Analyzing LF/VLF lightning waveforms to estimate D-region electron density profiles

Development of D-region electron and ion densities under various auroral conditions during the Energy Budget Campaign (EBC)

Abstract. The influence of atomic oxygen concentration on the height distribution of the main positive and negative ions and on electron density in the mesosphere is studied for the conditions prevailing during the solar proton event on 17 January 2005. It is shown by numerical modeling that the electron and ion density profiles are strongly dependent on the choice of the atomic oxygen profile. Experimental measurements of the electron density are used as the criterion for choosing the atomic oxygen profile in the mesosphere. With the help of modeling, the atomic oxygen profile in the daytime in the winter mesosphere is found to lead to a model electron density profile best matching the electron density profile obtained experimentally. As a result, with the help of modeling, we find the atomic oxygen profiles at various solar zenith angles in the winter mesosphere which lead to model electron density profiles matching the electron density profiles obtained experimentally. Alteration of the atomic oxygen concentration leads to a redistribution of the abundance of both positive and negative ion constituents, with changes in their total concentrations and transition heights. In consequence this results in changes of the electron density and effective recombination coefficient. For conditions of low concentration of atomic oxygen (during a solar proton event), the formation of cluster ions is the key process determining electron and ion densities at altitudes up to 77 km. The complex negative CO3− ion is formed up to about 74 km and the final NO3− ion, which is stable in relation to the atomic oxygen, is the dominant negative ion up to 74 km. As a result the transition heights between cluster ions and molecular ions and between negative ions and electron density are located at 77 km and 66 km, respectively.

Longitudinal behaviors of the IRI-B parameters of the equatorial electron density profiles retrieved from FORMOSAT-3/COSMIC radio occultation measurements

Traditional long-range lightning detection and location networks use Time-of-Arrival (TOA) differences, and a single timestamp to locate lightning events. For long propagation distances, the amplitude of ground waves decays faster with distance than sky waves as a result of the ground conductivity and the effects of Earth curvature (Caligaris et al., 2008, Cooray, 2009, Hou et al., 2018). This can lead the skywaves to interfere with their large amplitudes when locating lightning.Coherency, which is short for phase coherency of the analytic signal, is used here, which exhibits lightning characteristics (Bai & Fullekrug, 2022). This work introduces a simulation study to lay the foundation for new lightning location concepts. A novel interferometric method using coherency is presented here, which expands the use of more data points of recorded lightning sferics to map the lightning into an area in a long-range network. In this map, each pixel corresponds to a lightning location with different coherency and time of arrival differences, simulated by shifting the complex lightning waveforms. In long-range networks, the coherency of the 1st skywave is larger than the ground wave, and it is difficult to distinguish them due to the short time delay between them. One solution is to use a small network so that the recorded waveforms are associated with short propagation distances which can eliminate the interferences caused by the first skywave. Another solution is to filter the data such that a lightning waveform is represented by an impulse. In this case, only one maximum coherency area exists for each event at the lightning occurrence time.In the future, the data collected with a real-time lightning detection network will be analysed to map the lightning events using the complex interferometric method for use in long-range lightning location networks. ReferencesBai, X., & Füllekrug, M. (2022). Coherency of Lightning Sferics. Radio Sci., 57(5), e2021RS007347. doi: 10.1029/2021rs007347Caligaris, C., Delfino, F., & Procopio, R. (2008). Cooray–Rubinstein Formula for the Evaluation of Lightning Radial Electric Fields: Derivation and Implementation in the Time Domain. IEEE Trans. Electromagn. Compat., 50(1), 194-197. doi: 10.1109/temc .2007.913226Cooray, V. (2009). Propagation Effects Due to Finitely Conducting Ground on Lightning-Generated Magnetic Fields Evaluated Using Sommerfeld’s Integrals. IEEE Trans. Elec-tromagn. Compat., 51(3), 526-531. doi: 10.1109/temc.2009.2019759Hou, W., Zhang, Q., Zhang, J., Wang, L., & Shen, Y. (2018). A New Approximate Method for Lightning-Radiated ELF/VLF Ground Wave Propagation over Intermediate Ranges. Int. J. Antennas Propag., 2018(6), 1-10. doi: 10.1155/2018/9353294

Predicting the behaviour of the Earth’s ionosphere is crucial for the ground-based and spaceborne technologies that rely on it. This paper presents a novel way of inferring ionospheric electron density profiles and electron temperature profiles using machine learning. The analysis is based on the Nearest Neighbour (NNB) and Radial Basis Function (RBF) regression models. Synthetic data sets used to train and validate these two inference models are constructed using the International Reference Ionosphere (IRI 2020) model with randomly chosen years (1987–2022), months (1–12), days (1–31), latitudes (−60 to 60°), longitudes (0, 360°), and times (0–23 h), at altitudes ranging from 95 to 600 km. The NNB and RBF models use the constructed ionosonde-like profiles to infer complete ISR-like profiles. The results show that the inference of ionospheric electron density profiles is better with the NNB model than with the RBF model, while the RBF model is better at inferring the electron temperature profiles. A major and unexpected finding of this research is the ability of the two models to infer full electron temperature profiles that are not provided by ionosondes using the same truncated electron density data set used to infer electron density profiles. NNB and RBF models generally over- or underestimate the inferred electron density and electron temperature values, especially at higher altitudes, but they tend to produce good matches at lower altitudes. Additionally, maximum absolute relative errors for electron density and temperature inferences are found at higher altitudes for both NNB and RBF models.

To generate high-quality reconstructions of ionospheric electron density (IED), we propose an extended simultaneous algebraic reconstruction technique (ESART). The ESART method distributes the discrepancy between the actual GNSS TEC and the calculated TEC among the ray–voxels based on the contribution of voxels to GNSS TEC, rather than the ratio of the length of ray–voxel intersection to the sum of the lengths of all ray–voxel intersections, as is adopted by conventional methods. The feasibility of the ESART method for reconstructing the IED under different levels of geomagnetic activities is addressed. Additionally, a preliminary experiment is performed using the reconstructed IED profiles and comparing them with ionosonde measurements, which provide direct observations of electron density. The root mean square errors (RMSE) and absolute errors of the ESART method, the simultaneous algebraic reconstruction technique (SART) method, and the International Reference Ionosphere (IRI) 2016 model are calculated to evaluate the effectiveness of the proposed method. Compared to the conventional SART method of ionospheric tomography and the IRI-2016 model, the reconstructed IED profiles obtained using the ESART method are in better agreement with the electron density obtained from the ionosondes, especially for the peak electron densities (NmF2). In addition, a case study of an intense geomagnetic storm on 17–19 March 2015 shows that the spatial and temporal features of storm-related ionospheric disturbances can be more clearly depicted using the ESART method than with the SART method.

Electron density and temperature profiles in the mid latitude ionosphere are derived from the ”resonance cone” in the radiation pattern of high‐frequency point antennas aboard a sounding rocket. By comparing the shapes for reversed wave propagation direction it is possible to study electron drift motion and field‐aligned beams. The data from the COREX‐I (Cooperative Resonance Cone Experiment) experiment show close agreement of electron density and temperature profiles from resonance cones with results from independent instruments. There was no indication of a substantial temperature anomaly atEregion altitude during this flight. An unexpected electron drift can be interpreted as electron Hall current in the presheath of the negatively charged payload. Field‐aligned beams, which have been suggested in connection with the temperature anomaly, were not detected.

Ray tracing analysis and a first‐order treatment of the wave frequency and electron energy dependence of gyroresonant pitch angle scattering reveals the L shell dependence of the time‐integrated energetic electron (>150 keV) precipitation flux in the drift loss cone due to a single cloud‐to‐ground lightning stroke. Primary features of the L dependence are determined by the ray paths of magnetospherically reflecting nonducted whistler waves, as well as the refraction of the wave normal vectors due to ionospheric horizontal electron density gradients at low latitudes. Calculations for lightning source latitudes of 25°, 35°, and 45°, with an assumed “average” ionospheric electron density profile, as well as an International Reference Ionosphere model profile, indicate that horizontal ionospheric electron density gradients can cause the whistler wave energy to focus near the geomagnetic equator on the first magnetospheric traverse, resulting in a spatially narrow precipitation signature at low L shells (1.4 < L < 1.65). The magnitude of this low‐L peak is found to be sensitively dependent on the local ionospheric density gradients. Subsequent magnetospheric reflections result in a broad precipitation peak at higher L shells (1.8 < L < 4.2), which is robustly present, with magnitude being relatively insensitive to lightning source‐latitude and ionospheric horizontal gradients. The calculated L dependences were found to be in excellent agreement with drift‐loss cone fluxes measured aboard the SAMPEX satellite.

Faraday rotation changes the polarization plane of linearly polarized microwaves which propagate through the ionosphere. To correct for ionospheric polarization error, it is necessary to have electron density profiles on a global scale that represent the ionosphere in real time. We use raytrace through the combined models of ionospheric conductivity and electron density (ICED), Bent, and Gallagher models (RIBG model) to specify the ionospheric conditions by ingesting the GPS data from observing stations that are as close as possible to the observation time and location of the space system for which the corrections are required. To accurately calculate Faraday rotation corrections, we also utilize the raytrace utility of the RIBG model instead of the normal shell model assumption for the ionosphere. We use WindSat data, which exhibits a wide range of orientations of the raypath and a high data rate of observations, to provide a realistic data set for analysis. The standard single‐shell models at 350 and 400 km are studied along with a new three‐shell model and compared with the raytrace method for computation time and accuracy. We have compared the Faraday results obtained with climatological (International Reference Ionosphere and RIBG) and physics‐based (Global Assimilation of Ionospheric Measurements) ionospheric models. We also study the impact of limitations in the availability of GPS data on the accuracy of the Faraday rotation calculations.

The ionosphere of Mars is expected to be significantly affected by the solar wind because Mars does not possess a significant intrinsic magnetic field which deflects the solar wind. Despite a number of plasma measurements made near Mars, the nature of the solar wind-Mars interaction has not yet been fully understood. In order to self-consistently study the solar wind interaction with the ionosphere of Mars, a two-dimensional MHD model has been developed with an emphasis placed on the structure of the ionosphere of Mars. It is found that the modeled electron density profile in the upper ionosphere strongly depends on the solar wind dynamic pressure as well as the solar zenith angle. The ionosphere in the model tends to have an ionopause-like sharp drop of the electron density at some altitude for realistic solar wind dynamic pressures. Such behavior is not consistent with most of the observed electron density profiles, which exhibit relatively large and constant scale height in most of the dayside region. While the observed electron density profiles of the Venus ionosphere have been reproduced reasonably well by ionospheric models as well as recent three-dimensional MHD models, the electron density profiles of the Martian ionosphere have not been successfully reproduced by theoretical models including this study. This fact implies that processes not present in the Venus ionosphere, such as crustal magnetic fields and the rotation of the planet, may have significant effects on the structure and the dynamics of the ionosphere of Mars.

On the small-scale fluctuations in the peak electron density of Martian ionosphere observed by MEX/MARSIS

Space weather deposits energy into the high polar latitudes, primarily via Joule heating that is associated with the Poynting flux electromagnetic energy flow between the magnetosphere and ionosphere. One way to observe this energy flow is to look at the ionospheric electron density profile (EDP), especially that of the topside. The altitude location of the ionospheric peak provides additional information on the net field‐aligned vertical transport at high latitudes. To date, there have been few studies in which physics‐based ionospheric model storm simulations have been compared with topside EDPs. A rich database of high‐latitude topside ionograms obtained from polar orbiting satellites of the International Satellites for Ionospheric Studies (ISIS) program exists but has not been utilized in comparisons with physics‐based models. Of specific importance is that the Alouette/ISIS topside EDPs spanned the timeframe from 1962 to 1983, a period that experienced very large geomagnetic storms. We use a physics‐based ionospheric model, the Utah State University Time Dependent Ionospheric Model (TDIM), to simulate ionospheric EDPs for quiet and storm high‐latitude passes of ISIS‐II for two geomagnetic storms. This initial study finds that under quiet conditions there is good agreement between model and observations. During disturbed conditions, however, a large difference is seen between model and observations. The model limitation is probably associated with the inability of its topside boundary to replicate strong outflow conditions. As a result, modeling of the ionospheric outflows needs to be extended well into the magnetosphere, thereby moving the upper boundary much higher and requiring the use of polar wind models.

Determination of Electron Density Profiles and Area from Simulations of Undulating Membranes

Modelling of particle charging in the polar summer mesosphere: Part 2—Application to measurements

GPS radio occultation measurements establish the basis for a new remote sensing technique for vertical profile information on the electron density of the entire ionosphere from satellite orbit heights down to the bottomside. No other profiling technique such as vertical sounding or incoherent scatter, unifies vertical profiling through the entire ionosphere with global coverage. Inversion methods are described both for vertical profiling as well as for tree dimensional electron density reconstructions of the ionosphere. In three dimensional electron density reconstructions using signals from Global Navigation Satellite Systems (GNSS), the Ionospheric Radio Occultation (IRO) measurements provide vertical information which is complementary to the information obtained by ground based measurements. Assessment of achievable accuracy and spatial resolution are addressed by simulation studies. IRO measurements have been carried out onboard the German CHAMP satellite since 11 April 2001 on a routine basis. Assuming a spherically layered ionosphere, up to about 150 Electron Density Profiles (EDPs) per day are retrieved within a latency of 3 h. Validation results obtained by using independent data sources are reported. The validation with vertical sounding data in mid-latitudes indicates a small positive bias in the plasma frequency of up to about 0.5 MHz throughout the entire profile. Averages of the numerous EDPs show wellknown ionospheric phenomena such as the equatorial anomaly, the winter anomaly and the expansion of the profile with increasing solar energy input. It is concluded that CHAMP-IRO measurements have the potential to establish global data sets of vertical electron density profiles for developing and improving global ionospheric models and to provide operational space weather information.