ELF and VLF wave generation by HF heating: A comparison of AM and CW techniques

Antenna field 2 of the Tromsø Heating facility consists of six rows of six horizontal crossed full wave dipoles aligned with the rows running geographic east-west. In previous experiments on ELF/VLF wave generation it has been the practice to feed the rows in parallel, with HF radiation amplitude modulated at the ELF/VLF frequency it was desired to radiate (AM configuration). Here we describe how the antenna array was also configured so that it could be fed with a continuous wave (CW) input power but still carry information at an ELF/VLF frequency. To effect this the three southern most rows of the antenna array were driven with a CW signal at 4.04 MHz and the three northernmost rows with a CW signal at frequency greater than 4.04 MHz by the ELF/VLF frequency it was desired to radiate (CW configuration). Experiments were performed with modulation/difference frequencies of 565 and 2005 Hz and the signals were received at the Lycksele Geophysical Observatory, 500 km south of the heating facility. The signals were typically 11 dB greater in the AM than the CW configuration, despite the fact that the average power delivered to the ionosphere in the CW configuration was four times that in the AM configuration. Significant harmonic radiation (both odd and even) was produced in the AM configuration but no harmonic radiation was detected in the CW configuration. A simple theory has been developed to model the fields produced by HF heating using the two techniques (AM,CW). A good agreement has been obtained between the experimental observations and model computations. The model has been extended to show how the relative efficiency of generation, AM/CW, varies with the frequency of the ELF/VLF radiation and HF antenna element spacing. Radiation patterns for the ‘ionospheric ELF/VLF antenna’ have also been derived for the two generation techniques.     

Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway

We present an analysis of phenomena observed by HF distance-diagnostic tools located in St. Petersburg combined with multi-instrument observation at Tromsø in the HF modified ionosphere during a magnetospheric substorm. The observed phenomena that occurred during the Tromsø heating experiment in the nightside auroral E_s region of the ionosphere depend on the phase of substorm. The heating excited small-scale field-aligned irregularities in the E region responsible for field-aligned scattering of diagnostic HF waves. The equipment used in the experiment was sensitive to electron density irregularities with wavelengths 12–15 m across the geomagnetic field lines. Analysis of the Doppler measurement data shows the appearance of quasiperiodic variations with a Doppler frequency shift, f_d and periods about 100–120 s during the heating cycle coinciding in time with the first substorm activation and initiation of the upward field-aligned currents. A relationship between wave variations in f_d and magnetic pulsations in the Y-component of the geomagnetic field at Tromsø was detected. The analysis of the magnetic field variations from the IMAGE magnetometer stations shows that ULF waves occurred, not only at Tromsø, but in the adjacent area bounded by geographical latitudes from 70.5° to 68° and longitudes from 16° to 27°. It is suggested that the ULF observed can result from superposition of the natural and heater-induced ULF waves. During the substorm expansion a strong stimulated electromagnetic emission (SEE) at the third harmonic of the downshifted maximum frequency was found. It is believed that SEE is accompanied by excitation of the VLF waves penetrating into magneto-sphere and stimulating the precipitation of the energetic electrons (10–40 keV) of about 1-min duration. This is due to a cyclotron resonant interaction of natural precipitating electrons (1–10 keV) with heater-induced whistler waves in the magnetosphere. It is reasonable to suppose that a new substorm activation, exactly above Tromsø, was closely connected with the heater-induced precipitation of energetic electrons.     

利用射线追踪方法研究化学物质释放对高频加热的聚焦效应

电离层化学物质释放能导致释放区域电子密度的损耗,从而产生明显的电离层空洞现象.高频电波通过电离层空洞时,由于电子密度不同,对电波产生折射效果进而形成聚焦效应.本文利用射线追踪方法,评估高频电波通过空洞形成聚焦加热的效果.结果表明,释放水分子与SF6都能对电离层产生明显的空洞,空洞半径约为25～50 km,电子密度的损耗...     

SPEAR-induced field-aligned irregularities observed from bi-static HF radio scattering in the polar ionosphere

Experimental results from SPEAR HF heating experiments in the polar ionosphere are examined. Bi-static scatter measurements of HF diagnostic signals were carried out on the Pori (Finland)–SPEAR–St. Petersburg path at operational frequencies of 11,755 and 15,400 kHz and the London–SPEAR–St. Petersburg path at frequencies of 12,095 and 17,700 kHz, using a Doppler spectral method. The SPEAR HF heating facility generates heater-induced artificial field-aligned small-scale irregularities (AFAIs), which can be detected by HF diagnostic bi-static radio scatter techniques at St. Petersburg at a distance of about 2000 km. In accordance with the Bragg condition, HF bi-static backscatters were sensitive to small-scale irregularities having spatial sizes of the order of 9–13 m across the geomagnetic field line. The properties and behaviour of AFAIs have been considered in the winter and summer seasons under quiet magnetic conditions and under various status of the polar ionosphere (the presence of “thick” and “thin” sporadic Es layers, different structures of the F2 layer). The experimental results obtained have shown that AFAIs can be excited in the F as well as in the E regions of the polar ionosphere. The excitation of a very intense wide-band spectral component with an abrupt increase in the spectral width up to 16–20 Hz has been found in the signals scattered from striations. Along with a wide-band component, a narrow-band spectral component can be also seen in the Doppler sonograms and in the average spectra of the signals scattered from the SPEAR-induced striations. AFAIs were excited even when the HF heater frequency was up to 0.5 MHz larger than the critical frequency. A simulation of the ray geometry for the diagnostic HF radio waves scattered from AFAIs in the polar ionosphere has been made for the geophysical conditions prevailing during experiments carried out in both the winter and summer seasons.     

Observations of intensity variations and harmonics of heater induced VLF waves

Very low-frequency (VLF) radio waves have been generated in the ionosphere by the heating facility near Tromsø. VLF signals have been received at Porojärvi (69.17 ° N, 21.47 ° E, L = 6.1), in Northern Finland, about 100 km from the heating transmitter. Three heating experiments were performed under different geophysical conditions and different results were obtained. Signal intensities have been compared with magnetic variations. Correlations and anti-correlations were observed. Several harmonics of the generated signal have generally been found. In some cases when the intensity of the detected VLF signal was low the first harmonic was as intensive as the fundamental frequency. VLF response from Artificial Periodic Inhomogeneities (API) experiment was also observed.     

Observations of Pc 3–4 and Pi 2 geomagnetic pulsations in the low-latitude ionosphere

Day-time Pc 3–4 (≃5–60 mHz) and night-time Pi 2 (≃5–20 mHz) ULF waves propagating down through the ionosphere can cause oscillations in the Doppler shift of HF radio transmissions that are correlated with the magnetic pulsations recorded on the ground. In order to examine properties of these correlated signals, we conducted a joint HF Doppler/magnetometer experiment for two six-month intervals at a location near L = 1.8. The magnetic pulsations were best correlated with ionospheric oscillations from near the F region peak. The Doppler oscillations were in phase at two different altitudes, and their amplitude increased in proportion to the radio sounding frequency. The same results were obtained for the O- and X-mode radio signals. A surprising finding was a constant phase difference between the pulsations in the ionosphere and on the ground for all frequencies below the local field line resonance frequency, independent of season or local time. These observations have been compared with theoretical predictions of the amplitude and phase of ionospheric Doppler oscillations driven by downgoing Alfvén mode waves. Our results agree with these predictions at or very near the field line resonance frequency but not at other frequencies. We conclude that the majority of the observations, which are for pulsations below the resonant frequency, are associated with downgoing fast mode waves, and models of the wave-ionosphere interaction need to be modified accordingly.     

CUTLASS HF radar observations of high-latitude azimuthally propagating vortical currents in the nightside ionosphere during magnetospheric substorms

High-time resolution CUTLASS observations and ground-based magnetometers have been employed to study the occurrence of vortical flow structures propagating through the high-latitude ionosphere during magnetospheric substorms. Fast-moving flow vortices (800 m s^–1) associated with Hall currents flowing around upward directed field-aligned currents are frequently observed propagating at high speed (1 km s^–1) azimuthally away from the region of the ionosphere associated with the location of the substorm expansion phase onset. Furthermore, a statistical analysis drawn from over 1000 h of high-time resolution, nightside radar data has enabled the characterisation of the bulk properties of these vortical flow systems. Their occurrence with respect to substorm phase has been investigated and a possible generation mechanism has been suggested.     

Optical effects in the aurora caused by ionospheric HF heating

Ionospheric heating experiments were done by the EISCAT Heater in Tromsø on 15–19 November, 1993. A low-light TV camera was installed at the VLF receiving station at Porojärvi about 100 km to the south-east of Tromsø. The spectral analysis of the auroral luminosity variations showed that the brightness of the aurora varied at the modulation frequency of the heating wave. The results of this analysis and the numerical simulations of the auroral luminosity variations caused by the HF heating are shown. The variations of the optical emission intensity at the heating frequency occur during the auroral ionosphere modification. The observed intensity variation of the auroral green line during the interval of enhanced electron temperature is explained by a decreasing rate of the O₂⁺ ion dissociative recombination when the electron temperature increases. The brightness variation depends on the characteristic energy and the intensity of the auroral electron flux and the heating wave parameters. The artificial luminosity pulsations caused by HF heating are estimated.     

The ionospheric signature of transient dayside reconnection and the associated pulsed convection return flow

Three SuperDARN coherent HF radars are employed to investigate the excitation of convection in the dayside high-latitude ionosphere in response to transient reconnection occurring in the cusp region. This study demonstrates the existence of transient antisunward-propagating backscatter features at the expected location of the ionospheric footprint of the cusp region, which have a repetition rate near 10 min. These are interpreted as the ionospheric signature of flux transfer events. Moreover, transient sunward-propagating regions of backscatter are observed in the convection return flow regions of both the pre- and post-noon sectors. These patches are observed to propagate towards the noon sector from at least as far around the auroral zone as 07 MLT in the pre-noon sector and 17 MLT in the post-noon sector, travelling with a veloCity of approximately 1.5 to 2 km s⁻¹. These return flow patches have a repetition rate similar to that of the transient features observed at local noon. While providing supporting evidence for the impulsive nature of convection flow, the observation of sunward-propagating features in the return flow region is not consistent with current conceptual models of the excitation of convection.     

Letter to the editor First direct observations of the reduced striations at pump frequencies close to the electron gyroharmonics

It is well known that the ionospheric plasma response to high-power HF radio waves changes drastically as the heater frequency approaches harmonics of the electron gyrofrequency. These include changes in the spectrum of the stimulated electromagnetic emission, reduction in the anomalous absorption of low-power diagnostic waves propagating through the heated volume, and reduction in the large scale F-region heating. Theoretical models as well as previous experimental evidence point towards the absence of small-scale field-aligned plasma density irregularities at pump frequencies close to electron gyroharmonics as the main cause of these changes. Results presented in this paper are the first direct observations of the reduced striations at the 3rd gyroharmonic made by the CUTLASS radar. In addition, simultaneous EISCAT observations have revealed that the “enhanced ion-line” usually present in the EISCAT ion-line spectrum during the first few seconds after heater switch on, persisted at varying strengths while the heater was transmitting at frequencies close to the 3rd electron gyroharmonics.     

Results of Russian experiments dealing with the impact of powerful HF radiowaves on the high-latitude ionosphere using the EISCAT facilities

We present the results of complex experiments dealing with the impact of powerful HF radiowaves on the high-latitude ionosphere using the European Incoherent Scatter Scientific Association (EISCAT) facilities. During the ionospheric F-region heating by powerful extraordinary (X-mode) polarized HF radiowaves under the conditions of heating near the critical f _H frequency f _H ≈ f _x F2 of the extraordinary wave of the F2-layer, we were first to detect the excitation of intense artificial small-scale ionospheric irregularities (ASIs), accompanied by electron temperature increases by approximately 50%. The results of coordinated satellite and ground-based observations of the powerful HF radiowave impact on the high-latitude ionosphere are considered. During ionospheric F-region heating by powerful HF radiowaves of ordinary polarization (O-mode) during evening hours, the phenomenon of ion outflow accompanied by electron temperature increases and thermal plasma expansion was revealed. Concurrent DMSP-F15 satellite measurements at a height of about 850 km indicate an O⁺ ion density increase. The CHAMP satellite observations identified ULF emissions at the modulation frequency (3 Hz) of the powerful HF radiowave, generated during modulated emissions of the powerful HF radiowave of O-polarization and accompanied by a substantial increase in the electron temperature and ASI generation.     

Plasma inhomogeneities in the topside ionosphere in the region of the geomagnetic equator and wave radiation according to the APEX satellite data

The measurements of the broadband wave radiation in the topside ionosphere in the region of the geomagnetic equator (the APEX satellite experiment) are presented. The region of unstable plasma with increased density was observed in the nightside topside ionosphere. This region could be formed by heating of the ionosphere from below. An asymmetric distribution of the frequency band width and electrostatic radiation intensity relative to the geomagnetic equator was registered. It has been indicated that a substantial effect of the geomagnetic equator on plasma diffusion from the heating region could be related to the generation, propagation, and damping of electrostatic oscillations and large-scale (as compared to the Larmor ion radius) plasma vortices. The anisotropy in the temperature of the plasma electron component can increase in the regions where the transverse electric field of disturbances damps. The intensity of the electromagnetic radiation, caused by the external sources, apparently, of an artificial origin at frequencies higher than the local plasma frequency, decreases to the radiation detection threshold level in the region of increased plasma density.     

Peculiarities of artificial ionospheric radiation under the action of high-power HF radiowaves, emitted by the SPEAR facility, on the sporadic E layer of the polar ionosphere

The results of the experimental studies of the ionospheric effects originating under the action of high-power HF radiowaves, emitted by the SPEAR heating facility into the sporadic E _s layer of the polar ionosphere, are presented. The experiment was performed on March 2, 2007, simultaneously at two spaced points: Barentsburg (Spitsbergen, a distance of about 40 km from the SPEAR facility) and Gor’kovskaya observatory near St. Petersburg, located at a distance of about 2000 km from SPEAR. The distributions of the heating signal intensity in the 100 kHz frequency band were measured in Barentsburg. Bistatic backscatter of diagnostic HF signals by small-scale artificial ionospheric irregularities was observed at Gor’kovskaya observatory. Based on an analysis of the experimental data obtained in Barentsburg, it has been found out that a broadband noise-like component originated and additional maximums appeared in the heating signal spectrum. The broadband emission intensity was a factor of 1.5–3 as high as the noise level. The additional maximums were formed in the regions of the positive and negative frequency shift relative to the heating signal frequency and were observed when the heating frequency was lower than the critical frequency of the E _s layer; e.g., a high-power HF radiowave reflected from E _s . The expression for determining the frequency shift of the additional maximum in the heating signal spectrum at altitudes of the ionospheric E region, taking into account the ion-electron collision frequency, has been obtained. The heating signal spectrum registration was compared with the observations of small-scale artificial ionospheric irregularities and the trajectory modeling of signals scattered by the considered irregularities. The observation results have been analyzed and interpreted taking into account the magnetic and ionospheric data characterizing the background geophysical conditions.     

The generation of non aspect sensitive plasma density irregularities by field aligned drifts in the lower ionosphere

A theory of the generation of plasma density irregularities with virtually no aspect sensitivity, in the lower ionosphere at high latitudes, by electron drifts aligned with the geomagnetic field, is presented. The theory is developed through fluid equations in which the destabilising mechanism involves positive feedback from electron collisional heating. When field aligned electron drift speeds exceed a few km s^–1, this effect destabilises waves with wavelengths in excess of a few tens of metres in the lower E-region, where collisional effects are sufficiently large. Furthermore, the threshold conditions are almost independent of the wave propagation direction and the unstable waves propagate at speeds well below the ion acoustic speed. The role that this new instability may play in recent radar backscatter observations of short scale irregularities propagating in directions close to that of the geomagnetic field, in the lower E-region is also considered.     

Validation of the CUTLASS HF radar gravity wave observing capability using EISCAT CP-1 data

Quasi-periodic fluctuations in the returned ground-scatter power from the SuperDARN HF radars have been linked to the passage of medium-scale gravity waves. We have applied a technique that extracts the first radar range returns from the F-region to study the spatial extent and characteristics of these waves in the CUTLASS field-of-view. Some ray tracing was carried out to test the applicability of this method. The EISCAT radar facility at Tromsø is well within the CUTLASS field-of-view for these waves and provides a unique opportunity to assess independently the ability of the HF radars to derive gravity wave information. Results from 1st March, 1995, where the EISCAT UHF radar was operating in its CP-1 mode, demonstrate that the radars were in good agreement, especially if one selects the electron density variations measured by EISCAT at around 235 km. CUTLASS and EISCAT gravity wave observations complement each other; the former extends the spatial field of view considerably, whilst the latter provides detailed vertical information about a range of ionospheric parameters.     

Possible role of magnetosphere-ionosphere coupling in auroral arc generation

Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E × B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s^-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Polar Geophysical Institute, Apatity, Russia     

Synchrotron emission of weakly-relativistic electron beams at midlatitude ionospheric altitudes

Results of the spectral measurements of ionospheric noise in the meter band are presented. The events lasting several milliseconds (the emission maximum of which drifts upward (in frequency), is reflected (stops), and drifts downward) have been distinguished. Moreover, multiple harmonics are observed. The frequency-time structure of such events have been considered from the viewpoint of registration of the electron beam synchrotron emission harmonics at ionospheric altitudes in the geomagnetic field. The model calculations of the frequency-time structure of ionospheric radio noise bursts drifting in frequency have been performed taking into account the measurement conditions. It has been indicated that the model electron radio noise bursts agree with the measured bursts reflecting from the ionosphere at altitudes of 100–180 km. The model of the monoenergetic beam of electrons precipitating from the radiation belt (L ～ 2.0–2.8) into the ionosphere has been proposed.     

Atmosphere–ionosphere vertical electric coupling above thunderstorms of different intensity

The most important features of the quasi-electrostatic fields (QSFs) and currents, generated in the region between a thunderstorm (TS) and the ionosphere between lightning discharges, are theoretically investigated. They depend on different factors having large variability, related to the TS and to the atmospheric conductivity. These features are studied in order to understand better the conditions when QSFs cause modifications of the parameters and chemical balance in the lower ionosphere over TSs due to electron heating, as well as the generation of red sprites. For this purpose, an analytical model based on Maxwell's equations under conditions of curl-free electric fields is presented. The temporal and spatial behaviour of the QSFs is studied as a function of the parameters of lightning discharges and of atmospheric conductivity. The dependence of the QSF, mainly its peak values, on the charge moment change, the discharge time, and the horizontal extent of the discharged region, on the one hand, and of the conductivity profile, on the other, is studied. It is shown that the profile of the QSF time peak changes its scale height at the altitudes where the relaxation time becomes equal to the discharge time, and where the conductivity scale height is diminished; below these altitudes the peak QSFs decrease with time much slower than above them. Also, the QSF peak increases almost linearly with the charge height and depends little on the size of the discharged region. The total Maxwell and conduction currents, which flow from the TS to the ionosphere, are also studied. The peak current is proportional to the charge moment change, and actually does not depend on the frequency of lightning discharges.     

High-latitude HF Doppler observations of ULF waves: 2. Waves with small spatial scale sizes

The DOPE (Doppler Pulsation Experiment) HF Doppler sounder located near Tromsø, Norway (geographic: 69.6°N 19.2°E; L = 6.3) is deployed to observe signatures, in the high-latitude ionosphere, of magnetospheric ULF waves. A type of wave has been identified which exhibits no simultaneous ground magnetic signature. They can be subdivided into two classes which occur in the dawn and dusk local time sectors respectively. They generally have frequencies greater than the resonance fundamentals of local field lines. It is suggested that these may be the signatures of high-m ULF waves where the ground magnetic signature has been strongly attenuated as a result of the scale size of the waves. The dawn population demonstrate similarities to a type of magnetospheric wave known as giant (Pg) pulsations which tend to be resonant at higher harmonics on magnetic field lines. In contrast, the waves occurring in the dusk sector are believed to be related to the storm-time Pc5s previously reported in VHF radar data. Dst measurements support these observations by indicating that the dawn and dusk classes of waves occur respectively during geomagnetically quiet and more active intervals.     

ULF wave occurrence statistics in a high-latitude HF Doppler sounder

Ultra low frequency (ULF) wave activity in the high-latitude ionosphere has been observed by a high frequency (HF) Doppler sounder located at Tromsø, Norway (69.71°N, 19.2°E geographic coordinates). A statistical study of the occurrence of these waves has been undertaken from data collected between 1979 and 1984. The diurnal, seasonal, solar cycle and geomagnetic activity variations in occurrence have been investigated. The findings demonstrate that the ability of the sounder to detect ULF wave signatures maximises at the equinoxes and that there is a peak in occurrence in the morning sector. The occurrence rate is fairly insensitive to changes associated with the solar cycle but increases with the level of geomagnetic activity. As a result, it has been possible to characterise the way in which prevailing ionospheric and magnetospheric conditions affect such observations of ULF waves.