European Incoherent Scatter UHF Radar Research Articles

First Observations of Recurring HF‐Enhanced Topside Ion Line Spectra Near the Fourth Gyroharmonic

AbstractWe present first incoherent scatter radar observations of systematically recurring, high‐frequency (HF)‐enhanced ion line spectra at the topside F‐region ionosphere, during magnetic field aligned HF pumping in an O‐mode polarization. The European Incoherent Scatter UHF radar was directed in magnetic zenith on 9–11 March 2016 while stepping the pump frequency across the double resonance of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 3‐min‐on, 3‐min‐off pump cycle. Topside and bottomside enhancements occur at the respective plasma resonance altitude and seem to be asymmetrically conditioned by the relative proximity of the pump frequency to the double resonance frequency. Further, the topside HF‐induced ion line enhancements predominantly appear while the pump frequency is just below the double resonance frequency and only simultaneous to strong bottomside enhancements. A powerful, HF radio wave in O‐mode, transmitted in the direction of magnetic zenith is reflected a few kilometers below the plasma resonance altitude, where the pump frequency is equal to the local plasma frequency, on the bottomside F‐region in the ionosphere. Transionospheric propagation of the pump wave outside the radio window can be facilitated by density striations in the plasma, and we consider, in detail, the possible mechanisms proposed for propagation outside the standard radio window.

Comment on “Parametric Instability Induced by X‐Mode Wave Heating at EISCAT” by Wang et al. (2016)

AbstractIn their recent article Wang et al. (2016) analyzed observations from EISCAT (European Incoherent Scatter) Scientific Association Russian X‐mode heating experiments and claimed to explain the potential mechanisms for the parametric decay instability (PDI) and oscillating two‐stream instability (OTSI). Wang et al. (2016) claim that they cannot separate the HF‐enhanced plasma and ion lines excited by O or X mode in the EISCAT UHF radar spectra. Because of this they distinguished the parametric instability excited by O‐/X‐mode heating waves according to their different excitation heights. Their reflection heights were determined from ionosonde records, which provide a rough measure of excitation altitudes and cannot be used for the separation of the O‐ and X‐mode effects. The serious limitation in their analysis is the use of a 30 s integration time of the UHF radar data. There are also serious disagreements between their analysis and the real observational facts. The fact is that it is the radical difference in the behavior of the X‐ and O‐mode plasma and ion line spectra derived with a 5 s resolution, which provides the correct separation of the X‐ and O‐mode effects. It is not discussed and explained how the parallel component of the electric field under X‐mode heating is generated. Apart from the leakage to the O mode, results by Wang et al. (2016) do not explain the potential mechanisms for PDI and OTSI and add nothing to understanding the physical factors accounting for the parametric instability generated by an X‐mode HF pump wave.

Identification of scintillation signatures on GPS signals originating from plasma structures detected with EISCAT incoherent scatter radar along the same line of sight.

Ionospheric scintillation originates from the scattering of electromagnetic waves through spatial gradients in the plasma density distribution, drifting across a given propagation direction. Ionospheric scintillation represents a disruptive manifestation of adverse space weather conditions through degradation of the reliability and continuity of satellite telecommunication and navigation systems and services (e.g., European Geostationary Navigation Overlay Service, EGNOS). The purpose of the experiment presented here was to determine the contribution of auroral ionization structures to GPS scintillation. European Incoherent Scatter (EISCAT) measurements were obtained along the same line of sight of a given GPS satellite observed from Tromso and followed by means of the EISCAT UHF radar to causally identify plasma structures that give rise to scintillation on the co‐aligned GPS radio link. Large‐scale structures associated with the poleward edge of the ionospheric trough, with auroral arcs in the nightside auroral oval and with particle precipitation at the onset of a substorm were indeed identified as responsible for enhanced phase scintillation at L band. For the first time it was observed that the observed large‐scale structures did not cascade into smaller‐scale structures, leading to enhanced phase scintillation without amplitude scintillation. More measurements and theory are necessary to understand the mechanism responsible for the inhibition of large‐scale to small‐scale energy cascade and to reproduce the observations. This aspect is fundamental to model the scattering of radio waves propagating through these ionization structures. New insights from this experiment allow a better characterization of the impact that space weather can have on satellite telecommunications and navigation services.

First wind shear observation in PMSE with the tristatic EISCAT VHF radar

AbstractThe Polar Summer Mesosphere has the lowest temperatures that occur in the entire Earth system. Water ice particles below the optically observable size range participate there in the formation of strong radar echoes (Polar Mesospheric Summer Echoes, PMSE). To study PMSE we carried out observations with the European Incoherent Scatter (EISCAT) VHF and EISCAT UHF radar simultaneously from a site near Tromsø (69.58°N, 19.2272°E) and observed VHF backscattering also with the EISCAT receivers in Kiruna (67.86°N, 20.44°E) and Sodankylä (67.36°N, 26.63°E). This is one of the first tristatic measurements with EISCAT VHF, and we therefore describe the observations and geometry in detail. We present observations made on 26 June 2013 from 7:00 to 13:00 h UT where we found similar PMSE patterns with all three VHF receivers and found signs of wind shear in PMSE. The observations suggest that the PMSE contains sublayers that move in different directions horizontally, and this points to Kelvin‐Helmholtz instability possibly playing a role in PMSE formation. We find no signs of PMSE in the UHF data. The electron densities that we derive from observed incoherent scatter at UHF are at PMSE altitudes close to the noise level but possibly indicate reduced electron densities directly above the PMSE.

Height-dependent ionospheric variations in the vicinity of nightside poleward expanding aurora after substorm onset

AbstractHigh‐latitude ionospheric variations at times near auroral substorms exhibit large temporal variations in both vertical and horizontal extents. Statistical analysis was made of data from the European Incoherent Scatter UHF radar at Tromsø, Norway, and International Monitor for Auroral Geomagnetic Effects magnetometer for finding common features in electron density, ion and electron temperatures and relating these to currents and associated heating. This paper particularly focused on the height dependencies. Results show clear evidences of large electric field with corresponding frictional heating and Pedersen currents located just outside the front of the poleward expanding aurora, which typically appeared at the eastside of westward traveling surge. At the beginning of the substorm recovery phase, the ionospheric density had a large peak in the E region and a smaller peak in the F region. This structure was named as C form in this paper based on its shape in the altitude‐time plot. The lower altitude density maximum is associated with hard auroral electron precipitation probably during pulsating aurora. We attribute the upper F region density maximum to local ionization by lower energy particle precipitation and/or long‐lived plasma that is convected horizontally into the overhead measurement volume from the dayside hemisphere.

Variations of the neutral temperature and sodium density between 80 and 107 km above Tromsø during the winter of 2010–2011 by a new solid‐state sodium lidar

A new solid‐state sodium lidar installed at Ramfjordmoen, Tromsø (69.6°N, 19.2°E), started observations of neutral temperature together with sodium density in the mesosphere‐lower thermosphere (MLT) region on 1 October 2010. The new lidar provided temperature data with a time resolution of 10 min and with good quality between ∼80 and ∼105 km from October 2010 to March 2011. This paper aims at introducing the new lidar with its observational results obtained over the first 6 months of observations. We succeeded in obtaining neutral temperature and sodium density data of ∼255.5 h in total. In order to evaluate our observations, we compared (1) the sodium density with that published in the literature, (2) average temperature and column sodium density data with those obtained with Arctic Lidar Observatory for Middle Atmosphere Research Weber sodium lidar, and (3) the neutral temperature data with those obtained by Sounding of the Atmosphere with Broadband Emission Radiometry/Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. For the night of 5 October 2010, we succeeded in conducting simultaneous observations of the new lidar and the European Incoherent Scatter UHF radar with the tristatic Common Program 1 (CP‐1) mode. Comparisons of neutral and ion temperatures showed a good agreement at 104 km between 0050 and 0230 UT on 6 October 2010 when the electric field strength was smaller, while significant deviations (up to ∼25 K) are found at 107 km. We evaluated contributions of Joule heating and electron‐ion heat exchange, but derived values seem to be underestimated.

The thresholds of ionospheric plasma instabilities pumped by high‐frequency radio waves at EISCAT

We test the existing theories regarding the thresholds for the parametric decay instability (PDI), the oscillating two‐steam instability (OTSI), and the thermal parametric instability (TPI) using the European Incoherent Scatter (EISCAT) facility's ionospheric heater. In these processes, the pump wave can couple to various electrostatic waves in the F layer ionosphere, which can be observed using the EISCAT UHF radar (PDI and OTSI) or by HF radar (TPI). On 19 October 2012, the heater power was stepped from ∼0.5 MW to ∼100 MW effective radiated power in seven steps using a 1 min on, 1 min off cycle. We use an electric field model, taking into account D region absorption, to compare theory with our observations. In all three cases, we find good agreement. In addition, the growth of striations formed during the TPI causes anomalous absorption of the heater wave, which we observe as decreased UHF ion line and plasma line backscatter power. We show evidence that heating for a prolonged period of time reduces the UHF ion line intensity throughout the experiment.

High-precision measurement of satellite range and velocity using the EISCAT radar

Abstract. This paper is a continuation of an earlier work by Nygrén et al. (2012), where the velocity of a hard target was determined from a set of echo pulses reflected by the target flying through the radar beam. Here the method is extended to include the determination of range at a high accuracy. The method is as follows. First, the flight time of the pulse from the transmitter to the target is determined at an accuracy essentially better than the accuracy given by the sampling interval. This method makes use of the fact that the receiver filtering creates slopes at the phase flips of the phase modulated echo pulse. A precise flight time is found by investigating the echo amplitude within this slope. A value of velocity is calculated from each echo pulse as explained in the earlier paper. Next, the ranges together with velocities from a single beam pass are combined to a measurement vector for a linear inversion problem. The solution of the inversion problem gives the time-dependent range and velocity from the time interval of satellite flight through the radar beam. The method is demonstrated using the EISCAT (European Incoherent Scatter) UHF radar and radio pulses reflected by a satellite. The achieved standard deviations of range are about 5–50 cm and those of velocity are about 3–25 mm s−1.

Regular features of the polar ionosphere characteristics from Digisonde measurements over Norilsk

Regular features of the polar ionosphere have been studied using its local empirical model of the electron density distribution in the bottomside ionosphere. The local empirical model was derived from the hand-scaled ionogram data recorded by DPS-4 Digisonde at Norilsk, Russia (69.4N, 88.1E; 60N GLAT, 166E GLON) for a 6-year period from December, 2002 to December, 2008. The paper describes the technique used to build the local empirical model and discusses its diurnal, seasonal, and solar activity specifications in comparison with the standard IRI-2007 climatological model for the same period of time, long-term observations from the European Incoherent Scatter UHF radar (1988–1999), and the high-latitude ionosondes data. Primary focus of the paper is behavior of the three F2 layer parameters: the F2 peak density (NmF2), the peak height (hmF2) and the bottomside thickness (B0). Special emphasis of the paper is the analysis of the winter anomaly manifestation at Norilsk and the peculiar diurnal–seasonal behavior of hmF2 under low solar activity, named as a “polar day effect”.

Localized mesosphere-stratosphere-troposphere radar echoes from theEregion at 69°N: Properties and physical mechanisms

[1] We present the first observations, to our knowledge, of a new class of high-latitude mesosphere-stratosphere-troposphere radar echoes from the E region as observed with the Arctic Lidar Observatory for Middle Atmosphere Research wind radar during the period 2004–2008. These echoes occur primarily during the summer months and in the altitude range from 93 to 114 km, with a pronounced peak of maximum occurrence at about 100 km. The echoes are rather short with typical durations of ∼20 min, with some examples lasting as long as 3 h. The echoes typically cover only a few hundred meters in the vertical and show both small Doppler velocities (±1–2 m/s) as well as very narrow spectral widths (just a few meters per second when converted to Doppler velocities). The echoes are highly aspect sensitive indicative of a specular-scattering mechanism and reveal a distinct diurnal variation with maxima of occurrence around noon and midnight. The latter is related to the semidiurnal tidal components of the zonal and meridional wind where times of occurrence correspond to large values of corresponding vertical wind shears. Considering possible physical mechanisms, turbulence with large Schmidt number scatter is likely ruled out as is auroral backscatter. Finally, a strong case for a close correspondence of the echoes to sporadic E layers is presented on the basis of comparisons to ionosonde data, occurrence patterns of sporadic layers, simultaneous and common volume lidar measurements of a sporadic Fe layer, as well as simultaneous measurements of sporadic E layers with the European Incoherent Scatter UHF radar at a horizontal distance of 130 km. Applying the theory of partial reflections to the observed electron density gradients, we are able to demonstrate that the observed echo strengths can likely be explained on the basis of this scattering mechanism.

Statistical relationships between cosmic radio noise absorption and ionospheric electrical conductances in the auroral zone

Statistical models expressing the Hall and Pedersen conductances and their ratio as functions of cosmic noise absorption (CNA) are derived for five intervals of magnetic local time (MLT). The models are based on simultaneous measurements of electron densities from the European Incoherent Scatter UHF radar at Tromsø (69.6°N, 19.2°E) and absorption from the imaging riometer at Kilpisjärvi (69.1°N, 20.8°E). The Hall conductance and the conductance ratio are found to be rather strongly related to CNA, whereas the Pedersen conductance is less so. The Hall conductance‐CNA relationship is strongly dependent on MLT. These results are interpreted as being the consequence of the particular sensitivity of CNA to the typical energy of electron precipitation, the latter changing as a function of MLT as the electrons drift around the Earth. The models are compared to a previous study which did not use simultaneous measurements or take into account the MLT dependence. There is a significant difference between that study and the results presented here.

An ion drag contribution to the lower thermospheric wind in the summer polar region

Using the European incoherent scatter (EISCAT) Svalbard radar located in Longyearbyen (78.2°N, 16.0°E) and with the EISCAT UHF radar located in Tromsø (69.6°N, 19.2°E), we have analyzed the data obtained for six consecutive days from 1000 UT on 1 July 1999 to 1000 UT on 7 July 1999 to investigate the importance of ion drag on the lower thermospheric wind dynamics in the summer polar region. The electric field observed at Longyearbyen exhibited a stable diurnal variation primarily occurring throughout the first 3 days of observations, while, during the latter 3 days, the diurnal variation was not clearly identified. The electric field observed at Tromsø did not exhibit such a stable variation. On the basis of the difference of the electric field variation observed at Longyearbyen, we divided the data sets into two intervals with a length of 3 days each. We compared the horizontal velocities of the ions and the neutral gases. At upper heights (∼110–120 km) on the first 3 days, the difference between the ion and neutral motion was clearly identified at Longyearbyen, while it was not at Tromsø. We compared the diurnal tidal amplitudes and phases of the two intervals derived from the Longyearbyen data. The amplitude of the meridional diurnal tide for the first 3 days was larger than that of the latter 3 days over the height region between 100 and 118 km at Longyearbyen. Furthermore, we compared ion drag accelerations with total accelerations. We concluded that the ion drag played an important role in the diurnal amplitude at Longyearbyen on the first 3 days of the observations. In addition, we compared the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) predictions with the observations. The TIME‐GCM predictions also indicated that the diurnal tide is influenced by the convection electric field.

Climatological features of electron density in the polar ionosphere from long-term observations of EISCAT/ESR radar

Abstract. In this paper, climatological features of the polar F2-region electron density (Ne) are investigated by means of statistical analysis using long-term observations from the European Incoherent Scatter UHF radar (called EISCAT in the following) and the EISCAT Svalbard radar (ESR) during periods of quiet to moderate geomagnetic activity. Field-aligned measurements by the EISCAT and ESR radars operating in CP-1 and CP-2 modes are used in this study, covering the years 1988–1999 for EISCAT and 1999–2003 for ESR. The data are sorted by season (equinox, summer and winter) and solar cycle phase (maximum, minimum, rising and falling). Some novel and interesting results are presented as follows: (1) The well-known winter anomaly is evident during the solar maximum at EISCAT, but it dies out at the latitude of the ESR; (2) The daytime peaks of Ne at EISCAT for all seasons during solar maximum lag about 1–2 h behind those at ESR, with altitudes about 10–30 km lower. (3) In addition to the daytime peak, it is revealed that there is another peak just before magnetic midnight at ESR around solar maximum, especially in winter and at equinox. The day-time ionization peak around magnetic noon observed by ESR can be attributed to soft particle precipitation in the cusp region, whereas the pre-midnight Ne maximum seems likely to be closely related to substorm events which frequently break out during that time sector, in particular for the winter case. (4) Semiannual variations are found at EISCAT during solar minimum and the falling phase of the solar cycle; at the rising phase, however, the EISCAT observations show no obvious seasonal variations.

A comparison study of the auroral lower thermospheric neutral winds derived by the EISCAT UHF radar and the Tromsø medium frequency radar

A comparative study of the neutral wind in the polar upper mesosphere/lower thermosphere is conducted using two radars, the European Incoherent Scatter (EISCAT) UHF radar (ν = 931 MHz) and the Tromsø MF radar (ν = 2.8 MHz) colocated in northern Scandinavia. Comparisons of winds are made in case studies as well as on a statistical basis. On the basis of simultaneous observations obtained by the two radars, comparisons over a height range from ∼90 to ∼100 km are made for about 20 days between February and October 1999. In these comparisons we directly compare temporal wind variations. On the other hand, mean winds as well as diurnal and semidiurnal components are derived using 1‐month‐averaged wind data obtained by the medium frequency (MF) radar for 1999. They are compared with those derived from 56 days of EISCAT data studied by Nozawa and Brekke [1999a, 1999b]. In the case studies, we have also utilized altitude profiles of electron density obtained by the EISCAT radar at and above 62 km height to determine the total reflection height as well as to estimate the effect of group retardation for 2.8 MHz radio wave. It can be seen that the effect of particle precipitation sometimes penetrates into the D region (down to ∼84 km). Generally, wind velocities derived by EISCAT with the monostatic method exhibit significant scatter with time below 100 km, while the number of the MF radar wind data sharply decreases above 90 km except for summer. These facts make the comparison difficult, but no significant departures between different radar winds are identified on an instantaneous basis. In the statistical studies, generally both altitude profiles are well connected, or continuous, but significant disagreements are observed for the mean wind and the tidal components for summer. On the basis of these comparisons, we raise strong concerns regarding the use of summer wind data above 91 km obtained by MF radar located at high latitude under high solar activity conditions. The wind values during the winter and equinox observed by the two radars are complementary. Thus here is a good opportunity to utilize wind data from the two different radars through a year, which then enables us to cover a wide height range (approximately 70–120 km). This wide range gives us good opportunities to examine the mesosphere/lower thermosphere coupling processes based on observational data.

A case study of the auroral E region neutral wind on a quiet summer day: Comparison of the European Incoherent Scatter UHF radar for deriving the E region wind

Comparison study of three methods of the European Incoherent Scatter (EISCAT) radar for furnishing E region neutral wind data is performed on the basis of observational data. A Special Program by using the EISCAT UHF radar located in northern Scandinavia was conducted for 24 hours starting at 1000 UT on July 14, 1997. Two modes of EISCAT Common Program 1 and 2 are combined in this Special Program, so that the capability of the tristatic method and monostatic (or beamswinging) methods are compared on the basis of observations at four heights between 99 and 108 km under geomagnetic quiet conditions. The meridional neutral wind is derived by the field‐aligned method as well. Derived wind velocities by the three methods are compared, and fairly good agreement is found in terms of a relatively longer temporal variation (∼2–3 hours). The comparison of the derived velocities shows that the tristatic method (i.e., Common Program 1) is superior to the monostatic method (i.e., Common Program 2) in terms of an instantaneous measurement, while the monostatic method has an advantage on a tidal study. Regarding the meridional wind component, the filed‐aligned method has significant advantages compared to the other methods concerning temporal data coverage, error, and height coverage. The derived neutral wind in this Special Program is compared with that from a statistical study by Nozawa and Brekke [1999a]. Generally good agreement for mean and semidiurnal winds is found with the statistical result, but relatively strong amplitude (∼35 m s−1) of 8‐hour meridional component is found in this case study. Comparison of the zonal mean wind and mean temperature is made with the CIRA (1986) model as well. Good agreement is obtained for the temperature, while poor agreement is found for the zonal mean wind.

The EISCAT Svalbard radar: A case study in modern incoherent scatter radar system design

The EISCAT (European incoherent scatter) Svalbard radar (ESR) was officially inaugurated on August 22, 1996. This event marked the successful completion on schedule of the first phase of the EISCAT Svalbard radar project. In contrast to previous incoherent scatter radars, the ESR system design was adapted to make use of commercial off‐the‐shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum. Commercial hardware is also used in the digital signal processing system. Control and monitoring are performed by distributed, networked VME systems. Thanks to modern reflector antenna design methods and extreme efforts to reduce the receiver noise contribution, the system noise temperature is only 70 K, thus making the ESR about 30% faster than the much more powerful EISCAT UHF radar in F region experiments! Once the transmitter power is increased to 1 MW, it will become about 2–3 times faster than the UHF radar. State‐of‐the‐art exciter and receiver hardware has been developed in‐house to accommodate the special requirements introduced by operating the radar at the exceptionally high duty cycle of 25%. The RF waveform is generated by a system based on four switchable direct digital synthesizers. Continuous monitoring of the transmitted RF waveform by the receiver system allows removal of klystron‐induced spurious Doppler effects from the data. Intermediate‐frequency sampling at 7.5 MHz is employed, followed by fully digital channel separation, signal detection, and postdetection filtering in six parallel receiver channels. Radar codes for both E and F layer observation have been designed and perfected. So far, more than 40 hours of good quality ionospheric data have been collected and analyzed in terms of plasma parameters. While the tragic loss of the Cluster mission suddenly changed the plans and dispositions of a majority of the ESR user community, the radar has still been in high demand since its inauguration. It is now being operated by EISCAT staff on a campaign basis, to provide ground‐based support data for a number of other magnetospheric satellites, notably Polar and FAST, and will be opened to the EISCAT user community for special program operations later in 1997.

Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

Abstract. We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are short-lived and occur in single 2- or 10-s data dumps. They are offset by 125–130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental.

Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are short-lived and occur in single 2- or 10-s data dumps. They are offset by 125–130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental.

Strongly enhanced incoherent‐scatter plasma lines in aurora

Observations of incoherent‐scatter plasma‐line intensities, measured in the E‐region with the European Incoherent Scatter UHF radar, during auroral precipitation, are presented. Intensities up to 200 times the thermal level were observed in a restricted frequency interval from 5.5 to 6.5 MHz. Intensities were lower at both higher and lower frequencies. The intensities are compared with quantitative estimates based on model suprathermal electron fluxes for the prevailing conditions. The minimum in suprathermal electron fluxes between 2 and 4 eV, which is caused by the excitation of vibrational levels in N2, is found to result in a minimum in expected plasma‐line damping and a maximum in intensity between 5.5 and 6.5 MHz, for the EISCAT UHF wavelength. Good agreement is found between the calculated and observed intensities.

Meteor observations with the European Incoherent Scatter UHF Radar

The European Incoherent Scatter (EISCAT) UHF radar, which operates at a nominal frequency of 930 MHz, is introduced as a powerful meteor radar. Its high sensitivity is utilized to detect transient enhanced ionization trails caused by meteors of all orientations, in contrast to conventional HF and VHF backscatter radars, which observe only the meteor trails oriented approximately normal to the radar beam. A comparison shows that EISCAT observes almost as high hourly rates of meteors as meteor radars do, in spite of its beamwidth being only 0.5° compared to their 100°. Two different kinds of echoes are seen in the data. About three fourths of them are strongly Doppler‐shifted transient echoes, which we interpret as head echoes, since they move with meteor velocities. The remaining echoes are long‐lived. They come from the ionized trails left behind the meteors, which drift slowly or not at all. There are fundamental differences between the scattering process producing both types of echoes and the scattering from trails observed with the meteor radars. For example, we have never seen the trail echoes appear after a head echo. All our shower echoes are also strongly nonspecular. We postulate that some time is required for the trails to expand and for the ionization within them to approach the thermal equilibrium state in order to give detectable incoherent echoes. Due to the very small beamwidth our radar cannot follow the motion of an individual meteor, as the conventional radars can, but we can obtain a statistical profile of what happens at different heights as the meteoroids penetrate the atmosphere. The altitude distribution of ionized trails shows that they are observable over the whole measurement range from about 65 to 165 km, while the head echoes can be detected only within a narrow altitude range. The latter effect is probably related to the radio meteor ceiling effect.