The Experts below are selected from a list of 8880 Experts worldwide ranked by ideXlab platform
U S Inan - One of the best experts on this subject based on the ideXlab platform.
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long lastingd region ionospheric modifications caused by intense lightning in association with elve and sprite pairs
Geophysical Research Letters, 2012Co-Authors: C Haldoupis, U S Inan, M B Cohen, E Arnone, B R T CottsAbstract:[1] Observations show that intense +CG lightning discharges which trigger both an elve and a sprite are associated with long-lasting conductivity modifications in the upperD-region Ionosphere. They are observed as strong perturbations in VLF signals propagating through the disturbed region, manifested asLOng Recovery Early VLF events (LORE), which can last up to 30 minutes. These same ionospheric modifications are also responsible for step-like changes, seen mostly in off-storm VLF transmissions, which offset signal levels even for longer times. The evidence suggests that when a very intense positive cloud to ground lightning stroke leads to an elve and a high altitude sprite, and possibly a sprite halo as well, there is production of long lasting elevations in electron density at VLF reflection heights that cause LOREs and severe effects on VLF propagation. The present results confirm past predictions and postulations that elves may be accompanied by long-lasting electron density perturbations in the Lower Ionosphere.
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modeling vlf propagation in the earth Ionosphere waveguide using the discontinuous galerkin method
URSI General Assembly and Scientific Symposium, 2011Co-Authors: Forrest Foust, T F Bell, U S InanAbstract:Modeling of scattering of very low frequency (VLF, 3–30 kHz) waves from ionospheric disturbances remains a significant computational challenge due to the strong inhomogeneity and anisotropy naturally present at these frequencies. The discontinuous Galerkin (DG) method on unstructured grids can lead to to very efficient formulations in strongly inhomogeneous materials. We discuss a generic method to incorporate any linear, frequency-dependent permittivity or permeability in the nodal DG framework (including PMLs, ferrites, and cold plasmas) and apply the resulting scheme to modeling the VLF scattered field from a density perturbation in the Lower Ionosphere.
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on the generation of elf vlf waves for long distance propagation via steerable hf heating of the Lower Ionosphere
Journal of Geophysical Research, 2010Co-Authors: M B Cohen, U S Inan, Mark Golkowski, N G LehtinenAbstract:[1] ELF/VLF radio waves (300 Hz to 30 kHz) have been successfully generated via modulated HF (3–10 MHz) heating of the Lower Ionosphere in the presence of natural currents, most recently with the HAARP facility in Alaska. Generation is possible via amplitude modulation or via two techniques involving motion of the HF beam during the ELF/VLF cycle, known as beam painting and geometric modulation, described and measured by Cohen et al. (2010b). In this paper, we describe a theoretical model describing the HF heating and ionospheric responses, followed by a full-wave calculation of ELF/VLF propagation, and utilize this end-to-end model to derive the predicted radiated ELF/VLF pattern up to 1000 km from the HF heater in the Earth-Ionosphere waveguide. We quantitatively compare the generated ELF/VLF signals on the ground from various generation techniques and find it to be generally in agreement with earlier measurements. We apply a simplified ELF/VLF propagation model to quantify the contribution of the ELF/VLF phased array in the radiation pattern resulting from geometric modulation and find this contribution to be significant. We also use a limited HF heating model to quantify the degree to which the current power level of HAARP is sufficient for the beam painting technique, since this technique requires high HF power densities at high altitudes.
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elves and associated electron density changes due to cloud to ground and in cloud lightning discharges
Journal of Geophysical Research, 2010Co-Authors: R A Marshall, V. S. Glukhov, U S InanAbstract:[1] A 3-D finite difference time domain model is used to simulate the lightning electromagnetic pulse (EMP) and its interaction with the Lower Ionosphere. Results agree with the frequently observed, doughnut-shaped optical signature of elves but show that the structure exhibits asymmetry due to the presence of Earth's ambient magnetic field. Furthermore, in-cloud (horizontal) lightning channels produce observable optical emissions without the doughnut shape and, in fact, produce a much stronger optical output for the same channel current. Electron density perturbations associated with elves are also calculated, with contributions from attachment and ionization. Results presented as a function of parameters such as magnetic field direction, dipole current orientation, altitude and amplitude, and ambient ionospheric density profile demonstrate the highly nonlinear nature of the EMP-Ionosphere interaction. Ionospheric effects of a sequence of in-cloud discharges are calculated, simulating a burst of in-cloud lightning activity and resulting in large density changes in the overlying Ionosphere.
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geometric modulation a more effective method of steerable elf vlf wave generation with continuous hf heating of the Lower Ionosphere
Geophysical Research Letters, 2008Co-Authors: M B Cohen, U S Inan, Mark GolkowskiAbstract:[1] ELF/VLF radio waves (300 Hz–30 kHz) are difficult to generate with practical antennae, because of their extraordinarily long (10–1000 km) wavelengths, and the lossy nature of the Earth's surface at these frequencies. ELF/VLF waves have been successfully generated via amplitude modulated (AM) HF (2–10 MHz) heating of the Lower Ionosphere. Through the temperature-dependent conductivity of the Lower ionospheric plasma, a patch of the ionospheric current becomes a large radiating ‘antenna’. We implement a new method of ELF/VLF wave generation, herein named ‘geometric modulation’, involving scanning the HF heating beam in a geometric pattern without modulating its power. Utilizing results from the upgraded 3.6 MW radiated HAARP HF antenna array, we show that geometric modulation can enhance ELF/VLF wave generation by up to ∼11 dB over the conventional AM method. Geometric modulation also allows directional launching of the signal into the Earth-Ionosphere waveguide, forming an unprecedented steerable large-element ELF/VLF ionospheric phased array.
Ute Ebert - One of the best experts on this subject based on the ideXlab platform.
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a time scale for electrical screening in pulsed gas discharges
Journal of Physics D, 2014Co-Authors: Jannis Teunissen, Anbang Sun, Ute EbertAbstract:The Maxwell time is a typical time scale for the screening of an electric field in a medium with a given conductivity. We introduce a generalization of the Maxwell time that is valid for gas discharges: the ionization screening time, that takes the growth of the conductivity due to impact ionization into account. We present an analytic estimate for this time scale, assuming a planar geometry, and evaluate its accuracy by comparing with one- and three-dimensional numerical simulations. We investigate the minimum plasma density required to prevent the growth of streamers with local field enhancement, and we discuss the effects of photoionization and electron detachment on ionization screening. Our results can help to understand the development of pulsed discharges, for example nanosecond pulsed discharges at atmospheric pressure or halo discharges in the Lower Ionosphere.
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a time scale for electrical screening in pulsed gas discharges
arXiv: Plasma Physics, 2014Co-Authors: Jannis Teunissen, Anbang Sun, Ute EbertAbstract:The Maxwell time is a typical time scale for the screening of an electric field in a medium with a given conductivity. We introduce a generalization of the Maxwell time that is valid for gas discharges: the \emph{ionization screening time}, that takes the growth of the conductivity due to impact ionization into account. We present an analytic estimate for this time scale, assuming a planar geometry, and evaluate its accuracy by comparing with numerical simulations in 1D and 3D. We investigate the minimum plasma density required to prevent the growth of streamers with local field enhancement, and we discuss the effects of photoionization and electron detachment on ionization screening. Our results can help to understand the development of pulsed discharges, for example nanosecond pulsed discharges at atmospheric pressure or halo discharges in the Lower Ionosphere.
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emergence of sprite streamers from screening ionization waves in the Lower Ionosphere
Nature Geoscience, 2009Co-Authors: Alejandro Luque, Ute EbertAbstract:Sprite discharges above thunderclouds, at altitudes of 40–90 km, are usually created by a strong positive cloud-to-ground lightning flash. A numerical discharge model of the process suggests that sprite streamers are generated through the collapse of a downward-propagating screening-ionization wave in the Lower Ionosphere.
R A Marshall - One of the best experts on this subject based on the ideXlab platform.
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very low frequency subionospheric remote sensing of thunderstorm driven acoustic waves in the Lower Ionosphere
Journal of Geophysical Research, 2014Co-Authors: R A Marshall, J B SnivelyAbstract:We present observations of narrowband subionospheric VLF transmitter signals on 20 March 2001, exhibiting coherent fluctuations of over 1 dB peak to peak. Spectral analysis shows that the fluctuations have periods of 1-4 min and are largely coherent. The subionospheric propagation path of the signal from Puerto Rico to Colorado passes over two regions of convective and lightning activity, as observed by GOES satellite imagery and National Lightning Detection Network lightning data. We suggest that these fluctuations are evidence of acoustic waves launched by the convective activity below, observed in the 80-90 km altitude range to which nighttime VLF subionospheric remote sensing is sensitive. These observations show that VLF subionospheric remote sensing may provide a unique, 24 h remote sensing technique for acoustic and gravity wave activity. We reproduce this event in simulations using a fluid model of gravity and acoustic wave propagation to calculate the ionospheric disturbance, followed by an electromagnetic propagation model to calculate the perturbation amplitude at the location of the VLF receiver. Simulation results show that a very large and coherent convective source is required to produce these amplitude perturbations.
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very low frequency subionospheric remote sensing of thunderstorm driven acoustic waves in the Lower Ionosphere
Journal of Geophysical Research, 2014Co-Authors: R A Marshall, J B SnivelyAbstract:We present observations of narrowband subionospheric VLF transmitter signals on 20 March 2001, exhibiting coherent fluctuations of over 1 dB peak to peak. Spectral analysis shows that the fluctuations have periods of 1-4 min and are largely coherent. The subionospheric propagation path of the signal from Puerto Rico to Colorado passes over two regions of convective and lightning activity, as observed by GOES satellite imagery and National Lightning Detection Network lightning data. We suggest that these fluctuations are evidence of acoustic waves launched by the convective activity below, observed in the 80-90 km altitude range to which nighttime VLF subionospheric remote sensing is sensitive. These observations show that VLF subionospheric remote sensing may provide a unique, 24 h remote sensing technique for acoustic and gravity wave activity. We reproduce this event in simulations using a fluid model of gravity and acoustic wave propagation to calculate the ionospheric disturbance, followed by an electromagnetic propagation model to calculate the perturbation amplitude at the location of the VLF receiver. Simulation results show that a very large and coherent convective source is required to produce these amplitude perturbations.
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an improved model of the lightning electromagnetic field interaction with the d region Ionosphere
Journal of Geophysical Research, 2012Co-Authors: R A MarshallAbstract:[1] We present an improved time-domain model of the lightning electromagnetic pulse (EMP) interaction with the Lower Ionosphere. This improved model inherently accounts for the Earth's curvature, includes an arbitrary number of ion species, and uses a convolutional Perfectly Matched Layer (PML) boundary. We apply an improved model of electron heating due to the lightning EMP and electrostatic fields, and we include ionization, attachment, and detachment. In addition to modeling lightning, this model can be used for long-distance VLF wave propagation in the Earth-Ionosphere waveguide, heating of the Lower Ionosphere by VLF transmitters, and heating in the F-region Ionosphere by lightning. In this paper we present three initial results of this model. First, we compare results of ionospheric heating and electron density disturbances with and without electron detachment taken into account. We find that detachment is important only for the QE effects on time scales longer than 1 ms. Second, we find a simple explanation for the recently-reported “elve doublets”, which we find are an effect of the rise and fall times of the lightning waveform. In particular, we find that all elves are doublets, and the rise and fall times of the current pulse control the brightness and separation in time of the two successive halves of the elve. Third, we find a similar simple explanation for “ring” sprites, whole columns appear in a circle symmetric around the discharge axis. We find that ring sprites can be initiated for particular current waveforms, where the QE and EMP fields in the mesosphere produce a maximum reduced field away from the discharge axis.
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elves and associated electron density changes due to cloud to ground and in cloud lightning discharges
Journal of Geophysical Research, 2010Co-Authors: R A Marshall, V. S. Glukhov, U S InanAbstract:[1] A 3-D finite difference time domain model is used to simulate the lightning electromagnetic pulse (EMP) and its interaction with the Lower Ionosphere. Results agree with the frequently observed, doughnut-shaped optical signature of elves but show that the structure exhibits asymmetry due to the presence of Earth's ambient magnetic field. Furthermore, in-cloud (horizontal) lightning channels produce observable optical emissions without the doughnut shape and, in fact, produce a much stronger optical output for the same channel current. Electron density perturbations associated with elves are also calculated, with contributions from attachment and ionization. Results presented as a function of parameters such as magnetic field direction, dipole current orientation, altitude and amplitude, and ambient ionospheric density profile demonstrate the highly nonlinear nature of the EMP-Ionosphere interaction. Ionospheric effects of a sequence of in-cloud discharges are calculated, simulating a burst of in-cloud lightning activity and resulting in large density changes in the overlying Ionosphere.
M B Cohen - One of the best experts on this subject based on the ideXlab platform.
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the vlf fingerprint of elves step like and long recovery early vlf perturbations caused by powerful cg lightning em pulses
Journal of Geophysical Research, 2013Co-Authors: C Haldoupis, M B Cohen, E Arnone, B R T Cotts, Stefano DietrichAbstract:[1] Subionospheric VLF recordings are investigated in relation with intense cloud-to-ground (CG) lightning data. Lightning impacts the Lower Ionosphere via heating and ionization changes which produce VLF signal perturbations known as early VLF events. Typically, early events recover in about 100 s, but a small subclass does not recover for many minutes, known as long-recovery early events (LORE). In this study, we identify LORE as a distinct category of early VLF events, whose signature may occur either on its own or alongside the short-lived typical early VLF event. Since LORE onsets coincide with powerful lightning strokes of either polarity (±), we infer that they are due to long-lasting ionization changes in the uppermost D region Ionosphere caused by electromagnetic pulses emitted by strong ± CG lightning peak currents of typically > 250 kA, which are also known to generate elves. The LORE perturbations are detected when the discharge is located within ~250 km from the great circle path of a VLF transmitter-receiver link. The probability of occurrence increases with stroke intensity and approaches unity for discharges with peak currents ≥ ~300 kA. LOREs are nighttime phenomena that occur preferentially, at least in the present regional data set, during winter when strong ± CG discharges are more frequent and intense. The evidence suggests LORE as a distinct signature representing the VLF fingerprint of elves, a fact which, although was predicted by theory, it escaped identification in the long-going VLF research of lightning effects in the Lower Ionosphere.
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long lastingd region ionospheric modifications caused by intense lightning in association with elve and sprite pairs
Geophysical Research Letters, 2012Co-Authors: C Haldoupis, U S Inan, M B Cohen, E Arnone, B R T CottsAbstract:[1] Observations show that intense +CG lightning discharges which trigger both an elve and a sprite are associated with long-lasting conductivity modifications in the upperD-region Ionosphere. They are observed as strong perturbations in VLF signals propagating through the disturbed region, manifested asLOng Recovery Early VLF events (LORE), which can last up to 30 minutes. These same ionospheric modifications are also responsible for step-like changes, seen mostly in off-storm VLF transmissions, which offset signal levels even for longer times. The evidence suggests that when a very intense positive cloud to ground lightning stroke leads to an elve and a high altitude sprite, and possibly a sprite halo as well, there is production of long lasting elevations in electron density at VLF reflection heights that cause LOREs and severe effects on VLF propagation. The present results confirm past predictions and postulations that elves may be accompanied by long-lasting electron density perturbations in the Lower Ionosphere.
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on the generation of elf vlf waves for long distance propagation via steerable hf heating of the Lower Ionosphere
Journal of Geophysical Research, 2010Co-Authors: M B Cohen, U S Inan, Mark Golkowski, N G LehtinenAbstract:[1] ELF/VLF radio waves (300 Hz to 30 kHz) have been successfully generated via modulated HF (3–10 MHz) heating of the Lower Ionosphere in the presence of natural currents, most recently with the HAARP facility in Alaska. Generation is possible via amplitude modulation or via two techniques involving motion of the HF beam during the ELF/VLF cycle, known as beam painting and geometric modulation, described and measured by Cohen et al. (2010b). In this paper, we describe a theoretical model describing the HF heating and ionospheric responses, followed by a full-wave calculation of ELF/VLF propagation, and utilize this end-to-end model to derive the predicted radiated ELF/VLF pattern up to 1000 km from the HF heater in the Earth-Ionosphere waveguide. We quantitatively compare the generated ELF/VLF signals on the ground from various generation techniques and find it to be generally in agreement with earlier measurements. We apply a simplified ELF/VLF propagation model to quantify the contribution of the ELF/VLF phased array in the radiation pattern resulting from geometric modulation and find this contribution to be significant. We also use a limited HF heating model to quantify the degree to which the current power level of HAARP is sufficient for the beam painting technique, since this technique requires high HF power densities at high altitudes.
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geometric modulation a more effective method of steerable elf vlf wave generation with continuous hf heating of the Lower Ionosphere
Geophysical Research Letters, 2008Co-Authors: M B Cohen, U S Inan, Mark GolkowskiAbstract:[1] ELF/VLF radio waves (300 Hz–30 kHz) are difficult to generate with practical antennae, because of their extraordinarily long (10–1000 km) wavelengths, and the lossy nature of the Earth's surface at these frequencies. ELF/VLF waves have been successfully generated via amplitude modulated (AM) HF (2–10 MHz) heating of the Lower Ionosphere. Through the temperature-dependent conductivity of the Lower ionospheric plasma, a patch of the ionospheric current becomes a large radiating ‘antenna’. We implement a new method of ELF/VLF wave generation, herein named ‘geometric modulation’, involving scanning the HF heating beam in a geometric pattern without modulating its power. Utilizing results from the upgraded 3.6 MW radiated HAARP HF antenna array, we show that geometric modulation can enhance ELF/VLF wave generation by up to ∼11 dB over the conventional AM method. Geometric modulation also allows directional launching of the signal into the Earth-Ionosphere waveguide, forming an unprecedented steerable large-element ELF/VLF ionospheric phased array.
Sandip K Chakrabarti - One of the best experts on this subject based on the ideXlab platform.
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Theoretical study of Lower ionospheric response to solar flares: sluggishness of D-region and peak time delay
Astrophysics and Space Science, 2015Co-Authors: Sourav Palit, Tamal Basak, Sujay Pal, Sandip K ChakrabartiAbstract:The rates of ion production and loss processes in the Lower Ionosphere during solar and other astronomical ionizing events vary with height. This variations influence the time lags of the response in different ionospheric layers. Very Low Frequency (VLF) signals reflected from any of these layers follow this time lag or delay during a transient cosmic events. One of the easiest ways to study this property is to observe the shift in the peak of VLF signal amplitude with respect to the peak of solar flares. We numerically model to find ion densities and resulting VLF signal perturbations during some solar flares. We clearly find from the model that the delay in the peak of the electron densities (with respect to peak of the ionizing event) in the Lower Ionosphere varies from height to height. The result also depends on the properties of events, such as peak intensity and sharpness. We investigate analytically how the delay of electron density peak should depend on height varying chemical rate parameters as well as the nature of transient events. Our capability is demonstrated using three classes (namely, X, M and C) of solar flares. The work is a step forward in our goal to employ Ionosphere as a natural detector for astronomical observations.
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on the use of very low frequency transmitter data for remote sensing of atmospheric gravity and planetary waves
Advances in Space Research, 2015Co-Authors: Sujay Pal, Sandip K Chakrabarti, Suman ChakrabortyAbstract:Abstract Continuous ground-based monitoring of Very Low Frequency (VLF) transmitter signals is an efficient remote sensing tool for studying of the Lower Ionosphere (60–90 km). Here, we present the use of VLF radio data to study short-period (∼min–hrs) atmospheric gravity waves and long-period (∼days) planetary waves. We analyse VLF data from several receiving stations obtained by ICSP-VLF network during the total solar eclipse of July, 2009 to show the existence of short-period atmospheric gravity waves. We find dominant wave periods range from 10 min to 1 h around the time of maximum eclipse phase which could be associated with atmospheric gravity waves excited due to the eclipse. We also analyse VLF amplitude data of 2007 received at ICSP, Kolkata from VTX (18.2 kHz) transmitter for planetary wave-type oscillations in the mesosphere–Lower Ionosphere system. Fourier and wavelet analysis show presence of periodic structures with periodicity in the range of 5–27 days. We compare VLF planetary spectrum with spectrum obtained from total column density of Ozone and mesospheric average temperature data which may indicate vertical coupling between the stratosphere and Ionosphere in winter to early spring time.
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modeling of very low frequency vlf radio wave signal profile due to solar flares using the geant4 monte carlo simulation coupled with ionospheric chemistry
Atmospheric Chemistry and Physics, 2013Co-Authors: Sourav Palit, Tamal Basak, Sandip K Chakrabarti, S K MondalAbstract:Abstract. X-ray photons emitted during solar flares cause ionization in the Lower Ionosphere (~60 to 100 km) in excess of what is expected to occur due to a quiet sun. Very low frequency (VLF) radio wave signals reflected from the D-region of the Ionosphere are affected by this excess ionization. In this paper, we reproduce the deviation in VLF signal strength during solar flares by numerical modeling. We use GEANT4 Monte Carlo simulation code to compute the rate of ionization due to a M-class flare and a X-class flare. The output of the simulation is then used in a simplified ionospheric chemistry model to calculate the time variation of electron density at different altitudes in the D-region of the Ionosphere. The resulting electron density variation profile is then self-consistently used in the LWPC code to obtain the time variation of the change in VLF signal. We did the modeling of the VLF signal along the NWC (Australia) to IERC/ICSP (India) propagation path and compared the results with observations. The agreement is found to be very satisfactory.
