Reflection Height Research Articles

Virtual Reflection Height of Nighttime Equatorial Ionosphere Estimated With Low‐Frequency Magnetic Sferics Measured in Malacca

AbstractThe return stroke of cloud‐to‐ground (CG) lightning is an impulsive radiator of very low‐frequency/low‐frequency (VLF/LF) electromagnetic signals allowing for the remote sensing of lower ionosphere over large spatial coverage. In this study, we examined the LF magnetic fields measured in Malacca, Malaysia, to probe reflection heights of the lower ionosphere near the equator on three different nights in 2021. The results show that the virtual ionospheric height at nighttime typically ranged from 82.0 to 90.0 km, with a mean value of 85.3 km. Our measurements also revealed significant variations in the virtual ionospheric height across different regions over a spatial scale of about 800 km. The maximum height difference was about 5.0 km. Moreover, the fluctuation characteristics are observed in both estimated ionospheric height and calculated peak reflection ratio during similar periods. This fluctuation may be related to atmospheric gravity waves in the nighttime ionosphere. In addition, we compared the virtual ionospheric height estimated from CG strokes of different polarities, and the results showed that the virtual reflection height for positive CG strokes is lower than that for negative ones.

Long-term oscillations and trends of the mesosphere derived from 60 Years of standard phase-heights measurements over Europe: An update

The time series of standard phase-height (SPH) and plasma scale-height (PSH) have been updated from a 60-year long-radio-wave measurement of the broadcasting station Allouis (France, 162 kHz). The signal was received at Kühlungsborn (54° N, 12° E, Mecklenburg, Northern Germany).The statistical analysis of the SPH series shows a significant overall trend with a decrease of 116 m/decade indicating a subsidence of the long-radio wave reflection height of about 700 m. With consideration of a stratopause altitude trend (-70 m/decade) follows an overall mesospheric shrinking of about 300 m over Western Europe.As expected the time series of SPH shows in its spectrum dominant modes which are typical for the solar cycle, ENSO and for QBO bands indicating solar and lower atmospheric influences. Solar cycle and ENSO (-QBO)-like band-pass show a growing increase of SPH up to 1987, followed by a decrease afterward. We found a strong reduction in the amplitude of the solar cycle band due to the weak solar cycle 24, but an increase in the ENSO band.For summer months during solar minimum years, and without stratopause altitude trend, a thickness temperature trend of the mesosphere is significant with a trend value of −0.47 ± 0.43 K/decade. The long-term solar variability and the stratopause altitude trend were excluded to determine a more realistic intrinsic mesospheric thickness temperature trend. The overall cooling of the intrinsic mesospheric temperature during 60 years of observation is in the order of 3 K.The long-term solar variability including the decreasing maximum of last solar cycle, and the stratopause altitude trend have to be excluded in order to determine an intrinsic mesospheric temperature trend, which may be caused by greenhouse gas increase in the middle atmosphere.

Fusion of Land-Based and Satellite-Based Localization Using Constrained Weighted Least Squares.

Combining multiple devices for localization has important applications in the military field. This paper exploits the land-based short-wave platforms and satellites for fusion localization. The ionospheric reflection height error and satellite position errors have a great impact on the short-wave localization and satellite localization accuracy, respectively. In this paper, an iterative constrained weighted least squares (ICWLS) algorithm is proposed for these two kinds of errors. The algorithm converts the nonconvex equation constraints to linear constraints using the results of the previous iteration, thus ensuring convergence to the globally optimal solution. Simulation results show that the localization accuracy of the algorithm can reach the corresponding Constrained Cramér-Rao Lower Bound (CCRLB). Finally, the localization results of the two methods are fused using Kalman filtering. Simulations show that the fused localization accuracy is improved compared to the single-means localization.

Why Does the October Effect Not Occur at Night?

AbstractThe 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.

Solar Cycle Variation of Radiated Electric Field and Ionospheric Reflection Height Over NWC Transmitter During 2005–2009: DEMETER Spacecraft Observations and Simulations

AbstractBesides the significant effects of a variety of naturally occurring magnetospheric waves on the electron dynamics in the magnetosphere, the important contribution of ground‐based very‐low‐frequency (VLF) transmitter waves also has been gradually discovered. The VLF transmitter's wave penetrating into the topside ionosphere is its energy source injected into the magnetosphere and has been extensively investigated. In the VLF wave trans‐ionospheric propagation, the main energy attenuation occurs in the lower ionosphere which is controlled by solar short‐wave radiation. However, the investigation on the variation of the VLF transmitters' energy in the topside ionosphere and ionospheric reflection height with solar activity is lacking. We use 4 years electric field measurements performed by DEMETER satellite and full‐wave simulations to address these concerns. The results show the electric field radiated from NWC was relatively similar from May 1 to July 31 in 2006, 2008, and 2009 in daytime and nighttime, stronger than that in 2005, because the solar activity was similar and extremely low in these years compared with that in 2005. The nighttime and daytime ionospheric reflection heights are also relatively similar in these 3 years, with about 6 km higher than that in 2005. The difference in the simulated electric field based on the electron density profile from the IRI‐2016 model between 2005 and 2009 is lower compared with the observation results. However, considering 6 km added in ionospheric reflection height in 2009, the simulation results are much more consistent with the observation results both in daytime and nighttime.

Variation in the Reflection Height of VLF/LF Transmitter Signals in the D‐Region Ionosphere and the Possible Source: A 2018 Meteoroid in Hokkaido, Japan

AbstractSeveral studies have examined ionospheric variation associated with meteorites, meteoroids, or meteors based on Global Satellite Navigation System total electron content observations. However, there have been few quantitative studies of the D‐region of the ionosphere (60–90 km), which is associated with meteoroids. We investigated variation in the D‐region during the passage of a meteoroid over northeastern Hokkaido, Japan, at 11:55:55 UT on 18 October 2018, using very low‐frequency (VLF, 3–30 kHz) and low‐frequency (LF, 30–300 kHz) signals observed by three transmitters [JJY (40 kHz), JJY (60 kHz), and JJI (22.2 kHz)], at Rikubetsu, Japan. Periodic variation of 100–200 s was observed in the VLF and LF amplitudes upon arrival of the acoustic wave. The vertical seismic velocity of Hi‐net and F‐net data also showed acoustic waves. Although the main period of the acoustic wave was 0.1–0.5 s in the seismic data, a longer period component (100–200 s) remained during propagation up to the D‐region ionosphere. The estimated velocity of the acoustic waves was ∼340 m/s on the ground according to the Hi‐net seismic data. The acoustic wave originated near the endpoint (25 km altitude) of the meteoroid trajectory. Based on the observed propagation time of the acoustic waves and ray tracing results, the acoustic waves propagated obliquely from near the endpoint of the meteoroid trajectory up to a D‐region height (about ∼90 km altitude), south of the Rikubetsu receiver.

Peculiarities of leakage of the electromagnetic waves from striations

It is well-known that an ordinary (O-mode) high frequency (HF) electromagnetic (EM) wave radiated into the ionosphere generates in the F-region upper-hybrid resonance (UHR) oscillations trapped in small-scale elongated plasma depletions (striations). Striations are not ideal resonators for UHR oscillations. There is some leakage of the HF energy from striations in the form of Z-mode waves. This result obtained in the quasi-electrostatic approximation is partly reconsidered in the present paper. It is shown in the 2D case that two EM waves (instead of one Z-mode) propagate from each side of a striation. The lowest eigenmodes of the electrostatic oscillations trapped in a striation excite at particular heights Z-mode waves that propagate significantly higher than the reflection height of the initial O-mode wave. This result exists for different angles of the O-mode radiation with respect to the vertical (both more and less than the critical angle).

Long-term trends of midlatitude horizontal mesosphere/lower thermosphere winds over four decades

Abstract. We analyse 43 years of mesosphere/lower thermosphere (MLT) horizontal winds obtained from a joint analysis of low frequency (LF) spaced receiver lower ionospheric drift measurements from late 1978 through 2008 and VHF meteor radar wind observations since summer 2004 at Collm (51.3∘ N, 13.0∘ E). Due to limitations of the earlier LF measurements, we restrict ourselves to the analysis of monthly mean winds near 90 km, which represents the meteor peak height as well as mean LF reflection heights in the MLT. We observe mainly positive trends of the zonal prevailing wind throughout the year, while the meridional winds tend to decrease in magnitude in both summer and winter. Furthermore, there is a change in long-term trends around the late 1990s, which is most clearly visible in summer MLT winds. We compare these measurements with long-term partial reflection radar observations of winds at 81–85 km over Juliusruh (54.6∘ N, 13.4∘ E) since 1990, and find general qualitative agreement of trends except for summer. The latter can be explained by the different altitudes considered, and by the latitude dependence of the summer mesospheric jet.

Influence of the Neutral Atmosphere Model on the Correctness of Simulation the Electron and Ion Concentrations in the Lower Ionosphere

AbstractThe work is devoted to modeling and verification of the dynamics of charged components of the Earth's lower ionosphere, obtained using different values of the background neutral atmosphere parameters. Long‐term experimental data on temperature and concentration of neutral components from the Aura satellite and Mass Spectrometer Incoherent Scatter Radar (MSIS) model were used as input data. The average relative difference between the electron concentration values obtained using Aura and MSIS data (11 June 2014, 6–18 UT, h = 50–90 km, 41°N 10°E) was 34%. Obtained results were verified with use of very low frequency (VLF) signals propagation data, which were collected on Mikhnevo Geophysical Observatory (GPO) and A118 station from GBZ, GQD, DHO, TBB, and ICV transmitters. Based on the results of the verification, it was found that the response of the lower ionosphere to X‐ray flares, that occurred on 9–11 June 2014, is better described by the Aura satellite data in 4 out of 5 cases. Consequently, they are more preferable for predicting the state of ionospheric parameters below the reflection height of signals of the considered frequencies (∼75 km). It is also shown that taking into account the ion concentration for calculation the amplitude‐phase characteristics of the VLF‐signals makes it possible to describe the dynamics of parameters better.

Ionospheric Response on Solar Flares through Machine Learning Modeling

Following solar flares (SF), the abrupt increase in X-radiation and EUV emission generates additional ionization and higher absorption of, e.g., electromagnetic waves in the sunlit hemisphere of the Earth’s ionosphere. The modeling of the ionosphere under solar flares are motivated by new observations with spacecrafts, satellites, and ground-based measurements. The estimation of modeling parameters for the ionospheric D-region during SF events poses a significant challenge, typically requiring a trial-and-error approach. This research presents a machine learning (ML) methodology for modeling the sharpness (β) and reflection height (H′) during SF events occurred from 2008 to 2017. The research methodology was divided into two separate approaches: an instance-based approach, which involved obtaining SF parameters during the peak SF, and a time-series approach, which involved analyzing time-series data during SFs. The findings of the study revealed that the model for the instance-based approach exhibited mean absolute percentage error (MAPE) values of 9.1% for the β parameter and 2.45% for the H′ parameter. The findings from the time-series approach indicated that the model exhibited lower error rates compared to the instance-based approach. However, it was observed that the model demonstrated an increase in β residuals as the predicted β increased, whereas the opposite trend was observed for the H′ parameter. The main goal of the research is to develop an easy-to-use method that provides ionospheric parameters utilizing ML, which can be refined with additional and novel data as well as other techniques for data pre-processing and other algorithms. The proposed method and the utilized workflow and datasets are available at GitHub.

Variation of Ionospheric Shortwave Absorption of the Extraordinary Wave

AbstractIn this study, associating with the A‐H formulation, we have developed a comprehensive ionospheric absorption model by incorporating the latest version of geomagnetic model, the atmospheric and ionospheric model as the background. Based on the absorption model, the ionospheric stratification theory is utilized to explore the trend of the total absorption for extraordinary (X) wave with frequency and elevation angle. We found that the factor of local time has a greater impact on the trend of total absorption variation than other factors—solar activity, season, and latitude, attributing to the variation of electron collision frequency. During the two different local times (daytime and night), the trends of the total absorption have distinct differences. In the daytime, it is worth noting that the reflection heights and the peaks of the absorption coefficient both mainly occur in Layers D and E. Therefore, the non‐deviative absorption dominates the variation of the total absorption. On the other hand, the occurrence of Pedersen ray results in inflexion points arising in the curves of absorption versus frequency or elevation angle. Another interesting phenomenon is the dominant role of deviative absorption at night. At night, the reflection heights mainly occur in Layer F, and the absorption coefficient of Layer F begins to exceed the peak levels of Layers D and E when the frequency or elevation angle is larger than certain values. Hence, the total absorption will grow as a result of the increase in deviative absorption with the increase in reflection height and absorption coefficient.

Automatic Detection of VLF Tweek Signals Based on the YOLO Model

Tweek signals are a special type of VLF (very low frequency) pulse, originally produced by lightning discharge, which becomes dispersive after repetitive bounces within the waveguide between the Earth’s surface and lower ionosphere. As such, tweek signals carry critical information about the region near the reflection height of the VLF waves, namely the D-region ionosphere. Although tweek measurements have been widely utilized in studies of the D-region ionosphere and lightning discharge, few statistical studies have been conducted, mainly due to the difficulty of manually identifying tweek signals from the enormous amount of VLF data with heavy noise. Considering the importance of tweek signals and the lack of a high-precision detection model, in this study, we propose a method to automatically and accurately pick out tweek signals from VLF measurements. This method is explicitly developed based on the you only look once (YOLO) model and a post-tracing process. Using a total of 2495 randomly selected VLF spectrogram images as the testing set, we evaluated the performance of this method. The precision and recall are found to be 92.0% and 89.2% for the first-order mode, and 97.5% and 86.7% for the first-two-order mode tweek, respectively. The time needed to process 10-s VLF measurements with a cadence of 4 μs is only 6.5 s, allowing for identifying the tweek signals from continuous VLF measurements in real time. Therefore, this method represents a reliable means to automatically detect tweek signals and enables the opportunity to statistically investigate the D-region ionosphere and lightning discharge via these signals.

Modeling of Solar Eclipse effects on the sub-ionospheric VLF/LF signals observed by multiple stations over Japan

We present the numerical modeling of field amplitude perturbations for two VLF/LF transmitters (JJI: 22.2 kHz, JJY: 40.0 kHz) signals recorded at six places (UEC-VLF network) in Japan during the Annular Solar Eclipse of 2012 (ASE-2012). We determine the characteristics of the D-region ionospheric perturbation according to the antumbra and penumbral shadow of the annular solar eclipse. A space–time dependent 2-Dimensional Elliptical Gaussian distribution Model (2DEGM) is introduced to get the effect of ASE-2012 on the Wait’s D-region parameters, such as effective VLF/LF reflection height (h′) and sharpness factor (β). The Long Wave Propagation Capability (LWPC) code is used to simulate the VLF signal amplitude profile with the help of modelled Wait’s parameters. A method of comparison of observed and LWPC simulated VLF signal amplitude across all the considered signal propagation paths is introduced to estimate final 2DEGM related parameters and hence, to model D-region electron density perturbations during ASE-2012. Using 2DEGM + LWPC, (i) we consistently simulate the observed VLF signal amplitude profiles for almost all the propagation paths of UEC-VLF network and (ii) evaluate the percentage change in D-region electron density due to ASE-2012.

Day to night shift in reflection height of VLF radio waves derived from IRI model electron density models

Day to night shift in reflection height of VLF radio waves derived from IRI model electron density models

Influence of strong solar proton events on propagation of radio signals in the VLF range in a high-latitude region

In this paper, we examine the features of RSDN-20 signal propagation in a high-latitude Earth–ionosphere waveguide during solar proton events, using computational experiment methods. We have analyzed two proton ground-level enhancement (GLE) events of December 13, 2006 (GLE70) and September 10, 2017 (GLE72). Electron density profiles were constructed using the Global Dynamic Model of Ionosphere (GDMI) and the RUSCOSMICS model, developed at PGI. We present estimated phase and amplitude changes in RSDN-20 signals during precipitation of high-energy protons in the high-latitude region of the Earth–ionosphere waveguide. From the results of computational experiments and the analysis of the electromagnetic signal attenuation based on analytical Maxwell’s equation system solution in magnetized ionospheric plasma, we have found a pattern in the signal attenuation frequency dependence associated simultaneously with the signal reflection height, electron density profiles, and the collision frequency of electrons with neutral particles and ions. We discuss limitations of the computational experiment method and compare simulation results with data from Lovozero and Tuloma observatories.

Влияние сильных солнечных протонных событий на распространение радиосигналов в диапазоне ОНЧ в области высоких широт

In this paper, we examine the features of RSDN-20 signal propagation in a high-latitude Earth–ionosphere waveguide during solar proton events, using computational experiment methods. We have analyzed two proton ground-level enhancement (GLE) events of December 13, 2006 (GLE70) and September 10, 2017 (GLE72). Electron density profiles were constructed using the Global Dynamic Model of Ionosphere (GDMI) and the RUSCOSMICS model, developed at PGI. We present estimated phase and amplitude changes in RSDN-20 signals during precipitation of high-energy protons in the high-latitude region of the Earth–ionosphere waveguide. From the results of computational experiments and the analysis of the electromagnetic signal attenuation based on analytical Maxwell’s equation system solution in magnetized ionospheric plasma, we have found a pattern in the signal attenuation frequency dependence associated simultaneously with the signal reflection height, electron density profiles, and the collision frequency of electrons with neutral particles and ions. We discuss limitations of the computational experiment method and compare simulation results with data from Lovozero and Tuloma observatories.

Analysis of influencing factors and clinical value of anterior peritoneal reflection for patients with rectal cancer

Objectives: To investigate the factors influencing the height of anterior peritoneal reflection (APR) for patients with rectal cancer, and to analyze the relationship between the APR and the lateral lymph node metastasis. Methods: Clinical data of 432 patients with tumor located within and below APR were retrospectively collected from the rectal cancer database at the Department of General Surgery, Peking Union Medical College Hospital from August 2020 to September 2022. Ninty-eight non-rectal cancer patients were also enrolled as a control group. There were 308 males and 124 females in the tumor group, aged (M(IQR)) 62 (16) years (range: 24 to 85 years) and 53 males and 45 females in the control group, aged 60 (22) years (range: 27 to 87 years). The APR height, pelvis, and tumor-related parameters were measured by MRI. A multifactor linear regression model was established to analyze the dependent correlation factors of APR height. These factors of the two groups were matched by propensity score matching and their APR heights were compared after matching. An ordinal Logistic regression model was established to explore the relationship between APR-related parameters and radiographic lateral lymph node metastasis. Results: The APR height of the tumor group was (98.7±14.4) mm (range: 43.3 to 154.0 mm) and the control group was (95.1±12.7) mm (range: 68.0 to 137.9 mm). Multivariable linear regression revealed that the greater the weight (B=0.519, 95%CI: 0.399 to 0.640, P<0.01), the anterior pelvic depth (B=0.109, 95%CI: 0.005 to 0.213, P=0.039) and the smaller the bi-ischial diameter (B=-0.172, 95%CI:-0.294 to -0.049, P=0.006), the higher the APR height. The tumor group had a higher APR height than the control group after propensity score matching ((98.3±14.2) mm vs. (95.1±12.7) mm, t=-1.992, P=0.047). Ordinal Logistic regression indicated that the longer segment of the tumor invade the nonperitoneal rectum was an independent influencing factor of radiographic lateral lymph node metastasis (OR=1.016, 95%CI: 1.002 to 1.030, P=0.021), while the distance between the anal verge and the tumor was not (OR=0.986, 95%CI: 0.972 to 1.000, P=0.058). Conclusions: The higher the weight, the deeper and narrower the pelvis, the higher the APR height. There is a certain relationship between APR and lateral lymph node metastasis on imaging.

Влияние высыпающихся релятивистских электронов на характеристики сигналов системы РСДН-20 во время события 28 марта 2017

Based on the experimental data of the EISCAT radar and numerical modeling methods, the influence of the fluxes of subrelativistic (relativistic) electrons of the Earth's outer radiation belt on the operation of the RSDN-20 long-range navigation system operating at frequencies of 11.905 kHz, 12.649 kHz and 14.881 kHz has been studied. As a result of computational experiments, a slight decrease in attenuation under perturbed conditions was obtained at a frequency of 14.881 kHz with a slight, less than an order of magnitude increase in the electron concentration in the D- and E-layers of the ionosphere. At lower frequencies and in the case of an increase in concentration in the reflection region, an increase in signal attenuation is observed. It is shown that the violation of monotonicity in the frequency dependence of signal attenuation is associated with the position of the boundary between regions with different types of signal attenuation and the height of the signal reflection from the ionosphere. In the case when the intersection point of the high-altitude profile of the electron collision frequency υe with the profile of the doubled plasma frequency 2ωe is inside the signal reflection region, a nonlinear type of attenuation dependence on frequency is observed. The phases of RSDN-20 signals are weakly affected by the precipitation of relativistic energy electrons into the atmosphere. A comparison of the amplitudes of the signals obtained in computational experiments and the signals recorded at the observatories of the Polar Geophysical Institute Lovozero and Barentsburg showed partial agreement in most of the changes recorded during the precipitation of subrelativistic electrons in the polar ionosphere. The presented estimates can be used on short radio paths of 100–200 km in the high latitude region to monitor the state of the ionosphere.

A Comparative Study of VLF Transmitter Signal Measurements and Simulations during Two Solar Eclipse Events

To monitor the Very-Low-Frequency (VLF) environment, a VLF detection system has been installed in Suizhou, China, a location with the longitude almost identical to that of the NWC transmitter in Australia. In the years 2019 and 2020, two solar eclipses crossed the NWC–Suizhou path at different locations. Each solar eclipse event represents a naturally occurring controlled experiment, but these two events are unique in that similar levels of electron density variation occurred at different locations along the VLF propagation path. Therefore, we conducted a comparative study using the VLF measurements during these two eclipses. Previous studies mostly estimated a pair of the reflection height (h′) and sharpness parameter (β) using the Long Wavelength Propagation Capability code, whereas, in this study, we use the VLF amplitude and phase as constraints in order to find the electron density change that best explains the VLF measurements. The eclipse measurements could be best explained if the path-averaged β value was 0.56 and 0.62 km−1 for the 2019 and 2020 eclipse, respectively. The VLF reflection height increased from 71.5 to 73.3 km for the 2019 eclipse and from 71.1 to 72.8 km for the 2020 eclipse. The best-fit β values were consistent with the Faraday International Reference Ionosphere model and statistical studies, and the h′ change was also consistent with previous studies and theoretical calculations. Moreover, present results suggested that VLF signals collected by a single receiver were not sensitive to where the electron density change occurs along the propagation path but reflected the average path condition. Therefore, a network of VLF receivers is required in order to monitor in real time the spatial extent of the space weather events that disturb the lower ionosphere.

On the improvement of the single-site lightning distance estimation by using the time-delay correction

Due to the propagation effect, the arrival time of the ground wave peak is delayed when sferics propagate at long distances. In this work, we propose a new method combined with a numerical algorithm to correct the effect of the ground wave peak time delay and calculate the ionosphere height using the difference between the arrival time of the ground wave and the skywave. The results showed that, with each increase of the propagation distance by 100 km, the delay in arrival time of the ground wave peak increased by an average of 0.9 μs. For the first and second reflectance heights, the maximum heights observed at night were 86 km and 89 km, and the minimum heights during the day were 66 km and 69 km, respectively. Using the difference in arrival time between the ground wave and skywave and the ionosphere equivalent reflection height, a single site was used to estimate the distance of lightning occurring within 900 km. This method had an average relative error of 14.6%, an average absolute error of 128.8 km, and a median distance error of 52.6 km. Finally, the percentage of data with an estimated deviation within 10% increased from 52% to 65%.