF. The validation is evaluated by comparing the predicted MUF with the maximum observed frequency (MOF) at each time, as determined by experiments conducted at several ionospheric oblique sounding stations constructed throughout China. The MUF predictions from the combination of the IRI and INGV model are also calculated for comparison. Furthermore, the deviations at different times of the day for different links are analyzed. The results demonstrate that the average RMSE is 2.27 MHz and the RRMSE is 16.6 % over the entire experimental links nationwide. In summary, MUF prediction through the IRTAM&INGV model is simple to implement and of a certain applicability in China. (c) 2025 COSPAR. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Previous observations show that the daytime irregularities (the fossil of nighttime irregularities) are mostly located in the low latitude region and at 10-14 LT. In this study, daytime topside ionospheric irregularities were observed along the longitude of ti 94oE by Swarms satellite in the afternoon equatorial ionosphere (about 15:00 LT) on 18 June 2020. Simultaneously, the F3 layer were observed in the bottomside ionosphere. Occurrences of F3 layer shows that there are disturbances on the zonal electric field. Multi-satellite observations show that the topside ionospheric irregularities did not occur in the morning equatorial ionosphere. Results indicate that afternoon ionospheric irregularities might be associated with the electrodynamic process of the plasma in the equatorial ionosphere. Simulations show that the combined effects of the disturbed zonal electric field and velocity shears in zonal plasma drift can be favorable to increase the occurrence rate of daytime ionospheric irregularities in the topside equatorial ionosphere. (c) 2024 COSPAR. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

The observation of the ionosphere is of great significance for geophysics, radio propagation mechanisms, and related engineering applications. In response to the low range and Doppler resolution of the scattering function map obtained by traditional ionosonde with fewer coherent soundings, this letter proposes a super-resolution analysis method. By utilizing the coherence between the frequency components of the echo signal and using phase weighted summation, the super-resolution synthesis in the range dimension was achieved. In the frequency dimension, the Capon spectral super-resolution estimation method was employed. In this way, super-resolution is achieved both along the range and Doppler axis of the scattering function map simultaneously. It breaks the limitations of spatial and temporal sampling rates on observation resolution. Simulation and experiment results have verified the performance of this method. With a tenfold increase in resolution, it achieves the distinction of multi-layer echo traces and the identifies their subtle Doppler difference. This method provides a favorable means for efficient and accurate detection of the ionosphere.

Every day, millions of meteoroids enter the atmosphere and ablate, forming a long plasma trail. It is a strongly scattering object for electromagnetic waves and can be effectively detected by meteor radar at altitudes between 70 km and 140 km. Its echo typically has Fresnel oscillation characteristics. Most of the traditional detection methods rely on determining the threshold value of the signal-to-noise ratio (SNR) and solving parameters to recognize meteor echoes, making them highly susceptible to interference. In this paper, a neural network model, YOLOv8n-BP, was proposed for detecting the echoes of underdense meteors by identifying them from their echo characteristics. The model combines the strengths of both YOLOv8 and back propagation (BP) neural networks to detect underdense meteor echoes from Range-Time-Intensity (RTI) plots where multiple echoes are present. In YOLOv8, the n-type parameter represents the lightweight version of the model (YOLOv8n), which is the smallest and fastest variant in the YOLOv8 series, specifically designed for resource-constrained scenarios. Experiments show that YOLOv8n has excellent recognition ability for underdense meteor echoes in RTI plots and can automatically extract underdense meteor echoes without the influence of radio-frequency interference (RFI) and disturbance signals. Limited by the labeling error of the dataset, YOLOv8 is not precise enough in recognizing the head and tail of meteors in the radar echograms, which may result in the extraction of imperfect echoes. Utilizing the Fresnel oscillation properties of meteor echoes, a BP network based on a Gaussian activation function is designed in this paper to enable it to detect meteor head and tail positions more accurately. The YOLOv8n-BP model can quickly and accurately detect and extract underdense meteor echoes from RTI plots, providing correct data for meteor parameters such as radial velocities and diffusion coefficients, which are used to allow wind field calculations and estimate atmospheric temperature.

Radio waves in the ionosphere propagate in two modes: ordinary (O) and extraordinary (X). Accurate parameter inversion necessitates their separation. This letter proposes a self-calibrating O/X-mode separation method. By focusing on the higher-frequency branches beyond the overlap point of the F2 layer O/X traces, a direct iterative optimization method is used to determine phase-shift and amplitude compensation parameters for mode synthesis. By tracking diurnal variation characteristics of ionograms and extrapolating predictions, the parameters across the whole frequency bands can be determined. Comparing with the traditional separation method, this method is independent of antenna type and alignment, requires no internal or external calibration source, and can provide real-time calibration updates.

Ionospheric plasma density disturbances significantly affect radio wave propagation, impacting communication and planetary exploration. At Earth, sporadic E layers (Es) are commonly observed. They are characterized by thin, high-density plasma structures in the E-region ionosphere, primarily formed by electromagnetic forces induced by horizontal wind shears. Similar sharp vertical electron density changes have also been observed in the topside of the Martian ionosphere by missions like Mars Global Surveyor, Mars Express, and Mars Atmosphere and Volatile EvolutioN. Recent studies suggest that such structures observed on Mars can be attributed to a sporadic E-like mechanism induced by magnetic shear, which can be referred to as Es-like structures. While wind shear theoretically contributes to such events, no observational evidence currently reports wind shear as an explanation for these density gradients on Mars. In this study, we report a novel observation of an Es-like topside layer in the Martian ionosphere, characterized by both density layer and rift (an abrupt depletion in density), where wind and magnetic shear are simultaneously observed. Statistical analysis of 305 orbits with wind speed observations revealed wind shear is observed in nine Es-like events, primarily occurring in strong crustal magnetic field regions. Simulations suggest that the observed rift and layer can be explained either by “wind shear” or by “wind and magnetic shear” under strong crustal magnetic field. This study provides new observations of Es-like events on Mars and serves as a credible candidate for the contribution of wind shear to the formation of an Es-like topside layer.

Aiming to mitigate the substantial dispersion in arrival angle estimation due to colored and white noise interference, which may seriously affect the accuracy of short-wave single-station positioning, this paper introduces an approach to optimizing angles based on the statistical features. By utilizing the extraction of the main peak area of the probability density distribution of the measured angle, as well as the two-dimensional Gaussian fitting and confidence ellipse bounding, the angle measurement results affected by colored noise interference and the noise points with large deviations can be sequentially filtered out. Combining experimental scenarios and confirmed by actual measurement data, the dispersion of arrival angle estimation results has been significantly constrained, and, correspondingly, the positioning accuracy has also been significantly improved by about 3%.

A study of occurrence characteristics and morphology of daytime spread F (DSF) irregularities was conducted using ionosonde from 2013 to 2020 over low-latitude Puer station in the China sector. In this study, a day with two or more consecutive F-region diffuse echo ionograms within one hour from local sunrise to sunset at an altitude of 250 km is considered a DSF day. The results show that the DSF irregularities observed in different solar activity years have a similar seasonal distribution, manifesting as frequent occurrences in June solstice with a maximum occurrence near sunrise. Furthermore, the morphology of most DSFs on the ionograms is predominantly frequency spread F, with a few observed near sunrise as MSF and RSF. In contrast to in high solar activity years (HSAY, 2013-2015), an interesting phenomenon of DSF irregularities in low solar activity years (LSAY, 2016-2020) is presented. That is, in the late afternoon hours (15-18 LT), DSFs have an unexpectedly high occurrence rate during the December solstice. By analyzing the virtual height variations for iso-frequency plots of the DSF event occurring in the afternoon, these DSF irregularities were found to be likely associated with daytime medium-scale traveling ionospheric disturbances (MSTIDs). Compared with the observed characteristics of the nighttime spread F, the fossils of nighttime plasma bubbles irregularities might play a key role in the development of DSF occurring in the early morning. At the same time, the influence of daytime MSTIDs on DSF irregularities occurring in the late afternoon hours cannot be ignored. Equatorial Spread F have been widely known as a large-scale plasma density depletions (bubbles) over the magnetic equator. The bubbles usually decay rapidly near sunrise due to strong photoionization. Due to the rarity of daytime spread F (DSF), and the difficulty in determining when and where these daytime F-region irregularities are initially generated, their occurrence characteristics and generation mechanisms remain unresolved well. In this study, we made the first long-term continuous observations of the DSF echoes at the low-latitude Puer ionosonde station from 2013 to 2020. The observations show that DSFs occur frequently in June solstice with a maximum occurrence near sunrise, and mostly appear on the ionogram in frequency spread F morphology. An interesting phenomenon of DSF irregularities in low solar activity years (LSAY) is that an unexpectedly high occurrence rate in the late afternoon hours (15-18 LT). The research results indicate that these DSFs could be associated with daytime medium-scale traveling ionospheric disturbances (MSTIDs). Our results indicate that the DSF irregularities in the low-latitude F region were not totally the continuations of nighttime plasma bubbles. The daytime MSTID is also responsible for the effects of DSFs that appear in the late afternoon hours during LSAY. DSFs show similar seasonal patterns in different solar activities: a high occurrence during the June solstice, peaking around sunriseThe morphology of most DSFs on the ionograms is mainly frequency spread F, with a few observed near sunrise as mix spread F (MSF) and range spread F (RSF) typesThe surprisingly high DSF occurrences at 15-18 LT during low solar activity years may be associated with daytime medium-scale traveling ionospheric disturbances

Radar detection technology has been used to observe meteors since the last century. According to the echoes of electromagnetic waves scattered by meteor trails, the drift velocity of meteors is calculated to research the atmospheric dynamics characteristics in their distribution height. In this paper, complementary code sequences are applied to a Very High Frequency (VHF) ionospheric sounding system. Unlike the common VHF meteor radar, the system parameters can be set with a pulse duty cycle, allowing for flexible adjustment of detection range. At the same time, the detection frequency can also be adjusted within a certain range, breaking through the traditional fixed frequency detection mode. Moreover, by employing a miniaturized antenna array composed of an orthogonal dipole antenna for the VHF receiving, the system takes account of the requirements of the inversion algorithms of meteor radar and the detection need for the irregularities sounding concurrently, achieving the function of detecting multiple targets. Therefore, the system is called the Wuhan VHF Comprehensive Sounding (WHCS) system. This system has the function of integrating transmission and reception, which can be directly detected by a single station or synchronously detected through the separation of transmission and reception. This system has the function of internal channel calibration to compensate for the phase error of meteor echoes. Through further analysis of the detection data, the height distribution of meteors, as well as typical echoes of ionospheric irregularities, were obtained. The experiments verified the availability of the system scheme and its engineering application significance.

Spread F (SF) in the ionosphere can be observed frequently in mid-latitude regions. It is suggested that atmospheric gravity waves play a significant role for the seeding of mid-latitude SF. Previous research suggested that the source of travelling ionospheric disturbances (TIDs) over China is in the southeastern and northeastern edge of the Qinghai-Tibet Plateau, however, until now there have been no ground-based observations of the ionosphere in this region. Recently, an advanced digital ionosonde was installed at Zhangye station (39.2°N, 100.54°E, Dip Lat 29.6°N) in the northeastern edge of the Qinghai-Tibet Plateau. It is an opportunity to verify the effect of gravity waves on the formation of mid-latitude SF by comparing it with observations in other regions of the Chinese sector. In this study, statistical analysis of SF recorded at Zhangye station during 2017–2022 was carried out. Results show that diurnal, seasonal and solar cycle characteristics of the occurrence rate of SF are similar with previous studies. At Zhangye station, the maximum occurrence rate of SF is during the post-midnight period in summer and winter. The occurrence rate of SF events have a negative relationship with solar activity. There is no obvious relationship between the occurrence rate of SF and geomagnetic activity. Comparing observations of other stations in the mid-latitude region, we found that the occurrence rates of SF (the annual maximum rates are from 33.83% to 53.29%) are much higher at Zhangye station. Further studies show that ionospheric disturbances can be observed frequently at Zhangye station, especially in autumn and winter. Gravity waves/TIDs in the northeast of the Qinghai-Tibet Plateau are suggested to explain the abnormal higher occurrence rate of SF at Zhangye station.

For the past few decades, it has been demonstrated that gravity waves (GWs) and neutral winds can drive ionospheric irregularities on Earth. Still, as far as we know, the formation of ionospheric irregularities on Mars due to GWs has not been well studied. In this study, we use data from the NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission to show evidence of an irregularity event in the Martian ionosphere, potentially seeded by the GWs break (GWB). Statistical findings indicate that the observed ratio of GWB-related irregularity events varies from similar to 0.25 to similar to 0.47 each year, and the average ratio in 2015-2020 is similar to 0.37. We perform a numerical simulation to provide further insight into the processes behind irregularity formation, which employs neutral wind shear as a source of perturbation in the context of the GWB. The simulations yield results fundamentally aligned with the observed characteristics of ionospheric irregularities in the 2018 event by considering the wind shear as the disturbance source. This study provides supplementary insights into the perturbation sources involved in shaping irregularities within the Martian ionosphere and presents valuable information about the coupling between the Martian ionosphere and the lower atmosphere. Many remote and in situ observations of the Martian ionosphere have been made by landers, orbiting or flyby spacecrafts, for example, Viking, Mars Global Surveyor (MGS), Mars Express (MEX), and Mars Atmosphere and Volatile EvolutioN (MAVEN) missions. It gives us a great opportunity to know the basic ionospheric structure and composition of Mars. However, there are still many scientific questions about the Martian ionosphere that have yet to be studied, for example, the structure of the lower ionosphere, small-scale ionospheric irregularities, and the role of lower atmospheric effects on the ionosphere. Normally, the ionosphere is a stable stratified structure. However, in some situations, the ionosphere becomes unstable, generating perturbations in the electron density, leading to these so-called "irregularities." In this study, we report that the break of gravity waves potentially accounts for the formation of ionospheric irregularities observed in the Martian ionosphere by NASA's MAVEN spacecraft. The results of our statistical study support the notion that gravity waves are a crucial seeding source in the formation of irregularities. This investigation adds to the current understanding of perturbation sources responsible for irregularity formation in the Martian ionosphere, while also contributing to our knowledge of the coupling between the Martian ionosphere and the lower atmosphere. An example of a Martian ionospheric irregularity event potentially seeded by GWB (the break of gravity waves) is presented Statistical results show that the proportion ratio of GWB-related irregularities peaked at similar to 0.47 in 2018, reached a minimum of similar to 0.25 in 2017, and averaged similar to 0.37 over the six years The simulation shows that the formation of the irregularities can be fundamentally reproduced using the GWB-related wind shear as a perturbation source

Spread F is one of most widely observed nighttime ionospheric irregularities by ionosondes in the equatorial, low- and mid-latitude regions. One type of spread F, known as strong range spread F (backscatter echoes beyond the critical frequency of the F2 layer, foF2), has been observed in the equatorial and low-latitude regions but not at middle latitudes. This study reported for the first-time observations of strong range spread F at Zhangye Station (ZHY, Geographical latitude 39.4 degrees N, longitude 100.0 degrees E, Dip Lat 29.6 degrees N) on 29 November 2019 (under quiescent conditions). Observations show that strong range spread F occurred at first during midnight (similar to 16:30 UT), accompanied with mid-latitude nighttime ionospheric enhancements in foF2 and total electron content (TEC). Then, it becomes more pronounced when ionospheric collapse occurred. The measurements from satellite revealed that there are large opposite temperature and density gradients (temperature decreased sharply from similar to 3100 to similar to 2500 K, however, electron density increased sharply from similar to 0.3 e5 to similar to 1.2 e5 cm-3 around Zhangye Station) during this event. Results suggest that mid-latitude strong range spread F might be attributed to temperature gradient instabilities.

The Wuhan Ionospheric Sounding System (WISS), based on FPGA and DDS architecture with USB communication, was developed for studying the ionosphere by the Ionospheric Laboratory of Wuhan University. This letter presents an improved hardware system of WISS, which is based on network port communication and suitable for software defined radio. The output waveforms are more controllable and more diversified in the improved system than the previous version. The new version of ionospheric sounding system can not only conduct conventional ionospheric exploration, but also implement meteors sounding by VHF band. The system structure and the design scheme of each module are described in detail. Then, some typical experiment results are carried out to verify the performance of the improved system. Results show that the new version system performed well in the experiments of ionospheric meteors sounding.