Observation of ionospheric cavitons (original) (raw)
Artificial ionospheric layers driven by high‐frequency radiowaves: An assessment
Journal of Geophysical Research: Space Physics, 2016
High-power ordinary mode radio waves produce artificial ionization in the F region ionosphere at the European Incoherent Scatter (Tromsø, Norway) and High Frequency Active Auroral Research Program (Gakona, Alaska, USA) facilities. We have summarized the features of the excited plasma turbulence and descending layers of freshly ionized ("artificial") plasma. The concept of an ionizing wavefront created by accelerated suprathermal electrons appears to be in accordance with the data. The strong Langmuir turbulence (SLT) regime is revealed by the specific spectral features of incoherent radar backscatter and stimulated electromagnetic emissions. Theory predicts that the SLT acceleration is facilitated in the presence of photoelectrons. This agrees with the intensified artificial plasma production and the greater speeds of descent but weaker incoherent radar backscatter in the sunlit ionosphere. Numerical investigation of propagation of O-mode waves and the development of SLT and descending layers have been performed. The greater extent of the SLT region at the magnetic zenith than that at vertical appears to make magnetic zenith injections more efficient for electron acceleration and descending layers. At high powers, anomalous absorption is suppressed, leading to the Langmuir and upper hybrid processes during the whole heater on period. The data suggest that parametric upper hybrid interactions mitigate anomalous absorption at heating frequencies far from electron gyroharmonics and also generate SLT in the upper hybrid layer. The persistence of artificial plasma at the terminal altitude depends on how close the heating frequency is to the local gyroharmonic.
New phenomena observed by EISCAT during an RF ionospheric modification experiment
Radio Science, 1990
An ionospheric HF-modification experiment was carried out near TromsO, Norway, using the Max-Planck-Institut for Aeronomie (MPI) heating facility and the ElSCAT 933-MHz incoherent scatter radar (ISR). The MPI heater was normally operated at 4.04 MHz and modulated 20-s on, 40-s off. The ISR observed waves propagating parallel to B 0, and chirped as well as normal plasma line observations were performed. Heater-induced plasma lines were observed only in the first 10-s integration interval, indicating a strong overshoot. These lines are unusual in that multiple simultaneous lines were observed, normally originating within one kilometer of the critical region but sometimes from lower heights, and that the frequency of the most constant line is offset some 250 kHz from the heating frequency, with the other lines occurring at greater frequency offsets. The natural, photoelectron-enhanced plasma line was not observed; however, the background plasma was diagnosed via ion line observations and comparisons to chirped observations performed at EISCAT in May 1986 indicate that increased Landau damping may be responsible for both the strong induced-line overshoot and the lack of a distinct natural line. Finally, ion power profile observations show the existence of a topside enhanced ion line at the critical density corresponding to the heater frequency. We believe this is due to strong O to Z-mode coupling parallel to B0 and a low foF2. INTRODUCTION An RF ionospheric modification experiment was conducted in August 1986 near Troms½ using the Max-Planck-Institut f/Jr Aeronomie (henceforth MPI) heating facility and with the ElSCAT UHF incoherent scatter radar as the primary diagnostic tool. One of the main purposes of the experiment was to repeat and verify the observations made at Arecibo with the chirp technique [Birkmayer et al., 1986] which have consistently shown that the heater-enhanced plasma line comes from a higher altitude than the natural plasma line, and that this height is assumed within a few milliseconds of the RF heater turn-on [lsham et al., 1987]. It seemed
Direct access to plasma resonance in ionospheric radio experiments
Journal of Geophysical Research, 1984
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.
Observations of stimulated scattering of a strong high-frequency radio wave in the ionosphere
Physical Review Letters, 1982
Electromagnetic sideband spectra induced by a strong monochromatic high-frequency radio wave in an ovcrdense ionosphere have been observed for the first time by means of a new direct observational technique. The observed asymmetry of the spectra, a characteristic feature of stimulated scattering, is tentatively attributed to parametric and linear-mode-conversion processes occurring in the irradiated ionospheric-plasma volume.
Response of ionospheric electron density profile to the action of powerful HF radio-wave radiation
Solnechno-Zemnaya Fizika
Using electron density and temperature equations, we have modeled the dynamics of the electron density profile in the ionosphere due to the expulsion of plasma from localization regions of plasma waves, pumped by high-power HF radio waves, i.e. wave reflection and upper hybrid resonance regions. Causes of the ionospheric plasma expulsion are an increase in the gas-kinetic pressure due to the ohm heating of electrons by plasma waves, and the high-frequency pressure of plasma waves (ponderomotive expulsion). We have established that the ponderomotive expulsion develops more rapidly and is responsible for the formation of local regions of plasma density depletion near plasma resonances, whereas the gas-kinetic pressure increase is responsible for the formation of lower-density region, which is slower in time and more extended and smoother in height. The results obtained qualitatively agree with the data from the experiment conducted at the HAARP facility in 2014.
Ionospheric plasma by VHF waves
Pramana-journal of Physics, 2000
The amplitude scintillations of very high frequency electromagnetic wave transmitted from geo-stationary satellite at 244.168 MHz have been recorded at Varanasi (geom. lat. 14′ 55′N) during 1991 to 1999. The data are analyzed to determine the statistical features of overhead ionospheric plasma irregularities which are mostly of small duration <30 minutes and are predominant during pre-midnight period. The increase of solar activity generally increases the depth of scintillation. The auto-correlation functions and power spectra of scintillations predict that the scale length of these irregularities varies from 200–500 m having velocity of movement between 75 m/sec to 200 m/sec. These results agree well with the results obtained by other workers.
Artificial ionospheric cavity induced by the radiation from the “sura” facility
Radiophysics and Quantum Electronics, 1999
during experiments on the artificial modification of ionosphere by powerful HF radiation using spaced heating, we recorded twice a significant (tens of percent) decrease of the electron number density in the F-layer, synchronous with the operation of the heating transmitters. The critical frequency of this layer decreased by 10-20~. This points to the possibility of artificial generation of large-scale inhomogeneities of the ionospheric plasma density using spatially split heating.
Journal of Geophysical Research, 1989
Observations of very large poleward directed electric fields were obtained with a clustered set of instrumentation that included the Millstone Hill incoherent scatter radar, the Boston University Mobile Ionospheric Observatory, and the HILAT and Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites. In this paper we concentrate on data from the Millstone Hill incoherent scatter radar which was operated on selected evenings in a rapid azimuthal scan, centered on magnetic west. The mode was designed with the express purpose of measuring line-of-sight drift velocity and electron density as a function of latitude during events with large localized electric fields. During this same period, the Defense Nuclear Agency HILAT satellite made northem hemisphere measurements every 100 min of ion drift, density, and field-aligned currents across the equatorial boundary of the auroral oval. A detailed study of optical data in this region is provided in a companion paper. On the evenings of April 20 and 21, 1985, during an intense magnetic storm (K•, > 8+), large ionospheric electric fields (E > 80 mV/m) were detected along the edge of the auroral oval with the Millstone Hill incoherent scatter radar. These constitute the first definitive incoherent scatter observations of this phenomenon. An L shell-aligned (zero order) deep trough in electron density was colocated with these large electric fields at L shells as low as L = 2.8. These data indicate that the trough develops much more quickly than present theories predict, at least near the F peak. We also report elevated ion and electron temperatures in the trough and conjecture that these may contribute to the rapid decay. We also show that the associated field-aligned currents are very weak, as predicted by Banks and Yasuhara (1978) but that it is the F region structure which dominates the conductivity gradient rather than the E region emphasized by the earlier work. We also discuss the data set in light of competing theories for the production of large electric fields and for undulations of the edge of the diffuse aurora. In particular we discuss the importance of large radial ion temperature gradients indicated by the DMSP data we present. INTRODUCrlON In situ observations of large electric fields in the premidnight, high-latitude ionosphere were first published by Srniddy et al. [1977] using S3-2 satellite data, and subsequently by Maynard et al. [ 1978], Spiro et al. [1979], and Rich et al. [1980] using satellite-borne electric field detectors or particle drift meters. These observations have shown that very large localized poleward directed electric fields form along the edge of the auroral oval with strengths as high as 350 mV/m perpendicular to the magnetic field at altitudes around 1300 km, corresponding to westward directed flows of up to 10 km/s. Maynard et al. [1980] have also detected large localized electric fields directed radially outward in the equatorial plane near L = 4. Labelle et al. [ 1988] have reported a localized intense plasma flow region at 1500 local time at similar L values. To our Copyright 1989 by the American Geophysical Union. Paper number 88JA03874. 0148-0227/89/88JA-03874505.00 knowledge no examples of coincident large localized flow and large electric fields have been reported at magnetospheric altitudes. All of the above observations have been based on satellite data which typically consist of a few minutes of data spaced hours or days apart. In mid-April 1985, a multi-instrumented experiment was conducted at the Millstone Hill incoherent scatter radar facility in Westford, Massachusetts, for the purpose of obtaining the first continuous and extensive ground-based radar and optical measurements of these large electric fields and their effects on the ionospheric and magnetospheric plasma. HILAT and Defense Meteorological Satellite Program (DMSP) satellite data were also obtained during this period, which provided the first simultaneous measurements of these large localized electric fields by both satellite and groundbased incoherent scatter radar. This paper and the companion paper by Mendillo et al. [this issue] describe the radar and optical measurements and the geophysical conditions leading to the large fields, and provide some insights into the various mechanisms involved. These data provide information on magnetosphere-ionosphere 535O PROVIDAKES ET AL.: ]:•OU)• AND OPTICAl., i¾[EASURElVi•S OF IONOSPHERIC PROCESSF.,
An olden but golden EISCAT observation of a quiet-time ionospheric trough
Journal of Geophysical Research, 2010
1] Incoherent scatter measurements were carried out on 9 November 1987, showing the presence of an ionospheric trough in the F region. The experiment was made using the EISCAT UHF radar, and it consisted of an azimuthal scan with constant beam elevation and a meridional scan. Since the radar rotates with the Earth, beams with different directions from subsequent scans meet in the same MLT-CGMLat pixel in nonrotating frame. If the ionosphere is not too variable, these can be combined to give an average value of electron density and ion/electron temperature in each pixel. Furthermore, since different beams passing through the same pixel give different ion velocity components, it is also possible to obtain the velocity vector. The geomagnetic conditions during the observations were quiet enough for assuming a quasi-stationary ionosphere. It was found that both ion and electron temperatures have minima within the trough region and increase at the poleward wall. Ion velocity measurements, together with a convection model, suggest that the density depletion within the trough is due to recombination of F region plasma convecting for a long time in the dusk convection cell beyond the terminator. The northern edge of the trough is associated with soft particle precipitation. The southern edge is steeper than the northern edge, and is built by sunlit plasma brought to the trough region by corotation. The trough is thus a result of a combination of transport and precipitation processes rather than stagnation.
Focusing of HF radio-waves by ionospheric ducts
Journal of Atmospheric and Solar-Terrestrial Physics, 2011
This paper presents the first direct observations of HF focusing induced by natural and artificial ionospheric ducts along with a simple theoretical model. The experiments were conducted by injecting HF radio-waves using the Ionospheric Research Instrument of the High Frequency Active Auroral Research Program located in Gakona, Alaska and detecting them with instruments on the overflying French micro-satellite DEMETER. The latter observed a multiple frequency band structure, which is characteristic of a strong HF signal exceeding the detector's saturation level. Analysis of the O þ density measured by DEMETER along its orbit shows that the strong radio signal coincides with the presence of a ''negative'' duct in the ionosphere. ''Negative'' refers to the presence of a plasma density depletion with the peak depletion located near the center of the duct. Such ducts induce changes in the index of refraction leading to the focusing of HF waves in a manner equivalent to a ''thick'' plasma lens. Examination of the data along with a simple plasma lens model indicates the presence of focal node(s) in the vicinity of the overflying satellite. Two examples, one corresponding to focusing by a natural duct and one by an artificial one are presented.
Journal of Geophysical Research, 1998
Experimental evidence points toward magnetic-field-aligned small-scale plasma density striations as responsible for the long time features of stimulated electromagnetic emissions (SEE) in ionospheric radio modification experiments. This paper emphasizes that such striations act as resonators for the generation of trapped upper hybrid oscillations. The sidebands observed in long time SEE experiments are associated with nonstationary behavior, such as parametric instabilities, of these trapped oscillations. A theoretical model for this is formulated and studied numerically. Source spectra for SEE generation, consistent with the main properties of observed downshifted maximum and broad continuum features, will be presented. The first is associated with a trapped oscillations decay instability, while the latter is associated with cavitation in the pre-formed striations. 14,712
Physical Review Letters, 2009
Experimental results of secondary electromagnetic radiation, stimulated by high-frequency radio waves irradiating the ionosphere, are reported. We have observed emission peaks, shifted in frequency up to a few tens of Hertz from radio waves transmitted at several megahertz. These emission peaks are by far the strongest spectral features of secondary radiation that have been reported. The emissions are attributed to stimulated Brillouin scattering, long predicted but hitherto never unambiguously identified in highfrequency ionospheric interaction experiments. The experiments were performed at the High-Frequency Active Auroral Research Program (HAARP), Alaska, USA.
Radiophysics and Quantum Electronics, 2012
We present the results of observations of the wave disturbances in the ionospheric F region over the city of Kharkov, which accompanied the impact on the ionosphere by high-power radio emission of the Sura heating facility located about 960 km from the observation site. Enhancement of the wave activity in the heater operation intervals is detected. A 1.5-4-fold increase in the relative amplitude of the wave disturbance with a period of about 30 min close to the Sura operation mode at altitudes 160-235 km is revealed. The parameters of this disturbance are evaluated. It is shown that this effect can be due to the propagation of internal gravity waves in the atmospheric waveguide having about 200 km in height and 80-100 km in width. The efficiency of comprehensive analysis of the experimental data which we used to reveal and estimate the parameters of the wave disturbances with a small (a few percent) relative amplitude is demonstrated.
Ionospheric plasma density irregularities measured by stimulated electromagnetic emission
Journal of Geophysical Research, 2008
It is well known that ionospheric plasma turbulence can be conveniently generated by controlled injection of powerful high-frequency radio beams from the ground. Irradiation of the ionosphere with such radio waves leads to the formation of plasma density structures, striations, and the generation of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission (SEE). In this paper we present experimental results of the dependence of SEE on decreasing excitation levels of the striations. In the experiments the frequency of the injected radio beam was varied near the fifth harmonic of the local ionospheric electron gyro frequency. We use the SEE measurements to obtain transverse length scales of the striations involved in the generation of the SEE. Our results show that different spectral features of the SEE display different temporal dynamics, suggesting that they are related to striations with different transverse length scales (1 ] l ? ] 25 m).
Artificial modification of ionosphericE-layer by high power radio transmitters
Pure and Applied Geophysics, 1990
Some anomalous TV receptions in India on band I have been interpreted as being propagated via F-layer artificially modified by high power broadcast transmitters. In this paper, the possibility of VHF communication by artificially modified E-layer is discussed. The presented theoretical analysis shows that high-power broadcast transmitters can produce substantial changes in temperature and ionisation in the E-layer. The rate of energy transfer from the wave to the medium particles has also been computed. Model calculations are made for a transmitter located near Calcutta. The possibility of the extra-ionisation producing field-aligned structures to support VHF communication is discussed.
Magnetic field aligned super-small-scale irregularities in the concentration of charged particles are often observed in the Earth's ionosphere and magnetosphere. Earlier, the time evolution of such irregularities was studied with the help of the mathematical model, developed in the Polar Geophysical Institute. This model is based on a numerical solution of the Vlasov-Poisson system of equations. This mathematical model is used in the present paper. The purpose of the present paper is to examine numerically how high-power high-frequency radio waves, utilized for artificial heating experiments and pumped into the ionosphere by ground-based ionospheric heaters, influence on the time evolution of the super-small-scale irregularities present naturally in the F-layer ionospheric plasma. The results of simulation indicate that a presence of high-power high-frequency radio wave ought to influence essentially on the behavior of physically significant parameters of the plasma inside and in the vicinity of the irregularity.
A special experiment was carried out using a high powered transmitter and a high gain rhombic antenna to heat the ionosphere at a distance of 1300 km from the transmitter. A Digisonde ionospheric sounder was located at the midpoint to sense any changes that might occur in the ionosphere at the heating cycle period of ten minutes. The measured phase data was processed using spectrum analysis in an attempt to detect this ten minute period. DD,^, 1473 COITION OF 1 NOV 6S IS OBSOLETE SECURITY CUÄSSIFiCATlÖN OF THIS PAGE fWion Data Bnttod) IJNCTASSTFTT;T> UNCLASSIFIED «KCyWITY CLAStiriCATlOM QW TWIt gAjtggfcg O-Ktfnl) ^i 20. ABSTRACT (Continued) k The results are not conclusive but there is some indication that the heating period was detected only during the nighttime in agreement with theoretical prediction. fCa^