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

https://iopscience.iop.org/article/10.3367/UFNr.2016.07.037868/pdf

Dynamic properties of ionospheric plasma turbulence driven by high-power high-frequency radiowaves

: It was predicted (Carlson, 1987; 1993) that once HF radio waves achieved ionospheric energy densities comparable to that from the solar EUV, they could produce their own ionosphere. That work estimated a GW ERP of rf energy would produce an ionosphere half that from an overhead sun, assuming 15 efficiency conversion of rf energy to accelerated electron energy. [Until 2009 only one experimental estimate existed, 15 from Carlson, 1982.] The production mechanism proposed was impact ionization, by HF-accelerated electrons, to energies exceeding thermospheric ionization potentials. Solar EUV, aurora, and high-power HF radio-waves produce suprathermal electrons in the 15-100 eV energy range, yielding long-lived ionization in the ionosphere. Once suprathermal electrons are produced, the Aeronomy of production, transport, and recombination are in common. The key to understanding artificial ionization thus reduces to conversion efficiency of HF energy to ionization. By 2008 technology reached GW ERP. The prediction was tested, and confirmed [Pedersen et al, 2009, Blagoveschenskaya et al, 2009] at high latitudes.

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Creating space plasma from the ground

In this paper, a current density convolution finite-difference time-domain method is extended to study the electromagnetic wave propagation and absorption in inhomogeneous magnetized plasmas. First of all, we analyze the wave propagation in homogeneous magnetized plasmas for arbitrary magnetic inclination angle. Then, we studied three types of plasma absorption, including plasma resonance, electron cyclotron resonance, and upper hybrid resonance in homogeneous and inhomogeneous magnetized plasmas. This model is applied to simulate the heating mechanism during ionospheric modification experiments by high-power high-frequency radio waves. These results have important implications for the interpretation of physical mechanisms for the interaction of high power radio waves with magnetized ionospheric plasmas and design of future experiments.

FDTD Analysis of Propagation and Absorption in Nonuniform Anisotropic Magnetized Plasma Slab

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The Extending of Observing Altitudes of Plasma and Ion Lines During Ionospheric Heating

The Arecibo high-power, high-frequency (HF) facility and 430 MHz radar are used to examine the temporal development of the HF-induced Langmuir and ion turbulences from 1 ms to many minutes after the turn-on of the HF beam in the F region. All HF observations begin in a smooth, stratified, stable plasma. “Cold start” HF transmissions are employed to avoid remnant irregularities from prior HF transmissions. HF-excited plasma line (HFPL) and ion line echoes are used to monitor the evolution of the turbulence. In the evening/nighttime the HFPL develops in three reproducible stages. Over time scales of 0 to 10–20 ms (possibly 40 ms), the smooth plasma conditions are maintained, and the results are consistent with theoretical models of the excitation of strong Langmuir turbulence near HF reflection. This entails the initiation of the so-called “caviton production cycle.” The turbulence from the parametric decay instability is detected at lower altitudes where the radar wave vector matches those of the HF-enhanced waves. The data suggests that the two processes coexist in the region in between. After ~40 ms the “overshoot process” begins and consists of a downward extension of the HFPL from the HF reflection region to heights ~1.1 km below followed by a retreat back to the reflection region. The whole overshoot process takes place over a time scale of ~3 s. Thereafter the echo remains near HF reflection for 20–90 s after HF turn-on. The HFPL echo subsequently breaks up into patches because of the formation of large-scale electron density structures in the plasma. New kinetic models indicate that suprathermal electrons excited in the plasma by, for example, caviton burn-out serve to regulate plasma turbulence in the modified ionospheric volume.

F. T. Djuth

Papers

Temporal Development of HF-Excited Langmuir and Ion Turbulence at Arecibo