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 article, a brief overview of effects on the ionosphere of upward propagating waves from lower-lying regions is given, separately for the lower ionosphere, for the E-region ionosphere.
Abstract: Meteorological processes in the lower-lying layers, particularly in the troposphere, affect the ionosphere predominantly through the upward propagating waves and their modifications and modulations. Those waves are planetary waves, tidal waves, gravity waves, and almost forgotten infrasonic waves. A part of wave activity can be created in situ at ionospheric heights as primary (e.g., diurnal tide, gravity waves) or secondary waves (e.g., some gravity or planetary waves), but this paper is focused on the upward propagating waves from below the ionosphere. They propagate into the ionosphere mostly directly but the planetary waves can propagate upwards to the F region heights only indirectly, via various potential ways like modulation of the upward propagating tides. The waves may be altered during upward propagation via non-linear interactions, particularly in the MLT region. A brief overview of effects on the ionosphere of upward propagating waves from lower-lying regions is given, separately for the lower ionosphere, for the E-region ionosphere, and for the F-region ionosphere. The upward propagating waves of the neutral atmosphere origin are important both from the point of view of vertical coupling in the atmosphere–ionosphere system, and for applications in radio propagation/telecommunications, as they are responsible for a significant part of uncertainty of the radio wave propagation condition predictions.
TL;DR: In this paper, the amplitude of long-distance subionospheric VLF transmissions were found at night in association with whistlers, and both increases and decreases in signal strength were observed, depending on signal frequency and the receiving antenna.
Abstract: Sudden changes in the amplitude of long-distance subionospheric VLF transmissions were found at night in association with whistlers. Both increases and decreases in signal strength were observed, depending on signal frequency and orieniation of the receiving antenna. Sample observations at Eights Station in Antarctica of station NSS (Annapolis, Maryland) on 22.3 kHz showed increases in signal strength that averaged 3 db, with rise times of about 2 sec and durations of about 30 sec. Coincident with every rise was a midlatitude (L approximately 2.5) whistler originating in the northern hemisphere. To explain this association, it is suggested that the whistler dumps energetic (30-300 keV) electrons into the D region. The resulting ionization then alters the properties of the earthionosphere wave guide. The mechanism of precipitation is thought to be pitch angle scattering of trapped electrons that resonate with the magnetic field of the whistler wave near the magnetic equator. (auth)
TL;DR: A review of developments in ELF and VLF radio-wave propagation research over the last 50 years of the Journal of Atmospheric and Solar-Terrestrial Physics can be found in this paper.
Abstract: This review covers developments in ELF and VLF radio-wave propagation research over the last 50 years of the Journal of Atmospheric and Solar-Terrestrial Physics. A review of such a large field, over such a long period, cannot be fully comprehensive and the authors have therefore covered important areas which have they themselves have found interesting. The survey begins with a review of work on natural and man made sources of ELF and VLF radiation. This is followed by sections on experimental and theoretical studies of unperturbed (ambient) ELF and VLF radio propagation. Schumann resonance research, which is currently undergoing a renaissance, is then reviewed. A review of research into transient perturbations of ELF and VLF propagation follows, extending from the early work on nuclear explosions up to the current work on sprites. The review concludes with a brief summary of the VLF navigation systems of the USSR and USA, (Alpha and Omega) whose development and life-span covered most of the last 50 years.
TL;DR: In this article, the authors analyze ion temperature observations at 100-300 km height obtained by the Millstone Hill incoherent scatter radar (42.6°N, 288.5°E) and conclude that this variation is associated with stratospheric warming.
Abstract:  Sudden stratospheric warming (SSW) is a large-scale meteorological process in the winter hemisphere lasting several days or weeks. The Incoherent Scatter World Day campaign conducted on January 17–February 1, 2008 was arranged during a minor SSW event and focuses on studies of thermospheric and ionospheric response to stratospheric changes. We analyze ion temperature observations at 100–300 km height obtained by the Millstone Hill incoherent scatter radar (42.6°N, 288.5°E). Alternating regions of warming in the lower thermosphere and cooling above 150km altitude were observed by the radar. We use National Center for Environmental Prediction (NCEP) temperature data at 10hPa (∼30km) level and the F10.7 and Ap indices to identify any cause-effect relationship between observed variations in the temperature and stratospheric warming event. We conclude that the seasonal trend, solar flux and geomagnetic activity cannot account for the observed warming and cooling temperature variation and suggest that this variation is associated with stratospheric warming. This study demonstrates a link between the lower atmosphere and the ionosphere which has not been considered before and indicates that ionospheric variability as part of space weather should be considered in conjunction with stratospheric changes.
TL;DR: In this paper, the authors developed a model of sferic propagation which is based on an existing frequency domain subionospheric VLF propagation code and derived the electron density profile that most closely matched an observed sferric spectrum.
Abstract: Lightning discharges radiate the bulk of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands. This energy, contained in impulse-like signals called radio atmospherics or sferics, is guided for long distances by multiple reflections from the ground and lower ionosphere. This suggests that observed sferic waveforms radiated from lightning and received at long distances (>1000 km) from the source stroke contain information about the state of the ionosphere along the propagation path. The focus of this work is on the extraction of nighttime D region electron densities (in the altitude range of ∼70–95 km) from observed VLF sferics. In order to accurately interpret observed sferic characteristics, we develop a model of sferic propagation which is based on an existing frequency domain subionospheric VLF propagation code. The model shows that the spectral characteristics of VLF sferics depend primarily on the propagation path averaged ionospheric D region electron density profile, covering the range of electron densities from ∼100 to 103 cm−3. To infer the D region density from observed VLF sferics, we find the electron density profile that produces a modeled sferic spectrum that most closely matches an observed sferic spectrum. In most nighttime cases the quality of the agreement and the uncertainties involved allow the height of an exponentially varying electron density profile to be inferred with a precision of ∼0.2 km.
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01 Jan 1975