Ionospheric Heating Experiments Research Articles

The nature of electron density enhancement over a wide altitude range during ionosphere heating experiments at EISCAT

The nature of electron density enhancement over a wide altitude range during ionosphere heating experiments at EISCAT

Comment on Blagoveshchenskaya et al. Artificial Ducts Created via High-Power HF Radio Waves at EISCAT. Remote Sens. 2023, 15, 2300

The claim that incoherent scatter radar data show electron density enhancements of 50–80% during some ionospheric heating experiments in a recent paper is questioned. The backscatter from the monostatic radar can indeed be enhanced during these experiments, but the conclusion that a large electron density increase is the cause is almost certainly wrong. Some natural plasma line data are presented in support of our claim. Previously published studies of similar events and a possible explanation for the observed increases in backscattered power are pointed out.

ELF/VLF Wave Radiation Experiment by Modulated Ionospheric Heating Based on Multi-Source Observations at EISCAT

Ground-based high-frequency modulated waves can periodically heat the ionosphere and create “virtual antennas”, which can radiate extremely low frequency (ELF, 0.3–3 kHz) or very low frequency (VLF, 3–30 kHz) waves for long-distance communication. Ionospheric X-mode and O-mode heating experiments using amplitude and beat-wave (BW) modulations were conducted on 21 November 2019. Experimental results were analyzed from multiple perspectives based on data from Dynasonde, a magnetometer, stimulated electromagnetic emissions, an ELF/VLF signal receiver, and ultra-high-frequency radar. The strongest excited ELF/VLF signals in previous BW modulation heating experiments were around 8–12 kHz; however, in this experiment, no signal excited in this frequency range was observed, and the signal with the highest signal/noise ratio was at the frequency of 3517 Hz, which will aid in understanding the best communication frequency under different ionospheric backgrounds. It is well-accepted that the electron temperature changes periodically with the modulation frequency. However, we noted that the electron temperature had insufficient cooling during the O-mode modulated heating process and then increased again, resulting in a continuous electron temperature increase. We found that this was related to the change in ion composition after analyzing ion-line spectra, which will be helpful in studying the effect of modulation heating on the ionosphere background.

Analysis of Observations near the Fourth Electron Gyrofrequency Heating Experiment in EISCAT

We present the observations of the artificial ionospheric heating experiment of EISCAT (European Incoherent Scatter Scientific Association) on 22 February 2012 in Tromsø, Norway. When the pump is operating near the fourth electron gyrofrequency, the UHF radar observation shows some strong enhancements in electron temperature, electron density, ion line, and the outshifted plasma lines. Based on some existing theories, we find the following: first, Langmuir waves scattering off lower hybrid density fluctuations and strong Langmuir turbulence (SLT) in the Zakharov model cannot completely explain the outshifted plasma lines, but the data suggest that this phenomenon is related to the cascade of the pump wave and should be researched further; second, the spatiotemporal consistency between the enhancement in electron density/electron temperature reaches up to three to four times that of the undisturbed state and HF-enhanced ion lines (HFILs) suggest that SLT excited by parametric instability plays a significant role in superthermal electron formation and electron acceleration; third, some enhancements in HFILs and HF-induced plasma lines (HFPLs) are generated by parametric decay instability (PDI) during underdense heating in the third cycle, we suggest that this is due to the existence of a second cut-off in the upper hybrid dispersion relation as derived from a kinetic description.

The Mysterious Green Streaks Below STEVE

Strong thermal emission velocity enhancement (STEVE) is an optical phenomenon of the subauroral ionosphere arising from extreme ion drift speeds. STEVE consists of two distinct components in true‐color imagery: a mauve or whitish arc extended in the magnetic east–west direction and a region of green emission adjacent to the arc, often structured into quasiperiodic columns aligned with the geomagnetic field (the “picket fence”). This work employs high‐resolution imagery by citizen scientists in a critical examination of fine‐scale features within the green emission region. Of particular interest are narrow “streaks” of emission forming underneath field‐aligned picket fence elements in the 100‐ to 110‐km altitude range. The streaks propagate in curved trajectories with dominant direction toward STEVE from the poleward side. The elongation is along the direction of motion, suggesting a drifting point‐like excitation source, with the apparent elongation due to a combination of motion blur and radiative lifetime effects. The cross‐sectional dimension is <1 km, and the cases observed have a duration of ∼20–30 s. The uniform coloration of all STEVE green features in these events suggests a common optical spectrum dominated by the oxygen 557.7‐nm emission line. The source is most likely direct excitation of ambient oxygen by superthermal electrons generated by ionospheric turbulence induced by the extreme electric fields driving STEVE. Some conjectures about causal connections with overlying field‐aligned structures are presented, based on coupling of thermal and gradient‐drift instabilities, with analogues to similar dynamics observed from chemical release and ionospheric heating experiments.

Ionospheric HF Heating Experiment With Frequency Ramping‐up Sweep: Approach for Artificial Ionization Layer Generation

AbstractAn ionospheric high‐frequency (HF) heating experiment was conducted at the HAARP facility in Alaska by sweeping the heater frequency, near the third electron gyroharmonic, over a 145 kHz range with 4 min duration. A 446 MHz diagnostic radar was used to detect scattering from HF‐enhanced plasma waves corresponding to the HF‐enhanced ion lines (HFILs). Increasing frequency progressively moves the third gyroharmonic resonance layer downward, and this is evident in observations of both downgoing and upgoing HFILs dropping in height about 50 km. Artificial ionization was formed by the HF‐plasma interaction during this process and driven downward to about 150–155 km. Total electron content measurements oblique across the High Frequency Active Auroral Research Program transmit beam, above the altitude of primary ionization production, show variations that correspond to ionization production at lower heights. A theoretical interpretation of the experimental observations is presented to understand the HF‐plasma interaction process. It is shown that the double resonance condition enhances the excitation of the upper‐hybrid parametric decay instability (PDI). HF heater frequency ramping‐up sweep increases the spatial height span of the upper‐hybrid PDI generation region and the spectral width of generated upper‐hybrid waves, as well as provides a positive tuning frequency on the excited upper‐hybrid waves that keeps the continuous positive feedback on the Doppler‐shifted cyclotron harmonic resonance interaction. Resonant electrons gain energy from the upper‐hybrid waves and are generated in an extended spatial region; after a drift space, spatially distributed energetic electrons bunch together to build up a density structure.

Simulation studies of strongly turbulent stimulated Brillouin backscattering during ionospheric heating

ABSTRACTRecent experimental observations of stimulated electromagnetic emissions (SEE) during ionospheric heating have shown that the SEE spectrum produced as a result of the stimulated Brillouin backscatter (SBS) instability is structurally very different for low and high pump power regimes. Initial results from electromagnetic Particle-In-Cell (PIC) simulation studies conducted to investigate the difference in the SBS SEE spectrum for the two pump power regimes are presented. Simulation results presented here are used to gain insight into ionospheric heating experiments at the High Frequency Active Auroral Research Program (HAARP) facility in which the pump power either linearly increased from a low power to the maximum available power, or was maintained at the maximum available power during the heating cycle. The observations show (1) a distinct, narrow SEE spectral line downshifted from the pump frequency, due to SBS, for the low power case and (2) a broad, aggregate SBS SEE structure downshifted from the pump frequency for the high power heating. In general, the spectra of the backscattered EM field obtained from PIC simulations for the two pump power regimes show correspondence with the HAARP observational data. Simulations predict that the broad spectral structure for the high power case results from strong ion acoustic (IA) turbulence. Further characteristics of the strong turbulence include cavity development in the ion density in which (I) strong long lifespan EM waves are trapped (II) short-lived ES waves are trapped, and (III) high temperature electrons exist.

Nonlinear Upper Hybrid Waves Generated in Ionospheric HF Heating Experiments at HAARP

Excitation of nonlinear upper hybrid waves by O-mode HF heater in the ionospheric heating experiments was explored via HAARP digisonde operated in a fast mode. The observations are manifested by a bump in the virtual spread, which expands the ionogram echoes upward as much as 140 km and is located below the upper hybrid resonance frequency. The bump is similar to that, occurring in daytime ionograms, caused by the cusp at the E-F2 layer transition, indicating that there is a small ledge in the density profile similar to E-F2 layer transitions. The ionograms also show that the virtual height spread, which exceeds 50 km over a significant frequency range below the upper hybrid resonance frequency, is downward, rather than upward as observed in a natural Spread-F situation. It indicates that density irregularities along the geomagnetic field, rather than field-aligned, were generated. Theory shows that the upper hybrid waves excited by the parametric instabilities evolve into nonlinear periodic and solitary waves that enhance downward virtual height spread and generate density cavity to explain the experimental observations.

Relationship Between the Polar Electrojet Dynamics and the Amplitude of ELF/VLF Signal from the Ionospheric Source in the Modulated Ionospheric Heating Experiment

We consider the results of the experiment on the polar ionosphere heating by a high-power modulated HF wave conducted by the AARI at the EISCAT/heating facility in October 2016. The data on joint processing of the values of the equivalent polar electrojet current above the heated ionosphere region retrieved by the IMAGE network of ground-based stations and the amplitudes of a signal of the horizontal component of the magnetic field from an ionospheric source at the modulation frequency recorded in the PGI network of high-latitude stations are presented. The events with a strong positive correlation between the magnetic field amplitude variations and the polar electrojet current variations are considered. It is shown that if the direction of the current changes from eastward to northward, correlation between the current variations and magnetic field variations almost disappears at a modulation frequency of 3017 Hz. Temporal and spatial variations of the coefficient of linear regression of the amplitude of the horizontal magnetic-field component with respect to the electrojet current are analyzed. In the transition from day to night, which is accompanied by the electrojet current decrease, the regression slope increases, and it decreases with increasing distance between the heater and the observation point.

Effects of wave trapping and ducting inside plasma density depletions are studied in a laboratory magnetoplasma

A laboratory experiment creates a pulsed magnetoplasma and demonstrates that oblique propagation of upper hybrid waves is necessary to understand ionospheric heating experiments.

Effects of the Hall Conductivity in Ionospheric Heating Experiments

AbstractWe investigate the role of Hall conductivity in ionospheric heating experiments. Ionospheric heating by powerful X‐mode waves changes the Hall and Pedersen conductances in the E and D regions, which lead to the generation of ultra‐low frequency (ULF)/extremely‐low frequency/very low frequency waves, when the electric field exists in the ionosphere. The importance of the Hall currents in the magnetosphere‐ionosphere interactions, carried by ULF waves and field‐aligned currents, has been consistently overlooked in studies devoted to the active experiments. Simulations of the three‐dimensional two‐fluid magnetohydrodynamic (MHD) model, presented in this paper, demonstrate that the Hall conductivity changes (1) the growth rate and the amplitude of ULF waves generated by the heating and (2) the orientation and the direction of propagation of the generated waves. These findings provide insight in the experiments where the waves were generated with a geometric modulation technique and suggest a new and more efficient approach for conducting such experiments in the future.

Features of downshifted maximum spectra during a dual-pump ionospheric heating experiment

An ionospheric heating experiment was conducted using the dual-pump mode at the EISCAT/HEATING facility in Tromsø, Norway. Some new features were found in the downshifted maximum (DM) component of the stimulated electromagnetic emission (SEE) spectra. During the experiment, the DM1 generated by pump 1 was enhanced under the action of pump 2 with the peak intensity being increased by ∼4.8–9.8 dB to achieve maximum value, when the frequency of pump 2 was 4.100 MHz. The gyro resonance at the upper hybrid altitude played an important role in this phenomenon. It was also observed that the development time of DM2 generated by pump 2 was greatly shorter than that of DM1 due to the precondition provided by the artificial field-aligned irregularities (AFAIs) stimulated by pump 1. Additionally, the frequency offset and peak intensity of the DM1 spectrum showed a significant negative correlation, where the correlation coefficient reached a value of −0.91.

Artificial Aurora Produced by HAARP

AbstractWe present results from the ionospheric heating experiment conducted at the High Frequency Active Auroral Research Program (HAARP) facility, Alaska, on 12 March 2013. During the experiment, HAARP transmitted in the direction of the magnetic zenith X‐mode 4.57‐MHz wave. The transmitted power was modulated with the frequency of 0.9 mHz, and it was pointed on a 20‐km spot at the altitude of 120 km. The heating (1) generates disturbances in the magnetic field detected with the fluxgate magnetometer on the ground and (2) produces bright luminous spots in the ionosphere, observed with the HAARP telescope. Numerical simulations of the 3‐D reduced magnetohydrodynamic (MHD) model reveal that these effects can be related to the magnetic field‐aligned currents, excited in the ionosphere by changing the conductivity in the E region when the large‐scale electric field exists in the heating region.

Optimized analysis of ionospheric amplitude modulated heating parameters for excitation of very/extremely low frequency radiations

It is now well known that amplitude modulated (AM) high frequency (HF) radio wave transmissions into the ionosphere can be used to generate very/extremely low frequency (VLF/ELF) radio waves using the so-called ‘electrojet antenna’. Duty cycle and heating frequency are analyzed and discussed with the lower-ionosphere modulated heating model, so as to improve the radiation efficiency of VLF/ELF waves in AM ionospheric heating experiments. Based on numerical simulation, the ranges of parametric selectivity in optimal duty cycle and heating frequency (fHF) are derived. The International Reference Ionosphere 2015 (IRI-2015) model and two-parameter model are used to predict background electron density profiles, and optimized ranges of duty cycle for different density profiles are analyzed and compared. The influences of wave polarizations on optimal duty cycle are also discussed. It is shown that intensity of the VLF/ELF equivalent radiation source (M) firstly rises and then falls with the increase of duty cycle. When using the IRI model, M peaks at a duty cycle of 50%, optimally ranging from 40%−70%. For the two-parameter model case, an optimal duty cycle is 40% and the optimized ranges vary from 30%−60%. Heating with an X-mode polarization is more efficient than with the O-mode case in VLF/ELF wave generation. Nevertheless, an optimal duty cycle is almost independent of HF wave polarizations. To obtain better VLF/ELF generation, optional fHF may be 0.8−0.9 times of foE for the O-mode heating and 0.75−0.85 times for the X-mode polarization case. Finally, the variations of these two parameters in different latitudes are discussed.

Large-scale numerical simulation of ionospheric Langmuir turbulence excited by a radio frequency electromagnetic wave

A numerical simulation is presented concerning an L/O mode electromagnetic wave propagating normally into an overdense magnetised plasma with a smooth density gradient leading to excitation of Langmuir turbulence in the vicinity of the reflection point. The simulation parameters are chosen to represent an ionospheric radio frequency heating experiment but may have relevance to other situations. The simulation model is one-dimensional for large-scale electromagnetic waves and two-dimensional for short-scale electrostatic waves. This allows consideration of local modulational and parametric-decay instabilities as well as the larger scale depletion of the driver electromagnetic wave by anomalous absorption due to the excited turbulence. Simulated growth rates are shown to be in broad agreement with expected values and the evolution of the spatial distribution of the turbulence and driver field profile are presented along with simulated scatter radar spectra.

Effects of variations of geomagnetic field on VLF waves induced by beating of two HF waves

Beat wave (BW) high frequency (HF) ionospheric heating experiments were conducted to generate very low frequency (VLF) waves. The VLF waves were registered with a VLF receiver located ∼15 km east of the European Incoherent Scatter (EISCAT) heating facility in Tromsø, Norway. A fluxgate magnetometer was used to monitor auroral electrojet current, and ionospheric conditions were measured using a Dynasonde. Correlation coefficients between VLF amplitudes and the deviation of geomagnetic north–south components were calculated. Experimental results show that strong and positive correlation exists the majority of the time, but sometimes no correlation or even a negative correlation occurred. This is consistent with similar past experiments that took place with exclusively AM generation. These results therefore support the conclusion that BW generation of VLF waves is no different than with AM, likely occurring in the D or lower E ionospheric region.

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.

Threshold of parametric instability in the ionospheric heating experiments

Many observations in the ionospheric heating experiment, by a powerful high frequency electromagnetic wave with ordinary polarization launched from a ground-based facility, is attributed to parametric instability (PI). In this paper, the general dispersion relation and the threshold of the PI excitation in the heating experiment are derived by considering the inhomogeneous spatial distribution of pump wave field. It is shown that the threshold of PI is influenced by the effective electron and ion collision frequencies and the pump wave frequency. Both collision and Landau damping should be considered in the PI calculation. The derived threshold expression has been used to calculate the required threshold for excitation of PI for several ionospheric conditions during heating experiments conducted employing EISCAT high frequency transmitter in Tromsø, Norway, on 2nd October 1998, 8th November 2001, 19th October 2012 and 7th July 2014. The results indicate that the calculated threshold is in good agreement with the experimental observations.

First Observations of Narrowband Stimulated Electromagnetic Emissions at the Pump Frequency Second Harmonic During Ionosphere Interaction Experiments

AbstractReported herein are first observations of narrowband stimulated electromagnetic emissions (NSEEs) within ±30 Hz of the second harmonic of the pump wave frequency during ionospheric heating experiments at the High Frequency Active Auroral Research Program facility. The observations are juxtaposed with contemporaneous NSEEs observed within ±30 Hz of the pump wave frequency. The transmit power of the pump wave is linearly increased, and the pump wave frequency is stepped through frequencies near the third electron gyroharmonic frequency during the experiment. The first observation is that there is a transmit power threshold for the occurrence of NSEEs downshifted from the second harmonic of the pump wave frequency. Secondly, when downshifted NSEEs are present at both the pump frequency and its second harmonic, the transmit power threshold for the onset of both are comparable. Third, the frequency offset of the downshifted NSEEs spectral line from the second harmonic frequency is approximately twice the frequency offset of the downshifted NSEEs spectral line from the pump wave frequency when the downshifted NSEEs are present near both the pump and second harmonic frequency. Finally, the NSEEs at the second harmonic of the pump wave frequency are suppressed at pump frequencies very close to the third electron gyroharmonic frequency.

Simulation of Physical Phenomena in the Ionosphere and Magnetosphere of the Earth on Krot Plasma Device. Some Results and Prospects

Krot device is a unique installation of the scientific infrastructure of the Russian Federation. It is a source of highly uniform low-temperature plasma that takes up to several tens of cubic meters. The setup makes it possible to perform both scaling laboratory simulations of ionospheric and magnetospheric phenomena in the approximation of unbound plasma, as well as plasma tests of full-size samples of the onboard equipment of spacecraft (SC). The simulation results of the dynamics and interaction of small-scale thermal plasma irregularities occurring during ionospheric heating experiments are presented. The impedance of the small-size models of RESONANCE and STRANNIK SC electric antennas in plasma is measured. The possible use of free-space calibration of antennas in the magnetospheric portions of the orbit in the ELF and VLF ranges is confirmed. The efficiency of a new plasma parameters resonance sensor for the TRABANT SC in the ionospheric range of electron densities is shown.