EISCAT observations of large scale electron temperture and electron density perturbations caused by high power HF radio waves

In this paper EISCAT observations of the effect of artificial modification on the F-region electron temperature and electron density during several heating experiments at Tromsø are reported. During O-mode heating at full power (ERP = 240 MW) the electron temperature is increased by up to 55% of its ambient value at altitudes close to the heater interaction height. Measurements of the electron density have revealed both enhancements and depletions in the vicinity of the heater reflection height. These differences are indicative of variations in the balance between the transport and chemical effects. These results are compared with a time dependent numerical model developed from the perturbation equations of Vas'kov and Gurevich [(1975) Geomagn. Aeron.15, 51]. The results of numerical modelling of the electron temperature are in good agreement with the EISCAT observations, whereas there is less good agreement with regard to electron density.     

High-resolution measurements of magnetospheric electric fields

Observations made at EISCAT suggest that the plasma velocity measured in the F-region above Tromsø can vary substantially on a timescale of a minute or so. The high-resolution measurements made using alternating codes during the ERRRIS experiment have confirmed this result by showing that the rapid variations of plasma velocity measured directly correspond exactly to the variations of ion temperature in the rmupper-E and lower-F region caused by frictional heating, and the variations of electron temperature in the E-region, caused by wave turbulence heating.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

On Joule heating of the equatorial electrojet E-region

Simultaneous data on electron density, electron temperature and current density obtained from a rocket borne Langmuir probe, a glass-sealed Langmuir probe and a proton precession magnetometer flown from Thumba (geomag. lat. 0.99°S, geomag. long. 146.79°E, magnetic dip 0°47'S) have been used to calculate the Joule heating in order to assess whether it contributes significantly to the thermal imbalance in the E-region. It is envisaged that the changes in electron temperature are partially brought about by changes in collision frequency and the energy loss factor. It is found that the Joule heating alone is not sufficient to explain the observed differences in electron and neutral gas temperatures. The inclusion of photoelectron heating and adjustments of profiles of the collision frequency and the energy loss factor bring the computed temperature differences closer to the observed differences.     

Comparison of incoherent scatter observations of electron density,and electron and ion temperature at Millstone Hill with the International Reference Ionosphere

Millstone Hill incoherent scatter (IS) observations of electron density (N_e, electron temperature (T_e) and ion temperature (T_i) are compared with the International Reference Ionosphere (IRI-86) for both noon and midnight, for summer, equinox and winter, at both solar maximum (1979–1980) and solar minimum (1985–1986). The largest difference inN_e is found in the topside, where values of N_e given by IRI-86 are generally larger than those obtained from IS measurements, by a factor which increases with increasing height, and which has a mean value near two at 600 km. Apart from the bottom of the profile, which is tied to the CIRA neutral temperature, the IRI-86 T_e model has no solar cycle variation. However, the IS measurements during the summer reveal larger T_e at solar maximum than at solar minimum. At other seasons higher T_e at solar maximum occurs only during the daytime at the greater heights. Nighttime T_e is shown by the IS radar to be generally larger in winter than in summer, an effect not included in the IRI. This is apparently due to photoelectron heating during winter from the sunlit ionosphere conjugate to Millstone Hill. The day-night difference in T_i given by IRI-86 above 600km is not as large in the IS measurements.     

The relationship between electron temperature and electron density under quiet conditions in the auroral zone

A theoretical model is described which predicts electron temperature in the day-time F-region above EISCAT on geomagnetically quiet days, given the observed values of electron concentration, ion temperature and heat conduction, the daily average of solar radiation at 10.7cm and the MSIS-86 model of the neutral atmosphere. The values predicted by the model agree very closely with the observed temperatures, both for average conditions and for individual days. On two occasions the onset of a geomagnetic disturbance after a period of quiet conditions was accompanied by a growing divergence between the predicted and observed values, which corresponds to an additional source of electron heating such as would be provided by low energy particle precipitation.     

EISCAT incoherent scatter radar observations and model studies of day to twilight variations in the D-region during the PCA event of August 1989

An intense solar proton event causing enhanced ionization in the ionospheric D-region occurred on 12 August 1989. The event was partially observed during three successive nights by the EISCAT UHF incoherent scatter radar at Ramfjordmoen near Tromsa, Norway. Ion production rates calculated from GOES-7 satellite measurements of proton flux and a detailed ion chemistry model of the D-region are used together with the radar data to deduce electron concentration, negative ion to electron concentration ratio, mean ion mass and neutral temperature in the height region from 70 to 90 km, at selected times which correspond to the maximum and minimum solar elevations occurring during the radar observations. The quantitative interpretation of EISCAT data as physical parameters is discussed. The obtained temperature values are compared with nearly simultaneous temperature measurements at Andøya based on lidar technique.     

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm⁻¹, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O⁺ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O⁺ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O⁺ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.     

Observations of the high latitude trough using EISCAT

Data taken by EISCAT are presented as contours of electron density, ion and electron temperature and plasma velocity versus invariant latitude and local magnetic time.Three nights near midsummer were studied and in each case a trough in electron density occurred north of invariant latitude 64° shortly after local midnight (MLT 0200) and remained a prominent feature for about 3 h before moving poleward. The minimum in electron density was associated with a marked increase in ion temperature, but the electron temperature showed litttle change. In this respect the high latitude trough is clearly different from the mid-latitude trough.Full velocity measurements were not available for all three nights, but it seems that the appearance of the trough followed the start of a strong eastward plasma velocity combined with a strong upward velocity along the magnetic field line. The sudden change in plasma velocity causes frictional heating, which explains the increase in ion temperature. Upward plasma velocity is a major factor in the formation of the trough, with enhanced recombination making a smaller contribution.     

A study of gravity waves in ionospheric electron content at L = 4

Using satellite radio beacon transmissions, travelling ionospheric disturbances have been observed in the electron content at L = 4. Waves are a common feature at this latitude, present for at least 98% of all daylight hours. The amplitude is usually 1–4% of the mean electron content and periods range between 15 and 90 minutes. Simultaneous observation of two satellite beacons, giving an effective east-west separation of 350 km, indicated apparent east-to-west velocities of 200–700 m/s.A search was made for a likely source of the waves, using data from magnetometers and riometers, from incoherent scatter radar measurements of Joule heating, and from orbiting satellite measurements of electron influx, but no definite source could be established.It is also shown that travelling disturbances are closely related to occurrences of spread-F on ionograms at high latitudes.     

Simultaneous generation of electron temperature and density fluctuations by high power HF radio waves

The parametric interaction between right-hand circularly polarized electron cyclotron waves as well as non-resonant density and temperature perturbations is considered by taking into account the radiation pressure and the differential Joule heating nonlinearities. A nonlinear dispersion relation, which admits a new class of thermal parametric instabilities for the case in which Joule heating nonlinearity far exceeds the radiation pressure, is derived. It is found that the temperature and density fluctuations are rapidly driven when the pump frequency is close to the electron gyrofrequency. The relevance of our investigation to enhanced density and temperature fluctuations due to the action of high power HF radio waves in the Earth's ionosphere is pointed out.     

The influence of geomagnetic activity on the upper mesosphere/ lower thermosphere in the auroral zone. I. Vertical winds

A scanning Fabry-Perot spectrometer (FPS), located at Mawson station, Antarctica (672S, 63°E, invariant latitude 70°S) was used to obtain vertical wind, temperature, and emission intensity measurements from the λ558 nm emission of atomic oxygen. The measured temperature is used to assign an approximate emission height to the observations. A spaced-antenna partial-reflection radar was run concurrently with the FPS from which the presence of enhanced ionization in the D-region could be inferred from the return heights and strengths of the echoes. Large upwards winds of approximately 30 m s⁻¹, at altitudes less than 110 km, appear to be a direct response of the neutral atmosphere to intense auroral events. It is suggested that the observed upwelling is a result of particle heating at heights below the principal emission height. At higher altitudes, vertical winds of a similar magnitude are also measured during geomagnetically disturbed conditions, although here they do not appear to be associated with particular auroral events. In this case it is suggested that upwelling is produced by a combination of Joule and particle heating.     

The use of electron spin resonance spectroscopy to determine the thermal histories of cereal grains

The thermal histories of archaeological cereal grains have been explored with electron spin resonance spectroscopy by a quantitative study of pyrolysis-induced radical carbon. Comparison with modern samples of heated cereal grain has shown that the g value of the radical carbon signal is characteristic of exposure to a previous maximum temperature, and studies of other signal parameters suggest that further information, such as the duration of previous heating, may be deduced from such studies. The proposed method offers the possibility of quantitatively rigorous measurement in archaeological studies of cultural and technological processes that previously relied upon subjective assessment.     

A case study of a high latitude ionospheric electron density depletion

Incoherent scatter measurements inside and outside an ionospheric electron density depletion are described. The density depletion is probably caused by an enhancement of NO⁺-ions and subsequent dissociative recombination. The NO⁺-ions are increased because high electric fields present at the geographical location of the density depletion speed up the reaction O⁺ + N₂ → NO⁺ + N. The electron as well as the ion temperature within the density depletion are strongly enhanced, the latter due to Joule heating, also caused by the electric field.     

Winds,wind-shears and plasma densities during the initial phase of a magnetic storm from equatorial latitudes

Results of a sodium vapour release experiment carried out from SHAR (India), an equatorial rocket launching station, immediately after (⩽ 2 h) a storm sudden commencement (SSC) during the initial phase of a magnetic storm, followed by electron density measurements are presented. Many of the relevant atmospheric parameters, namely, neutral winds and their altitude variation, the magnitude of the shears in them, the neutral temperature with altitude by spectroscopic methods, diffusion measurements on the released trail, clues on the turbopause level and the electron densities including the structures (irregularities) in them were obtained. The results of the temperature measurements carried out independently on the sodium trail by means of a ground-based Fabry-Perot spectrometer, operating on the sodium D 1 line, resonantly scattered by the trail have already been reported by us (Ranjan Guptaet al., 1986). In this paper the effects of the excess temperature reported earlier and the rest of the measured parameters on the electron density profiles are evaluated using models and compared with the measurements.The formation of sharp layers of ionization have been explained on the basis of the electro-dynamical processes associated with the wind shears at a location, close to the edge of the equatorial electrojet belt. The significance of the changes in the neutral composition due to the enhanced neutral temperature and the low turbopause level, in increasing the base-level plasma densities by a factor of 3–5 are demonstrated and the possible role of plasma dynamics discussed.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

F-layer irregularities' formation at auroral latitudes: radio wave scintillation and EISCAT observations

A coordinated experiment involving scintillation observations using orbital satellite beacons and CP-3-F program measurements by means of the EISCAT ionospheric radar facility is described. The results reveal the location of patches, containing kilometre-scale irregularities, in the vicinity of a region of an electron density minimum and an electron temperature increase. In the daytime under quiet geomagnetic conditions, the region of scintillations coincided closely with the southwards gradient in electron density, while a plasma drift velocity was mainly westwards V_E-W ≲ 0.3 km/s. In the evening, the region of the most intense irregularities was transformed to the northwards sense of the electron density gradient simultaneously with the plasma drift velocity reverse and the arrival of a significant southwards component V_N-S ≲ (1.5−1.0) km/s. EISCAT data demonstrated the patches' location in regions of an electron temperature increase. Processes operating to create kilometer-scale irregularities were analysed and estimated according to the data obtained. The assessments suggest that irregularities with a cross-field scale, equal to or greater than 1 km, and a field-aligned scale, equal to or greater than 30 km, were the result of growth of the thermomagnetic instability.     

A model of the lower ionosphere above Red Lake,Canada, during the 26 February 1979 solar eclipse

Measurements of ionization sources, ionization profiles and minor atmospheric constituents were conducted during the 26 February 1979 solar eclipse above Red Lake, Canada. A model of the lower thermosphere has developed to describe the D- and E-regions of the ionosphere for this case with the model being guided by the measurements. During the eclipse a rather intense particle precipitation event was in progress. For this reason, an auroral deposition code was coupled to a chemical-kinetics code to calculate degraded primary and secondary electron fluxes, ionization rates, positive ion and electron densities. The model was calibrated with the experimental measurements of electron flux below 100 km and electron density between 70 and 150 km. This calculation not only satisfactorily described the ionization in the E-region but also the gross electron density characteristics of the D-region. Bursts in the observed electron flux were also simulated with the model to give electron density profiles that were remarkably consistent with small perturbations seen in the electron density measurements.     

Diurnal,semi-diurnal and terdiurnal Hough components of surface pressure

Diurnal and semi-diurnal Hough components of surface pressure are evaluated by classical tidal theory for previously presented profiles of water vapour and ozone heating. Values are compared with the observational results of Haurwitz and Cowley (1973) and show significant discrepancies which are considered to indicate the need for additional heating to be identified. The present calculations are in satisfactory agreement with those for semi-diurnal modes evaluated by Walterscheid et al. (1980).Profiles of terdiurnal heating due respectively to ozone and water vapour absorption of solar radiation are calculated for the (3,3,3) to (3,3,6) Hough modes and corresponding surface pressure oscillations are evaluated by classical tidal theory. Comparisons are made with earlier evaluations and with observationally derived results. The calculated solstitial (3, 3, 4) mode, which is the mode of the largest surface pressure amplitude, shows good agreement in phase with observation but underpredicts observed amplitudes by about 36% in contrast to earlier evaluations which were based on a now unacceptable basic temperature profile.     

An inversion procedure for obtaining collision frequency profiles from swept-frequency absorption measurements

A method is presented which inverts swept-frequency Al absorption data to obtain collision frequency profiles in the E- and F-regions of the ionosphere. The method gives consistent results from successive sets of measurements and the profiles obtained are consistent with other measurements of collision frequency. Accounting for D-region absorption is a difficulty affecting the accuracy of the collision frequencies obtained at the lowest heights, but model simulations show that values at higher heights are not affected seriously. The technique can be used to obtain results for the F1-region for which there are very few previous measurements.Comparison with theoretical calculations of collision frequency show agreement in the form of the altitude variation. That is, there is a rapid decrease with altitude through the E-region which becomes much less in the F-region so that the collision frequency becomes almost constant with height. This change is caused by electron-ion collisions becoming more dominant than electron—neutral collisions. However, consistent with other observers, we find a major discrepancy between the magnitude of the experimental and theoretical values. If the electron and ion temperatures are assumed equal, the experimental values are approximately five times greater. The discrepancy increases if Te >Ti in the theoretical calculations.