Search

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 (69.7°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.Key words. Ionosphere (auroral ionosphere; ionosphere -magnetosphere interactions) · Magnetospheric physics (MHD waves and instabilities)

In previous attempts to detect eclipse-induced AGW, it has always been difficult to establish a direct link between individual waves and a specific source. This study reports observations of travelling ionospheric disturbances made in the UK at the time of the total solar eclipse of 11 August 1999. The speed and direction of the waves were estimated by a four-station array using the HF Doppler technique. In addition, the wave observations were supported by two other propagation paths, one in the north of England close to the main array and the other further afield, between the UK and Sweden. The AGW activity following the eclipse totality was different to the background waves detected before this time in amplitude, speed and direction. The velocity vectors are consistent with a generating mechanism for the waves based on the supersonic passage of the cooled region of the atmosphere during the eclipse.

The performance of the HF radars such as the collaborative UK twin located auroral sounding system, depends critically on the propagation characteristics of HF radio waves in the ionosphere. The transmission paths through the ionosphere have been determined by ray tracing techniques applied to suitable models of the electron density height distribution. By launching waves at a range of elevation angles, it has been possible to calculate the skip distance and ground illumination flux of each radar for the operating frequency (10 MHz). The calculated and measured positions of the ground backscatter are found to be in excellent agreement, indicating that this modelling approach can predict the performance of the radar for any conditions for which a representative electron density height profile can be established. When waves of very high power are propagated through the ionosphere, artificial irregularities are produced which can be detected by the HF radars in a similar manner to those occurring naturally. The advantage of the heater-induced irregularities is that they can be generated at will and can form a ‘window’ through which many geophysical phenomena (e.g. ULF waves) can be investigated. Examples of the applications of this technique are presented, and it is evident that it has great potential for a range of geophysical investigations.

Experimental results are presented from ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar. Tomographic measurements on 10 October 1996, showing a narrow, field-aligned enhancement in electron density in the post-noon sector of the dayside auroral zone, are related to a temporal increase in the plasma concentration observed by the incoherent scatter radar in the region where the HF radar indicated a low velocity sunwards convection. The results demonstrate the complementary nature of these three instruments for polar-cap ionospheric studies.Key words. Ionosphere · Auroral ionosphere · Polar ionosphere · Radio science (ionospheric physics)

The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative Bz and positive By, to positive Bz and small By, has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (~2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within ~2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of ~7 and ~14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further ~6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change.Key words. Ionosphere (Auroral ionosphere) · Magnetospheric physics (Magnetopause · cusp · and boundary layers; Magnetosphere · ionosphere interaction)

Four high frequency propagation paths were provided by a transmitter located within the polar cap and four receivers located variously within the polar cap and at sub-auroral latitudes. Of these paths, one was contained entirely within the polar cap at all times, two were trans-auroral at all times, and one varied from trans-auroral during the day to polar cap during the night. Fourteen frequencies within the HF band were transmitted each hour for the duration of two 24 day experimental campaigns during the summer of 1988 and the winter of 1989. The received signals were analysed to determine signal recognition and signal strength. The mean quiet-time behaviour of the propagation was determined and compared with four periods during which the ionosphere was perturbed by (relatively mild) geomagnetic storm conditions. The daily occurrence of correctly recognised signals decreased during the disturbed periods, both on polar cap and trans-auroral paths. Several geophysical causes for this decrease are identified, including a decrease in F-region electron density, decreasing the MOF, as a consequence of negative storm effects and an extended mid latitude trough. On polar cap paths, an increase in the LOF caused by mild PCA events also decreased the available HF band. Storm-time auroral-E makes propagation possible on frequencies higher than expected during the night on trans-auroral paths, mitigating to some extent the degradation in propagation. The storm-time decrease in the available HF band is quantified, falling to 0.3–0.5 of undisturbed levels even during relatively small geomagnetic storms. During the two experimental campaigns, 50% of days are identified as being storm-disturbed.

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.

Results are presented from an experimental campaign in April 1996, in which the new CUTLASS (Co-operative UK twin-located Auroral Sounding System) coherent scatter radar was employed to observe artificial field aligned irregularities (FAI) generated by the EISCAT (European Incoherent SCATter) heating facility at Tromsø, Norway. The distribution of backscatter intensity from within the heated region has been investigated both in azimuth and range with the Finland component of CUTLASS, and the first observations of artificial irregularities by the Iceland radar are also presented. The heated region has been measured to extend over a horizontal distance of 170±50km, which by comparison with a model of the heater beam pattern corresponds to a threshold electric field for FAI of between 0.1 and 0.01V/m. Differences between field-aligned and vertical propagation heating are also presented.

Analyses of ground backscatter measurements recently available from the CUTLASS HF radars, show that waves with large spatial scales and periods in the range of 1 2 to 3 hours, are frequently present in the high-latitude ionosphere. These periods and spatial scales are consistent with travelling ionospheric disturbances caused by thermospheric gravity waves. Waves are most clearly seen during intervals of low geomagnetic activity and tend to travel southwards. Elevation angle studies, which indicate whether scatter is from the forwards or backwards direction relative to the radar beam, enable the direction of wave travel to be interpreted correctly. Initial attempts to simulate the effects of southward travelling gravity waves indicate structures similar to those found in CUTLASS observations.

In the coherent radar technique, bacsccatter is obtained from plasma irregularities even though the radar frequency can greatly exceed the ionospheric plasma frequency maximum. From the velocity spectrum of the received signals an estimate of the flow velocity can be obtained and hence the electric field determined. Information regarding the irregularity scattering cross section is obtained from the amplitude of the backscatter return. Current radar studies of a range of geophysical phenomena are presented. In addition, attention is drawn to the problems of interpreting the radar observations in terms of the underlying geophysical processes.

Results are presented from recent ionospheric F-region modification experiments in which the heater induced anomalous absorption and electron temperature changes were measured for a variety of heater frequencies which included the third and fourth harmonics of the electron gyro frequencies. The results indicate that there are strong minima in the responses of both anomalous absorption and electron heating in the vicinity of the two harmonic frequencies. These new observations provide evidence for the view that anomalous electron heating in the presence of small scale field aligned plasma irregularities dominates over collisional heating at high latitudes.

Experimental results concerning European incoherent scatter observations of heater-induced electron temperature enhancements, anomalous absorption of low-power HF probe waves, and the spectrum of stimulated electromagnetic emission (SEE) in the sidebands of a high-power HF electromagnetic wave are presented. For the experiments reported in this paper, an O mode pump wave was transmitted vertically into the F region above Tromsø, Norway, while the injected frequency was varied in small steps around the third harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the data obtained from all three diagnostics. Measurements of anomalous absorption, the downshifted maximum (DM) spectral feature, and heater-induced electron temperature enhancements all exhibited broad minima as the heater frequency approached the third harmonic of the electron gyrofrequency. In addition, the signal strength of the HF probe wave measured during heater off periods is also reduced at these and higher heater frequencies. The experimental findings suggest that at heater frequencies in the vicinity of the third gyroharmonic, small-scale field-aligned irregularities are not excited, whereas very small scale irregularities, of the order of a few electron cyclotron radii, which are responsible for the production of fast electrons, may be generated. The observed reduction in the diagnostic signal strength is then attributed to the ionized patches produced by these energetic electrons.

Results are presented from recent ionospheric modification experiments in which the EISCAT UHF radar measured the E-region temperature and density response to high power RF heating above Tromso. A variety of electrojet conditions were encountered during these experiments. In particular, the electron drift velocity varied considerably allowing the heating efficiency of the RF heater to be investigated as a function of electron flow velocity. These observations constitute the first direct investigation of electrojet temperature modifications by high power radio waves and provide a test of a recent theoretical model in which the combined effects of RF heating and of natural plasma turbulence associated with the Farley-Buneman instability have been considered.

Observations of the anomalous absorption of low-power HF probe waves caused by the action of a high-power HF ordinary mode radio wave are presented. At pump frequencies in the vicinity of harmonics of the electron gyrofrequency the anomalous absorption measured on the low-power probe waves exhibit local minima. Also at these frequencies, large-scale changes deduced from phase measurements of the low-power probe waves and heater self-absorption are reduced, whereas the growth time of the anomalous absorption is increased. The electron gyrofrequency estimated from the frequency of the absorption minimum compares favorably with that derived from the International Geomagnetic Reference Field (IGRF) magnetic field model.

Equatorward propagating bands of auroral radar backscatter of period 150–600 s are a commonly observed phenomenon on the SABRE VHF coherent radar, but until very recently were rarely observed on other coherent radar systems located at different latitudes. A recent statistical study (Mao Tian et al., 1991, Planet. Space Sci.39, 1239) provided strong evidence that such propagating bands on SABRE were geomagnetic pulsations with resonant structures on the plasmapause. Here a detailed analysis of such pulsations observed by SABRE, together with the SAMNET and EISCAT cross magnetometer arrays, is presented. The pulsations have rapid westward azimuthal phase propagation independent of local time and a latitudinal phase change in both geographic and geomagnetic coordinate systems which, on average, exceeds the 180° phase change predicted by field line resonance theory. Furthermore, the events exhibit a strong attenuation between their ionospheric signature and their ground magnetometer signature. These results are consistent with an excitation mechanism involving wave-particle interactions, similar to that postulated for storm-time Pc5 pulsation events. The azimuthal phase propagation of the pulsations, if used to calculate a drift velocity mapped out to the equatorial plane, is consistent with that of protons of energy 35–70 keV undergoing gradient-curvature drift. It is suggested that these particles drift westwards and interact with the dusk plasmapause region, producing a resonant wave which is detected by the coherent radar as an equatorward propagating pulsation event.

Enhancements in the F-region ion temperature, caused by frictional heating, are a notable manifestation of the interaction between the ionized and neutral atmospheres. Heating of this type arises whenever the ion and neutral populations move with different velocities. A velocity difference of order 1 km s−1 provides sufficient heat input to double the unperturbed ion temperature. This paper presents a synoptic study of ion heating events observed by the EISCAT UHF facility during the 2-year period from January 1985 to December 1986. The data set consists of observations obtained during the ‘Common Programme’ operation of EISCAT in the CP-1 and CP-2 modes. Only ion temperatures measured in the field-aligned direction have been considered, since in this direction the line-of-sight ion temperature can be obtained by assuming that the ion thermal velocity distribution remains Maxwellian. This assumption is known to break down progressively as the difference between the ion and neutral velocities increases, but the resulting discrepancies in the field-parallel ion temperature are smaller than those for other observing directions and do not significantly affect the conclusions presented here. A criterion previously applied to the examination of satellite data has been adopted to identify intervals of F-region frictional heating and employed to derive their diurnal distribution in 1000 h of EISCAT measurements. Almost equal numbers of heating events were encountered in both the dawn and dusk sectors. While this result contrasts with some earlier findings, the present study makes use of a much larger data set than previous investigations and is concerned with somewhat lower latitudes. The magnitudes of the ion temperature enhancements are similar for both the pre-midnight and post-midnight sectors, although the longest intervals of ion heating tended to be concentrated in the early morning hours. Many of the ion heating events occurred in association with intervals of intense and structured particle precipitation, with F-region ion temperatures and velocities being enhanced at the boundaries of auroral forms, but depressed above the regions of precipitation themselves. It is suggested that this behaviour can be explained by the presence of a polarization electric field within the auroral form, whose direction opposes that of the convection electric field. Similar observations of reduced electric fields above regions of particle precipitation have been reported by previous workers, who have classified such events as ‘anti-correlation’ or ‘asymmetric’ auroral arcs.

[1] Measurements of the direction-of-arrival of signals propagating on a long (4490 km) path along the midlatitude trough show that the azimuth can be deviated by up to 100° from the great circle bearing. In this paper an attempt has been made to model the shift in azimuth through a ray tracing simulation. Two possible mechanisms which lead to changes in azimuth have been investigated: (1) propagation along the density gradients which form the equatorward wall of the trough and (2) side scatter from regions of the sea well to the south of the trough. Of these two mechanisms, sea scatter gives results which are much closer to those observed.

The presence of horizontal electron density gradients and irregularities in the ionosphere cause HF radio signals to deviate from the great-circle path joining transmitter and receiver. Such deviations have serious implications for the operation of position location systems operating within the HF band. In order to investigate the nature of these large scale bearing deviations and to relate them to the mode content of the signal, various experiments have been undertaken over paths for which simultaneous bearing measurements and forward oblique soundings were available. Two types of measurement are examined in this paper. In the first type, the soundings were made over the full HF band at 30 minute intervals together with the bearing measurements at 90 second intervals at three fixed frequencies. These measurements were obtained over two paths in Canada. In the second type of experiment, the bearing measurements were made on the sounder signal itself (a so-called sweeping-DF measurement). These latter experiments are similar to those made with an interferometric DF system which have previously been reported by Black et al. (1993, 1995), however in this instance a goniometric DF system was employed.

Characteristic signatures are often observed in HF radar range-time-intensity plots when travelling ionospheric disturbances (TIDs) are present. These signatures, in particular the variation of the F-region skip distance, have been synthesised using a ray tracing model. The magnitude of the skip variation is found to be a function of the peak electron density perturbation associated with the TID and radar frequency. Examination of experimental observations leads to an estimate of the peak electron density perturbation amplitude of around 25% for those TIDs observed by the CUTLASS radar system. The advantage of using the skip variation over the radar return amplitude as an indicator of density perturbation is also discussed. An example of a dual radar frequency experiment has been given. The investigation of the effect of radar frequency on the observations will aid the optimisation of future experiments..Key words. Ionosphere (auroral ionosphere; ionosphere -atmosphere interactions; ionospheric disturbances)

The COSCAT system enables the detection of E-region auroral backscatter with the EISCAT remote receivers at magnetic aspect angles close to 90 °. This is achieved by utilising a low-power transmitter stationed in Oulu, Finland. Many important observations of E-region irregularities have been achieved with this simple experiment. Recent studies have attempted to push the COSCAT system to its experimental limits. Firstly, the CW signal has been phase-modulated with 13-bit Barker codes with baud lengths of 40, 70 and 100 µs. Interpretation of the received power allows the spatial distribution of the auroral scatterers to be determined. The second advance is in the use of a sophisticated correlator program which allows data to be buffered within the correlator at very high time resolution. This enables the coherent backscatter power to be sampled every 12.5 ms and the full auto-correlation function to be measured every 100 ms. These measurements allow the COSCAT system to be employed for the first time in an investigation of the growth and decay of the auroral irregularities.Key words. Ionosphere (ionospheric irregularities) · Radio science (Instruments and techniques)

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.Key words. Ionosphere (ionosphere – atmosphere interactions) · Meteorology and atmospheric dynamics (thermospheric dynamics) · Radio science (ionospheric propagations)

The performance of high frequency direction finding systems is known to be related to the mode content of the incoming signal and to the frequency of operation, both of which depend on the electron density distribution in the ionosphere. Due to tilts and gradients in the electron density distribution, signals often arrive at the receiver over paths well displaced from the great circle. Deviations of a few degrees are associated with tilts in the ionospheric electron density profile due, for example, to the solar terminator and to travelling ionospheric disturbances. Very large deviations are particularly prevalent in the high latitude regions where signals often arrive at the receiver with bearings displaced from the great circle direction by up to /spl plusmn/100/spl deg/ or more. A systematic study of these features has been undertaken and the magnitudes of the bearing errors observed in the various ionospheric regions and their likely causes are summarised. The paper also reviews some of the work undertaken at the University of Leicester on the direction of arrival (DOA) of HF signals propagated over two high latitude paths, one along the subauroral trough and the other contained within the polar cap. These two paths are illustrated together with an indication of the position of the subauroral trough produced by the model of Halcrow and Nisbet (1977).

Research has been undertaken at the University of Leicester for a number of years on large bearing errors (of up to around ±100°) which are frequently observed by HF direction finders when the signals are reflected from the high latitude ionosphere. In addition to gaining an understanding of the physical processes leading to these large errors, a major aim of the investigation is the development of a quality factor which may be assigned to individual bearing measurements. Such a quality factor should provide an indication, based on ionospheric propagation phenomena, of the reliability of a measured bearing as a indication of the great circle direction to the transmitter. Measurements of the bearings and signal characteristics of HF transmissions from Halifax, Nova Scotia and Iqaluit, North West Territories, have been made at a number of sites over a period of several years. This paper summarises, with supporting examples, the ionospheric propagation phenomena which lead to large deviations from the great circle for high latitude paths and relates these effects to various geophysical parameters. From this information, suggestions are given for the development of an appropriate HF-DF quality factor.

A statistical study of the occurrence of ground and ionospheric backscatter within the fields-of-view of the CUTLASS HF radars, at an operating frequency of 10 MHz, during the first 20 months of operation has been undertaken. The diurnal variation of the occurrence of backscatter and the range at which such backscatter is observed is found to be highly dependent on seasonal changes of the ionospheric electron density in both the E and F region, determined from ionosonde observations. In general, ionospheric backscatter is observed at far ranges during the local day in winter months and at near ranges during the local night in summer months. The Iceland radar observes more near-range E region backscatter than the Finland radar as a consequence of its more zonal look-direction. The dependence of the occurrence of backscatter on geomagnetic activity and radar operating frequency are also investigated. The occurrence of ground and ionospheric backscatter is discussed in terms of HF propagation modes and ionospheric electron densities as well as geophysical processes. A brief assessment of the possible impact of solar cycle variations on the observations is made and frequency management is discussed. Such a study, with its focus on the `instrumental' aspect of backscatter occurrence, is essential for a full interpretation of HF coherent radar observations.

Computer-based, long-term HF prediction methods employ numerical maps of the ionosphere which are frequently based on vertical ionospheric soundings. Furthermore, they assume that propagation occurs over the great circle path (GCP) between transmitter and receiver. Recent observations of signals propagated over two paths which lie tangential to, and equatorward of, the auroral oval indicate that propagation is commonly observed at times which differ appreciably from the predicted 'path open' times. Measurements of the direction of arrival of these signals indicate that at times when propagation is predicted to occur the signals arrive close to the great circle direction, whereas when propagation is not predicted the signals travel along off-great circle paths, sometimes with large bearing displacements of up to 100/spl deg/. At these times the signals are thought to reflect from the electron density gradients associated with the mid-latitude trough, a band of electron density depletion in the night-time ionosphere which forms just equatorward of the auroral oval. The days on which these effects occur depend on the formation and intensity of the trough which is associated with the level of geomagnetic activity.

Horizontal electron density gradients (tilts) in the ionosphere are responsible for the deviation of HF (330 MHz) radiowaves from the normal great circle propagation path (GCP). The resulting deviations are often far in excess of the instrumental accuracy ( approximately 0.1 degrees ) of direction finding (DF) systems and can amount to errors of over 50 degrees at high latitude sites where relatively large ionospheric tilts may occur. One region of the ionosphere in which strong electron density gradients occur is the mid-latitude trough. This trough is an F-region electron density depletion region a few degrees wide in latitude on the equatorward edge of the auroral oval in which the critical frequencies drop by a factor of 2-4 or more and the altitude of the electron density maximum rises by over 100 km. The mean positions of the poleward and equatorward walls of the trough and its extent in local time as a function of Kp have been calculated by means of the regression equations of Halcrow and Nisbet (1977). An example of the modelled trough positions for an equinox day is presented for various levels of geomagnetic activity.

Oblique ionograms received over northerly paths are often difficult to interpret, with many features not being of the form observed on the more familiar mid-latitude soundings. In particular, the ionograms contain unusual traces at frequencies close to and above the conventional or basic maximum usable frequency, i.e. above the junction frequency of the high and low angle modes. A series of experiments have been undertaken over a period of several years to investigate off-great-circle propagation mechanisms over high latitude and sub-auroral oval paths. In addition to measurements of the signal direction of arrival, oblique ionograms were also recorded over several of the paths. By these means, the characteristic ionogram signatures associated with off-great-circle propagation effects arising due to the presence of convecting patches of enhanced ionisation or sun-aligned arcs within the polar cap ionosphere, or to the presence of steep gradients to the side of the path within the mid-latitude trough region, have been identified.

Large electron density gradients in the ionosphere associated with the walls of the midlatitude trough produce tilted reflecting surfaces which lead to HF signals propagating over paths well displaced from the great circle. Measurements are presented of signals propagating over two paths along the midlatitude trough for which bearings deviations of up to 100° from the great-circle path, i.e. the expected direction of arrival, were frequently recorded. The times of occurrence and magnitude of these large deviations are consistent with the known features of the midlatitude ionospheric trough. Such large deviations of the measured bearing from the great-circle path, which far exceed the instrumental errors, have serious implications for the operation of position location systems operating within the HF band.

Simultaneous observations are presented of stimulated electromagnetic emission (SEE) spectra and the anomalous absorption of a diagnostic HF wave, caused by powerful radio waves generated by the Tromsø heating facility. The pump frequency was varied in small steps around the third, fourth, and fifth harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the anomalous absorption data and the entirety of SEE properties. The anomalous absorption exhibits a pronounced minimum at a certain pump frequency near (or at) the gyroharmonic in question. Of the SEE properties the strength and discreteness of the downshifted maximum and the width of the downshifted continuum minimize together with the anomalous absorption. Other SEE properties (namely the broad upshifted maximum, the downshifted peak, and the broad symmetrical structure) exist only in well-defined narrow pump frequency ranges near the absorption minimum. Significant quantitative and qualitative differences are found between the SEE properties around the third gyroharmonic on the one hand, and the fourth and fifth gyroharmonics on the other hand.

The effects of ionospheric heating on low-power diagnostic radio waves at Arecibo are reported. During O mode heating it was found that the signal strengths of the diagnostic waves were reduced by up to approximately 7 dB. The diagnostic signals are only significantly attenuated during heating at full power (effective radiated power of 80 MW). During X mode heating, no amplitude effects could be detected. The maximum attenuation of the diagnostic signals measured at Arecibo is lower than those typically observed at Tromso, which is consistent with the predictions of theory. These results are discussed in the context of theoretical calculations of the anomalous absorption due to mode coupling caused by small-scale field-aligned irregularities. From observations of the effects of heating on the amplitudes of several low-power diagnostics, the length of these irregularities along the geomagnetic field is estimated.

Copyright © 1991 IEE. This paper was published as: Microwaves, Antennas and Propagation, IEE Proceedings H, 1991, 138 (2), pp. 121-130. It is available from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=68343

Copyright © 1990 IEE. This paper was published as: Microwaves, Antennas and Propagation, IEE Proceedings H, 1990, 137 (1), pp. 25-30. It is available from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=52817

The bearings of a number of HF transmissions recorded continuously over many days are employed to investigate the large-scale horizontal gradients of electron density in the mid-latitude ionosphere (systematic ionospheric tilts or SITs). Although previous authors have explained these SITs and their associated bearing errors solely in terms of a dependence of ionospheric tilt on the variation of solar zenith angle, our observations indicate that plasma transport processes must also be taken into account. A 3D ray tracing technique is employed in conjunction with an ionospheric model based on predicted vertical-sounder data to simulate the experimental observations. Good agreement is obtained between predicted and observed bearing errors for south-to-north (S-N) transmission paths, although poor agreement is achieved for east-to-west (E-W) paths. The latter is due to the inability of our ionospheric model to yield accurate N-S ionospheric gradient information. Better agreement between computed and observed bearing errors for E-W paths is achieved by ray tracing through an ionospheric model based on measured median vertical-sounder data. A major limiting factor in the accurate simulation of SIT-induced bearing error is the day-to-day variation of ionospheric tilts.

An investigation is made of the adequacy of the simple corrugated reflector model to simulate the effects of large-scale travelling ionospheric disturbances (TIDs) on the bearings of HF transmissions. Model results are compared with the quasi-periodic variations of bearings measured simultaneously for a number of geographically spaced transmission paths and with results obtained from 3D ray tracing studies incorporating a more realistic TID model (the ‘'wave’ model). Although the bearing error signature is, at some times (particularly during the day), similar to that predicted by the corrugated reflector model, ray tracing calculations with the ‘wave’ model give generally better results and predict bearing error signatures which correspond much better to some of those observed in the experimental data than those predicted by the simpler model. However, the prediction accuracy of both models is found to be limited by temporal variation of both the TID waveform and the ambient ionosphere.

Observations of a Pc 5 micropulsation event, using the Wick half of the new SABRE auroral radar, are presented. During a 3-hour period in the early morning of September 18, 1981, a train of pulsations were recorded. Analysis revealed that there were three main events, rather than one continuous disturbance. The first event appeared as a perturbation of only one or two cycles, extending throughout the latitude of the viewing area. It was also visible in the STARE region, but the perturbation drift velocity was in the north-south direction as opposed to the east-west perturbation drift of a more conventional, toroidal mode field line resonance. The second disturbance was an isolated, one-cycle perturbation, also visible throughout both SABRE and STARE viewing areas and also having a dominant north-south velocity component. Finally, a very localized monochromatic event, lasting five cycles or more, with a period of about 5 min, was recorded. The characteristics of the third event were found to be consistent with those predicted by field line resonance theory. 18 references.

A high frequency radio Doppler experiment was deployed in the Antarctic Peninsula region, centred on Argentine Islands (65°15′S, 64°16′W; L = 2.3), to investigate the morphology and sources of ionospheric disturbances. The experiment consisted of a three-transmitter dual frequency network which permits horizontal and vertical propagation velocities to be estimated over a north-south baseline of 200 km and an east-west baseline of 100 km. A new class of ionospheric disturbance has been observed, in the period range 10 min−1 h. These disturbances are characterised by unusually good correlation between perturbations on all available Doppler signals, but are apparently non—propagating and occur simultaneously at each reflection point. Several of these events display large (2 Hz at about 5 MHz transmitted frequency) Doppler shifts, thus we have labelled them Large Simultaneous Disturbances (LSD s ). Criteria for identification of LSD s are established and the analysis of one event is described in detail. The occurrence statistics of the LSD s are presented, including their seasonal and diurnal distributions. There is no clear general relationship between LSD s and local geomagnetic field perturbations. However, examination of the magnetic indices AE and I RC indicates that there is a loose association between the occurrence and amplitude of LSD s and magnetic activity. Several possible mechanisms for the generation of LSD s at middle latitudes are reviewed. The most likely explanation is that high latitude electric fields penetrate to magnetic middle latitudes and drive the ionospheric plasma via the E × B drift.

The occurence of propagating wave-like disturbances in the atmosphere at ionospheric heights is well documented, but their causes and role in the energy balance of the atmosphere is poorly understood. This paper describes an experiment deployed in the Antarctic Peninsula region to investigate the morphology of the various classes of disturbance, with particular emphasis on the identification of their sources. Current knowledge of the phenomena is briefly reviewed and the reasons why observations in the peninsula region may be especially valuable are discussed. Some preliminary results from the first 3 months of operation are presented; these indicate the presence of waves with periods ranging from less than 1 min to more than 90 min. The short period waves (1-5 min) are unusually common in these data.

The anomalous self-absorption of a high-power radio wave measured during recent ionospheric modification experiments exhibits a hysteresis effect. These results are consistent with the assumption that the growth of field-aligned irregularities due to the interaction of a high-power pump wave and the ionospheric plasma occurs in two stages. During the first stage, striation growth is due to an instability which has a threshold independent of striation amplitude. When the striation level is sufficiently high, the threshold of the second stage is reached. The second-stage threshold is dependent on the striation amplitude. The state of background ionosphere is also found to be an important factor influencing the observed effects.

The heating facility at Ramfjordmoen near Tromso, Norway, is briefly described, and a survey is given of the experiments performed with this facility until now. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF, HF absorption and backscatter due to short-scale field-aligned irregularities, stimulated radio wave emission of the modified ionospheric plasma, short-time scale HF absorption due to the parametric decay instability, airglow modification, excitation of large-scale irregularities, and F -region cross modulation.

Some results of recent high-latitude ionospheric heating experiments are presented. Phase changes were induced in a diagnostic radio wave reflected from the lower F region of the ionosphere as it passed through a region of the ionosphere which was being illuminated by a high-power electromagnetic wave. From the results obtained there are indications that anomalous absorption contributes much more than deviative absorption to ionospheric heating at high latitudes.

Data from the Sweden and Britain Radar Auroral Experiment have been utilized in a statistical study of auroral backscatter due to 1-m-scale plasma irregularities and of plasma convection in the auroral E layer. An examination of approximately 15,000 hours of backscatter amplitude data indicates that (1) the strongest backscatter is invariably associated with the main electrojets, between 1300-1900 UT and 2300-0300 UT, (2) only weak backscatter occurs in the vicinity of the convection reversals, particularly in the morning sector, and (3) the backscatter intensity can exhibit an aspect angle attenuation of between 0 and 10 dB/deg depending on the absolute intensity measured. Average convection patterns obtained from approximately 1400 hours of two-station measurements reveal a well-defined two-cell structure for all magnetic conditions. As magnetic activity increases the Harang discontinuity moves toward earlier local times, the flow speed increases and the morning convection cell expands relative to the evening cell. 32 references.

An inversion technique is outlined for calculating the resolution and gross error associated with D-region electron density profiles obtained by modelling from a given set of VLF radiowave propagation parameters. Two profiles, representing quiet and disturbed ionospheric conditions, are examined in detail and the height resolution and accuracy obtainable from the amount of observational data used in constructing these profiles is determined.

Travelling ionospheric disturbances with periods in the range 10 τ −1 and may be classified as Medium Scale TIDs. An anticlockwise diurnal azimuth rotation is established, with waves tending to propagate in the (modelled) antiwindward direction. Waves associated with magnetically active intervals often have high speeds and do not generally conform to the simple azimuth variation described above. These differences are explained in terms of perturbed neutral wind patterns and the existence of different wave sources during active times. These observations are presented in the context of previous morphological wave studies. The geomagnetic dependence observed in Antarctica may explain some of the conflicting or ambiguous conclusions resulting from investigations at other locations.

From 1972 to 1975 F-region medium-scale travelling ionospheric disturbances (MSTIDs) were observed at Leicester, U.K. (52°32′N 1°8′W) by means of the HF Doppler technique. Most of the features of the disturbances previously reported in the literature are confirmed, with the exception of the apparent seasonal variation in the propagation direction. The measured wave azimuth rotates clockwise through 360° in 24 h, supporting theoretical predictions concerning the filtering effect of the neutral wind in the northern hemisphere. The most commonly observed direction of wave propagation, however, is displaced from the antiwind direction and is located at an azimuth of 130–140° relative to the wind. A periodic variation of the direction of wave propagation with respect to the anti-wind direction is evident, which may indicate that lower atmospheric winds can have a greater influence on waves at thermospheric heights than previously supposed. A synoptic survey of the data set reveals little correlation between wave occurrence and auroral processes, and it is unlikely that high-latitude sources are responsible for many of the MSTIDs observed at mid-latitudes.

High power radio waves can modify the ionospheric electron density distribution to produce field aligned plasma irregularities which give rise to the anomalous absorption of HF radio waves. The coefficient of anomalous absorption of a vertically propagated radio wave due to scattering from field aligned irregularities has been calculated, taking into account the effects of the geomagnetic field on electron motions. These results are compared with those of other theoretical models. Furthermore, the scale lengths of field aligned irregularities produced by a high power radio wave during recent high latitude modification experiments have been determined from measurements of the anomalous absorption by means of this theory.

The characteristics of medium scale travelling ionospheric disturbances (MSTIDS) have been determined from observations carried out between 1972 and 1975 at Leicester U.K. (52°32′N, 1°8′W) using the HF Doppler technique. By reverse ray tracing through a model atmosphere an estimate of the source locations of these waves can be obtained. Auroral sources do not appear to represent an important generation mechanism for MSTIDs observed at mid-latitudes. The majority of waves originate at tropospheric altitudes at ground ranges of less than 1500 km from the observation point, and a moderate correlation is found between the occurrence frequency of MSTIDs and the intensity of the meteorological jet stream.

EISCAT observations of ionospheric heating by means of high power HF radio waves are reported. These results indicate that the ionospheric response to heating involves a number of plasma instabilities with different characteristic time scales, ranging from a few milliseconds to several tens of seconds.

The phenomenon of wide band attenuation, i.e., anomalous absorption of an HF wave in the F region subjected to modification by a powerful HF wave, has been measured using the Tromso heating facility. Two types of results were found. In a quiet F region situation, the attenuation amounted to approximately 15 dB and did not change when the effective radiated heating power was lowered from 160 to 20 MW. In a variable F region, indicated by a high fading rate of the diagnostic wave, the absorption was considerably less than 15 dB and showed a strong dependence on the heating power. An explanation for both types of results is offered by extending a recent theory of Das and Fejer.

The reflection properties of typical D - and E -region models for radiowave frequencies varying from 16 kHz (VLF) to 3.0 MHz (HF) have been calculated by full wave, ray tracing and phase integral methods. The results of these calculations are compared with experimental observations of reflection coefficients in the frequency range 16–245 kHz, with oblique incidence absorption results (A3) at frequencies of 1178, 2614 and 2775 kHz and with vertical incidence measurements of absorption and equivalent height in the range 1.7–3.0 MHz. It was relatively easy to construct model ionospheres which yield calculated values of the propagation character- istics consistent with experimental results for any limited frequency range, but considerably more difficult to define profiles which account for measurements made throughout the frequency range from VLF to HF. Electron density distributions have been established which yield calculated values in good agreement at all frequencies with the monthly median of the mid-day observations during summer and equinox sun spot minimum conditions. It proved extremely difficult to construct an average profile for the winter period due to the great variability of the experimental data during this season.