Long Wave Propagation Capability Research Articles

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.

Quantification of Ionospheric Perturbations From Lightning Using Overlapping Paths of VLF Signal Propagation

AbstractLightning induced perturbations of the lower ionosphere are investigated with very low frequency (VLF) remote sensing on a unique overlapping propagation path geometry. The signals from two VLF transmitters (at different frequencies) are observed at a single receiver after propagation through a common channel in the Earth‐ionosphere waveguide. This measurement diversity allows for greater certainty in quantification of perturbations to the ionospheric D region. Changes in amplitude and phase are modeled with the Long Wave Propagation Capability (LWPC) software package to quantify changes in reference height and steepness of the two parameter D region electron density model. Since the nighttime D region profile prior to the perturbation is found to strongly affect the resulting quantification, and is highly variable and generally unknown at nighttime, an error minimization method for identifying the most likely ionospheric disturbance independent of the ambient profile is used. Analysis of 12 large lightning perturbations resulting from discharges with peak currents greater than 160 kA shows that the ionospheric reference height can change by 2–8 km. We investigate both early/fast events (direct ionization and heating from lightning) and lightning‐induced electron precipitation (LEP) events, induced by lightning hundreds of kilometer away. LEP events increase D region electron density while early/fast events can lead to a increase or decrease in electron density. Multi‐point observations along a VLF propagation path are needed to further improve ionospheric perturbation quantification with VLF remote sensing.

3-D FDTD Modeling of Long-Distance VLF Propagation in the Earth-Ionosphere Waveguide

Very-low-frequency (VLF) electromagnetic wave propagation is modeled for the first time over 1000 km scale distances using a 3-D finite-difference time-domain (FDTD) model of the Earth-ionosphere waveguide. Specifically, propagation paths of 2000 km in length are studied, with each simulation requiring 28k processing cores and over 45 h of real time and using ~3.9 TB. A variety of propagation scenarios are tested, including daytime and nighttime propagation paths, a realistic ground propagation path extending from the NAA VLF transmitter in Cutler, ME, USA, toward New Mexico, and day-to-night ionospheric transitions. The 3-D FDTD model results are compared with 2-D azimuthally symmetric FDTD and the long-wave propagation capability (LWPC) results to both validate the 3-D model and understand the impact a fully 3-D model can have on the propagation predictions. The results in this article identify under what conditions and also at what propagation distances, a fully 3-D model is most beneficial.

Ionospheric D Region Disturbances due to FAC and LEP Associated With Three Severe Geomagnetic Storms as Observed by VLF Signals

AbstractThe effects of one super and two intense storms on three European very low frequency (VLF) transmitter signals received at Algiers (Algeria) station, are presented. Two VLF transmitters (DHO and GQD) are located almost at the same mid‐latitude and third transmitter (NRK) at a high latitude, allowing us to study the latitudinal dependence of the storm effect on the VLF propagation. Two kind of ionospheric disturbances were considered: Direct effect of the field aligned currents (FACs) and the second concerning with the lightning‐induced electron precipitation (LEP) events. The FACs effect was clearly evident on the recorded signal amplitude during the daytime and nighttime. Our analysis of the short‐duration VLF perturbations due to LEP events showed that the ducted and non‐ducted LEPs are the majority of VLF perturbations and a small fraction of LEP events was associated with magnetospherically reflected waves on the high latitude transmitter path (NRK–Algiers). The coincident observation of LEP on the mid‐latitude transmitters (DHO and GQD) signals implies that the disturbance was large as much as the distance between the two transmitters (728 km). The modeling using Long‐Wave Propagation Capability (LWPC) code of LEP associated VLF perturbations considering Gaussian distribution of electron density enhancements, showed a decrease in the D‐region reference height from its usual value 87 km to 83.3 km for ducted LEP and 80.3 km for non‐ducted LEP event and that the non‐ducted event exhibited a wider disturbed region.

Quiet Ionospheric D-Region (QIonDR) Model Based on VLF/LF Observations

The ionospheric D-region affects propagation of electromagnetic waves including ground-based signals and satellite signals during its intensive disturbances. Consequently, the modeling of electromagnetic propagation in the D-region is important in many technological domains. One of sources of uncertainty in the modeling of the disturbed D-region is the poor knowledge of its parameters in the quiet state at the considered location and time period. We present the Quiet Ionospheric D-Region (QIonDR) model based on data collected in the ionospheric D-region remote sensing by very low/low frequency (VLF/LF) signals and the Long-Wave Propagation Capability (LWPC) numerical model. The QIonDR model provides both Wait’s parameters and the electron density in the D-region area of interest at a given daytime interval. The proposed model consists of two steps. In the first step, Wait’s parameters are modeled during the quiet midday periods as a function of the daily sunspot number, related to the long-term variations during solar cycle, and the seasonal parameter, providing the seasonal variations. In the second step, the output of the first step is used to model Wait’s parameters during the whole daytime. The proposed model is applied to VLF data acquired in Serbia and related to the DHO and ICV signals emitted in Germany and Italy, respectively. As a result, the proposed methodology provides a numerical tool to model the daytime Wait’s parameters over the middle and low latitudes and an analytical expression valid over a part of Europe for midday parameters.

D-region ionospheric disturbances associated with the Extremely Severe Cyclone Fani over North Indian Ocean as observed from two tropical VLF stations

The D-region ionospheric disturbances due to the tropical cyclone Fani over the Indian Ocean have been analysed using Very Low Frequency (VLF) radio communication signals from three transmitters (VTX, NWC and JJI) received at two low latitude stations (Kolkata-CUB and Cooch Behar-CHB). The cyclone Fani formed from a depression on 26th April, 2019 over the Bay of Bengal (Northeastern part of the Indian Ocean) and turned into an extremely severe cyclone with maximum 1-min sustained winds of 250 km/h on 2 May, 2019 which made landfall on 3 May, 2019. Out of six propagation paths, five propagation paths, except the JJI-CHB which was far away from the cyclone track, showed strong perturbations beyond 3σ level compared to unperturbed signals. Consistent good correlations of VLF signal perturbations with the wind speed and cyclone pressure have been seen for both the receiving stations. Computations of radio signal perturbations at CUB and CHB using the Long Wave Propagation Capability (LWPC) code revealed a Gaussian perturbation in the D-region ionosphere. Analysis of atmospheric temperature at different layers from the NASA’s TIMED satellite revealed a cooling effect near the tropopause and warming effects near the stratopause and upper mesosphere regions on 3 May, 2019. This study shows that the cyclone Fani perturbed the whole atmosphere, from troposphere to ionosphere and the VLF waves responded to the disturbances in the conductivity profiles of the lower ionosphere.

Solar flare effects on D-region ionosphere using VLF measurements during low- and high-solar activity phases of solar cycle 24

The effects of solar flares on the propagation of subionospheric VLF signals from NWC and NLK transmitter stations monitored at a low-latitude station, Suva (18.2°S, 178.4°E), Fiji, between December 2006 and December 2010 (an unprecedented solar minimum of solar cycles 23 and 24) and between January 2012 and December 2013 (moderate solar activity at the peak of solar cycle 24) have been analyzed to find solar flare time D-region changes. The amplitude and phase enhancements associated with solar flares were observed in the signals from both stations which are due to an increase in the electron density of the D-region as a result of extra ionization caused by the solar flares. The solar flare-induced perturbations in both the amplitude and phase of VLF signals were used to determine D-region ionospheric parameters: H′ (the ionospheric reflection height) and β (rate of increase in electron density with height) using Long Wave Propagation Capability (LWPC) version 2.1 modeling. A comparative analysis of the ionospheric D-region parameter changes carried out for this location shows a greater increase in β and decrease in H′ during low-solar activity period than during moderate-solar activity period, for the same class of flares. Our results also show greater differences in the values of β and H′ for strong flares in comparison with weak flares under both low- and moderate-solar activity conditions.

Finite-Difference Modeling of Very-Low-Frequency Propagation in the Earth-Ionosphere Waveguide

Simulations of Very-low-frequency (VLF) transmitter signals are conducted using three models: the long-wave propagation capability, a finite-difference (FD) time-domain model, and an FD frequency-domain model. The FD models are corrected using Richardson extrapolation to minimize the numerical dispersion inherent in these models. Using identical ionosphere and ground parameters, the three models are shown to agree very well in their simulated VLF signal amplitude and phase, to within 1 dB of amplitude and a few degrees of phase, for a number of different simulation paths and transmitter frequencies. Furthermore, the three models are shown to produce comparable phase changes for the same ionosphere perturbations, again to within a few degrees. Finally, we show that the models reproduce the phase data of existing VLF transmitter-receiver pairs reasonably well, although the nighttime variation in the measured phase data is not captured by the simplified characterization of the ionosphere.

Space weather effects on lower ionosphere: First investigation from Bharati station during 34th Indian scientific expedition to Antarctica

We investigate the solar flare effects on the D-region of the ionosphere with the help of VLF (Very Low Frequency) radio waves using a portable E-field system from Antarctica during the summer period of 34th Indian scientific expedition. Two GPS time synchronized VLF receivers, one located at Bharati, Antarctica (geographical latitude 69.40°S, longitude 76.18°E) and another located at Tripura, India (geographical latitude 23.84°N, longitude 91.28°E) were operated simultaneously to infer common mode changes in the lower ionosphere for a number of solar flares events. The two systems constantly monitored the carrier amplitude and phase of the MSK (Minimum Shift Keying) modulated navy transmitter located in Australia (Callsign: NWC, 19.8kHz, geographical latitude 21.88°S, longitude 114.13°E), around 5.6Mm great circle distance from the two receivers. The results are interpreted in terms of Earth-ionosphere wave-guide characteristics. A Long Wave Propagation Capability (LWPC) model study is also performed to infer the changes in the daytime electron density in polar D-region ionosphere during the solar flares. The exponential fit of the modeled electron density change with average X-ray flux change shows an excellent correlation (R2 value 0.95). The exponential fit is utilized to infer the daytime electron density change in the polar ionosphere during solar flare events. The analyses indicate that small solar flares of class ‘C’ can be very effectively detected with the portable antenna system even if the receiver is located in polar coastal region compared to equatorial region. The expedition results also demonstrate the feasibility of using portable VLF receivers from the coastal stations for monitoring the polar lower ionosphere from Antarctica and open up new opportunities for long term exploration.

Space weather effects on the low latitude D-region ionosphere during solar minimum

The effects of the solar flares and the geomagnetic storms (disturbance storm time (Dst) < −50 nT) during December 2006 to 2008, a period during the unprecedented solar minimum of solar cycles 23 and 24, have been examined on sub-ionospheric very low frequency (VLF) signals from NWC (19.8 kHz), NPM (21.4 kHz), VTX (18.2 kHz), and NLK (24.8 kHz) transmitters monitored at Suva (18.2° S, 178.4° E), Fiji. Apart from the higher class solar flares (C to X), a solar flare of class B8.5 also produced enhancements both on the amplitude and phase. The amplitude enhancements in NLK, NPM, and NWC signals as a function of peak solar flare X-ray flux in decibel (dB; relative to 1 μW/m2) shows that the relationship curve is steeper and quite linear between the flare power levels of 0 to 15 dB; below 0 dB, the curve gets less steep and flattens towards −5 dB flare power level, while it also gets less steep above 15 dB and almost flattens above 20 dB. In general, the level of amplitude enhancement for NLK signal is higher than that for NPM and NWC signals for all solar flares. The enhancement in the amplitude and phase of VLF signals by solar flares is due to the increase in the D-region electron density by the solar flare-produced extra ionization. The modeling of VLF perturbations produced by B8.5 and C1.5 classes of solar flares on 29 January 2007 using LWPC (Long Wave Propagation Capability) V2.1 codes show that reflection height (H') was reduced by 0.6 and 1.2 km and the exponential sharpness factor (β) was raised by 0.010 and 0.005 km−1, respectively. Out of seven storms with Dst < −50 nT, only the intense storm of 14 to 16 December 2006 with a minimum Dst of −145 nT has shown a clear reduction in the signal strength of NWC and NPM sub-ionospheric signals due to storm-induced reduction in the D-region electron density.

Response of the mid-latitude D-region ionosphere to the total solar eclipse of 22 July 2009 studied using VLF signals in South Korean peninsula

In this paper, we analyze VLF signals received at Busan to study the the D-region changes linked with the solar eclipse event of 22 July 2009 for very short (∼390km) transmitter–receiver great circle path (TRGCP) during local noon time 00:36–03:13UT (09:36–12:13KST). The eclipse crossed south of Busan with a maximum obscuration of ∼84%. Observations clearly show a reduction of ∼6.2dB in the VLF signal strength at the time of maximum solar obscuration (84% at 01:53UT) as compared to those observed on the control days. Estimated values of change in Wait ionospheric parameters: reflection height (h′) in km and inverse scale height parameter (β) in km−1 from Long Wave Propagation Capability (LWPC) model during the maximum eclipse phase as compared to unperturbed ionosphere are 7km and 0.055km−1, respectively. Moreover, the D-region electron density estimated from model computation shows 95% depletion in electron density at the height of ∼71km. The reflection height is found to increase by ∼7km in the D-region during the eclipse as compared to those on the control days, implying a depletion in the Lyman-α flux by a factor of ∼7. The present observations are discussed in the light of current understanding on the solar eclipse induced D-region dynamics.

Response of the Earth’s lower ionosphere to the Ground Level Enhancement event of December 13, 2006

In this study we analyze the Ground Level Enhancement Event No 70 observed on December 13, 2006, by correlating the observations from two research topics: Cosmic rays and Very Low Frequency (VLF<30kHz) wave propagation, as two ground based techniques for the detection of solar proton events, and their impact on the lower ionosphere. The observations have been endorsed from recordings of worldwide network ground based Neutron Monitors as well as by satellite data from the satellites GOES 12 (www.swpc.noaa.gov) and Pamela (www.pamela.roma2infn.it).We have evaluated the ionization rate for protons in the altitude range relevant to VLF propagation, and for galactic cosmic ray (GCR) background, finding that at energies up to ∼2GeV the ionization rate of solar protons exceeded the GCR ionization by 1.5 orders of magnitude.We have applied the Long Wave Propagation Capability (LWPC) code to evaluate the enhancement of the electron density from VLF signal perturbation and have inferred corresponding electron densities from the evaluated ionization rates and effective recombination coefficients from literature, to find the two independent sets in good agreement.

Modeling solar flare induced lower ionosphere changes using VLF/LF transmitter amplitude and phase observations at a midlatitude site

Abstract. Remote sensing of the ionosphere bottom using long wave radio signal propagation is a still going strong and inexpensive method for continuous monitoring purposes. We present a propagation model describing the time development of solar flare effects. Based on monitored amplitude and phase data from VLF/LF transmitters gained at a mid-latitude site during the currently increasing solar cycle no. 24 a parameterized electron density profile is calculated as a function of time and fed into propagation calculations using the LWPC (Long Wave Propagation Capability). The model allows to include lower ionosphere recombination and attachment coefficients, as well as to identify the relevant forcing X-ray wavelength band, and is intended to be a small step forward to a better understanding of the solar–lower ionosphere interaction mechanisms within a consistent framework.

Latitudinally dependent Trimpi effects: Modeling and observations

Modeling studies show that the exclusion of the propagating VLF wave from the ionospheric region results in the decline of Trimpi magnitude with patch altitude. In large models such as Long Wave Propagation Capability (LWPC) this exclusion does not occur inherently in the code, and high‐altitude precipitation modeling can produce results that are not consistent with observations from ground‐based experiments. The introduction to LWPC of realistic wave attenuation of the height gain functions in the ionosphere solves these computational problems. This work presents the first modeling of (Born) Trimpi scattering at long ranges, taking into account global inhomogeneities and continuous mode conversion along all paths, by employing the full conductivity perturbation matrix. The application of the more realistic height gain functions allows the prediction of decreasing Trimpi activity with increasing latitude, primarily through the mechanism of excluding the VLF wave from regions of high conductivity and scattering efficiency. Ground‐based observations from Faraday and Rothera, Antarctica, in September and October 1995 of Trimpi occurring on the NPM (Hawaii) path provide data that are consistent with these predictions. Latitudinal variations in Trimpi occurrence near L = 2.5, with a significant decrease of about 70% occurrence between L = 2.4 and L = 2.8, have been observed at higher L shell resolution than in previous studies (i.e., 2 &lt; L &lt; 3).