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<a target="_blank" rel="noopener" href="https://fatcat.wiki/container/ttyjhd5f3bhsxdbytjxgelqmr4" style="color: black;">Space Science Reviews</a>
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<span class="external-identifiers"> <a target="_blank" rel="external noopener noreferrer" href="https://doi.org/10.1007/s11214-018-0549-7">doi:10.1007/s11214-018-0549-7</a> <a target="_blank" rel="external noopener" href="https://fatcat.wiki/release/zclepocmjvazvgbsj7mwpfckr4">fatcat:zclepocmjvazvgbsj7mwpfckr4</a> </span>
more »... 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. Active ionospheric experiments involving the use of high-power, high-frequency (HF) transmitters to heat small regions of the ionosphere have a tremendous potential to reveal a wealth of information about plasma processes. The Earth's ionosphere is strongly coupled to the sun, less strongly coupled to the earth, is highly variable and exerts a wide array of effects on radio frequency (RF) propagation. Conventional ionospheric investigations involve remote sensing with RF transmitters on the ground or in space, or in-situ measurements with sounding rockets or satellites. To investigate a particular ionospheric phenomenon with conventional techniques, the experimenter must wait for that phenomena to appear. Active experiments can create a desired phenomenon on demand, effectively turning the overhead ionosphere into a plasma laboratory without walls. This review is concerned with active experiments involving heating of the ionosphere with HF transmitters from the ground and does not address other classes of active experiments involving chemical releases or in-situ plasma discharges or beam injections. High-power, HF radio transmitters can disturb plasma in the Earth's ionosphere and magnetosphere providing a unique opportunity to study interaction between electromagnetic waves and particles without the limited spatial scale-size and chamber edge effects that can be encountered while performing plasma experiments in a laboratory. By modulating the transmitted power in time, space or frequency, the ionosphere can effectively become an antenna for the generation of lower frequency waves. These lower frequency waves (ELF, VLF, ULF) provide opportunities to study a wide array of electromagnetic wave-particle interactions. The resulting interaction between the EM "pump" wave and the ionospheric plasma can then be observed via a number of channels: UHF/VHF incoherent scatter radar measures the plasma density and temperature; optical instruments observe the visible-spectrum of optical emissions produced via suprathermal electron collisions with neutrals; radio receivers and spectrum analysers monitor the stimulated electromagnetic emission (SEE) signal emerging from the heated region. The topic of active experiments by high power radio waves has generated enormous interest with thousands of publications including review articles by Gurevich , Leyser , and Leyser and Wong . The focus of this review is to a) report on the theoretical and experimental results from primarily the last decade in the areas of field-aligned irregularities, instabilities, ducts, ionization layers, optical emissions and ULF/ELF wave generation and propagation; b) discuss in detail satellite missions which have directly improved our understanding of new phenomena such as the formation of ducts; and c) to highlight science topics to be explored and propose experiments to address outstanding questions in this field. Section 2 describes the experimental facilities with section 2.1 giving a brief history and details of the four currently active ground-based heaters together with their main diagnostic instruments. Satellites and rockets have provided unique and important in-situ measurements of the heated volume and disturbances propagating from it to the upper ionosphere and magnetosphere. They are mentioned in some of the sections describing each HF facility. The most recent and planned 10 complex spatially and temporally variable antenna patterns down to elevations angles of 30º from the zenith. A photo of the antenna array is shown in Figure 2 .1. Because HAARP employs a phased array antenna, energy can be concentrated along variable directions, producing an effective radiated power (ERP) in the few GW range allowing a wide range of unique experiments. Other key instruments at HAARP include an ionosonde, GPS receivers, magnetometers, riometers, optical instruments and the MUIR radar. The HAARP program was initiated in 1989 and managed by the Air Force Research Laboratory (AFRL) and the Office of Naval Research (ONR). The facility was enhanced with additional funding from the Defense Advanced Research Projects Agency (DARPA), AFRL and ONR. In 2007 HAARP began operating at its current power levels. The ONR interest for HAARP was primarily focused on making the heated ionosphere a many-kilometer long antenna to generate and propagate extremely low frequency (ELF) signals for submarine communications. AFRL interest included studies of over the horizon radar capabilities and using the ionosphere to generate and inject ultra-low-frequency, extremely-low-frequency and very-low-frequency (ULF, ELF, VLF) waves along magnetic field lines into the magnetosphere. The goal was to use these waves to modify the pitch-angle distributions of trapped high energy electrons and increase their precipitation rates in order to reduce their fluxes in the radiation belts. Additional potential applications of HAARP include the use of the facility for: ionospheric imaging and solar corona/wind sounding; global HF communication and emergency broadcast messages; communication with submarines; detection of the sub-surface cavities; and as a transmitting element of an over-the-horizon radar (OTHR) system. In 2013 the Space Studies Board of the National Research Council conducted a Workshop to assess the scientific viability of HAARP. The Workshop resulted in a report entitled "The Role of High-Power, High Frequency Transmitters in Advancing Ionospheric/Thermospheric Research." That report described the scientific potential of HAARP to address science topics which are described in Section 2.1.5. SURA Ionospheric modification experiments in Nizhny Novgorod, Russia have been performed by the Radio Physical Research Institute (NIRFI) since 1973 at the Zimenki heating facility, located 20 km to the east of Nizhny Novgorod, Russia. This facility was operated at two pump wave frequencies f 0 = 5750 and 4600 kHz with effective radiated powers, P eff , of 20 and 12 MW respectively. The experimental results obtained were so impressive that it was decided to build a new more powerful heating facility (SURA facility) near the settlement of Vasil'sursk, 100 km to the east of Nizhny Novgorod (56.15° N, 46.1° E; magnetic dip angle I = 71°). The SURA facility was put into operation in November 1980. Since then it has been used for ionosphere modification by HF radio-waves to investigate a range of science topics listed in section 2.1.5.
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