Steerable ELF/VLF radiation produced by an array of ionospheric dipoles generated from HF heating

Solar flares, energetic particles, and Earth‐impacting coronal mass ejections enhance ionization in the lower ionosphere, inhibiting radio wave propagation in the Earth‐ionosphere waveguide (EIWG). This enhanced ionization is observed locally by ionosondes and GPS/GNSS receivers, but spatial coverage of these observations is limited by receiver location. Very low frequency (VLF) propagation studies have previously been performed to assess the impact of space weather on the EIWG; however, these studies are typically limited by small numbers of fixed VLF transmitters and receivers and observe only the region of the EIWG along propagation paths between transmitters and receivers. Here, we use global lightning as a VLF source and an existing lightning detection network as a receiver. By mapping sferic propagation paths between lightning strokes and numerous network stations and considering how this distribution of paths changes during solar events, we can identify attenuation regions in the EIWG caused by space weather. We describe the VLF response in the EIWG to two X‐class solar flares and compare mapped attenuation regions with those provided by the NOAA D‐Region Absorption Prediction model. The identified attenuation regions associated with these flares match the D‐Region Absorption Prediction‐predicted regions well in both spatial extent and onset timing. Measurements of VLF attenuation caused by solar flares can provide ground‐truth confirmation of modeled attenuation and can inform the detection efficiency of lightning location networks. This analysis also paves the way for real‐time VLF attenuation mapping in the EIWG.

Extremely low frequency (ELF) and very low frequency (VLF) observations have formed the cornerstone of measurement and interpretation of effects of lightning discharges on the overlying upper atmospheric regions, as well as near‐Earth space. ELF (0.3–3 kHz) and VLF (3–30 kHz) wave energy released by lightning discharges is often the agent of modification of the lower ionospheric medium that results in the conductivity changes and the excitation of optical emissions that constitute transient luminous events (TLEs). In addition, the resultant ionospheric changes are best (and often uniquely) observable as perturbations of subionospherically propagating VLF signals. In fact, some of the earliest evidence for direct disturbances of the lower ionosphere in association with lightning discharges was obtained in the course of the study of such VLF perturbations. Measurements of the detailed ELF and VLF waveforms of parent lightning discharges that produce TLEs and terrestrial gamma ray flashes (TGFs) have also been very fruitful, often revealing properties of such discharges that maximize ionospheric effects, such as generation of intense electromagnetic pulses (EMPs) or removal of large quantities of charge. In this paper, we provide a review of the development of ELF and VLF measurements, both from a historical point of view and from the point of view of their relationship to optical and other observations of ionospheric effects of lightning discharges.

Many patients with severe obstructive sleep apnea (OSA) do not complain of excessive daytime sleepiness (EDS), possibly due to increased sympathetic nervous activity (SNA) and accompanying heightened alertness. We hypothesized that in patients with OSA, those without subjective EDS (Epworth Sleepiness Scale, ESS score < 11) would have higher very low frequency (VLF) heart rate variability (HRV) during sleep, reflecting greater sympathetic heart rate modulation than patients with an ESS score ≥ 11. Patients with severe OSA (AHI ≥ 30: 26 with and 65 without heart failure) were divided into those with and without EDS. Heart rate (HR) signals were acquired in stage 2 sleep during periods of recurrent apneas and hypopneas and submitted to coarse graining spectral analysis, which extracts harmonic, neurally mediated contributions to HRV from total spectral power. Because the apnea-hyperpnea cycle entrains muscle SNA at VLF (0 to 0.04 Hz), VLF power was our principal between-group comparison. Subjects without EDS had higher harmonic VLF power (944 ± 839 vs 447 ± 461 msec(2), p = 0.003) than those with EDS, irrespective of the presence or absence of heart failure (1218 ± 944 vs 426 ± 299 msec(2), p = 0.043, and 1029 ± 873 vs 503 ± 533 msec(2), p = 0.003, respectively). ESS scores correlated inversely with VLF power in all (r = -0.294, p = 0.005) and in heart failure subjects (r = -0.468, p = 0.016). Patients with severe OSA but without EDS have higher VLF-HRV than those with EDS. This finding suggests that patients with severe OSA but without EDS have greater sympathetic modulation of HRV than those with EDS that may reflect elevated adrenergically mediated alertness. Taranto Montemurro L; Floras JS; Picton P; Kasai T; Alshaer H; Gabriel JM; Bradley TD. Relationship of heart rate variability to sleepiness in patients with obstructive sleep apnea with and without heart failure.

Satellite launch vehicle effect on the Earth’s lower ionosphere: A case study

Beat‐wave generation of very low frequency (VLF) waves by two HF heaters in the ionosphere is formulated theoretically and demonstrated experimentally. The heater‐induced differential thermal pressure force and ponderomotive force, which dominate separately in the D and F regions of the ionosphere, drive an electron current for the VLF emission. A comparison, applying appropriate ionospheric parameters shows that the ponderomotive force dominates in beat‐wave generation of VLF waves. Three experiments, one in the nighttime in the absence of D and E layers and two in the daytime in the presence of D and E layers, were performed. X mode HF heaters of slightly different frequencies were transmitted at CW full power. VLF waves at 10 frequencies ranging from 3.5 to 21.5 kHz were generated. The frequency dependencies of the daytime and nighttime radiation intensities are quite similar, but the nighttime radiation is much stronger than the daytime one at the same radiation frequency. The intensity ratio is as large as 9 dB at 11.5 kHz. An experiment directly comparing VLF waves generated by the beat‐wave approach and by the amplitude modulation (AM) approach was also conducted. The results rule out the likely contribution of the AM mechanism acting on the electrojet and indicate that beat‐wave in the VLF range prefers to be generated in the F region of the ionosphere through the ponderomotive nonlinearity, consistent with the theory. In the nighttime experiment, the ionosphere was underdense to the HF heaters, suggesting a likely setting for effective beat‐wave generation of VLF waves by the HF heaters.

Very low frequency (VLF) propagation in the earth-ionosphere waveguide has been of interest for many decades. Most computational capabilities have been performed for a single frequency, with time-dependent results (such as those applied to lightning) obtained by inverse-Fourier-transforming these single-frequency components over a spectrum of frequencies. But if time-of-arrival (TOA) of a wave pulse is of interest, a less computationally-intensive method can be employed. Since a single spectral field component in the waveguide may be represented as a sum of modal fields, and since the group velocity is different for each mode, only the mode with the largest group velocity – the lowest order mode – need be calculated to obtain TOA information. This principle is demonstrated in detail for a simple waveguide, and applied to the earth-ionosphere waveguide.

In this review paper, analytical methods are used to determine the electric field and current induced in the conducting human body when this is exposed to an electromagnetic field at extremely low frequencies (ELFs) or very low frequencies (VLFs). This is done by treating it as a parasitic antenna when this is modeled successively as a sphere, an ellipsoid, and a cylinder. Because the body is electrically very short at ELF and VLF, the axial current depends primarily on the length of the body. Comparison with the ellipsoidal shape shows that the induced current is virtually independent of the cross-sectional shape. It is concluded that the axial current induced in the cylinder is a good approximation of the current induced in an actual body with the same length and mean cross sectional area. References to persons standing on the earth and with the arms raised are given. The significance of the accurate knowledge of induced currents and fields for biomedical purposes is discussed.

Recent satellite observations1 have revealed details on the strength of the pulsing extremely low frequency (ELF) hiss in the equatorial region which maps down to the auroral zone. It is shown here to be insufficient to produce pitch–angle scattering of energetic electrons into the loss cone at a sufficiently high rate to explain the characteristics of the measured pitch-angle distributions in pulsating auroras, within the framework of the theory proposed by Coroniti and Kennel2. It is suggested that wave–particle interactions involving both broadband incoherent ELF waves and narrow band coherent very low frequency (VLF) waves may be able to produce a sufficiently high rate of pitch-angle scattering.

A narrowband receiver was installed at the SANAE IV base in Antarctica to monitor specific very low frequency (VLF) radio signals from transmitters around the world. VLF waves propagating through the Earth–Ionosphere Waveguide are excellent probes of the varying properties of the lower region of the ionosphere. This paper describes the set-up of the narrowband system and demonstrates its capabilities with data from a set of solar flares on 08 February and 12 February 2010.

The electromagnetic field amplitude of the subionospheric Very Low Frequency (VLF) propagation is sensitive to the lower ionospheric conditions. Accordingly, VLF waves have been proposed to study and monitor the lower ionosphere (D/E region). In this paper, the NARXNN (Nonlinear Autoregressive with Exogenous Input Neural Network) is used as a method for predicting the daily nighttime mean amplitude of VLF transmitter signals indicating the ionospheric perturbation around the transmitter-receiver path. The NARXNN has a good accuracy in predicting time series data and thus are more suitable for dynamic modeling. The NARX constructed model, which was built based on daily input variables of various physical parameters with the time interval from 1 January 2011 to 4 February 2013 such as stratospheric and mesospheric temperatures, cosmic rays, total column ozone, F10.7, Kp, AE, and Dst indices. We used the constructed model to predict high-(NLK-CHF), middle-(NPM-CHF) and low-latitude (NWC-CHF) paths. As a result, the constructed models are capable of performing reasonably good 5-day ahead predictions of the daily nighttime of VLF electric field amplitude for NPM-CHF path with the Pearson correlation coefficient (r) of 0.84 and with Root Mean Square Error (RMSE) of 3.12 dB, NLK-CHF (r = 0.80, RMSE = 3.57 dB) and NWC-CHF (r = 0.79, RMSE = 2.60 dB). We conclude that the constructed NARX NN model is capable of predicting the VLF electric field amplitude variation for different latitude paths.

Using the full-wave solution model, the numerical simulation experiments are carried out to study the spatial distribution of electromagnetic field energy in the earth-ionosphere waveguide and ionosphere excited by ground VLF(very low frequency) radiation sources with different frequency, and the absorption of electromagnetic radiation energy in the ionospheric D\E region. The simulation results show that the attenuation of VLF radiation in the waveguide is affected by the frequency of the radiation source, the attenuation in the waveguide decreases with the increase of the frequency, the absorption of the ionospheric D\E region increases with the increase of the frequency, and the total attenuation increases with the increase of the frequency.

Subject index

In this letter, Wait's exponential (EXP) model and international reference ionosphere (IRI) model are combined to study the diurnal variation characteristics of very low frequency (VLF) wave propagation. The EXP model is used below the IRI model to compensate for the subionosphere electron density lack of the IRI. The major EXP parameters are reconstructed from the lower IRI electron density for different times of the day. Being capable of characterizing the subionosphere details, the combined IRIEXP model gets higher accuracy than the pure IRI model for VLF diurnal variation prediction. The measurement result is replicated by using the finite-difference time-domain method to validate the presented model.

We have studied the parametric interaction between a fast magnetosonic (FM) mode and a lower oblique resonance (LOR) mode in a cold, magnetized plasma using a kinetic, three dimensional Particle-in-Cell simulation code called the Large Scale Plasma. The FM mode is excited with a loop antenna driven at a frequency below the lower hybrid frequency (ωLH), while the LOR is excited at a frequency above ωLH. For historical purposes explained in the Introduction, we call the antennas which drive the FM mode and LOR mode Extremely Low Frequency (ELF) and Very Low Frequency (VLF) antennas, respectively. The antennas are modeled as magnetic dipoles (ρant = 0) and are assigned a time varying current density within a finite sized current loop. The VLF and ELF antennas are driven at 10 A and 3 A, respectively. The parametric interaction is excited with a combined ELF/VLF antenna (which we call a parametric antenna) and includes both antennas driven simultaneously in the same simulation domain. We show that the parametric antenna non-linearly excites electromagnetic (EM) whistler waves to a greater extent than the VLF antenna alone. We also show that the parametric excitation of EM whistler waves leads to greater emitted EM power (measured in Watts) compared with a VLF antenna alone.

The present paper is the investigation of the semi‐diurnal tide (SDT) and gravity waves (GW) through coupling effects on propagating Very low frequency (VLF) signal. One year of recorded VLF data is used. The first modulation of the VLF signal with SDTs and GWs is presented. The structures associated with SDT and GW are analyzed, and the daily and monthly variations are determined. Wavelet and spectrum analysis is applied to the data to determine the origin of the SDT. The corresponding period of the observed SDT is matched with solar SDT (SSDT). Seasonal variability of the SSDT in the lower ionosphere has been investigated using the VLF radio sounding for the first time. Developmental, evolution, and dissipation phases of GW with daytime and month are resolved. A plan to deploy 8 VLF stations to probe the shoreline and characterize the generation and development of SDT and GWs along the coastline is discussed.