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21 pages, 6995 KiB

Open AccessArticle

An Updating of the IONORT Tool to Perform a High-Frequency Ionospheric Ray Tracing

by Marco Pietrella, Michael Pezzopane, Alessandro Pignatelli, Alessio Pignalberi and Alessandro Settimi

Remote Sens. 2023, 15(21), 5111; https://doi.org/10.3390/rs15215111 - 25 Oct 2023

Cited by 1 | Viewed by 825

Abstract

This paper describes the main updates characterizing the new version of IONORT (IONOsperic Ray Tracing), a software tool developed at Istituto Nazionale di Geofisica e Vulcanologia to determine both the path of a high frequency (HF) radio wave propagating in the ionospheric medium, [...] Read more.

This paper describes the main updates characterizing the new version of IONORT (IONOsperic Ray Tracing), a software tool developed at Istituto Nazionale di Geofisica e Vulcanologia to determine both the path of a high frequency (HF) radio wave propagating in the ionospheric medium, and the group time delay of the wave itself along the path. One of the main changes concerns the replacement of a regional three-dimensional electron density matrix, which was previously taken as input to represent the ionosphere, with a global one. Therefore, it is now possible to carry out different ray tracings from whatever point of the Earth’s surface, simply by selecting suitable loop cycles thanks to the new ray tracing graphical user interface (GUI). At the same time, thanks to a homing GUI, it is also possible to generate synthetic oblique ionograms for whatever radio link chosen by the user. Both ray tracing and homing GUIs will be described in detail providing at the same time some practical examples of their use for different regions. IONORT software finds practical application in the planning of HF radio links, exploiting the sky wave, through an accurate and thorough knowledge of the ionospheric medium. HF radio waves users, including broadcasting and civil aviation, would benefit from the use of the IONORT software (version 2023.10). Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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18 pages, 8899 KiB

Open AccessArticle

Analysis of Winter Anomaly and Annual Anomaly Based on Regression Approach

by Kaixin Wang, Jiandi Feng, Zhenzhen Zhao and Baomin Han

Remote Sens. 2023, 15(20), 4968; https://doi.org/10.3390/rs15204968 - 15 Oct 2023

Viewed by 844

Abstract

Studying the temporal and spatial dependence of ionospheric anomalies using total electron content (TEC) can provide an important reference for developing empirical ionospheric models. In this study, winter anomaly, annual anomaly, and the contributions of winter anomaly to annual anomaly were investigated during [...] Read more.

Studying the temporal and spatial dependence of ionospheric anomalies using total electron content (TEC) can provide an important reference for developing empirical ionospheric models. In this study, winter anomaly, annual anomaly, and the contributions of winter anomaly to annual anomaly were investigated during solar cycle 24 (2008–2018) by using the global ionosphere maps of the Center for Orbit Determination in Europe during the geomagnetic activity quiet period (Kp ≤ 5) based on a regression approach. Our detailed analysis shows the following: (1) Winter anomaly is more significant at 11:00–13:00 local time (LT), and the region of winter anomaly extends from North America to the Far East with increasing solar activity levels. (2) The minimum level of solar activity corresponding to the occurrence of winter anomaly was calculated at each grid point, which can provide a reference for single-point ionospheric modeling. (3) The annual anomaly reaches its maximum at 12:00 LT when the TEC in December is 34.4% higher than in June. (4) At 12:00 LT, the winter anomaly contributes up to 32% to the annual anomaly (at this time, the winter hemisphere contributes 57% to the annual anomaly). Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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18 pages, 14118 KiB

Open AccessArticle

Analysis of Ionospheric Anomalies before the Tonga Volcanic Eruption on 15 January 2022

by Jiandi Feng, Yunbin Yuan, Ting Zhang, Zhihao Zhang and Di Meng

Remote Sens. 2023, 15(19), 4879; https://doi.org/10.3390/rs15194879 - 09 Oct 2023

Cited by 5 | Viewed by 1675

Abstract

In this paper, GNSS stations’ observational data, global ionospheric maps (GIM) and the electron density of FORMOSAT-7/COSMIC-2 occultation are used to study ionospheric anomalies before the submarine volcanic eruption of Hunga Tonga–Hunga Ha’apai on 15 January 2022. (i) We detect the negative total [...] Read more.

In this paper, GNSS stations’ observational data, global ionospheric maps (GIM) and the electron density of FORMOSAT-7/COSMIC-2 occultation are used to study ionospheric anomalies before the submarine volcanic eruption of Hunga Tonga–Hunga Ha’apai on 15 January 2022. (i) We detect the negative total electron content (TEC) anomalies by three GNSS stations on 5 January before the volcanic eruption after excluding the influence of solar and geomagnetic disturbances and lower atmospheric forcing. The GIMs also detect the negative anomaly in the global ionospheric TEC only near the epicenter of the eruption on 5 January, with a maximum outlier exceeding 6 TECU. (ii) From 1 to 3 January (local time), the equatorial ionization anomaly (EIA) peak shifts significantly towards the Antarctic from afternoon to night. The equatorial ionization anomaly double peak decreases from 4 January, and the EIA double peak disappears and merges into a single peak on 7 January. Meanwhile, the diurnal maxima of TEC at TONG station decrease by nearly 10 TECU and only one diurnal maximum occurred on 4 January (i.e., 5 January of UT), but the significant ionospheric diurnal double-maxima (DDM) are observed on other dates. (iii) We find a maximum value exceeding NmF2 at an altitude of 100~130 km above the volcanic eruption on 5 January (i.e., a sporadic E layer), with an electron density of 7.5 × 10⁵ el/cm³. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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25 pages, 15557 KiB

Open AccessArticle

IonosphericTotal Electron Content Changes during the 15 February 2018 and 30 April 2022 Solar Eclipses over South America and Antarctica

by Juan Carlos Valdés-Abreu, Marcos Díaz, Manuel Bravo and Yohadne Stable-Sánchez

Remote Sens. 2023, 15(19), 4810; https://doi.org/10.3390/rs15194810 - 03 Oct 2023

Viewed by 952

Abstract

This is one of the first papers to study the ionospheric effects of two solar eclipses that occurred in South America and Antarctica under geomagnetic activity in different seasons (summer and autumn) and their impact on the equatorial ionization anomaly (EIA). The changes [...] Read more.

This is one of the first papers to study the ionospheric effects of two solar eclipses that occurred in South America and Antarctica under geomagnetic activity in different seasons (summer and autumn) and their impact on the equatorial ionization anomaly (EIA). The changes in total electron content (TEC) during the 15 February 2018 and 30 April 2022 partial solar eclipses will be analyzed. The study is based on more than 390 GPS stations, Swarm-A, and DMSP F18 satellite measurements, such as TEC, electron density, and electron temperature. The ionospheric behaviors over the two-fifth days on both sides of each eclipse were used as a reference for estimating TEC changes. Regional TEC maps were created for the analysis. Background TEC levels were significantly higher during the 2022 eclipse than during the 2018 eclipse because ionospheric levels depend on solar index parameters. On the days of the 2018 and 2022 eclipses, the ionospheric enhancement was noticeable due to levels of geomagnetic activity. Although geomagnetic forcing impacted the ionosphere, both eclipses had evident depletions under the penumbra, wherein differential vertical TEC (DVTEC) reached values <−40%. The duration of the ionospheric effects persisted after 24 UT. Also, while a noticeable TEC depletion (DVTEC ∼−50%) of the southern EIA crest was observed during the 2018 eclipse (hemisphere summer), an evident TEC enhancement (DVTEC > 30%) at the same crest was seen during the eclipse of 2022 (hemisphere autumn). Swarm-A and DMSP F18 satellite measurements and analysis of other solar eclipses in the sector under quiet conditions supported the ionospheric behavior. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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21 pages, 10331 KiB

Open AccessArticle

Unveiling the Core Patterns of High-Latitude Electron Density Distribution at Swarm Altitude

by Giulia Lovati, Paola De Michelis, Tommaso Alberti and Giuseppe Consolini

Remote Sens. 2023, 15(18), 4550; https://doi.org/10.3390/rs15184550 - 15 Sep 2023

Viewed by 641

Abstract

The ionosphere has distinctive characteristics under different solar and geomagnetic conditions, as well as throughout the seasons, and has a direct impact on our technological life in terms of radio communication and satellite navigation systems. In the pursuit of developing highly accurate ionospheric [...] Read more.

The ionosphere has distinctive characteristics under different solar and geomagnetic conditions, as well as throughout the seasons, and has a direct impact on our technological life in terms of radio communication and satellite navigation systems. In the pursuit of developing highly accurate ionospheric models and/or improving existing ones, understanding the various physical mechanisms that influence electron density dynamics is critical. In this study, we apply the Multivariate Empirical Mode Decomposition (MEMD) method to the electron density distribution in the mid-to-high latitude (above 50

^{°}

magnetic latitude) regions in order to identify the dominant scales at which these mechanisms operate. The data were collected by the Swarm mission in the Northern Hemisphere. MEMD allows us to separate the main intrinsic modes and assess their relative contributions to the original one, thereby identifying the most important modes and the spatial scales at which they exert influence. Our study spanned the period from 1 January 2016 to 31 December 2021, which was characterized by low solar activity levels. This choice allowed for a more focused investigation of other variables influencing electron density distribution under similar solar activity conditions. We specifically examined the variations of the resulting modes in relation to different seasons and geomagnetic activity conditions, providing valuable insights into the complex behavior of the ionosphere in response to various external factors. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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11 pages, 5709 KiB

Open AccessCommunication

A New Algorithm for Ill-Posed Problem of GNSS-Based Ionospheric Tomography

by Debao Wen, Kangyou Xie, Yinghao Tang, Dengkui Mei, Xi Chen and Hanqing Chen

Remote Sens. 2023, 15(7), 1930; https://doi.org/10.3390/rs15071930 - 04 Apr 2023

Cited by 1 | Viewed by 1115

Abstract

Ill-posedness of GNSS-based ionospheric tomography affects the stability and the accuracy of the inversion results. Truncated singular value decomposition (TSVD) is a common algorithm of ionospheric tomography reconstruction. However, the TSVD method usually has low inversion accuracy and reconstruction efficiency. To resolve the [...] Read more.

Ill-posedness of GNSS-based ionospheric tomography affects the stability and the accuracy of the inversion results. Truncated singular value decomposition (TSVD) is a common algorithm of ionospheric tomography reconstruction. However, the TSVD method usually has low inversion accuracy and reconstruction efficiency. To resolve the above problem, a truncated mapping singular value decomposition (TMSVD) algorithm is presented to improve the reconstructed accuracy and computational efficiency. To authenticate the effectiveness and the advantages of the TMSVD algorithm, a numerical test scheme is devised. Finally, ionospheric temporal–spatial variations of the selected reconstructed region are studied using the GNSS observations under different geomagnetic conditions. The reconstructed results of TMSVD can accurately reflect semiannual anomalies, diurnal variations, and geomagnetic storm effects. In contrast with the ionosonde data, it is found that the reconstructed profiles of the TMSVD method are more consistent with than those of the IRI 2016. The study suggests that TMSVD is an efficient algorithm for the tomographic reconstruction of ionospheric electron density (IED). Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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14 pages, 4702 KiB

Open AccessArticle

An Explainable Dynamic Prediction Method for Ionospheric foF2 Based on Machine Learning

by Jian Wang, Qiao Yu, Yafei Shi, Yiran Liu and Cheng Yang

Remote Sens. 2023, 15(5), 1256; https://doi.org/10.3390/rs15051256 - 24 Feb 2023

Cited by 5 | Viewed by 1316

Abstract

To further improve the prediction accuracy of the critical frequency of the ionospheric F2 layer (foF2), we use the machine learning method (ML) to establish an explanatory dynamic model to predict foF2. Firstly, according to the ML modeling process, the [...] Read more.

To further improve the prediction accuracy of the critical frequency of the ionospheric F2 layer (foF2), we use the machine learning method (ML) to establish an explanatory dynamic model to predict foF2. Firstly, according to the ML modeling process, the three elements of establishing a prediction model of foF2 and four problems to be solved are determined, and the idea and concrete steps of model building are determined. Then the data collection is explained in detail, and according to the modeling process, foF2 dynamic change mapping and its parameters are determined in turn. Finally, the established model is compared with the International Reference Ionospheric model (IRI-2016) and the Asian Regional foF2 Model (ARFM) to verify the validity and reliability. The results show that compared with the IRI-URSI, IRI-CCIR, and ARFM models, the statistical average error of the established model decreased by 0.316 MHz, 0.132 MHz, and 0.007 MHz, respectively. Further, the statistical average relative root-mean-square error decreased by 9.62%, 4.05%, and 0.15%, respectively. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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16 pages, 3035 KiB

Open AccessArticle

Statistical Analysis of SF Occurrence in Middle and Low Latitudes Using Bayesian Network Automatic Identification

by Jian Feng, Yuqiang Zhang, Shuaihe Gao, Zhuangkai Wang, Xiang Wang, Bo Chen, Yi Liu, Chen Zhou and Zhengyu Zhao

Remote Sens. 2023, 15(4), 1108; https://doi.org/10.3390/rs15041108 - 17 Feb 2023

Cited by 1 | Viewed by 1304

Abstract

Spread-F (SF) is one of the most important types of the ionospheric irregularities as it causes ionospheric scintillation which can severely affect the performance and reliability of communication, navigation, and radar systems. The ionosonde provides the most effective and economical way to study [...] Read more.

Spread-F (SF) is one of the most important types of the ionospheric irregularities as it causes ionospheric scintillation which can severely affect the performance and reliability of communication, navigation, and radar systems. The ionosonde provides the most effective and economical way to study the ionosphere and SF. However, the manual identification of SF from an ionogram is boring and hard work. To automatically identify SF on the ionogram and extend the study of SF into the middle and low latitudes of East Asia, this paper presents a statistical analysis of SF in this region, based on the naïve Bayesian classifier. The results showed that the accuracy of automatic identification reached up to 97% on both the validation datasets and test datasets composed of Mohe, I-Cheon, Jeju, Wuhan, and Sanya ionograms, suggesting that it is a promising way to automatically identify SF on ionograms. Based on the classification results, the statistical analysis shows that SF has a complicated morphology in the middle and low latitudes of East Asia. Specifically, there is a peak of occurrence of SF in the summer in I-Cheon, Jeju, Sanya, and Wuhan; however, the Mohe station has the highest occurrence rate of SF in December. The different seasonal variations of SF might be due to the different geographic local conditions, such as the inland-coastal differences and formation mechanism differences at these latitudes. Moreover, SF occurs more easily in the post-midnight hours when compared with the pre-midnight period in these stations, which is consistent with the previous results. Furthermore, this paper extracts the frequency SF (FSF) index and range SF (RSF) index to characterize the features of SF. The results shows that the most intense FSF/RSF appeared in the height range of 220–300 km/1–7 MHz in these stations, although there are different magnitude extensions on different season in these regions. In particular, strong spread-F (SSF) reached its maximum at the equinox at Sanya, confirming the frequent SSF occurrence at the equinox at the equator and low latitudes. These results would be helpful for understanding the characteristics of SF in East Asia. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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24 pages, 12885 KiB

Open AccessArticle

Solar Flare Effects Observed over Mexico during 30–31 March 2022

by Maria A. Sergeeva, Olga A. Maltseva, Artem M. Vesnin, Donat V. Blagoveshchensky, Victor J. Gatica-Acevedo, J. Americo Gonzalez-Esparza, Aleksandr G. Chernov, Isaac D. Orrala-Legorreta, Angela Melgarejo-Morales, Luis Xavier Gonzalez, Mario Rodriguez-Martinez, Ernesto Aguilar-Rodriguez, Ernesto Andrade-Mascote and Pablo Villanueva

Remote Sens. 2023, 15(2), 397; https://doi.org/10.3390/rs15020397 - 09 Jan 2023

Cited by 1 | Viewed by 1759

Abstract

Manifestations of two solar flares of March 2022 were studied over Mexico. The flare effects in the lower ionosphere had a ~3 min delay from the X1.3-flare onset and ~5 min from the M9.6-flare onset. The maximal impact on the HF signal amplitude [...] Read more.

Manifestations of two solar flares of March 2022 were studied over Mexico. The flare effects in the lower ionosphere had a ~3 min delay from the X1.3-flare onset and ~5 min from the M9.6-flare onset. The maximal impact on the HF signal amplitude was ~(14–15) min after the onset of both flares. The X1.3-flare provoked the shortwave fadeout during ~6 min. The effects in the lower ionosphere lasted longer than the flares and the effects at the F2 region and higher altitudes only during the flares. The interpretation of results showed the following. (1) Based on the absorption level estimated with minimum frequency and signal amplitude on ionograms, the major role of X-ray radiation in the electron concentration increase in the lower ionosphere was confirmed. At the same time, the EUV radiation impact on the lower ionosphere cannot be totally discarded. The lower ionosphere recovery began before and lasted after the X1.3-flare end, being more rapid at Eglin than in Mexico. During M9.6-flare, the responses at the two observation points were rather synchronized due to the more similar illumination conditions at the two meridians. (2) According to the dI variations characterizing the F2 region and higher, the M9.6-flare provoked medium-scale and the X1.3-flare provoked both medium- and small-scale ionospheric irregularities. The response duration corresponded to the dI series filtered with (10–20) min windows. The dI curve during the flares was characterized by the И-form and depended more on the active region position and the flare class than on the solar zenith angle. The available data do not allow us to unambiguously identify the reason for the negative dI: the applied filtering procedure or the physical effect. (3) During both flares, the major EUV impact on the lower ionosphere was by the flux at 133.5 nm and on the F2 region and higher altitudes at 25.6 nm. In addition, during the M9.6-flare, EUV 28.4, 30.4 and 121.6 nm spectral bands also played an important role in the F2 response. During the X1.3-flare, the EUV 25.6 nm flux and X-ray flux impacts on the F2 region were of the same level. The weakest impact was caused by the emission in the EUV 28.4 nm spectral band on the absorption in the lower ionosphere during both flares and on the electron density in the F2 region and higher during the X1.3-flare. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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18 pages, 5009 KiB

Open AccessArticle

Modeling and Forecasting Ionospheric foF2 Variation in the Low Latitude Region during Low and High Solar Activity Years

by Cheng Bi, Peng Ren, Ting Yin, Zheng Xiang and Yang Zhang

Remote Sens. 2022, 14(21), 5418; https://doi.org/10.3390/rs14215418 - 28 Oct 2022

Cited by 3 | Viewed by 1465

Abstract

Prediction of ionospheric parameters, such as ionospheric F2 layer critical frequency (foF2) at low latitude regions is of significant interest in understanding ionospheric variation effects on high-frequency communication and global navigation satellite system. Currently, deep learning algorithms have made a striking accomplishment in [...] Read more.

Prediction of ionospheric parameters, such as ionospheric F2 layer critical frequency (foF2) at low latitude regions is of significant interest in understanding ionospheric variation effects on high-frequency communication and global navigation satellite system. Currently, deep learning algorithms have made a striking accomplishment in capturing ionospheric variability. In this paper, we use the state-of-the-art hybrid neural network combined with a quantile mechanism to predict foF2 parameter variations under low and high solar activity years (solar cycle-24) and space weather events. The hybrid neural network is composed of a convolutional neural network (CNN) and bidirectional long short-term memory (BiLSTM), in which CNN and BiLSTM networks extracted spatial and temporal features of ionospheric variation, respectively. The proposed method was trained and tested on 5 years (2009–2014) of ionospheric foF2 observation data from Advanced Digital Ionosonde located in Brisbane, Australia (27°53′S, 152°92′E). It is evident from the results that the proposed model performs better than International Reference Ionosphere 2016 (IRI-2016), long short-term memory (LSTM), and BiLSTM ionospheric prediction models. The proposed model extensively captured the variation in ionospheric foF2 feature, and better predicted it under two significant space weather events (29 September 2011 and 22 July 2012). Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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16 pages, 3344 KiB

Open AccessArticle

Statistical Study of the Ionospheric Slab Thickness at Yakutsk High-Latitude Station

by Jian Feng, Yuqiang Zhang, Na Xu, Bo Chen, Tong Xu, Zhensen Wu, Zhongxin Deng, Yi Liu, Zhuangkai Wang, Yufeng Zhou, Chen Zhou and Zhengyu Zhao

Remote Sens. 2022, 14(21), 5309; https://doi.org/10.3390/rs14215309 - 24 Oct 2022

Cited by 1 | Viewed by 1056

Abstract

The ionospheric equivalent slab thickness (EST, also named τ) is defined as the ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2), and it is a significant parameter representative of the ionosphere. This paper presents a comprehensive statistical [...] Read more.

The ionospheric equivalent slab thickness (EST, also named τ) is defined as the ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2), and it is a significant parameter representative of the ionosphere. This paper presents a comprehensive statistical study of the ionospheric slab thickness at Yakutsk, located at the high latitude of East Asia, using the GPS-TEC and ionosonde NmF2 data for the years 2010–2017. The results show that the τ has different diurnal and seasonal variations in high- and low-solar-activity years, and the τ is greatest in the winter, followed by the equinox, and it is smallest in the summer in both high- and low-solar-activity years, except during the noontime of low-solar-activity years. Specifically, the τ in inter of high-solar-activity year shows an approximate single peak pattern with the peak around noon, while it displays a double-peak pattern with the pre-sunrise and sunset peaks in winter of the low-solar-activity years. Moreover, the τ in the summer and equinox have smaller diurnal variations, and there are peaks with different magnitudes during the sunrise and post-sunset periods. The mainly diurnal variation of τ in different seasons of high- and low-solar-activity years can be explained within the framework of relative variation of TEC and NmF2 during the corresponding period. By defining the disturbance index (DI), which can visually assess the relationship between instantaneous values and the median, we found that the geomagnetic storm would enhance the τ at Yakutsk. An example on 7 June 2013 is also presented to analyze the physical mechanism. It should be due to the intense particle precipitation and expanded plasma convection electric field during the storm at high-latitude Yakutsk station. The results would improve the current understanding of climatological and storm-time behavior of τ at high latitudes in East Asia. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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15 pages, 5065 KiB

Open AccessTechnical Note

Experimental Determination of the Ionospheric Effects and Cycle Slip Phenomena for Galileo and GPS in the Arctic

by S.S. Beeck, C.N. Mitchell, A.B.O. Jensen, L. Stenseng, T. Pinto Jayawardena and D.H. Olesen

Remote Sens. 2023, 15(24), 5685; https://doi.org/10.3390/rs15245685 - 11 Dec 2023

Viewed by 655

Abstract

The ionosphere can impair the accuracy, availability and reliability of satellite-based positioning, navigation and timing. The Arctic region is particularly affected by strong ionospheric gradients and phase scintillation, posing a safety issue for critical infrastructure and operations. Ionospheric warning and impact maps can [...] Read more.

The ionosphere can impair the accuracy, availability and reliability of satellite-based positioning, navigation and timing. The Arctic region is particularly affected by strong ionospheric gradients and phase scintillation, posing a safety issue for critical infrastructure and operations. Ionospheric warning and impact maps can provide support to Arctic operations, but to produce such maps threshold values have to be determined. This study investigates how such thresholds can be derived from the GPS and Galileo satellite signals. Rapid changes in total electron content (TEC) or scintillation-induced receiver tracking errors could result in cycle slips or even loss of lock. Cycle slips and data outages are used as a measure of impact on the receiver in this paper. For Galileo, 73.6% of the impacts were cycle slips and 26.4% were outages, while for GPS, 29.3% of the impacts were cycle slips and 70.7% were outages. Considering the sum of cycle slips and outages, it is worth noting that the sum of impacts for Galileo signals is larger than for GPS. A range of possible explanations have been examined through hardware-in-the-loop simulations. The simulations showed that the GPS L2 signal was not adequately tracked during rapid TEC changes and TEC changes were underestimated, thus the GPS cycle slips, derived from L1 and L2 derived TEC changes, were not all registered. These results are important in designing threshold values for TEC and for scintillation impact maps as well as for the operation of GNSS equipment in the Arctic. In particular, the results show that ionospheric changes could be underestimated if GPS L1 and L2 were used in isolation from other dual frequency combinations. It is the first time this analysis has been made for Greenland and the first time that the dual frequency derivation of ionospheric delay using GPS L1 and L2 has been shown to underestimate large TEC gradients. This has important implications for informing GNSS operations that rely on GPS to provide reliable estimates of the ionosphere. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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14 pages, 4351 KiB

Open AccessTechnical Note

High-Resolution Observation of Ionospheric E-Layer Irregularities Using Multi-Frequency Range Imaging Technology

by Bo Chen, Yi Liu, Jian Feng, Yuqiang Zhang, Yufeng Zhou, Chen Zhou and Zhengyu Zhao

Remote Sens. 2023, 15(1), 285; https://doi.org/10.3390/rs15010285 - 03 Jan 2023

Viewed by 1739

Abstract

E-region field-aligned irregularities (FAIs) are a hot topic in space research, since electromagnetic signal propagation through ionospheric irregularities can undergo sporadic enhancements and fading known as ionospheric scintillation, which could severely affect communication, navigation, and radar systems. However, the range resolution of very-high-frequency [...] Read more.

E-region field-aligned irregularities (FAIs) are a hot topic in space research, since electromagnetic signal propagation through ionospheric irregularities can undergo sporadic enhancements and fading known as ionospheric scintillation, which could severely affect communication, navigation, and radar systems. However, the range resolution of very-high-frequency (VHF) radars, which is widely used to observe E-region FAIs, is limited due to its bandwidth. As a technology that is widely used in atmosphere radars to improve the range resolution of pulsed radars by transmitting multiple frequencies, this paper employed the multifrequency radar imaging (RIM) technique in a Wuhan VHF radar. The results showed that the range resolution of E-region FAIs greatly improved when compared with the results in traditional single-frequency mode, and that finer structures of E-region FAIs can be obtained. Specifically, the imaging results in multifrequency mode show that E-region FAIs demonstrate an overall descending trend at night, and it could be related to the tides or gravity waves due to their downward phase velocities or even driven by downwind shear. In addition, typical quasi-periodic (QP) echoes with a time period of around 10 min could be clearly seen using the RIM technique, and the features of the echoes suggest that they could be modulated by gravity waves. Furthermore, the RIM technique can be used to obtain the fine structure of irregularities within a short time period, and the hierarchical structure of E-region FAIs can be easily found. Therefore, the multifrequency imaging RIM technique is suitable for observing E-region FAIs and their evolution, as well as for identifying the different layers of E-region FAIs. Combined with the RIM technique, a VHF radar provides an effective and promising way to observe the structure of E-region FAIs in more detail to study the physical mechanism behind the formation and evolution of ionospheric E-region irregularities. Full article

(This article belongs to the Special Issue Ionosphere Monitoring with Remote Sensing II)

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