Active ionospheric experiments using high-power, high-frequency transmitters, “heaters”, to study plasma processes in the ionosphere and magnetosphere continue to provide new insights into understanding plasma and geophysical proceses. This review describes the heating facilities, past and present, and discusses scientific results from these facilities and associated space missions. Phenomena that have been observed with these facilities are reviewed along with theoretical explanations that have been proposed or are commonly accepted. Gaps or uncertainties in understanding of heating-initiated phenomena are discussed together with proposed science questions to be addressed in the future. Suggestions for improvements and additions to existing facilities are presented including important satellite missions which are necessary to answer the outstanding questions in this field.

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Past, Present and Future of Active Radio Frequency Experiments in Space

[1] 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.

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On the generation of ELF/VLF waves for long‐distance propagation via steerable HF heating of the lower ionosphere

[1] The ionospheric heating facility of the High Frequency Active Auroral Research Program (HAARP) has been used extensively in the last 3 years for injection of ELF/VLF waves into the magnetosphere via modulated heating of the overhead auroral electrojet currents. Of particular interest are waves that are observed to be nonlinearly amplified after interaction with hot plasma electrons in the Earth's radiation belts. Past results have shown HAARP to be an effective platform for controlled studies of wave particle interactions in the Earth's magnetosphere. A summary of the experimental results is provided in the context of dependencies on geomagnetic conditions and transmitter parameters. It is deduced that the primary variable that is associated with successful ground observations of HAARP-induced magnetospheric amplification is availability of magnetospheric wave guiding structures. Such structures are found to be most prevalent under quiet geomagnetic conditions following a disturbance when the plasmapause extends to the latitude of the HAARP facility or higher. Strong electrojet currents and high amplitudes of generated ELF/VLF signals observed on the ground are poor indicators of observation probability on a day to day basis although variation of these variables can be important on minute and second timescales. Frequency-time formats with continuously increasing ELF/VLF frequency show preferential amplification as predicted by nonlinear theory of electron trapping. Amplification of signals is also found to be possible for signals with noncoherent bandwidths of up to 30 Hz.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JA016261

On the occurrence of ground observations of ELF/VLF magnetospheric amplification induced by the HAARP facility

High-frequency heating of the ionosphere is effective for generating extremely low frequencies (ELF, 3–3000 Hz) through modulation of the auroral electrojet current. While the amplitudes of the resulting ELF waves depend on the auroral electrojet current strength, the polarization of their horizontal magnetic field remains relatively stable. In this work, we determined that at the distance of several wavelengths from an ionospheric ELF source created by two HF heating waves separated by an ELF frequency, polarization parameters are influenced by the Earth-ionosphere waveguide. Previous experiments in the vicinity of the ionospheric ELF source have determined that the right-hand polarization of the magnetic field measured at the ground typically prevails, whereas in this paper we demonstrate that at the distance of 660 km to the east of the European Incoherent Scatter, a circular left-hand polarization dominates. We interpret this effect as a result of “trapping” of the left-hand mode between the upper and lower boundaries of the Earth-ionosphere waveguide, while the right-hand or whistler mode leaks into the ionosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013RS005336

Polarization of ELF waves generated during “beat‐wave” heating experiment near cutoff frequency of the Earth‐ionosphere waveguide

[1] Extremely low frequency/very low frequency (ELF/VLF) radio waves are difficult to generate with conventional antennas. Ionospheric high frequency (HF) heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural currents such as the auroral electrojet creates an antenna in the sky when heating is modulated at ELF/VLF frequencies. We present a summary of nearly 100 days of ELF/VLF wave generation experiments at the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, Alaska, at a variety of ELF/VLF frequencies, seasons, and times of day. We present comprehensive statistics of generated ELF/VLF magnetic fields observed at a nearby site, in the 500–3500 Hz band. Transmissions with a specific HF beam configuration (3.25 MHz, vertical beam, amplitude modulation) are isolated so the data comparison is self-consistent, across nearly 5 million individual measurements of either a tone or a piece of a frequency-time ramp. There is a minimum in the average generation close to local midnight. It is found that generation during local nighttime is on average weaker but more highly variable, with a small number of very strong generation periods. Signal amplitudes from day to day may vary by as much as 20–30 dB. Generation strengthens by ∼5 dB during the first ∼30 min of transmission, which may be a signature of slow electron density changes from sustained HF heating. Theoretical calculations are made to relate the amplitude observed to the power injected into the waveguide and reaching 250 km. The median power generated by HAARP and injected into the waveguide is ∼0.05–0.1 W in this baseline configuration (vertical beam, 3.25 MHz, amplitude modulation) but may have generated hundreds of watts for brief durations. Several efficiency improvements have improved the ELF/VLF wave generation efficiency further.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgra.50558

100 days of ELF/VLF generation via HF heating with HAARP

[1] Generation of ELF/VLF radio waves (300 Hz to 10 kHz) is achievable via modulation of natural currents in the lower ionosphere with high-power HF (2–10 MHz) heating Recently, Cohen et al (2008b) put forth an alternative to conventional amplitude HF power modulation, therein referred to as geometric modulation, in which the HF ionospheric heating beam is geometrically steered at the desired ELF/VLF frequency, and found 7–11 dB enhanced amplitudes, and ∼14 dB directional dependence for the thus generated ELF/VLF waves, compared to vertical amplitude modulation In this paper, we quantitatively compare amplitude modulation, geometric modulation, and a previously proposed technique known as beam painting, wherein the HF beam is rapidly moved over a wide area during the on portion of amplitude modulation in order to create a larger heated region in the ionosphere We experimentally analyze both the total generation and the directionality, ie, the suitability of each technique to direct signals along a chosen azimuth Among the three methods, geometric modulation is found to be uniquely well suited for both goals We also conduct experiments to investigate two particular physical effects and their role in generation efficacy: that of heat-cool duty cycle and the oblique angle of the HF heating beam It is found that both duty cycle and the oblique angle of the beam have small but counteracting impacts, consistent with the notion that the primary physical process responsible for generation enhancement in geometric modulation is that of formation of an effective multielement phased array

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009JA014410

ELF/VLF wave generation via ionospheric HF heating: Experimental comparison of amplitude modulation, beam painting, and geometric modulation

[1] Modulated high frequency (HF, 3–10 MHz) heating of the ionosphere in the presence of the auroral electrojet currents is an effective method for generating extremely low frequency (ELF, 3–3000 Hz) and very low frequency (VLF, 3–30 kHz) radio waves. The amplitudes of ELF/VLF waves generated in this manner depend sensitively on the auroral electrojet current strength, which varies with time. In an effort to improve the reliability of ELF/VLF wave generation by ionospheric heating, recent experiments at the Highfrequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, have focused on methods that are independent of the strength of the auroral electrojet currents. One such potential method is so-called “beat-wave” ELF/ VLF generation. Recent experimental observations have been presented to suggest that in the absence of a significant D-region ionosphere (60–100 km altitude), an ELF/VLF source region can be created within the F-region ionosphere (150–250 km altitude). In this paper, we use a time-ofarrival analysis technique to provide direct experimental evidence that the beat-wave source region is located in the D-region ionosphere, and possibly the lower E-region ionosphere (100–120 km altitude), even when ionospheric diagnostics indicate a very weak D-layer. These results have a tremendous impact on the interpretation of recent experimental observations. Citation: Moore, R. C., S. Fujimaru, M. Cohen, M. Gookowski, and M. J. McCarrick (2012), On the altitude of the ELF/VLF source region generated during “beat-wave” HF heating experiments, Geophys. Res. Lett., 39, L18101,

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On the altitude of the ELF/VLF source region generated during “beat‐wave” HF heating experiments

[1] 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.

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M. J. McCarrick

Papers

ELF/VLF wave generation via ionospheric HF heating: Experimental comparison of amplitude modulation, beam painting, and geometric modulation

On the altitude of the ELF/VLF source region generated during “beat‐wave” HF heating experiments

HF beam parameters in ELF/VLF wave generation via modulated heating of the ionosphere