Ionospheric Modification Research Articles

An Ionospheric Modification Facility for the Magnetic Equator

An Ionospheric Modification Facility for the Magnetic Equator

Observations of Ionospheric Disturbances Produced by a Powerful Very-Low-Frequency Radio Signal in the Magnetic Conjugated Region Respect the Transmitter

We investigate four observational cases over NWC magnetically conjugate region by DEMETER spacecraft and CSES satellite. The cases of DEMETER on 23 February 2008 and CSES on 22 March 2020 show the evident ionospheric heating effects, in which the electron density and electron temperature suggest simultaneous enhancements associated with the intense spectra of the VLF electric and magnetic field above the conjugate region. This indicates that the heating effects associated with ionospheric modification are indeed triggered by the VLF signal transmitted by the NWC transmitter. In other words, the strong disturbances induced directly above the transmitter propagate along the magnetic field lines and extend into the magnetic conjugated region. Differently, the cases on 11 February 2010 (DEMETER) and 15 February 2019 (CSES) show obvious increases in electron densities while having no significant elevation in electron temperatures. The presence of enhanced energetic electron spectra at higher L-values, rather than directly above the conjugate region, suggests precipitation events induced by the VLF transmitter.

Особенности возбуждения искусственной ионосферной турбулентности при О- и Х-нагреве вблизи критической частоты слоя F2

We present experimental results from the studies of large-scale inhomogeneities along the external magnetic field with increased electron density, electron temperature, and excitation of elongated plasma waves (Langmuir and ion-acoustic), induced by the ordinary (O-mode) and extraordinary (X-mode) HF heating near the F2-layer critical frequency, in the high-latitude ionospheric F-region. The experiments have been carried out at the EISCAT/Heating facility (Tromsø, Norway). Powerful HF radio waves radiated towards the magnetic zenith through a step change in the effective radiated power at frequencies fH near and below the F2-layer critical frequency fₒF2. The EISCAT incoherent scatter radar (930 MHz), co-located with the EISCAT/Heating facility, was utilized for diagnostics of ionospheric modification effects. We calculated the electric field of a powerful HF radio wave near the reflection altitude, taking into account the non-deflective absorption along the propagation path. We determined the conditions for electric field generation and its threshold (minimum) values required for electron density enhancements in a wide altitude range, excitation of Langmuir and ion-acoustic plasma waves under fH~fₒF2 and fH<fₒF2. The possible generation mechanisms for the electron density enhancements above the reflection altitude of the powerful HF radio wave of O- and X-polarization are discussed.

Особенности возбуждения искусственной ионосферной турбулентности при О- и Х-нагреве вблизи критической частоты слоя F2

We present experimental results from the studies of large-scale inhomogeneities along the external magnetic field with increased electron density, electron temperature, and excitation of elongated plasma waves (Langmuir and ion-acoustic), induced by the ordinary (O-mode) and extraordinary (X-mode) HF heating near the F2-layer critical frequency, in the high-latitude ionospheric F-region. The experiments have been carried out at the EISCAT/Heating facility (Tromsø, Norway). Powerful HF radio waves radiated towards the magnetic zenith through a step change in the effective radiated power at frequencies fH near and below the F2-layer critical frequency fₒF2. The EISCAT incoherent scatter radar (930 MHz), co-located with the EISCAT/Heating facility, was utilized for diagnostics of ionospheric modification effects. We calculated the electric field of a powerful HF radio wave near the reflection altitude, taking into account the non-deflective absorption along the propagation path. We determined the conditions for electric field generation and its threshold (minimum) values required for electron density enhancements in a wide altitude range, excitation of Langmuir and ion-acoustic plasma waves under fH~fₒF2 and fH<fₒF2. The possible generation mechanisms for the electron density enhancements above the reflection altitude of the powerful HF radio wave of O- and X-polarization are discussed.

The 14 December 2020 Total Solar Eclipse Effects on Geomagnetic Field Variations and Plasma Density Over South America

AbstractWe discuss the effects in the geomagnetic field variations and ionospheric plasma density modifications caused by the Total Solar Eclipse that occurred on 14 December 2020 over the South American sector. We used ground‐based magnetometer data and the Total Electron Content maps derived from the Global Navigation Satellite System to evaluate these changes. The results show that the geomagnetic field daily variation weakens between the first and last solar eclipse penumbra contact. Additionally, we observed a significant reduction of about 52.33 nT on the Equatorial Electrojet strength at Jicamarca (11.95°S, 76.88°W), where the solar obscuration reached 16.67% approximately. This behavior indicates that the solar eclipse in the equatorial region has possibly affected electric conductivities, altering the E region dynamo electric field. Consequently, it weakens the equatorial plasma fountain, affecting the Equatorial Ionization Anomaly development. Additionally, the ionospheric dynamics variations over Jicamarca during the solar eclipse event are analyzed using ionosonde data. We observe that the solar eclipse also caused a modification in the sporadic E layer and F region dynamics, indicating possible evidence of the gravity wave occurrence. Therefore, the results found here provide a better understanding of how the solar eclipse passage in the equatorial region affects the electron density in the low‐latitude regions.

GPS observations of ionospheric TEC variations over Nepal during 22 July 2009 solar eclipse

As the study of ionospheric behavior during various solar activities is an important task, various studies of ionospheric changes during eclipse events have been widely performed in the different regions of the globe. This paper investigates the ionospheric responses to the solar eclipse on 22 July 2009 over Nepal using the total electron content (TEC) measured by dual-frequency Global Positioning System (GPS) receivers. The time-averaged Vertical TEC (vTEC) of ten GPS stations from Nepal is analyzed and it is found that the value of ionospheric TEC decreases due to the reduction of ionizing radiation. In addition, the deviation in the TEC value on eclipse day from the mean vTEC value of the top five quietest days is found to lie in the range ~1–5 TECu at those regions which were associated with the partial eclipse shadow. On the other hand, the region with the total eclipse (BRN2 and RMTE) faced ~6–7 TECu on average reduction in the TEC value. Considering that the eclipse of 22 July 2009 occurred just at sunrise in the Nepalese zone, a maximum reduction of about 5 TECu is very significant. Higher deviation in TEC is therefore linked with the path of totality and the obscuration rate. This study reveals that the ionospheric TEC over Nepal was altered by wave-like energy and momentum transport, as well as obscuration of the solar disc due to the partial and total solar eclipse. Furthermore, the cross-correlation results presented similar type signatures of the eclipse-induced ionospheric modification over Nepal. This research work serves a crucial future reference for the comparative study of change of ionospheric TEC variability over the Nepal region during Eclipse events

The Apparent Behavior of Electron Density during an Alternating O/X-Mode Heating Experiment

We present the observations of the artificial ionospheric modification experiment of EISCAT on 18 October 2012 in Tromsø, Norway. When the pump of alternating O mode and X mode is switched on, the UHF radar observation shows some strong enhancements in electron density, ion lines and plasma lines. Based on some existing theories, we find the following: First, during the experiment, the frequency of plasma line (fL), ion line (fia) and pump (fh) matches fL = fh − 3fia and = fh − 5fia occasionally demonstrated that the cascade process occurred. Second, through quantitative calculation, we found that the O-mode component mixed in X-mode wave satisfies the thresholds of the parametric decay instability and the oscillation two-stream instability, from which we infer that the HF-induced plasma lines (HFPLs) and HF-enhanced ion lines (HFILs) observed in X-mode pulse could have been caused by the O-mode component mixed in X-mode wave. Third, the UHF radar observation shows some apparent enhancements over a wide altitude range (from approximately the reflection altitude to ~670 km) in electron density during X-mode pulse, which also does not, in fact, correspond to a true increase in electron density, but due to the enhancement in ion line or the enhancement in radar backscatter induced by some unknown mechanism.

Ionospheric Response Over Brazil to the August 2018 Geomagnetic Storm as Probed by CSES‐01 and Swarm Satellites and by Local Ground‐Based Observations

AbstractThe geomagnetic storm that occurred on 25 August 25 2018, that is, during the minimum of solar cycle 24, is currently the strongest ever probed by the first China Seismo‐Electromagnetic Satellite (CSES‐01). By integrating the in situ measurements provided by CSES‐01 (orbiting at altitude of 507 km) and by Swarm A satellite (orbiting at ca., 460 km) with ground‐based observations (ionosondes, magnetometers, and Global Navigation Satellite System receivers), we investigate the ionospheric response at lower‐ and mid‐latitudes over Brazil. Specifically, we investigate the electrodynamic disturbances driven by solar wind changes, by focusing on the disturbances driving modifications of the equatorial electrojet (EEJ). Our proposed multisensor technique analysis mainly highlights the variations in the topside and bottomside ionosphere, and the interplay between prompt penetrating electric fields and disturbance dynamo electric fields resulting in EEJ variations. Thanks to this approach and leveraging on the newly available CSES‐01 data, we complement and extend what recently investigated in the Western South American sector, by highlighting the significant longitudinal differences, which mainly come from the occurrence of a daytime counter‐EEJ during both 25 and 26 August at Braziliian longitudes and during part of 26 August only in the Peruvian sector. In addition, the increased thermospheric circulation driven by the storm has an impact on the EEJ during the recovery phase of the storm. The observations at the CSES‐01/Swarm altitudes integrated with the ground‐based observation recorded signatures of equatorial ionospheric anomaly crests formation and modification during daytime coupled with the positive ionospheric storm effects at midlatitude.

The Generation of ULF/ELF/VLF Waves in the Ionosphere by Modulated Heating

One of the most important effects of ionospheric modification by high power, high frequency (HF) waves is the generation of ultra low frequency/extremely low frequency/very low frequency (ULF/ELF/VLF) waves by modulated heating. This paper reviews the scientific achievements of the past five decades regarding the main mechanisms of excitation of ULF/ELF/VLF waves and discusses their characteristics, such as their electrojet dependency, the location of the source region, continuous and discontinuous waves, the number of HF arrays, and the suitable range of the modulation frequency for different proposed mechanisms. Finally, the outlook for future research in this area is presented.

ДИСТАНЦИОННЫЕ МЕТОДЫ ДИАГНОСТИКИ ЭФФЕКТОВ ВОЗДЕЙСТВИЯ ВЫСОКОШИРОТНЫХ НАГРЕВНЫХ КОМПЛЕКСОВ

The results of long-term studies of the effects of the high-latitude ionosphere modification by powerful high-frequency (HF) radio waves generated by the EISCAT/Heating facility based on remote diagnostic methods are presented. The brief description of remote diagnostic instruments and methods is given. The excitation conditions and characteristics of small-scale artificial field-aligned irregularities induced by the O- and X-mode HF pumping are considered using the bi-static scatter method. The results of the spectral analysis of HF heater signals under various radiation modes are provided. It is shown that the X-mode HF pumping induces narrowband artificial radio emission of the ionosphere recorded at a distance of more than 1000 km from the heater.

Plasma fluctuations in the presence of random density and temperature irregularities

A theoretical approach for finding spectra of electrostatic fluctuations in a plasma with random density and temperature irregularities is developed. The approach is valid for nonequilibrium plasmas, in particular, when electrons and ions have different temperatures. Using these results, we calculated the spectrum of electron density fluctuations, which is necessary to interpret incoherent scattering experiments. It is shown that ignoring the random irregularities can introduce errors into the measurements of main plasma parameters, such as ambient density and temperatures. The calculated spectra are applied to experiments on the artificial ionosphere modification by high-power radio waves. A possible experimental method for estimating the intensity of random temperature irregularities in the artificially modified ionosphere is discussed.

Ionosphere modification and topology of ionospheric magnetically-oriented irregularities

One of the perspective directions of active impact on the ionosphere is the for-mation of electron concentration irregularities. Such irregularities can be used to increase the efficiency of radar systems and signal transfer in an interlayer ionospheric waveguide. Experiments on Alaska-Antarctica path showed that signal level at a receiving site is de-termined by heating wave parameters. In this case we did not study the character of its level change with time, i.e. possible fading, that is very important for radio-engineering systems. It is known that fading is determined by dimensions and space distribution of irregularity. Observations over EISCAT experiments make it possible, to some degree, to show the character of a scattered signal and the topology of ionospheric magnetically-oriented irregularities in the area of active impact on the ionosphere.

Power threshold and saturation of parametric decay instabilities near the upper hybrid resonance in plasmas

Parametric decay instabilities (PDIs) occur for large-amplitude waves in quadratically nonlinear media, where they provide a limit of validity of linear theories and allow efficient coupling between different, well-defined wave modes. We investigate PDIs near the upper hybrid resonance in plasmas by injection of high-power electron cyclotron (EC) waves at the ASDEX Upgrade tokamak. Our measurements of PDIs have an unprecedented frequency resolution, far below the ion cyclotron frequency, allowing the first observations of secondary and tertiary PDIs during the saturation phase in a controlled laboratory setting. Furthermore, we are for the first time able to systematically compare theoretical predictions of the EC wave power thresholds, which must be exceeded to excite such PDIs, with experimental observations, validating the theory. Our findings are relevant for EC wave heating and current drive in tokamaks and stellarators, including future fusion power plants, as well as in low-temperature laboratory and industrial plasmas, inertial confinement fusion, and ionospheric modification experiments.

NSEE Yielding Electron Temperature Measurements at the Arecibo Observatory

AbstractNarrowband stimulated electromagnetic emissions (NSEE), a component of radio emissions created during high‐frequency (HF) radiowave ionospheric modification experiments, occur within 1 kHz of the HF pump frequency. NSEE was observed and studied for the first time at the HAARP (High Frequency Active Auroral Research Program) facility in Alaska (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004). Magnetized stimulated Brillouin scatter (MSBS) is a component of NSEE, which was also first observed at HAARP (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004) and later at European Incoherent Scatter Scientific Association (Fu et al., 2015, https://doi.org/10.5194/angeo-33-983-2015). Ion‐acoustic and electrostatic ion cyclotron modes are the daughter products of MSBS and can be used for the determination of the electron temperature and of the presence of minor ion species, respectively, in the HF‐modified ionosphere. Here we present the first observations of the MSBS process at magnetic midlatitudes, excited during radio wave ionospheric modification experiments at the Arecibo Observatory. The NSEE observations, in combination with a theoretical model and the wave matching conditions, are used to estimate background ionospheric parameters. A qualitative comparison of the MSBS component of the NSEE spectrum with the thermal ion line measured by incoherent scatter radar is presented.

Investigating Transport and Dissipation in the Subauroral E Region With Ionospheric Modification Experiments and Very High Frequency Radar Backscatter

AbstractIonospheric modification experiments have been performed at the High‐Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, using a Very High Frequency (VHF) coherent scatter radar in Homer, Alaska, for experimental diagnostics. The experiments were intended to determine the threshold pump electric field required to initiate thermal parametric instability in the E region. The pump power level was ramped systematically to determine the threshold, and the experiment was repeated at four closely spaced pump frequencies. This provided threshold estimates at four E region altitudes. The theory for thermal parametric instability based on the work of Dysthe et al. (1983, https://doi.org/10.1063/1.863993) has been modified for application in the E region. The theory considers magneto‐ionic effects on the pump mode, linear mode conversion theory for upper hybrid wave generation, wave heating, and the effects of transport and dissipation based on fluid theory. The theory amounts to an eigenvalue problem where the eigenvalue is the threshold pump electric field for instability. The theory shows how the threshold depends on ionospheric transport coefficients and on the fractional cooling rate for inelastic electron‐neutral collisions. The theoretical predictions for threshold are roughly consistent with experimental values although the latter are probably affected by excess ionospheric absorption.

Energetic Electron Precipitation Associated With Pulsating Aurora Observed by VLF Radio Propagation During the Recovery Phase of a Substorm on 27 March 2017

AbstractRecent studies suggest that electrons with energies up to several hundred keV precipitate into the atmosphere associated with pulsating aurora (PsA). It is debated the highest energy of precipitating electrons associated with PsA. Here we report for the first time that the energy extends to relativistic energies. PsA was observed by THEMIS all‐sky imagers during a substorm that occurred on 27 March 2017. Energetic electron precipitation was detected by very low frequency subionospheric propagation. We found similar time variations between the auroral intensity and perturbation of the received radio signal intensity when the PsA occurred on the radio path. The perturbation showed a short recovery time of ~2 s. The recovery time indicates relaxation from ionospheric modification due to energetic electron precipitation and depends on the stopping altitude of the electrons. The recovery time required a stopping altitude of 50–60 km and indicates that the PsA is accompanied by relativistic electron precipitation.

Spatiotemporal Characteristics of HF‐Induced Ionospheric Turbulence Revealed by Diagnostic Stimulated Electromagnetic Emission and Test Radio Waves at HAARP

AbstractThis paper describes the dynamic characteristics of ionospheric plasma turbulences excited by high‐power high‐frequency radio waves during ionospheric modification experiments at the High‐frequency Active Auroral Research Program heating facility, Gakona, Alaska. The data come from measurements of high‐frequency‐induced stimulated electromagnetic emission (SEE) and anomalous absorption of test radiowaves. In addition to quasi‐continuous pump (QCP) waves that excite the “standard” SEE, short pulses of diagnostic waves at frequencies shifted from the QCP frequency have been injected to generate diagnostic SEE. This experimental setup allowed us to study the spectra and damping rates of the plasma waves related to both diagnostic and QCP waves, as well as the QCP‐related small‐scale magnetic field‐aligned irregularities at different altitudes, ±25 km around the center of the heated volume. The results of numerical simulations of the upper hybrid‐related SEE and diagnostic SEE under a “double‐transformation” scheme with the use of an empirical model of the irregularities' spatial spectrum and its dynamics are in good agreement with the observations.

Density disturbance of small-scale field-aligned irregularities in the ionosphere heating experiments

A theoretical model which describes the small-scale irregularities excited by powerful high frequency (3–30 MHz) electromagnetic wave in ionosphere heating is investigated quantitatively in this paper. The model is based on the transport equation in magnetic plasma and mode conversion from electromagnetic wave to electrostatic wave in ionospheric modification. Threshold electric field for exciting small-scale (meter scale) irregularities and spatial spectra of irregularities are analytically calculated by this model. The results indicate that background electron density and geomagnetic field play an important role for the threshold electric field and the spatial scale of the electron density irregularities. The results demonstrate that the electric field threshold increases with the decrease of the spatial scale of the irregularities. For exciting meter scale irregularities, the threshold electric field is about tens of mV m−1. The theoretical results are consistent with those of the experiments.

Experimental studies on radio frequency sources for ionospheric heaters

The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be affected by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, high frequency source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. For developing a mobile heater with a much smaller antenna array, a new highly efficient megawatt-class Radio Frequency (RF) source is required to reduce the overall power demands on a fully deployable system. The concept of such a source has been described previously [Beaudoin et al., J. Electromagn. Waves Appl. 31(17), 1786–1801 (2017)]. Here, experimental results using an electron beam produced by a gridded thermionic electron gun to drive an external lumped element circuit for a high efficiency RF generation are reported. The gun produces an electron beam bunched at the driving frequency with a narrow phase angle spread that is then collected by an external circuit for resonant impedance matching to the load. The results showed that effects, such as the internal resistance of the inductor and deflection of the beam electrons by the induced RF voltages on the beam collector, are important considerations to be included in the design of a practical device using this configuration for high efficiency RF generation.

Asymmetry in Stimulated Emission Polarization and Irregularity Evolution During Ionospheric Electron Gyroharmonic Heating

AbstractThe first report is made of a holistic investigation of stimulated electromagnetic emission (SEE) during ionospheric electron gyroharmonic heating using a diagnostic approach at the High Frequency Active Auroral Research Program facility. The evolution of SEE polarization and plasma irregularity development near the third electron gyroharmonic 3fce is investigated, which provides new insights into SEE generation mechanisms by the associated parametric decay instabilities. New more complex SEE spectral line behavior is clearly observed for varying transmitter beam angles. New SEE spectral emissions are observed at ∼75 kHz when pumping below 3fce. The high‐frequency radar echoes and SEE polarimetry appear asymmetric for pumping above 3fce, which implies that the broad upshifted maximum spectral line formation involves plasma irregularities scattered by high‐frequency radar echoes. This has important implications for understanding artificial ionization layer generation during ionospheric modification.