Proceedings ArticleDOI

# Modeling of VLF network observations due to lower ionospheric perturbation during a solar eclipse

01 Mar 2019-

TL;DR: In this paper, the temporal dependences of very low frequency (VLF) electric amplitude perturbation from two Japanese VLF transmitters (JJI (22.2kHz) and JJY (40.0kHz) were analyzed.

AbstractA large part of the path of the Annular Solar Eclipse of May 20,2012 (magnitude 0.9439) (ASE-2012) was over southern Japan. The D-region ionospheric changes associated with the ASE-2012, led to several degree of observable perturbations of sub-ionospheric very low frequency (VLF) radio signal. The solar eclipse associated signal changes were identified in VLF several receiving stations $(R_{x})$ simultaneously for the VLF signals coming from both Japanese and US VLF transmitters ($T_{x}$). In this work, we have analyzed temporal dependences of VLF electric amplitude perturbation $(\Delta A_{ecl,obs}(t))$ from two Japanese VLF transmitters (JJI (22.2kHz) and JJY (40.0kHz)), and the spatio-temporal characteristics of respective subionospheric perturbations are studied in detail. We consider the 2-parameter D-region ionospheric model with the exponential electron density profile. To model the shadow effect on the D-region ionosphere due to obscuration of solar disk, we assume a generalized space-time dependent 2-Dimensional Elliptical Gaussian distribution Model (2DEGM) for ionospheric parameters, such as, effective reflection height $(h')$ and sharpness factor $(\beta )$. In the vicinity of the eclipse zone, we compute the subionospheric VLF signal propagation for several signal propagation paths. In the simulation, we obtain the perturbation of VLF signal amplitude ($\Delta A_{ecl,LWPC}(t))$ at each station and compare with its observtaional counterpart $(\Delta A_{ecl,obs}(t))$.

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01 Jul 2018
TL;DR: In this paper, the effects of the total solar eclipse on the VLF signal were investigated using the knowledge of the lower ionospheric chemical and physical properties, which is not well studied till date.
Abstract: The variation in the solar Extreme Ultraviolet (EUV) radiation flux by any measure is the most dominant natural source to produce perturbations or modulations in the ionospheric chemical and plasma properties. A solar eclipse, though a very rare phenomenon, is similarly bound to produce a significant short time effect on the local ionospheric properties. The influence of the ionizing solar flux reduction during a solar eclipse on the lower ionosphere or, more precisely, the D-region, can be studied with the observation of Very Low Frequency (VLF) radio wave signal modulation. The interpretation of such an effect on VLF signals requires a knowledge of the D-region ion chemistry, which is not well studied till date. Dominant parameters which govern the ion chemistry, such as the recombination coefficients, are poorly known. The occurrence of events such as a solar eclipse provides us with an excellent opportunity to investigate the accuracy of our knowledge of the chemical condition in this part of Earth’s atmosphere and the properties which control the ionospheric stability under such disturbances. In this paper, using existing knowledge of the lower ionospheric chemical and physical properties we carry out an interpretation of the effects obtained during the total solar eclipse of 22 of July 2009 on the VLF signal. Data obtained from a week long campaign conducted by the Indian Centre for Space Physics (ICSP) over the Indian subcontinent has been used for this purpose. Both positive and negative amplitude changes during the eclipse were observed along various receiver locations. In this paper, data for a propagation path between a Indian Navy VLF transmitter named VTX3 and a pair of receivers in India are used. We start from the observed solar flux during the eclipse and calculate the ionization during the whole time span over most of the influenced region in a range of height. We incorporate a D-region ion-chemistry model to find the equilibrium ion density over the region and employ the LWPC code to find the VLF signal amplitude. To tackle the uncertainty in the values of the recombination coefficients we explore a range of values in the chemical evolution model. We achieve two goals by this exercise: First, we have been able to reproduce the trends, if not the exact signal variation, of the VLF signal modulations during a solar eclipse at two different receiving stations with sufficient accuracy purely from theoretical modeling, and second our knowledge of some of the D-region ion-chemistry parameters is now improved.

9 citations

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06 Oct 2018

446 citations

### "Modeling of VLF network observation..." refers methods in this paper

• ...We modeled the [h′,β ]ecl(lon, lat, t) parameters of 2component exponential Wait’s lower ionospheric model [7] according to the elliptical nature of the penumbral shadow of ASE-2012....

[...]

• ...So, in this case, Wait’s 2-component exponential ionospheric model [7] is capable to represent it, because it is related with ‘Ne’ and ‘νe’....

[...]

Journal ArticleDOI

TL;DR: In this article, the amplitude and phase of four VLF transmitters in the frequency range 16-24 kHz were measured during the total solar eclipse observed in Europe on August 11, 1999.
Abstract: During the total solar eclipse observed in Europe on August 11, 1999, measurements were made of the amplitude and phase of four VLF transmitters in the frequency range 16–24 kHz Five receiver sites were set up, and significant variations in phase and amplitude are reported for 17 paths, more than any previously during an eclipse Distances from transmitter to receiver ranged from 90 to 14,510 km, although the majority were 10,000 km Negative phase changes were observed on most paths, independent of path length Although there was significant variation from path to path, the typical changes observed were ∼3 dB and ∼50° The changes observed were modeled using the Long Wave Propagation Capability waveguide code Maximum eclipse effects occurred when the Wait inverse scale height parameter β was 05 km−1 and the effective ionospheric height parameter H′ was 79 km, compared with β=043 km−1 and H′=71 km for normal daytime conditions The resulting changes in modeled amplitude and phase show good agreement with the majority of the observations The modeling undertaken provides an interpretation of why previous estimates of height change during eclipses have shown such a range of values A D region gas-chemistry model was compared with electron concentration estimates inferred from the observations made during the solar eclipse Quiet-day H′ and β parameters were used to define the initial ionospheric profile The gas-chemistry model was then driven only by eclipse-related solar radiation levels The calculated electron concentration values at 77 km altitude throughout the period of the solar eclipse show good agreement with the values determined from observations at all times, which suggests that a linear variation in electron production rate with solar ionizing radiation is reasonable At times of minimum electron concentration the chemical model predicts that the D region profile would be parameterized by the same β and H′ as the LWPC model values, and rocket profiles, during totality and can be considered a validation of the chemical processes defined within the model

81 citations

### "Modeling of VLF network observation..." refers background in this paper

• ...[3] presented the results of total solar eclipse (TSE) in Europe on 11th Aug 1999....

[...]

• ...Though for a different eclipse, [3] has shown the same ‘-ve’ anomaly for shorter paths like, GBZ-Cambridge, FTA2-Cambridge etc....

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• ...But, it was proved at several occasions of short and long propagation path analysis, that 2-component exponential ionospheric model, which is simplified and at the same time approximated, works well [3]....

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• ...[3] explained them with LWPC analysis....

[...]

Journal ArticleDOI
TL;DR: In this article, the authors used the continuity equation for the electron component and the amplitude time delay to estimate the effective recombination coefficient for the D-region during solar flares, which was found to be in good agreement with measurements by different techniques, as well as with the independent estimates of β and H ′.
Abstract: VLF amplitude enhancements of the NAA/24.0 kHz signal (Maine, USA), registered by the Belgrade AbsPAL facility during solar flares, have been related to solar X-ray fluxes measured by the GOES-12 satellite. The observed regular appearance of the VLF amplitude/phase peak behind the peak of the corresponding X-ray flux has been characterized by the time delay assigned to each of the 97 out of 114 flare-induced distinct amplitude/phase enhancement events analysed. The measurements reported pertain to the period May–August of the years 2004 and 2005 and have been used to model the electron density enhancements of the D-region during solar flares. On the grounds of the continuity equation for the electron component and the amplitude time delay, the effective recombination coefficient has been estimated to range from 10 - 13 m 3 / s to 10 - 11 m 3 / s , depending on the X-ray maximum flux. By integrating the continuity equation, the time variation of the electron density during solar flare occurrence has been restored. Enhancements up to two orders of magnitude, for X-class flares, with respect to undisturbed conditions were predicted. The results obtained have been found to be in good agreement with measurements by different techniques, as well as with the independent estimates of β and H ′ . Errors introduced by the applied approach have been critically examined to place the uncertainty of the results arrived at between 10% and 20%.

76 citations

### "Modeling of VLF network observation..." refers methods in this paper

• ...We choose the daytime ionosphere according to [4, 8, 1], i....

[...]

Journal ArticleDOI
TL;DR: In this paper, a week-long campaign was conducted in the Indian sub-continent to study the low-latitude D-region ionosphere using the very low frequency (VLF) signal from the Indian Navy transmitter (call sign: VTX3) operating at 18.2 kHz.
Abstract: During the total solar eclipse of 2009, a week-long campaign was conducted in the Indian sub-continent to study the low-latitude D-region ionosphere using the very low frequency (VLF) signal from the Indian Navy transmitter (call sign: VTX3) operating at 18.2 kHz. It was observed that in several places, the signal amplitude is enhanced while in other places the amplitude is reduced. We simulated the observational results using the well known Long Wavelength Propagation Capability (LWPC) code. As a first order approximation, the ionospheric parameters were assumed to vary according to the degree of solar obscuration on the way to the receivers. This automatically brought in non-uniformity of the ionospheric parameters along the propagation paths. We find that an assumption of 4 km increase of lower ionospheric height for places going through totality in the propagation path simulate the observations very well at Kathmandu and Raiganj. We find an increase of the height parameter by h ′ = + 3.0 km for the VTX-Malda path and h ′ = + 1.8 km for the VTX-Kolkata path. We also present, as an example, the altitude variation of electron number density throughout the eclipse time at Raiganj.

36 citations

### "Modeling of VLF network observation..." refers background in this paper

• ...According to [6], the first level approximation of ‘linear relationship’ between hecl and βecl , and the obscuration effect works well for VLF-ionosphere interaction modeling....

[...]

01 Jul 2018
TL;DR: In this paper, the effects of the total solar eclipse on the VLF signal were investigated using the knowledge of the lower ionospheric chemical and physical properties, which is not well studied till date.
Abstract: The variation in the solar Extreme Ultraviolet (EUV) radiation flux by any measure is the most dominant natural source to produce perturbations or modulations in the ionospheric chemical and plasma properties. A solar eclipse, though a very rare phenomenon, is similarly bound to produce a significant short time effect on the local ionospheric properties. The influence of the ionizing solar flux reduction during a solar eclipse on the lower ionosphere or, more precisely, the D-region, can be studied with the observation of Very Low Frequency (VLF) radio wave signal modulation. The interpretation of such an effect on VLF signals requires a knowledge of the D-region ion chemistry, which is not well studied till date. Dominant parameters which govern the ion chemistry, such as the recombination coefficients, are poorly known. The occurrence of events such as a solar eclipse provides us with an excellent opportunity to investigate the accuracy of our knowledge of the chemical condition in this part of Earth’s atmosphere and the properties which control the ionospheric stability under such disturbances. In this paper, using existing knowledge of the lower ionospheric chemical and physical properties we carry out an interpretation of the effects obtained during the total solar eclipse of 22 of July 2009 on the VLF signal. Data obtained from a week long campaign conducted by the Indian Centre for Space Physics (ICSP) over the Indian subcontinent has been used for this purpose. Both positive and negative amplitude changes during the eclipse were observed along various receiver locations. In this paper, data for a propagation path between a Indian Navy VLF transmitter named VTX3 and a pair of receivers in India are used. We start from the observed solar flux during the eclipse and calculate the ionization during the whole time span over most of the influenced region in a range of height. We incorporate a D-region ion-chemistry model to find the equilibrium ion density over the region and employ the LWPC code to find the VLF signal amplitude. To tackle the uncertainty in the values of the recombination coefficients we explore a range of values in the chemical evolution model. We achieve two goals by this exercise: First, we have been able to reproduce the trends, if not the exact signal variation, of the VLF signal modulations during a solar eclipse at two different receiving stations with sufficient accuracy purely from theoretical modeling, and second our knowledge of some of the D-region ion-chemistry parameters is now improved.

9 citations