Beating HF waves to generate VLF waves in the ionosphere

Ground‐based Pc5 ultralow frequency (ULF) wave power in multiple ground‐based meridians is compared to the very low frequency (VLF) wave amplitude proxy, derived from Polar Operational Environmental Satellite (POES) precipitation, for the 33 storms studied by Li et al. (2015, https://doi.org/10.1002/2015GL065342). The results reveal common L‐shell and time profiles for the ULF waves and VLF proxy for every single storm, especially at L ≤ 6, and identical discrimination between efficient and inefficient radiation belt electron acceleration. The observations imply either ULF waves play a role in driving precipitation, which is falsely interpreted as VLF wave power in the proxy, ULF waves drive VLF waves (the reverse being energetically unfeasible), or both have a common driver with nearly identical L‐shell and time dependence. Global ground‐based ULF wave power coherence implies that a small number of meridians can be used to estimate storm time radial diffusion coefficients. However, the strong correspondence between ULF wave power and VLF wave proxy complicates causative assessments of electron acceleration.

The physical mechanisms usually applied to explain the relativistic electron enhancement have been delved into to elucidate non-adiabatic electron acceleration resulting in the ultra-relativistic electron population observed in the outer radiation belt. We considered multisatellite observations of the solar wind parameters, magnetospheric waves, and particle flux to report an unusual local acceleration of ultra-relativistic electrons under a prolonged high-speed solar wind stream (HSS). A corotating interaction region reaches the Earth’s bowshock on August 3, 2016, causing a minor geomagnetic storm. Following this, the magnetosphere was driven for 72 h by a long-term HSS propagating at 600 km/s. During this period, the magnetosphere sustained both ultra-low frequency (ULF) and very-low frequency (VLF) waves in the outer radiation belt region. Besides the waves, the relativistic and ultra-relativistic electron fluxes were enhanced with different time lags regarding the magnetic storm main phase. The efficiency of wave-particle interaction in enhancing ultrarelativistic electrons is evaluated by the diffusion coefficient rates, considering both ULF and VLF waves together with phase space density analyses. Results show that local acceleration by whistler mode chorus waves can occur in a time scale of 2–4 h, whereas ULF waves take around 10’s of hours and magnetosonic waves take a time scale of days. This result is confirmed by the phase space density analysis. Accordingly, it shows that peaks of local acceleration of 1 MeV electrons are consistent with the observation of the highest chorus wave amplitude at the same L-shell and MLT. Thus, we argue that whistler mode chorus waves interacting with relativistic electrons are the main physical mechanisms leading to ultra-relativistic electron enhancement, while ULF and fast magnetosonic waves are found as secondary physical processes. Lastly, our analysis contributes to understanding how whistler and ULF waves can contribute to ultra-relativistic electrons showing up in the inner magnetosphere under the HSS driver.

The physical mechanisms usually applied to explain the relativistic electron enhancement have been delved into to elucidate non-adiabatic electron acceleration resulting in the ultra-relativistic electron population observed in the outer radiation belt. We considered multisatellite observations of the solar wind parameters, magnetospheric waves, and particle flux to report an unusual local acceleration of ultra-relativistic electrons under a prolonged high-speed solar wind stream (HSS). A corotating interaction region reaches the Earth’s bowshock on August 3, 2016, causing a minor geomagnetic storm. Following this, the magnetosphere was driven for 72 hours by a long-term HSS propagating at 600 km/s. During this period, the magnetosphere sustained both ultra-low frequency (ULF) and very-low frequency (VLF) waves in the outer radiation belt region. Besides the waves, the relativistic and ultra-relativistic electron fluxes were enhanced with different time lags regarding the magnetic storm main phase. The efficiency of wave-particle interaction in enhancing ultrarelativistic electrons is evaluated by the diffusion coefficient rates, considering both ULF and VLF waves together with phase space density analyses. Results show that local acceleration by whistler mode chorus waves can occur in a time scale of 2 to 4 hours, whereas ULF waves take around 10’s of hours and magnetosonic waves take a time scale of days. This result is confirmed by the phase space density analysis. Accordingly, it shows that peaks of local acceleration of 1 MeV electrons are consistent with the observation of the highest chorus wave amplitude at the same L-shell and MLT. Thus, we argue that whistler mode chorus waves interacting with relativistic electrons are the main physical mechanisms leading to ultra-relativistic electron enhancement, while ULF and fast magnetosonic waves are found as secondary physical processes. Lastly, our analysis contributes to understanding how whistler and ULF waves can contribute to ultra-relativistic electrons showing up in the inner magnetosphere under the HSS driver.

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.

The nighttime lower ionosphere behavior between 2007 to 2011 is studied estimating the nighttime reflection height (hN) from Very Low Frequency (VLF) waves propagation recorded at Brazilian Antarctic Station Comandante Ferraz (EACF). We focus on the long-term solar activity effect on hN. The results show that hN decrease while the solar activity starts to increase suggesting a control of the solar radiation on the nighttime lower ionosphere, and hence, on the electron density at night. Preliminary results show that the nighttime lower ionosphere might be influenced by the 27-days solar rotation and by long-term atmospheric oscillations as Semiannual (SAO) and Annual (AO) oscillations.

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.

Very low frequency (VLF) waves (about 3–30 kHz) in the Earth’s magnetosphere interact strongly with energetic electrons and are a key element in controlling dynamics of the Van Allen radiation belts. Bistatic very low frequency (VLF) transmission experiments have recently been conducted in the magnetosphere using the high-power VLF transmitter on the Air Force Research Laboratory’s Demonstration and Science Experiments (DSX) spacecraft and an electric field receiver onboard the Japan Aerospace Exploration Agency’s Arase (ERG) spacecraft. On 4 September 2019, the spacecraft came within 410 km of each other and were in geomagnetic alignment. During this time, VLF signals were successfully transmitted from DSX to Arase, marking the first successful reception of a space-to-space VLF signal. Arase measurements were consistent with field-aligned propagation as expected from linear cold plasma theory. Details of the transmission event and comparison to VLF propagation model predictions are presented. The capability to directly inject VLF waves into near-Earth space provides a new way to study the dynamics of the radiation belts, ushering in a new era of space experimentation.Graphical

A very low frequency (VLF) or extremely low frequency (ELF) dipole source has been created within the lower ionosphere by modulating the atmospheric dynamo currents with a ground-based high power HF source from the Arecibo Observatory. The authors and their colleagues have demonstrated that ELF or VLF generated in this way and injected into the earth-ionosphere waveguide could be received a few thousand kilometers away. The injection properties due to an array of ionospheric dipoles as a function of array geometry and element currents that will allow steerable ELF/VLF radiation within the earth-ionosphere waveguide are investigated theoretically. The ionospheric array factors for a linear and a planar array of Hertzian dipole sources are developed and their properties examined. The principle of pattern multiplication is then applied to include the effect of the ionospheric array element. This provides a means for predicting the field strengths at a remote receiving site due to a steerable linear or planar array of ionospheric sources generated by high power HF periodic plasma heating.

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.

Space weather phenomena can threaten space technologies. A hazard among these is the population of relativistic electrons in the Van Allen radiation belts. To reduce the threat, artificial processes can be introduced by transmitting very-low-frequency (VLF) waves into the belts. The resulting wave-particle interactions may deplete these harmful electrons. However, when transmitting VLF waves in space plasma, the antenna, plasma, and waves interact in a manner that is not well-understood. We conducted a series of VLF transmission experiments in the radiation belts and measured the power and radiation impedance under various frequencies and conditions. The results demonstrate the critical role played by the plasma-antenna-wave interaction around high-voltage space antennae and open the possibility to transmit high power in space. The physical insight obtained in this study can provide guidance to future high-power space-borne VLF transmitter developments, laboratory whistler-mode wave injection experiments, and the interpretation of various astrophysical and optical phenomena.

In a coral reef system, the reef barrier protects the lagoon from incoming ocean waves favoring the development of a relatively calm ecosystem in the middle of a much more energetic domain. Incident sea-swell waves (SS) are filtered and transformed while passing over the reef ; and simultaneously long waves (with usually wave periods greater than 20 s in such a context) are generated. These long waves are important drivers for marine submersion along low-lying islands. Thus, for several years, the scientific community has increased its effort in the understanding of reef-lagoon wave energy spectral distribution. In particular, works have focused on the origin of infragravity waves (IG) classically observed in the frequency band [0.004 ; 0.04] Hz. It has been shown that IG are forced mainly by wave groups, either through the release of incoming bound waves or through the oscillation of the breaking point. Waves and IG overpassing the reef might also drive the emergence of much longer waves termed VLF (Very Low Frequency) waves, some of which being possibly resonant waves (Gawehn et al., 2016), a category of long waves that are still poorly understood in reef-lagoon context. The aim of this study is to explore a set of pressure time series measured on a reef-lagoon system in French Polynesia in order to characterize the spatial, frequency and temporal distribution of IG / VLF energy. An additional purpose is to create a methodology capable of identifying, in pressure time series, any type of long wave developing in the IG / VLF bands.

We present a tomographic imaging technique for the D‐region electron density using a set of spatially distributed very low frequency (VLF) remote sensing measurements. The D‐region ionosphere plays a critical role in many long‐range and over‐the‐horizon communication systems; however, it is unreachable by most direct measurement techniques such as balloons and satellites. Fortunately, the D region, combined with Earth's surface, forms what is known as the Earth‐Ionosphere waveguide allowing VLF and low frequency (LF) radio waves to propagate to global distances. By measuring these signals, we can estimate a path measurement of the electron density, which we assume to be a path‐averaged electron density profile of the D region. In this work, we use path‐averaged inferences from lightning‐generated radio atmospherics (sferics) with a tomographic inversion to produce 3D models of electron density over the Southeastern United States and the Gulf of Mexico. The model begins with two‐dimensional great circle path observations, each of which is parameterized so it includes vertical profile information. The tomography is then solved in two dimensions (latitude and longitude) at arbitrarily many altitude slices to construct the 3D electron density. We examine the model's performance in the synthetic case and determine that we have an expected percent error better than 10% within our area of interest. We apply our model to the 2017 “Great American Solar Eclipse” and find a clear relationship between sunlight percentage and electron density at different altitudes.

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.

Study of Very Low Frequency (VLF) electromagnetic wave is very important for knowing the behavior of the Ionospheric layers due to Sunrise‐Sunset, Earthquakes, Solar flares, Solar eclipses and other terrestrial and extra terrestrial radiations. We study the properties of the variation of the VLF signal strength theoretically all over Indian sub‐continent. As an example, we concentrate on the VLF signal transmitted by Indian Naval Transmitter VTX at Vijayanarayanam (Latitude 08°23’, Longitude 77°45’) near the southern tip of Indian subcontinent. As has been noticed, several receiving stations placed during the VLF campaign in all over India, the VLF signal strength varies significantly with place and time. To understand the diurnal and seasonal variation of the received signal, a complete knowledge of physics of intensity distribution of the VLF signal is essential. The spatial variation of VLF signal plays an important role in selecting future VLF stations. In the wave‐hop theoretical model presented here, horizontally stratified ionospheric layers have been considered. The VLF wave emitted by the transmitter has both the ground wave and the sky wave components. The ground wave attenuates during propagation. The sky wave component experiences reflections by the ionosphere on its way to the receiver and its attenuation depends on the degree of ionization. Intensity variation occurs at a given receiver location for interference among singly and multiply reflected waves. This has been simulated considering some simplified and justifiable assumptions. This spatial variation wave‐hop theoretical model developed here has been compared with LWPC code generated results.