Ionospheric Heating Experiments Research Articles

Cyclotron harmonic effect on the thermal oscillating two‐stream instability in the high latitude ionosphere

A thermal oscillating two‐stream instability as the generation mechanism of electron Bernstein/upper hybrid waves by the o‐mode HF heater wave in the high‐latitude ionosphere is investigated. The purely growing decay mode is described by the fluid equations, while the kinetic equations are employed to derive the coupled mode equations of the electron Bernstein/upper hybrid sidebands. It is found that the instability can be excited in the altitude regions both above and below the upper hybrid resonance layer. The parametric excitation of electron Bernstein waves is in general not effective. On the other hand, the upper hybrid sidebands of the instability excited in the “below” region have been considered by Leyser (1991) to be the pumps for the parametric excitation of downshifted maximum (DM) lines of the stimulated electromagnetic emissions (SEEs) observed in the ionospheric heating experiments. The detail analysis of the proposed work shows that the instability zone of upper hybrid waves below the upper hybrid resonance layer becomes very small when the heater frequency fnof;0 is operated near three times of the local electron cyclotron frequency fnof;c. Thus the source wave of the DM lines is effectively suppressed. This result explains the phenomenon of quenching of the DM feature in the SEE spectrum as fnof;0 is adjusted near 3fnof;c in the ionospheric modification experiments.

Introduction to ionospheric heating experiments at Tromsø—II. Scientific problems

In the first section the theory of parametric decay instabilities (PDI) is briefly described. Then results of joint heating/incoherent scatter observations at Tromsø are presented. It turns out that most of the observed features are in good agreement with that theory, while some others remain still unexplained. Among the latter features the most striking is the existence of a ‘space-time blob structure’, which means that the time variation of scattered power from adjacent altitudes seems to be correlated. Experiments at Arecibo often lead to results different from ours. Some scientists in the field explain these observations in terms of a ‘strong turbulence’ in which also caviton formation is involved. We think that most of the Tromsø results can be adequately explained by the parametric decay process.

Introduction to ionospheric heating at Tromsø—I. Experimental overview

The HF ionospheric modification (heating) facility at Ramfjordmoen will become a part of the EISCAT association from January 1993. This paper, which is intended for the new user, describes the technical capabilities of the facility and the broad range of geophysical and plasma physical experiments which are possible. An overview is presented of the physical effects that a powerful HF electromagnetic wave incident on the ionosphere can produce on timescales ranging from tens of microseconds to minutes in height regions ranging from 50 to hundreds of km. Emphasis is placed on the practical implementation of ionospheric heating experiments using the EISCAT incoherent scatter radars as the main diagnostic, but other diagnostic techniques using ground-based radars, radio links, radio receivers, photometers, rocket and satellite instrumentation are also described. A companion paper presents in greater depth some of the current scientific issues being addressed in ionospheric modification research.

High‐latitude F region ionospheric interchange modes in the presence of powerful radio waves

We examine the possibility of influencing medium scale size (0.1 ∼ 10 km) interchange modes in the high‐latitude F region ionosphere by a high‐power, high‐frequency (HF) electromagnetic (“pump”) wave launched from the ground. The parametric coupling of interchange modes with the pump wave will occur near the critical layer where the pump frequency matches the local plasma frequency with the nonlinear Joule heating providing the dominant coupling mechanism. We find that the parametric excitation (or suppression) of the interchange process at high latitudes is possible, in the local approximation, for short wavelengths (k⊥LN ≫ 1, where k⊥ and LN are the transverse wavenumber of the interchange mode and density gradient scalelength, respectively) using HF power densities (≳10−6 W/m²) that are attainable by current ionospheric heater facilities. These power density estimates for longer wavelength interchange modes, i.e., for k⊥LN ≲ 1, are increased. The results are discussed in light of previous and possible future ionospheric heating experiments.

A source mechanism producing HF‐induced plasma lines (HFPLS) with up‐shifted frequencies

A nonlinear scattering process has been analyzed as a source mechanism producing the frequency up‐shifted HF‐enhanced plasma lines (HFPLs) observed in the Arecibo ionospheric heating experiments. Parallelly propagating Langmuir waves generated by the parametric decay instability of the HF pump near the reflection height scatter off the background lower hybrid density fluctuations to produce the up‐going and down‐going Langmuir waves with frequencies greater than the pump frequency by several tens of KHz. The frequency up‐shifted nature of excited Langmuir waves stems from the fact that the natural frequencies of the obliquely propagating Langmuir waves are higher than those of the parallelly propagating Langmuir waves at the same altitude. A physical picture is thus offered to explain how Langmuir waves with frequencies greater than the HF heater wave frequency can be produced in the heating experiments and be detected by incoherent radars as frequency up‐shifted HFPLs. Since the considered scattering process occurs in a region near the reflection height, it explains why the frequency up‐shifted HFPLs should originate from the altitude near the reflection height as observed. Moreover, the theory also shows that the amount of frequency up‐shift is inversely proportional to the frequency of the HF heater and increases linearly with the electron temperature. The quantitative analysis of the theory shows a good agreement with the experimental results.

Stimulated electromagnetic emissions from magnetized and inhomogeneous plasma

We model the radio emission spectrum for an electromagnetically pumped, magnetized and inhomogeneous plasma and compare the results to observations of upper sideband features of stimulated electromagnetic emissions (SEE) in ionospheric heating experiments when the heater frequency, ω0, is near harmonics of the ionospheric electron cyclotron frequency, Ωe. Our model involves solving the equations of motion for electrons in an external electromagnetic pump wave and geomagnetic field to calculate, for an inhomogeneous plasma, a current which excites electron Bernstein and lower hybrid waves. We reinsert this current into the equations of motion and continue in an iterative manner. The result is a current which depends on density gradients across the geomagnetic field. The current intensifies with decreasing density gradient scale. It has a frequency ω = 2ω0 ‐ pΩe, where p is an integer, is resonant for ω0 ≈ pΩe and for which there is a cutoff in the spectra at about 10 kHz above the pump frequency. Our results agree with observations made in SEE experiments.

Laboratory simulation of Ionospheric Heating Experiment

Resonant absorption of an electromagnetic wave (f=3.5 GHz) in a multipolar discharge (ne ≃ 1011cm−3) with an adjustable density gradient is studied. Around the resonant location, nonlinear effects appear, associated with the ponderomotive force arising from the plasma wave. These effects are modelled by the Zakharov equations. Experimental and numerical results both exhibit a transition from a nonlinear steady state regime toward chaos when the amplitude of the electromagnetic wave or the gradient length is increased.

Parametric interaction between upper hybrid and lower hybrid waves in heating experiments

A two‐fluid model of the parametric decay of an upper hybrid wave into an electromagnetic wave and a lower hybrid wave [Murtaza and Shukla, 1984] is applied to conditions in the ionospheric F region. It is found that the instability threshold can be exceeded in ionospheric high frequency heating experiments. Features of the growth rate can explain hitherto unexplained properties of a commonly observed prominent spectral structure of stimulated electromagnetic emission. The stimulated radiation can then be used to determine the magnitude of the geomagnetic field where the upper hybrid frequency matches electron cyclotron harmonics in the ionosphere.

Mapping of the polar electrojet current down to ionospheric D region altitudes

The high‐power auroral stimulation (HIPAS) heating facility has been used to modulate D region ionospheric currents at high latitudes, producing extremely low frequency (ELF) radio wave emissions. The behavior of these ionospheric currents can be deduced from a comprehensive study of the ELF signals received at a local field site. This paper examines the mapping of the polar electrojet current from the E region down through the D region, where it can then be modulated by the heater beam. Results have been obtained using a simple Cowling model of the electrojet. These results indicate that for an electrojet flowing at an altitude of 110 km with a scale size in excess of 100 km, the mapping of the horizontal components of the current can be completely characterized in terms of the Pedersen and Hall conductivities. This is attributed to the fact that the mapping becomes independent of scale sizes which exceed 100 km. Several ionospheric heating experiments are proposed, including beam steering for localized ELF generation in the mapped region below electrojets.

Upper hybrid mode conversion and resonance excitation of Bernstein modes in ionospheric heating experiments

We investigate mode conversion of propagating upper hybrid waves to resonantly excited nonpropagating electrostatic electron cyclotron harmonic (Bernstein) modes and show that it can explain the absence of stimulated emission of sideband electromagnetic radiation from the ionosphere in the kilohertz frequency range less than the input pump frequency when the latter is very close to or at harmonics (third and above) of the electron cyclotron frequency. The results are in good agreement with the ionospheric heating experiments conducted at Tromsø (Norway) as well as at Arecibo (Puerto Rico).

Stimulated scattering of large amplitude waves in the ionosphere

The nonlinear coupling of a large amplitude electromagnetic pump wave with low-frequency collisional modes in the ionosphere is considered. It then turns out that the threshold values for stimulated electromagnetic emissions are exceeded in ionospheric heating experiments.

Tromsø Heating Experiments: Stimulated emission at HF pump harmonic and subharmonic frequencies

Observations of electromagnetic emissions on the HF heater subharmonic and several harmonic frequencies are presented. Often the second harmonic emissions show characteristic amplitude variation, frequency shift, and spectral broadening that clearly have their origin in ionospheric plasma processes. The strongest candidate is a combined parametric decay and Raman up‐conversion process. The subharmonic emission is believed to be evidence of absolute stimulated Raman scattering and/or two plasmon decay processes which have not been observed before in ionospheric heating experiments.

Airglow enhancements associated with plasma cavities formed during Ionospheric Heating Experiments

Optical measurements made at the Arecibo Observatory during the 1987 heating campaign showed large temporal and spatial variations in 630.0‐nm airglow enhancements during times of continuous power transmissions of high‐power radio waves. Photometric data displayed fluctuations of 60 R or more in the red‐line (630.0 nm) emission from atomic oxygen. These fluctuations were associated with heater‐induced cavities which drifted and evolved in the modified ionosphere. Data from the Arecibo incoherent scatter radar were used in conjunction with airglow images to provide a physical interpretation of the modification process. Electrons were accelerated by large amplitude Langmuir waves excited by parametric decay instabilities occurring near the wave reflection points inside the density cavities. Inelastic collisions with oxygen atoms produced excited states which yielded enhanced 630.0‐nm and 557.7‐nm emissions. A numerical model has been used to relate the enhanced airglow intensities to the energy spectrum of the accelerated electrons. The measured airglow could have been produced by an isotropic source at 340 km altitude that accelerated 0.01% of the ambient electrons into a suprathermal Maxwellian distribution with a temperature of 2.05 eV. Experimental and theoretical studies suggest that airglow clouds were directly coupled to plasma density cavities because (1) these cavities trapped the HF radio beam and (2) electrons accelerated out into regions of reduced plasma concentration were less effectively thermalized and, consequently, were more effective for collisional excitation of neutral species.

Artificial Airglow Excited by High-Power Radio Waves

High-power electromagnetic waves beamed into the ionosphere from ground-based transmitters illuminate the night sky with enhanced airglow. The recent development of a new intensified, charge coupled-device imager made it possible to record optical emissions during ionospheric heating. Clouds of enhanced airglow are associated with large-scale plasma density cavities that are generated by the heater beam. Trapping and focusing of electromagnetic waves in these cavities produces accelerated electrons that collisionally excite oxygen atoms, which emit light at visible wavelengths. Convection of plasma across magnetic field lines is the primary source for horizontal motion of the cavities and the airglow enhancements. During ionospheric heating experiments, quasi-cyclic formation, convection, dissipation and reappearance of the cavites comprise a major source of long-term variability in plasma densities during ionospheric heating experiments.

Physical processes of ionospheric heating experiments

Plasma instabilities, resulting from the presence of a powerful radio wave in ionospheric heating experiments, are reviewed. First the linear theories of the parametric decay instability and of the oscillating two-stream instability are considered. The saturation theories of these instabilities for weak pump waves are then outlined and their results are shown to be in reasonably good agreement with observations. Different theories are needed for excitation by strong pump waves. Such theories generally invoke the formation of cavitons. Observations suggesting the formation of cavitons are outlined. The differences between caviton properties derived from observations and the properties derived from existing theories are pointed out. The effect of plasma instabilities on the creation or modification of a population of energetic electrons in ionospheric heating experiments is demonstrated by indirect observations of the temporal evolution of energetic electron fluxes after the heating transmitter is switched on.

Spatial and temporal evolution of 630.0 nm airglow enhancement during ionospheric heating experiments

Images of 630.00 nm enhancements have been recorded during the January, 1986 ionospheric heating campaign at Arecibo. The artificial airglow clouds convected eastward, vanished, and then reappeared at the zenith of the HF heater. Occasionally, the airglow patches are bifurcated. The structure and motion of the airglow clouds is an indication of the dynamic behavior of the modified ionosphere.

Parametric excitation of collisional modes in the high‐latitude ionosphere

This paper points out that the nonlinear coupling of a large‐amplitude electromagnetic pump wave with convective plasma fluid instabilities is significantly enhanced by pump‐induced temperature changes. It should thus be easy to exceed the threshold values for parametric instabilities in several ionospheric heating experiments.

Detection of radiation from a heated and modulated equatorial electrojet current system

In May 1983, ionospheric heating experiments were conducted using the Jicamarca very high frequency (VHF) radar facility located ∼11 km east of Lima, Peru. Experiments involving high frequency (HF) heating of the lower D-region of the ionosphere were successfully conducted during 1982 using the Arecibo, Puerto Rico HF transmitter located 5km east at Islote, Puerto Rico. These local experiments had characterized the signal radiated from a heated and modulated ionospheric current system near the mid-latitudes. A long-path signal had also been received in September 1982 at Salinas, Puerto Rico from a mid-day equatorial electrojet, heated and modulated by the Jicamarca facility. We have investigated the characteristics of the local signal that would be radiated from a strong equatorial electrojet when heated and modulated, and report here that at the geomagnetic equator they were similar to, but less intense than, those observed at Arecibo, Puerto Rico due to parameter differences. This radiation is believed to be the first detected from a heated and modulated equatorial electrojet current system in the Western Hemisphere.

Direct access to plasma resonance in ionospheric radio experiments

The concept of linear conversion of radio waves into electrostatic (ES) waves is adapted to ionospheric radio heating experiments. It is identified as access to the plasma resonance through the radio window. By means of existing heating facilities, large concentrations of electrostatic wave energy can be generated. The ES waves are confined to a restricted region in space, horizontally displaced in relation to the vertical over the heating transmitter along the magnetic meridian, the displacement being southward on the northern hemisphere. Ray‐tracing studies of the location of the resonance region relative to the transmitter are presented, and the horizontal dimension is estimated.

Excitation of upper-hybrid waves by a thermal parametric instability

A purely growing instability characterized by a four-wave interaction has been analysed in a uniform, magnetized plasma. Up-shifted and down-shifted upper-hybrid waves and a non-oscillatory mode can be excited by a pump wave of ordinary rather than extraordinary polarization in the case of ionospheric heating. The differential Ohmic heating force dominates over the ponderomotive force as the wave–wave coupling mechanism. The beating current at zero frequency produces a significant stabilizing effect on the excitation of short-scale modes by counterbalancing the destabilizing effect of the differential Ohmic heating. The effect of ionospheric inhomogeneity is estimated, showing a tendency to raise the thresholds of the instability. When applied to ionospheric heating experiments, the present theory can explain the excitation of field-aligned plasma lines and ionospheric irregularities with a continuous spectrum ranging from metre-scale to hundreds of metre-scale. Further, the proposed mechanism may become a competitive process to the parametric decay instability and be responsible for the overshoot phenomena of the plasma line enhancement at Arecibo.