Ionospheric Modification Facility Research Articles

An Ionospheric Modification Facility for the Magnetic Equator

An Ionospheric Modification Facility for the Magnetic Equator

VHF Radar Images of Artificial Field‐Aligned Ionospheric Irregularities in the Subauroral E Region

AbstractArtificial E region field‐aligned plasma density irregularities (AFAIs) have been generated using the High‐frequency Active Auroral Research Program (HAARP) ionospheric modification facility in Gakona and observed with a 30 MHz coherent scatter radar imager in Homer, Alaska. The AFAIs were generated using a distinctive, twisted‐beam antenna pattern that illuminated a particularly broad volume overhead. The broad beam facilitates studies of natural sporadic E layer patches when they are present. The center of the pattern was pointed at different angles between zenith and magnetic zenith to examine the effects on the AFAI morphology. Radar images of AFAIs generally resemble the radiation pattern of the high‐frequency source, but the irregularities are strongest within a narrow range of zenith angles bounded approximately by the Spitze angle. A number of factors which might influence AFAI generation and detection are examined. The most important is most likely the requirement for the pump mode to have a standing‐wave component for thermal parametric instability to operate.

Ionospheric electron number densities from CUTLASS dual‐frequency velocity measurements using artificial backscatter over EISCAT

AbstractUsing quasi‐simultaneous line‐of‐sight velocity measurements at multiple frequencies from the Hankasalmi Cooperative UK Twin Auroral Sounding System (CUTLASS) on the Super Dual Auroral Radar Network (SuperDARN), we calculate electron number densities using a derivation outlined in Gillies et al. (2010, 2012). Backscatter targets were generated using the European Incoherent Scatter (EISCAT) ionospheric modification facility at Tromsø, Norway. We use two methods on two case studies. The first approach is to use the dual‐frequency capability on CUTLASS and compare line‐of‐sight velocities between frequencies with a MHz or greater difference. The other method used the kHz frequency shifts automatically made by the SuperDARN radar during routine operations. Using ray tracing to obtain the approximate altitude of the backscatter, we demonstrate that for both methods, SuperDARN significantly overestimates Ne compared to those obtained from the EISCAT incoherent scatter radar over the same time period. The discrepancy between the Ne measurements of both radars may be largely due to SuperDARN sensitivity to backscatter produced by localized density irregularities which obscure the background levels.

First observation of the anomalous electric field in the topside ionosphere by ionospheric modification over EISCAT

AbstractWe have developed an active ground‐based technique to estimate the steady state field‐aligned anomalous electric field (E*) in the topside ionosphere, up to ~600 km, using the European Incoherent Scatter (EISCAT) ionospheric modification facility and UHF incoherent scatter radar. When pumping the ionosphere with high‐power high‐frequency radio waves, the F region electron temperature is significantly raised, increasing the plasma pressure gradient in the topside ionosphere, resulting in ion upflow along the magnetic field line. We estimate E* using a modified ion momentum equation and the Mass Spectrometer Incoherent Scatter model. From an experiment on 23 October 2013, E* points downward with an average amplitude of ~1.6 μV/m, becoming weaker at higher altitudes. The mechanism for anomalous resistivity is thought to be low‐frequency ion acoustic waves generated by the pump‐induced flux of suprathermal electrons. These high‐energy electrons are produced near the pump wave reflection altitude by plasma resonance and also result in observed artificially induced optical emissions.

Observations of unusually broadened HF radar spectra from heater-induced artificial plasma irregularities

[1] Cooperative UK Twin-Located Auroral Sounding System (CUTLASS) HF backscatter targets may be artificially produced using the European Incoherent Scatter (EISCAT) HF ionospheric modification facility at Tromso, Norway. Plasma irregularities created in this way are known to be highly field aligned and usually give rise to radar echoes possessing spectral widths that are much lower than those seen in natural radar auroral data. In this study we present CUTLASS Finland and Iceland observations of heater-induced irregularities; these irregularities gave rise to enhanced spectral width in the Iceland radar backscatter signal, while the width observed by the Finland radar, pointing almost orthogonal to the Iceland beams, remained low. The observations coincided with a period of disturbed ionospheric conditions when faster-flowing natural irregularities were present, poleward of the heated volume, as well as a flow gradient across the heated volume itself. A latitudinal variation of the Doppler velocity measurements was exhibited in both the Finland and Iceland radar observations. We suggest that this flow inhomogeneity, taken together with the geometry of the observing radar beams, would explain not only the broad spectrum of flow components across the heated volume detected by the Iceland radar but also the asymmetry between the spectral widths measured at Iceland and Finland.

First incoherent scatter radar observations of ionospheric heating on the second electron gyro-harmonic

We report first results from a unique experiment performed at the HIPAS ionospheric modification facility in conjunction with the Poker Flat incoherent scatter radar in Alaska. High-power radio waves at 2.85 MHz, which corresponds to the second electron gyro-harmonic at ~245 km altitude, were transmitted into the nighttime ionosphere. Clear evidence of F-region ionospheric electron temperature enhancements were found, for the first time at this pump frequency, maximizing when the pump frequency is close to the second gyro-harmonic and double resonance. This is consistent with previous pump-enhanced artificial optical observations. We estimate the plasma heating efficiency to be approximately double that for higher pump frequencies.

Application of an imaging HF riometer for the observation of scintillations of discrete cosmic sources

Scintillations of discrete cosmic radio sources have been investigated theoretically and experimentally for radiation scattered from natural and artificial inhomogeneities in the auroral zone ionosphere. The observations were performed at a frequency of 38.2 MHz with the multibeam phased array antenna of the imaging riometer located at Poker Flat, Alaska. For quiet ionospheric conditions and weak scintillations, the spatial spectrum and transverse motion velocities of the Fresnel inhomogeneities have been recovered. A theoretically based approach has been developed to estimate the time coherence interval of the radiation for the case of saturated scintillations and strong ionospheric turbulence. Examples of scintillation records are analyzed for the case of scattering from artificial ionospheric irregularities induced by high‐power HF radiation from the high‐power auroral simulation (HIPAS) ionospheric modification facility. The effect of aspect‐sensitive scattering of the cosmic source radiation by artificial field‐aligned small‐scale irregularities has been predicted and detected. Prospects for routine observations of the effects of scintillations and aspect‐sensitive scattering are discussed for special operation modes of the ionospheric modification facility.

Imaging HF‐induced large‐scale irregularities above HAARP

The University of Massachusetts‐Lowell digisonde is used with the HAARP high‐frequency (HF), ionospheric modification facility to obtain radio images of artificially‐produced, large‐scale, geomagnetic field‐aligned irregularities. F region irregularities generated with the HAARP beam pointed in the vertical and geomagnetic field‐aligned directions are examined in a smooth background plasma. It is found that limited large‐scale irregularity production takes place with vertical transmissions, whereas there is a dramatic increase in the number of source irregularities with the beam pointed parallel to the geomagnetic field. Strong irregularity production appears to be confined to within ∼5° of the geomagnetic zenith and does not fill the volume occupied by the HF beam. A similar effect is observed in optical images of artificial airglow.

Ionospheric Modification at Twice the Electron Cyclotron Frequency

In 2004, a new transmission band was added to the HAARP high-frequency ionospheric modification facility that encompasses the second electron cyclotron harmonic at altitudes between approximately 220 and 330 km. Initial observations indicate that greatly enhanced airglow occurs whenever the transmission frequency approximately matches the second electron cyclotron harmonic at the height of the upper hybrid resonance. This is the reverse of what happens at higher electron cyclotron harmonics. The measured optical emissions confirm the presence of accelerated electrons in the plasma.

Optical remote sensing of the thermosphere with HF pumped artificial airglow

Optical emissions excited by high‐power radio waves in the ionosphere can be used to measure a wide variety of parameters in the thermosphere. Powerful high‐frequency (HF) radio waves produce energetic electrons in the region where the waves reflect in the F region. These hot or suprathermal electrons collide with atomic oxygen atoms to produce localized regions of metastable O(1D) and O(1S) atoms. These metastables subsequently radiate 630.0 and 557.7 nm, respectively, to produce clouds of HF pumped artificial airglow (HPAA). The shapes of the HPAA clouds are determined by the structure of large‐scale (≈10 km) plasma irregularities that occur naturally or that develop during ionospheric heating. When the HF wave is operated continuously, the motion of the airglow clouds follows the E × B drift of the plasma. When the HF wave is turned off, the airglow clouds decay by collisional quenching and radiation, expand by neutral diffusion, and drift in response to neutral winds. Images of HPAA clouds, obtained using both continuous and stepped radio wave transmissions, are processed to yield the electric fields, neutral wind vectors, and diffusion coefficients in the upper atmosphere. This technique is illustrated using data that were obtained in March 1993 and 1995 at the ionospheric modification facility near Nizhny Novgorod, Russia. Analysis of HPAA clouds yields zonal plasma drifts of 70 m s−1 eastward at night. On the basis of artificial airglow from energetic electrons generated at 260 km the zonal neutral wind speed was estimated to be 96 m s−1 and the O(1D) diffusion coefficient was determined to be between 0.8 and 1.4 × 1011 cm2 s−1. The quenched lifetime of the O(1D) was determined to be 29.4 s. The diffusion and quenching rates are directly related to the atomic and molecular concentrations in the thermosphere. Improvements in the remote‐sensing technique may be obtained if the intensity of the artificial airglow emissions is increased. High‐power radio transmissions employing pulse sequences and tuning near electron cyclotron harmonics were attempted to increase the optical emissions. Both of these, however, produced reduced intensity, and consequently, continuous transmission at frequencies away from electron gyro harmonics is the preferred heating regime.

Effect of ionospheric self‐conditioning and preconditioning on the broad upshifted maximum component of stimulated electromagnetic emission

Stimulated electromagnetic emissions (SEE) are high‐frequency radio emissions that are generated during high‐power, high‐frequency, ordinary mode (O‐mode), radiowave heating of the ionosphere. These emissions are particularly useful in ionospheric heating experiments because they provide a way of monitoring space plasma processes remotely and passively. In order to utilize these emissions for diagnostic purposes, it is necessary to understand the space plasma processes involved in their generation. The purpose of these experiments was to observe the responses of a particular component of the SEE, the broad upshifted maximum (BUM), to a variety of heating stimuli in an effort to understand the factors involved in its development. Heating experiments were conducted at the Radiophysical Research Institute SURA Ionospheric Modification Facility in Russia. Experiments consisted of single‐pump, two‐pump, and single‐pump power‐stepping experiments. The single‐pump and two‐pump transmissions were organized into groups of pulses of varying widths and spacings to facilitate the investigation of self‐conditioning, preconditioning, and two‐pump‐interaction conditioning effects. The major findings of these experiments are that the action of a pump can have a conditioning effect on the medium that affects the time development of the BUM. The result of the conditioning process is the formation of an overshoot in the temporal development of the BUM. A residual conditioning effect is sustained after the end of a pump pulse for a period of time (∼30 s). The residual conditioning acts as preconditioning for the BUM of a subsequent pump pulse. A second O‐mode pump (pump2), at a frequency a few hundred kilohertz above that of the first pump (pump1), is observed to cause additional suppression of the pump1 BUM, implying an enhanced conditioning effect. Time constants for the buildup and decay of the conditioning effects are estimated. During power‐stepping experiments, the BUM spectrum was observed to evolve from a weak, narrow spectrum at an effective radiated power (ERP) of ≈5 MW, to a strong, broad spectrum with a ramp‐like spectral tail at an ERP of ≈150 MW. Other features noted during power stepping include (1) strong BUM transients at pump power level transitions, (2) BUM amplitude asymmetry between power‐up and power‐down sides of a power stepping cycle, (3) reduction of the BUM spectral peak offset from the pump frequency with increasing pump power, and (4) power law dependence of BUM power on pump power (exponent ∼2). Results of these experiments are used in an attempt to assess the relevance of small‐scale irregularity generation and electron heating mechanisms to the observed conditioning effects.

First CUTLASS-EISCAT heating results

The EISCAT ionospheric modification facility (heater), together with the EISCAT UHF radar, lie within the viewing area of the newly constructed HF radar system, CUTLASS. Results are reported from the first experiments in which CUTLASS has been employed to observe artificial small scale field aligned plasma irregularities induced by the EISCAT heater. Observations include the first simultaneous CUTLASS-EISCAT UHF measurements of heating effects associated with heater induced field aligned irregularities, the first determination of the spatial distribution of these irregularities and the first examination of their motion.

Tristatic EISCAT‐UHF measurements of the HF modified ionosphere for low background electron temperatures

The effects of a powerful high‐frequency electromagnetic wave transmitted in the ordinary mode from the European Incoherent Scatter (EISCAT) ionospheric modification facility up to the daytime ionospheric F region have been studied with the EISCAT‐UHF radar system. This is one of the first tristatic EISCAT‐UHF incoherent scatter experiments, in which the excited Langmuir and ion acoustic fluctuations are measured simultaneously at scattering angles near parallel (Tromsø site), ∼22° (Kiruna site), and ∼31° (Sodankylä site) to the geomagnetic field. We present measurements of the spectral structure and temporal evolution with a maximum resolution of 13 ms, at the different scattering angles. For the first time, up to five cascade lines in addition to the decay line in the Tromsø HF‐enhanced plasma line and well‐developed cascade type plasma lines in Sodankylä were observed. In spite of low background electron temperature, and hence negligible electron Landau damping parallel to the geomagnetic field, the excited plasma turbulence varied considerably.

Downshifted maximum features in stimulated electromagnetic emission spectra

New and detailed experimental and theoretical results concerning the prominent downshifted maximum (DM) feature in spectra of stimulated electromagnetic emissions are reported. The experimental results were obtained at the Sura ionospheric modification facility in Russia by transmitting a powerful high‐frequency ordinary mode pump wave into the ionospheric F region. We present detailed experimental results of the dependence of the DM on the pump frequency. Different frequency components of the DM have slightly different growth times after pump turn‐on and are suppressed in different pump frequency ranges at harmonics of the electron cyclotron frequency. The pump frequency range near the cyclotron harmonics in which the DM does not develop, decreases significantly with increasing harmonic, from several kilohertz at the fourth harmonic to an extremely narrow range of 0.2 kHz or less at the seventh harmonic. We discuss conditions for plasma wave propagation to explain this dependence on the cyclotron harmonics.

Spectral structure of stimulated electromagnetic emissions between electron cyclotron harmonics

Experimental results concerning the spectrum of stimulated electromagnetic emissions (SEE) in the sidebands of a powerful high‐frequency electromagnetic ordinary mode pump wave are presented. The pump wave was vertically injected into the ionospheric F region from the Sura ionospheric modification facility in Russia. We report detailed measurements of the commonly observed continuum, downshifted maximum, and broad upshifted maximum emissions for pump frequencies ranging from the fourth to above the seventh electron cyclotron harmonic in the ionospheric plasma as well as observations of a new wideband emission occurring primarily in the upper sideband. The dependence of the SEE spectral structure on the pump frequency in relation to electron cyclotron harmonics is emphasized. All spectral features in the sidebands of the pump exhibit cyclotron harmonic effects.

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.

HF‐enhanced plasma lines in the lower ionosphere

The ionospheric modification facility at Arecibo, Puerto Rico, has been used to study Langmuir waves excited in the lower ionosphere by a high‐power HF radio wave. Measurements of HF‐enhanced plasma lines have been made in the lower F region (< 160 km altitude) and in the E region using the 430‐MHz radar at Arecibo Observatory. These measurements complement observations made in the past at higher altitudes. Enhanced plasma line spectra observed in the lower F region peak at the so‐called “decay line” and contain additional spectral structure similar to that found in the upper F region. In the E region the spectra exhibit narrow peaks located at 430 MHz ±ƒHF, where ƒHF is the frequency of the modifying HF wave. While strong plasma line enhancements are commonly observed in sporadic E, only weak enhancements have been detected in the normal daytime E layer. Candidate mechanisms for the E region enhancements include the oscillating two‐stream instability and direct conversion of the HF wave into Langmuir waves.

Observations of stimulated electromagnetic emissions in ionospheric heating experiments

Induced sideband modes, due to stimulated electromagnetic emissions in the ionosphere, have been observed around the frequency of a high‐intensity HF radio wave transmitted from the Heating ionospheric modification facility near Tromsø, Norway. The new observational technique used amounts to analyzing the received HF spectrum directly without utilizing any diagnostic radar. It appears that we have in this way been able to identify the parameteric decay instability and, possibly, the stimulated Brillouin backscattering of the strong HF pump wave. A series of narrow spectral peaks induced in the upper sideband by Heating transmission in extraordinary mode may indicate the excitation of ion Bernstein modes. At times, very strong spectral features, upshifted from the 5.4‐MHz pump by typically 20–45 kHz, are observed. These features are still lacking a convincing theoretical explanation. By monitoring a few fixed frequencies in both the lower and the upper sidebands of the ionospherically reflected modifying wave, we have found that the intensity of the stimulated emissions exhibits a conspicuous long‐term as well as short‐term variability.

Observations of the HF‐enhanced plasma line with a 46.8‐MHz Radar and reinterpretation of previous observations with the 430‐MHz radar

During 5.1‐MHz HF transmissions below the F region penetration frequency by a nearby ionospheric modification facility, radar echoes coming from F region heights at a frequency of 46.8 + 5.1 = 51.9 MHz were received while a VHF radar using the 305 m spherical reflector at Arecibo was transmitting pulses coherently on 46.8 MHz. The bandwidth of the echoes was less than 10 Hz. The so‐called decay line due to the parametric decay instability was expected at a frequency lower by about 300 Hz, but it was not observed. Density variations caused by the ponderomotive force resulting from the standing wave pattern at 5.1 MHz are tentatively invoked to explain the observations. Poorly understood aspects of some previous observations with the 430‐MHz radar can be similarly explained.

Nonlinear reflectivity of high-power radio waves in the ionosphere

An important result of earlier ionospheric modification experiments performed at Platteville, Colorado, was the discovery that the reflectivity of the ionosphere was greatly reduced in the reflection region of a high-power radio (pump) wave1. Because the reflectivity of low-power diagnostic waves passing through the modified region with frequencies from just below the pump wave frequency up to the F region critical frequency were reduced, the phenomenon was termed wide-band absorption (WBA). Subsequent experimental and theoretical work (for review see ref. 2) has led to the conclusion that WBA is due to the scattering of radio waves from field-aligned irregularities into plasma waves. These irregularities are generated by the interaction of the pump wave and the ionospheric plasma but the mechanism by which this occurs is not completely understood. It is evident, however, that in the process the pump wave itself undergoes WBA. In a recent campaign using the Max-Planck Institute ionospheric modification facility at Ramfjordmoen, near Tromso, Norway, the existence of WBA at high latitudes was confirmed by experiments using low-power diagnostic waves passing through the reflection region of the high-power radio wave3,4. As we report here, during an experiment to measure the effect of WBA on the pump wave itself, the essentially nonlinear nature of the interaction between the high-power radio waves and the ionosphere was demonstrated in a new and striking way.