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We investigate the attenuation of whistler waves generated by hypotetical Venusian lightning occurring at the altitude of the cloud layer under different ionospheric conditions. We use the Stanford full-wave method for stratified media of Lehtinen and Inan (2008) to model wave propagation through the ionosphere of Venus. This method calculates the electromagnetic field created by an arbitrary source in a plane-stratified medium (i.e., uniform in the horizontal direction). We see that the existence of holes in electronic densities and the magnetic field configuration caused by solar wind play an important role in the propagation of electromagnetic waves through the Venusian ionosphere. ©2017. American Geophysical Union. All Rights Reserved.

Electrostatic fields generated by thunderclouds can significantly heat and modify the lower ionospheric electrons at altitudes of 70-80km. These fields can map to higher altitudes along the geomagnetic field lines and have been proposed as the mechanism for generation of whistler ducts. Previous 2-D modeling of these fields have been limited to azimuthally symmetric cases which requires a vertical magnetic field. We have developed a 3-D model of the electrostatic thundercloud fields which allows the consideration of effects of the geomagnetic field dip angle on the mapping of the fields to high altitudes. The results show stronger electric fields at altitudes of 70-110km with an equatorward and eastward shift of tens of kilometers at lower geomagnetic latitudes. These stronger fields are mapped into the magnetosphere and may therefore be important for whistler duct generation. The fields also indicate a more significant contribution of the quiescent heating on VLF early/fast events., Defense Advanced Research Projects Agency

Approximately 90% of natural lightning occurs over land, but recent observations, using Global Lightning Detection (GLD360) geolocation peak current estimates and satellite optical data, suggested that cloud-to-ground flashes are on average stronger over the ocean. We present initial statistics from a novel experiment using a Low Frequency (LF) magnetic field receiver system installed aboard the National Oceanic Atmospheric Agency (NOAA) Ronald W. Brown research vessel that allowed the detection of impulsive radio emissions from deep-oceanic discharges at short distances. Thousands of LF waveforms were recorded, facilitating the comparison of oceanic waveforms to their land counterparts. A computationally efficient electromagnetic radiation model that accounts for propagation over lossy and curved ground is constructed and compared with previously published models. We include the effects of Earth curvature on LF ground wave propagation and quantify the effects of channel-base current risetime, channel-base current falltime, and return stroke speed on the radiated LF waveforms observed at a given distance. We compare simulation results to data and conclude that previously reported larger GLD360 peak current estimates over the ocean are unlikely to fully result from differences in channel-base current risetime, falltime, or return stroke speed between ocean and land flashes., Defense Advanced Research Project Agency (DARPA)

Accurate models of trans-ionospheric propagation are needed to assess the role of Earth-originating very low frequency (VLF) electromagnetic waves in radiation belt dynamics. Recent studies have called the relatively crude early trans-ionospheric models into question, finding that they underestimate the attenuation by 20–100 dB. A full wave model that includes all of the relevant physics has recently become available and experimentally verifed to within a few decibels via comparison to more extensive satellite data. Using this model, we discuss the importance of wave polarization, incidence angle, bearing, ground conductivity, horizontal distance from the source, and the ionospheric pro?le, all of which are demonstrated to play a signifcant role in the trans-ionospheric propagation. Trans-ionospheric attenuation estimates are provided both for the case of vertical incidence of a whistler-mode wave and for the case of magnetospheric injection from a dipolar terrestrial VLF source. These, AFRL; NSF

The effects of ground-based very low frequency (VLF) transmitters on the lower ionosphere are investigated. Controlled modulation experiments are performed with the 21.4 kHz, 424 kW VLF transmitter NPM in Lualualei, Hawaii, and physical effects of the NPM transmissions are studied with a subionospherically propagating VLF probe signal. Observed perturbations to the probe signal are consistent neither with expectations from transmitter-induced electron precipitation nor to off-path scattering from a concentrated heating region near the transmitter but rather appear to be the result of scattering from extended lateral heating of the ionosphere by the NPM transmitter. A large-scale computational modeling framework confirms theoretically that this form of ionospheric heating can account for the observed probe signal modulations, establishing that the lateral extent of ionospheric heating due to VLF transmitters is several thousand kilometers, significantly greater than previously recognized., AFRL

Ground based Very Low Frequency (VLF, 3-30 kHz) radio transmitters play a role in precipitation of energetic Van Allen electrons. Initial analyses of the contribution of VLF transmitters to radiation belt losses were based on early models of trans-ionospheric propagation known as the Helliwell absorption curves, but some recent studies have found that the model overestimates (by 20-100 dB) the VLF energy reaching the magnetosphere. It was subsequently suggested that conversion of wave energy into electrostatic modes may be responsible for the error. We utilize a newly available extensive record of VLF transmitter energy reaching the magnetosphere, taken from the DEMETER satellite, and perform a direct comparison with a sophisticated full wave model of trans-ionospheric propagation. Although the model does not include the effect of ionospheric irregularities, it correctly predicts the average total power injected into the magnetosphere within several dB. The results, particularly at nighttime, appear to be robust against the variability of the ionospheric electron density. We conclude that the global effect of irregularity scattering on whistler mode conversion to quasi-electrostatic may be no larger than 6 dB. Citation: Cohen, M. B., N. G. Lehtinen, and U. S. Inan (2012), Models of ionospheric VLF absorption of powerful ground based transmitters, Geophys. Res. Lett., 39, L24101, doi:10.1029/2012GL054437., AFRL award

ELF/VLF (0.3–30 kHz) wave generation is achievable via modulated HF (3–30 MHz) heating of the lower ionosphere in the presence of natural currents such as the auroral electrojet. Using the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, AK, we investigate the effect of HF frequency and beam size on the generated ELF/VLF amplitudes, as a function of modulation frequency, and find that generation in the Earth-ionosphere waveguide generally decreases with increasing HF frequency between 2.75–9.50 MHz. HAARP is also capable of spreading the HF power over a wider area, and we find that a larger beam area yields larger generated amplitudes on the ground. Measurements are shown to generally agree with a theoretical model, which is then applied to also predict the effect of HF beam parameters on magnetospheric injection with HAARP., ONR; AFRL

Experiments for generating extremely low frequency (ELF) radio waves using modulated HF heating of the auroral ionosphere have been conducted and refined at the High Frequency Active Auroral Research Program (HAARP) facility at Gakona, Alaska. Because this technique is dependent on strength of the naturally generated electrojet current system, the amplitude of the generated ELF changes with geophysical conditions. Past work has shown that electrojet current strength as measured by magnetometers often correlates with generated ELF amplitude, but there are periods of poor or negative correlation. We attempt to use additional diagnostics from a radar, riometer, ionosonde, and magnetometer chain to understand how ionospheric conditions affect ELF generation. We then present the results of a statistical model that shows that ELF amplitude is roughly proportional to magnetometer measurements for a fixed value of riometer absorption and that the proportionality constant decreases as riometer absorption increases. Theoretical simulations of modulated heating are conducted for a variety of ionospheric density profiles to verify that denser profiles result in smaller gains for ELF generation as a function of electrojet current at a given electric field., HAARP; Office of Naval Research; Air Force Research Laboratory; Defense Advanced Projects Research Agency, via ONR; Stanford Graduate Fellowship

ELF/VLF waves have been generated via steerable HF heating of the lower ionosphere. The temperature-dependent conductivity of the lower ionospheric plasma enables HF heating (and subsequent recovery) to modulate natural current systems such as the auroral electrojet, thus generating an antenna embedded in the ionospheric plasma. We apply a realistic three-dimensional model of HF heating and ionospheric recovery, as well as ELF/VLF wave propagation in and below the ionosphere, to derive the radiation pattern into the magnetosphere as a result of steerable HF heating. It is found that modulated HF heating preferentially directs signals upward into space because of the phasing effect of the upward HF wave propagation. We find that the steering techniques such as the geometric modulation "circle sweep" enhances the total ELF/VLF power injected into the magnetosphere by 5-7 dB compared to amplitude modulated heating, with a few dB enhancement in the peak magnetic field value. Another technique known as beam painting enhances the total injected power by 1-3 dB but produces weaker peak magnetic fields due to the power being spread over a larger area. Observations on the DEMETER spacecraft are presented and compared with theoretical predictions. DEMETER observations show that the signal produced with geometric modulation can be stronger than the signal from AM under the same conditions., HAARP; Office of Naval Research (ONR); Air Force Research Laboratory (AFRL); Defense Advanced Research Programs Agency (DARPA)

The production of terrestrial gamma ray flashes (TGFs) requires a seed energetic electron source and a strong electric field. Lightning leaders naturally provide seed electrons by cold runaway and strong electric fields by charge accumulation on the channel. We model possible TGF production in such fields by simulating the charges and currents on the channel. The resulting electric fields then drive simulations of runaway relativistic electron avalanche and photon emission. Photon spectra and directional distributions produced by the model agree qualitatively with observations. Simulations with a variety of initial conditions indicate sufficient electric fields are produced if an unbranched channel supports a current pulse of at least 100 kA such as occurs if the channel is at least 1 km long and embedded in an ambient electric field of at least 100 kV m(-1). The mechanism does not strongly depend on altitude as friction and characteristic electric field strengths scale similarly. Seed particle production is not directly simulated, but estimates of seed production rates suggest current pulse activity of similar to 1 ms duration can account for TGF-scale emission., National Science Foundation

Terrestrial gamma ray flashes (TGFs) are brief bursts of photons with energies up to 20 MeV typically observed in association with lightning. Such energetic photons may undergo photonuclear reactions with nontrivial cross section in the vicinity of the giant dipole resonance. Pulses of neutrons have been observed experimentally in coincidence with lightning, suggesting such reactions are observable. We present simulations of expected photoneutron production based on initial conditions inferred from observations of TGFs. We predict an average of similar to 10(12) neutrons produced per TGF and give energy, time, and space distributions of neutrons produced and neutrons reaching ground and satellite altitude. The simulation results are consistent with some observations and suggest further experiments may be profitable., National Science Foundation

This study explores the manner in which the plasmapause is responsible for dictating which magnetospheric source regions of ELF/VLF chorus are able to propagate to and be received by midlatitude stations on the ground. First, we explore the effects of plasmapause extent on ground-based observations of chorus via a 3 month study of ground-based measurements of chorus at Palmer Station, Antarctica (L = 2.4, 50 degrees S geomagnetic latitude), and data on the plasmapause extent from the IMAGE EUV instrument. It is found that chorus normalized occurrence peaks when the plasmapause is at L similar to 2.6, somewhat higher than Palmer's L shell, and that this occurrence peak persists across a range of observed chorus frequencies. Next, reverse ray tracing is employed to evaluate the portion of the equatorial chorus source region, distributed in radial distance and wave normal, from which chorus is able to reach Palmer Station via propagation in a nonducted mode. The results of ray tracing are similar to those of observations, with a peak of expected occurrence when the plasmapause is at L similar to 3. The exact location of the peak is frequency dependent. This supports the conclusion that the ability of chorus to propagate to low altitudes and the ground is a strong function of instantaneous plasmapause extent and that peak occurrence of chorus at a given ground station may occur when the L shell of the plasmapause is somewhat beyond that of the observing station. These results also suggest that chorus observed on the ground at midlatitude stations propagates predominantly in the nonducted mode., National Science Foundation; Office of Naval Research

Recent observations by Starks et al. (2008) from multiple spacecraft suggest that the actual nighttime intensity of VLF transmitter signals in the radiation belts is approximately 20 dB below the level that is assumed in the model developed by Abel and Thorne (1998) and approximately 10 dB below model values during the day. In the present work, we discuss one experimentally established mechanism which might be responsible for some of this intensity discrepancy, linear mode coupling as electromagnetic whistler mode waves propagate through regions containing small-scale (2-100 m) magnetic field-aligned plasma density irregularities. The scattering process excites quasi-electrostatic whistler mode waves, which represents a power loss for the input waves. Although the distribution and amplitude of the irregularities is not well known at present, we construct plausible models in order to use numerical simulations to determine the characteristics of the mode coupling mechanism and the conditions under which the input VLF waves can lose significant power to the excited quasi-electrostatic whistler mode waves. For short propagation paths of approximately 15 km, the full-wave model predicts power losses ranging from -3 dB (25% probability) to -7 dB (2% probability). For longer propagation paths of approximately 150 km, the full-wave model predicts power losses ranging from -4 dB (25% probability) to over -10 dB (2% probability). We conclude that for the irregularity models investigated, the mode coupling mechanism can result in significant power loss for VLF electromagnetic whistler mode waves., Office of Naval Research

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., Office of Naval Research (ONR); Air Force Research Laboratory; Defense Advanced Research Programs Agency, via ONR

Terrestrial gamma ray flashes (TGFs) are brief bursts of gamma rays observed by satellites, typically in coincidence with detectable lightning. We incorporate TGF observations and the key physics behind current TGF production theories with lightning physics to produce constraints on TGF production mechanisms. The combined constraints naturally suggest a mechanism for TGF production by current pulses in lightning leader channels. The mechanism involves local field enhancements due to charge redistribution in current pulses and draws seed energetic particles from cold runaway in breakdown processes. The mechanism can reproduce the observed TGF time scale including multipulse TGFs, is in agreement with existing production altitude estimates, and may help explain TGF spectra by naturally producing unbeamed emissions., National Science Foundation

The MF data recorded by the low-altitude satellite DEMETER have been used to survey the MF waves around the Earth. A global map of the MF emissions indicates that there exists a wave activity in the frequency range 2-2.5 MHz above the main powerful VLF ground-based transmitters operating in the frequency range 18-50 kHz. It is shown that this is due to the high-frequency part of whistlers induced by the thunderstorm activity. They can penetrate trough the ionosphere at the locations of the transmitters because these transmitters induce large ionospheric perturbations. This means that an integrated map over several months is able to show these MF emissions above all main VLF transmitters. The discrepancy between intensities of the emissions in winter and summer (in the Northern Hemisphere) is explained by considering the geographic variations of the plasma frequency below the satellite. It is shown that the MF waves spread in longitude in the hemisphere opposite to the VLF transmitters., Centre National d'Etudes Spatiales (CNES); MURI; Stanford University