Remote sensing of the ignorosphere: Need for a complete earth-ionosphere radio wave propagation model
01 Jan 2018-Vol. 53, pp 527-543
TL;DR: In this article, a short review on retrieval mechanism of the D-region ionospheric plasma using sub-ionospheric VLF/LF data is presented, where the authors discuss importance of VLFs/LFs observation techniques and significant earth-ionosphere propagation models to diagnose electron-ion distribution in the lower ionosphere.
Abstract: We present a short review on retrieval mechanism of the D-region ionospheric plasma using sub-ionospheric VLF/LF data. First, we discuss importance of VLF/LF observation techniques and significant earth-ionosphere propagation models to diagnose electron-ion distribution in the lower ionosphere. Then we discuss about VLF/LF perturbations due to different geophysical phenomena and corresponding numerical simulations applied to retrieve the state of the D-region ionosphere.
Citations
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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.
14 citations
TL;DR: In this article, the D-region ionospheric disturbances due to the tropical cyclone Fani over the Indian Ocean have been analyzed 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).
Abstract: 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-minute 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 3 σ 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.
7 citations
01 Dec 2006
TL;DR: In this article, a new class of early/fast VLF events with recoveries of up to 20 min was introduced, much longer than typical Early/fast and Lightning-induced Electron Precipitation (LEP) events which recover to pre-event levels in ≲200 s.
Abstract: [1] We introduce a new class of Early/fast VLF events with recoveries of up to 20 min, much longer than typical Early/fast and Lightning-induced Electron Precipitation (LEP) events which recover to pre-event levels in ≲200 s. Three distinct types of long recovery events are observed, each exhibiting different characteristics, with the observed features of at least some of the event types consistent with the possibility of persistent ionization at altitudes below 60 km as put forth by Lehtinen and Inan (2007).
5 citations
TL;DR: In this article, the authors reported disturbance in the mid-latitude sub-ionospheric VLF radio signals due to the super geomagnetic storm which began on 17 March 2015.
Abstract: This paper reports disturbance in the mid-latitude sub-ionospheric VLF radio signals due to the super geomagnetic storm which began on 17 March 2015. Narrow-band signals from the NAA transmitter are studied for the storm period recorded at eight mid-latitude receiving stations spread over the Europe and USA. Daytime signals amplitude at all places showed a disturbing pattern after 17 March. Fluctuation in the nighttime signals significantly increased in the succeeding nights. As a primary effect of the storm, the entire diurnal signals in the transoceanic west to east long propagation paths enhanced by 3–5 dB, which gradually decreased over the period of ~ 10 days following the storm recovery. A different behavior was observed in the east to west short propagation paths over the landmass, where during the peak storm the daily variations of the VLF amplitude reduced to 20–25% of a normal day and, after ~ 10 days the signals returned to the pre-storm condition. Modeling of the radio waves in the west to east paths shows that the D-region electron density was increased by ~ 8-fold and varied up to 10 days. Electron density variations in the D-region closely follows the variations of precipitated electron flux as observed by the POES satellite over the region. The elevated electron density in the D-region ionosphere caused by the extension of the auroral precipitation to the mid-latitudes along with interference among the various waveguide modes in the earth-ionosphere waveguide during the storm is suggested for the cause of observed VLF signals behaviors.
3 citations
References
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TL;DR: In this paper, the authors examined the quality and propagation characteristics of the Very Low Frequency (VLF) radio waves in a very long propagation path, Indian Centre for Space Physics, Kolkata, participated in the 27th Indian scientific expedition to Antarctica during 2007-2008.
Abstract: To examine the quality and propagation characteristics of the Very Low Frequency (VLF) radio waves in a very long propagation path, Indian Centre for Space Physics, Kolkata, participated in the 27th Indian scientific expedition to Antarctica during 2007–2008. One Stanford University made AWESOME VLF receiving system was installed at the Indian Antarctic station Maitri and about five weeks of data were recorded successfully from the Indian transmitter VTX and several other transmitting stations worldwide. The quality of the signal from the VTX transmitter was found to be very good, consistent and highly stable in day and night. The signal shows the evidences of the presence of the 24 h solar radiation in the Antarctic region during local summer. Here we report the both narrow band and broadband VLF observations from this site. The diurnal variations of VTX signal (18.2 kHz) are presented systematically for Antarctica path and also compared the same with the variations for a short propagation path (VTX-Kolkata). We compute the spatial distribution of the VTX signal along the VTX-Antarctica path using the most well-known LWPC model for an all-day and all-night propagation conditions. The calculated signal amplitudes corresponding to those conditions relatively corroborate the observations. We also present the attenuation rate of the dominant waveguide modes corresponding to those propagation conditions where the effects of the Antarctic polar ice on the attenuation of different propagating waveguide modes are visible.
11 citations
TL;DR: In this paper, the authors investigated land-sea propagation at VLF frequencies for an Earth-ionosphere waveguide bounded above by an anisotropic, horizontally stratified ionosphere.
Abstract: Land-sea propagation at VLF frequencies is investigated for an Earth-ionosphere waveguide which is bounded above by an anisotropic, horizontally stratified ionosphere. Exact boundary conditions are used but lateral waves are neglected. The calculation of mode reflection and transmission coefficients is discussed and Kirchhoff's approximation used to obtain explicit formulae for the transmission coefficients. Some numerical results are given which illustrate the dependence of the transmission coefficients on the type of mode being scattered and on other parameters.
11 citations
TL;DR: In this article, the effects of the occultation of a solar flare was observed through lunar occultation and that too during a partial solar eclipse was quantitatively compared by using analogies with previous observations and found best fitting parameters for the time when the flare was occulted.
Abstract: Very Low Frequency (VLF) radio waves propagate through the Earth-ionosphere waveguide. Irregularities caused by excess or deficient extreme ultra-violet and X-rays, which otherwise sustain the ionosphere, change the waveguide properties and hence the signals are modified. We report the results of monitoring of the NWC transmitter (19.8 kHz) by a receiver placed at Khukurdaha (22°27′N, 87°45′E) during the partial solar eclipse (75 %) of 15th January, 2010. The propagation path from the transmitter to the receiver crosses the annular eclipse belt. We got a clear depression in the data during the period of the eclipse. Most interestingly, there was also a X-ray flaring activity in the sun on that day which reached its peak (C-type) right after the time when the eclipse reached its maximum. We saw the effects of the occultation of this flare in our VLF signal since a part of the X-ray active region was clearly blocked by the moon. We quantitatively compared by using analogies with previous observations and found best fitting parameters for the time when the flare was occulted. We then reconstructed the VLF signal in the absence of the occulted flare. To our knowledge, this is the first such incident where the solar flare was observed through lunar occultation and that too during a partial eclipse.
10 citations
TL;DR: In this article, 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.
10 citations
10 Jul 2000
TL;DR: In this article, a survey of the VLF-LF spectrum evolution during the 11 August 1999 solar eclipse in the vicinity of the totality region was conducted, which consisted of amplitude spectrum acquisitions in the frequency band 10-80 kHz with a repetition period of 1 min.
Abstract: A survey of the VLF-LF spectrum evolution has been conducted during the 11 August 1999 solar eclipse in the vicinity of the totality region. The measurements consisted of amplitude spectrum acquisitions in the frequency band 10-80 kHz with a repetition period of 1 min. The field strength received from various transmitters could thus be monitored both during the eclipse and control days. The time signatures of the eclipse appear to be highly variable according to the frequency and radio path characteristics. The main features of the eclipse signatures are interpreted by using a waveguide propagation model.
10 citations