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.
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.
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.
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:  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).
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.
TL;DR: In this paper, the effects of X-ray solar flares on very low frequency (VLF) wave propagation along the Earth-ionosphere waveguide are detected from an observed VLF signal phase and amplitude perturbations, taking place during X-Ray solar flares.
Abstract: . The classification of X-ray solar flares is performed regarding their effects on the Very Low Frequency (VLF) wave propagation along the Earth-ionosphere waveguide. The changes in propagation are detected from an observed VLF signal phase and amplitude perturbations, taking place during X-ray solar flares. All flare effects chosen for the analysis are recorded by the Absolute Phase and Amplitude Logger (AbsPal), during the summer months of 2004–2007, on the single trace, Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E) with a distance along the Great Circle Path (GCP) D≈2000 km in length. The observed VLF amplitude and phase perturbations are simulated by the computer program Long-Wavelength Propagation Capability (LWPC), using Wait's model of the lower ionosphere, as determined by two parameters: the sharpness (β in 1/km) and reflection height (H' in km). By varying the values of β and H' so as to match the observed amplitude and phase perturbations, the variation of the D-region electron density height profile Ne(z) was reconstructed, throughout flare duration. The procedure is illustrated as applied to a series of flares, from class C to M5 (5×10−5 W/m2 at 0.1–0.8 nm), each giving rise to a different time development of signal perturbation. The corresponding change in electron density from the unperturbed value at the unperturbed reflection height, i.e. Ne(74 km)=2.16×108 m−3 to the value induced by an M5 class flare, up to Ne(74 km)=4×1010 m−3 is obtained. The β parameter is found to range from 0.30–0.49 1/km and the reflection height H' to vary from 74–63 km. The changes in Ne(z) during the flares, within height range z=60 to 90 km are determined, as well.
TL;DR: In the course of an investigation designed to explore different aspects of a possible connection between tropospheric and ionospheric changes as indicated by a previous study, an interesting effect was noted as mentioned in this paper.
Abstract: In the course of an investigation designed to explore different aspects of a possible connection between tropospheric and ionospheric changes as indicated by a previous study,1 an interesting effect was noted. During 1954 and 1955, four hurricanes—Hazel (1954), and Connie, Diane, and lone (1955)—came close to Washington, D.C., for which meteorological and geophysical data were analyzed. Figure 1 shows the smoothed paths of these hurricanes according to the U.S. Weather Bureau Hurricane Tracking Chart.2
TL;DR: In this article, the authors explore the dynamical coupling of the stratosphere and mesosphere during the 2002 Southern Hemisphere major stratospheric sudden warming and suggest the possibility of feedback interactions between the planetary wave forcing and the mesospheric/stratospheric mean state changes.
Abstract:  NCEP data and a NCAR TIME-GCM simulation are used to explore the dynamical coupling of the stratosphere and mesosphere during the 2002 Southern Hemisphere major stratospheric sudden warming. The analyses suggest the possibility of feedback interactions between the planetary wave forcing and the mesospheric/stratospheric mean state changes. Multiple strong planetary waves before the warming penetrate into the mesosphere and weaken the polar jet. They alter the wave transmission condition in favor of more upward-poleward propagation of the wave energy at progressively lower altitudes. The jet reversal and the planetary wave surf zone also descend from the mesosphere down to the stratosphere, making wave breaking more likely at decreasing altitudes with each wave episode. These changes in the wave transmission and breaking conditions in the mesosphere and stratosphere may be critical for the extensive major stratospheric warming.
TL;DR: In this article, a computer program based on formalism developed by Budden is used to evaluate the adequacy of some approximations that have been used in VLF waveguide propagation studies.
Abstract: A computer program based on formalism developed by Budden will be used to evaluate the adequacy of some approximations that have been used in VLF waveguide propagation studies. The program makes allowance for the inhomogeneity and anisotropy of the lowest part of the ionosphere, allows for earth curvature, and may be used for arbitrary direction of propagation at any latitude. The results of this program will be compared with calculations of Wait and Spies who used a model ionosphere consisting of an exponential variation for both electron density and collision frequency. This comparison will allow us to demonstrate a close numerical equivalence between the modified refractive index formulation and the spherical coordinate formulation as implemented by Wait and Spies. Also, it will be shown that an approximation introduced by Wait and Spies during the course of applying the effective surface impedance concept, though quite valid for first-order mode calculations, is inadequate for daytime second-order mode calculations.
TL;DR: In this article, a new model of the physical processes associated with subionospheric VLF signal perturbations caused by lightning-induced electron precipitation (LEP) bursts is developed to diagnose the state of the lower ionosphere (e.g., electron number density and rate coefficients for various chemical reactions) on the basis of measurements of VlF recovery signatures.
Abstract: A new model of the physical processes associated with subionospheric VLF signal perturbations caused by lightning-induced electron precipitation (LEP) bursts is developed to diagnose the state of the lower ionosphere (e.g., electron number density and rate coefficients for various chemical reactions) on the basis of measurements of VLF recovery signatures. The model accounts for the energy spectrum of the electron bursts precipitated by lightning-generated whistlers, the chemical relaxation of enhanced secondary ionization in the nighttime D region due to LEP bursts, and quantitatively treats the resultant effects on propagation of the VLF signal in the Earth-ionosphere waveguide. Application of the model to experimental data obtained for the VLF propagation path from NPM station (Hawaii) to Palmer station (Antarctica) indicates that effective electron detachment rate γ, enhanced secondary ionization profile (e.g., energy content of LEP bursts), as well as the ambient electron density distribution, may be estimated using observed subionospheric VLF recovery signatures. The effective detachment rate was identified as ∼10−18 N s−1, where N is total number density of neutrals. Model indicates in particular that the attachment-detachment processes play the dominant role in recovery of subionospheric VLF signal perturbations on timescales ∼ 100 s, and that the observed perturbations of the NPM-Palmer signal correspond to the LEP bursts consisting of relatively soft (< 250 keV) electrons.
Related Papers (5)
VLF帯大地-電離圏導波管伝搬を用いた電子密度同定問題における適切なパラメタ選択に関する検討;VLF帯大地-電離圏導波管伝搬を用いた電子密度同定問題における適切なパラメタ選択に関する検討;A Consideration on Appropriate Parameters for Identifying Electron Density Profile in the Lower Ionosphere by using VLF Wave Propagation in the Earth-ionosphere Waveguide
01 Jan 1975