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  1. Article ; Online: Observations of Ionospheric Disturbances Produced by a Powerful Very-Low-Frequency Radio Signal in the Magnetic Conjugated Region Respect the Transmitter

    Ting Feng / Moran Liu / Chen Zhou

    Remote Sensing, Vol 15, Iss 3307, p

    2023  Volume 3307

    Abstract: We investigate four observational cases over NWC magnetically conjugate region by DEMETER spacecraft and CSES satellite. The cases of DEMETER on 23 February 2008 and CSES on 22 March 2020 show the evident ionospheric heating effects, in which the ... ...

    Abstract We investigate four observational cases over NWC magnetically conjugate region by DEMETER spacecraft and CSES satellite. The cases of DEMETER on 23 February 2008 and CSES on 22 March 2020 show the evident ionospheric heating effects, in which the electron density and electron temperature suggest simultaneous enhancements associated with the intense spectra of the VLF electric and magnetic field above the conjugate region. This indicates that the heating effects associated with ionospheric modification are indeed triggered by the VLF signal transmitted by the NWC transmitter. In other words, the strong disturbances induced directly above the transmitter propagate along the magnetic field lines and extend into the magnetic conjugated region. Differently, the cases on 11 February 2010 (DEMETER) and 15 February 2019 (CSES) show obvious increases in electron densities while having no significant elevation in electron temperatures. The presence of enhanced energetic electron spectra at higher L-values, rather than directly above the conjugate region, suggests precipitation events induced by the VLF transmitter.
    Keywords conjugate ; heating effect ; precipitation ; DEMETER ; CSES ; Science ; Q
    Subject code 530
    Language English
    Publishing date 2023-06-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  2. Article ; Online: Assimilation and Inversion of Ionospheric Electron Density Data Using Lightning Whistlers

    Tian Xiang / Moran Liu / Shimin He / Chen Zhou

    Remote Sensing, Vol 15, Iss 3037, p

    2023  Volume 3037

    Abstract: The data assimilation algorithm is a common algorithm in space weather research. In this paper, the time-frequency information in the dispersion spectrum of lightning whistlers received by the ZH-1 satellite is used as the observed value, and the ... ...

    Abstract The data assimilation algorithm is a common algorithm in space weather research. In this paper, the time-frequency information in the dispersion spectrum of lightning whistlers received by the ZH-1 satellite is used as the observed value, and the international reference ionospheric model serves as the background model to construct the calculation model of the propagation time of lightning whistlers in the ionosphere. Kalman filtering is adopted to assimilate the electron density distribution along the propagation path of lightning whistlers. The results show that the situation where the electron density of the background model deviates greatly from the true value is significantly improved through data assimilation. The electron density after assimilation is in good agreement with the true value, which effectively helps realize the process of using observed values to correct the background value. On this basis, the influence of the frequency difference on the assimilation inversion effect is studied, and the results show that the assimilation effect is worse when the frequency difference between frequency points is less than 1 kHz.
    Keywords data assimilation ; electron density ; lightning whistlers ; dispersion spectrum ; Science ; Q
    Subject code 541
    Language English
    Publishing date 2023-06-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  3. Article ; Online: CAiTST

    Guozhen Xia / Moran Liu / Fubin Zhang / Chen Zhou

    Remote Sensing, Vol 14, Iss 4223, p

    Conv-Attentional Image Time Sequence Transformer for Ionospheric TEC Maps Forecast

    2022  Volume 4223

    Abstract: In recent years, transformer has been widely used in natural language processing (NLP) and computer vision (CV). Comparatively, forecasting image time sequences using transformer has received less attention. In this paper, we propose the conv-attentional ...

    Abstract In recent years, transformer has been widely used in natural language processing (NLP) and computer vision (CV). Comparatively, forecasting image time sequences using transformer has received less attention. In this paper, we propose the conv-attentional image time sequence transformer (CAiTST), a transformer-based image time sequences prediction model equipped with convolutional networks and an attentional mechanism. Specifically, we employ CAiTST to forecast the International GNSS Service (IGS) global total electron content (TEC) maps. The IGS TEC maps from 2005 to 2017 (except 2014) are divided into the training dataset (90% of total) and validation dataset (10% of total), and TEC maps in 2014 (high solar activity year) and 2018 (low solar activity year) are used to test the performance of CAiTST. The input of CAiTST is presented as one day’s 12 TEC maps (time resolution is 2 h), and the output is the next day’s 12 TEC maps. We compare the results of CAiTST with those of the 1-day Center for Orbit Determination in Europe (CODE) prediction model. The root mean square errors (RMSEs) from CAiTST with respect to the IGS TEC maps are 4.29 and 1.41 TECU in 2014 and 2018, respectively, while the RMSEs of the 1-day CODE prediction model are 4.71 and 1.57 TECU. The results illustrate CAiTST performs better than the 1-day CODE prediction model both in high and low solar activity years. The CAiTST model has less accuracy in the equatorial ionization anomaly (EIA) region but can roughly predict the features and locations of EIA. Additionally, due to the input only including past TEC maps, CAiTST performs poorly during magnetic storms. Our study shows that the transformer model and its unique attention mechanism are very suitable for images of a time sequence forecast, such as the prediction of ionospheric TEC map sequences.
    Keywords transformer ; ionospheric TEC maps ; global prediction ; Science ; Q
    Language English
    Publishing date 2022-08-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  4. Article ; Online: Lightning Detection and Imaging Based on VHF Radar Interferometry

    Wenjie Yin / Weizheng Jin / Chen Zhou / Yi Liu / Qiong Tang / Moran Liu / Guanyi Chen / Zhengyu Zhao

    Remote Sensing, Vol 13, Iss 2065, p

    2021  Volume 2065

    Abstract: In this study, detection and three-dimensional (3D) imaging of lightning plasma channels are presented using radar interferometry. Experiments were carried out in Leshan, China with a 48.2 MHz VHF radar configured with an interferometric antenna array. ... ...

    Abstract In this study, detection and three-dimensional (3D) imaging of lightning plasma channels are presented using radar interferometry. Experiments were carried out in Leshan, China with a 48.2 MHz VHF radar configured with an interferometric antenna array. The typical characteristics of lightning echoes are studied in the form of amplitude, phase, and doppler spectra derived from the raw in-phase/quadrature (I/Q) data. In addition, the 3D structure of lightning channels is reconstructed using the interferometry technique. The localization results of lightning are verified with the locating results of lightning detection networks operating at VLF ranges, which indicate the feasibility of using VHF radar for lightning mapping. The interpretation of the observational results is complicated by the dendric structure of lightning channel and the overlap between passive electromagnetic radiations and return echoes. Nevertheless, some parts of the characteristics of lightning are still evident. The observational result of return echoes shows good consistency with the overdense assumption of lightning channels. The transition from the overdense channel to the underdense channel in the form of amplitude and phase is clearly observed. This technique is very promising to reveal the typical characteristics of lightning return echoes and structure of lightning propagation processes.
    Keywords VHF lightning radar interferometer ; 3D lightning imaging ; lightning ; Science ; Q
    Language English
    Publishing date 2021-05-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  5. Article ; Online: Numerical Study of Global ELF Electromagnetic Wave Propagation with Respect to Lithosphere–Atmosphere–Ionosphere Coupling

    Zhuangkai Wang / Chen Zhou / Shufan Zhao / Xiang Xu / Moran Liu / Yi Liu / Li Liao / Xuhui Shen

    Remote Sensing, Vol 13, Iss 4107, p

    2021  Volume 4107

    Abstract: Before and after earthquakes, abnormal physical and chemical phenomena can be observed by gathering ground-based and satellite data and interpreted by the lithosphere–atmosphere–ionosphere coupling (LAIC) mechanism. In this study, we focused on the ... ...

    Abstract Before and after earthquakes, abnormal physical and chemical phenomena can be observed by gathering ground-based and satellite data and interpreted by the lithosphere–atmosphere–ionosphere coupling (LAIC) mechanism. In this study, we focused on the mechanism of LAIC electromagnetic radiation and investigated the seismic electromagnetic (EM) wave generated in the lithosphere by earthquakes and its global propagation process from the lithosphere through the atmosphere and into the bottom of ionosphere, in order to analyze the abnormal disturbance of ground-based and space-based observation results. First, analytic formulas of the electrokinetic effect were used to simulate the generation and propagation process of the seismic EM wave in the lithosphere, interpreted as the conversion process of the seismic wave and EM wave in porous media. Second, we constructed a three-dimensional Earth–ionosphere waveguide by applying the finite-difference time-domain (FDTD) algorithm to model the global propagation process of the seismic EM wave into the atmosphere and cavity between the bottom of the ionosphere and the surface of the Earth. By combining the model of the electrokinetic effect in the lithosphere with the numerical model of the Earth–ionosphere waveguide in the atmosphere and ionosphere, we numerically simulated the global transmission process of extremely low-frequency (ELF: 3 Hz–3000 Hz) EM waves which are related to earthquakes. The propagation parameters of coseismic ELF EM waves with different duration times and center frequencies were analyzed and summarized. The simulation results demonstrate that the distribution characteristics of an electric field along longitude, latitude and altitude with time are periodic and the time interval during which an EM wave travels around the whole Earth is approximately 0.155 s when adopting the conductivity of the knee profile. We also compared the observation data with the simulation results and found that the attenuating trends of the ELF electric field are consistent. ...
    Keywords lithosphere–atmosphere–ionosphere coupling ; ELF electromagnetic radiation ; multi-layer electromagnetic wave model ; numerical modeling ; wave propagation ; electrical properties ; Science ; Q
    Subject code 535
    Language English
    Publishing date 2021-10-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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    Inter-library loan at ZB MED

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