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: A 12-hour sequence of perturbations of subionospheric VLF signals observed in association with lightning provided preliminary evidence that the ionospheric regions perturbed in these events may be confined to within ∼150 km of the lightning discharges, and that intracloud flashes as well as cloud-to-ground lightning may be important in producing the perturbation.
Abstract: A 12-hour sequence of perturbations of subionospheric VLF signals observed in association with lightning provided preliminary evidence that the ionospheric regions perturbed in these events may be confined to within ∼150 km of the lightning discharges, and that intracloud flashes as well as cloud-to-ground lightning may be important in producing the perturbations. High-resolution analysis of event signatures indicated the presence of two different classes of events. For one set of events, observed during the most active central 6 hours of the observation period, a ∼0.6-s delay between the causative lightning and VLF event onset and a ∼1-s onset duration was observed, consistent with previously suggested models of the gyroresonant whistler-particle interaction that leads to particle precipitation and perturbation of the Earth-ionosphere waveguide. However, another set of events, observed during the first 2 hours of the observation period, exhibited a very different temporal signature, characterized by a much smaller (<50 ms) delay and sometimes also very short (<50 ms) rise times. Such events are possibly related to previously reported cases of similarly early/fast events and may involve a more direct coupling between the lightning discharge and the lower ionosphere.
TL;DR: In this article, the electron densities of the D-region were determined from observations of VLF subionospheric amplitude changes and these enhancements were then related to the magnitudes of the X-ray fluxes measured by the GOES satellites.
Abstract: Enhancements of D-region electron densities caused by solar flares are determined from observations of VLF subionospheric amplitude changes and these enhancements are then related to the magnitudes of the X-ray fluxes measured by the GOES satellites. The electron densities are characterised by the two traditional parameters, H′ and β (being measures of the ionospheric height and the rate of increase of electron density with height, respectively), which are found by VLF radio modelling of the observed amplitudes using the NOSC Earth-ionosphere waveguide programs (LWPC and Modefinder) mainly on two paths, one short and one long. The short path measurements were made near Cambridge, UK, on the 18.3 kHz signals from the French transmitter 617 km to the south while the long path measurements were made near Dunedin, NZ, on the 24.8 kHz signals from NLK in Seattle, USA, 12.3 Mm across the Pacific Ocean. The observations include flares up to a magnitude of about M5 (5×10 −5 W m −2 at 0.1– 0.8 nm ) which gave VLF amplitude enhancements up to about 8 dB ; these corresponded, under near solar maximum conditions (1992), to a reduction in H′ from about 71 km down to about 63 km and an increase in β from 0.43 km −1 up to about 0.49 km −1 . The increased values of β during a flare are caused by the solar X-rays dominating all sources of ionisation during the flare in contrast with the normal unperturbed daytime values of β which are significantly lower than for a single solar UV or X-ray source due to the extra electrons from the normal galactic cosmic ray ionisation in the lowest parts of the D-region. This steady, normal (unperturbed) cosmic ray influence on β, and hence unperturbed VLF attenuation, is more marked at times of reduced solar Lyman-α flux in the D-region such as at solar minimum, high latitudes or early or late in the day, thus explaining the normal (unperturbed) higher VLF attenuation rates previously reported in these conditions.
TL;DR: In this paper, the physical processes leading to sprites also alter the conductivity of the lower ionosphere, and the data constitutes the first evidence that the physical process leading to the sprites also alters the conductivities of the ionosphere.
Abstract: VLF perturbations on signals propagating along great-circle-paths (GCP) through electrically active midwest thunderstorms are associated with luminous high altitude glows (referred to as sprites) observed from aircraft or ground. The data constitutes the first evidence that the physical processes leading to sprites also alter the conductivity of the lower ionosphere.
TL;DR: In this article, a quantitative model of the relaxation of transient lower ionospheric (D region) disturbances caused by lightning-induced electron precipitation is developed, taking advantage of known particular features of the lightning induced disturbances, such as the fact that they are produced in typically <1 s and decay over 10-100 s.
Abstract: A quantitative model of the relaxation of transient lower ionospheric (D region) disturbances caused by lightning-induced electron precipitation is developed, taking advantage of known particular features of the lightning-induced disturbances, such as the fact that they are produced in typically <1 s and decay over 10–100 s. The model represents the nighttime D region as consisting of only four kinds of charged particles (electrons, positive ions, negative ions, and positive cluster ions) and is particularly suited for description of the detailed behavior of the electron density. Application of the model to some previously modeled disturbances indicates that some of the least known chemical reaction rates in the nighttime D region altitudes may be measurable using subionospheric VLF data. In the production of secondary ionization by precipitating electron bursts, the model calculations indicate the presence of a saturation effect such that the number density of the secondary electrons is not simply equal to the ion pair production rate times the burst duration. In some cases involving precipitation of ∼1-MeV electrons, the model predicts the formation of new layers of ionization at 50–70 km altitude that represent a different attachment-detachment quasi-equilibrium value from that of the unperturbed ambient. Such new layers may exist for up to ∼105 s following electron precipitation bursts.
TL;DR: In this article, the Trimpi effect was used to detect lightening-induced electron precipitation (LEP) events using a single waveguide mode theory, in which the precipitation-induced ionization enhancements in the lower ionosphere (D region) give rise to rapid perturbations of subionospheric VLF signals.
Abstract: Lightning-induced electron precipitation (LEP) events are studied using the Trimpi effect, in which the precipitation-induced ionization enhancements in the lower ionosphere (D region) give rise to rapid perturbations of subionospheric VLF signals. In 1983,the phase and amplitude of signals from the NPM transmitter in Hawaii (23.4 kHz) and the Omega transmitter in Argentina (12.9 kHz) were measured at Palmer, Antarctica (L-2.4), together with the magnetospheric whistler background. The long baseline and over-sea great circle paths from these two sources make it possible for the observed perturbations to be interpreted using a single waveguide mode theory. Analytical expressions are used to relate the magnitude of the phase perturbations to differential changes in ionospheric reflection height along a segment of the propagation path. The predicted relationship between relative perturbation sizes on the two different signals is compared with measurements. From this information, the whistler-induced flux levels are inferred to be in the 10/sup -4/--10/sup -2/ erg cm/sup -2/ s/sup -1/ range and the precipitation regions are inferred to be roughly ''circular'' in shape, rather than elongated along L shells. Measured amplitude changes tended to be small (--0.5 dB) and negative, as expected from a single-mode theory, but the ratios of simultaneous amplitudemore » and phase perturbations were slightly larger than the theory predicts, probably due to the effects of an additional mode(s). An assessment of the relative detectability of amplitude versus phase perturbations favors phase perturbations by -- 10 dB, irrespective of the detection scheme used. copyright American Geophysical Union 1987« less
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