Evidence for non-Maxwellian ion velocity distributions in the F-region

A study has been made of data taken with EISCAT using the Common Program CP-3-C (F-region meridian scan) which shows that regions of enhanced ion temperature (in excess of 3000K at all three EISCAT stations) are found on most days when K_p exceeds 2 or 3, usually accompanied by ion drift velocities of more than 1 km s⁻¹. These periods are often accompanied by anisotropy of the ion temperature and abnormally low apparent electron temperature, consistent with the presence of a non-Maxwellian ion velocity distribution such as would result from large but not exceptional ion drifts. Data for a selected period have been fitted using theoretical ion velocity distributions based on the relaxation collision model and assuming that the ion composition is 100% O⁺. The results confirm the presence of non-Maxwellian distributions, but a detailed comparison with theory reveals some discrepancies, indicating that the analysis may need to be extended to include effects due to, for example, molecular ions and instabilities.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

Comparison of plasma flow and thermospheric circulation over northern Scandinavia using EISCAT and a Fabry-Perot interferometer

The EISCAT incoherent scatter radar, operating in a full tristatic mode, provided data on the ionospheric plasma drift above northern Scandinavia, during the 24 h period, 11 UT 25 November to 11 UT 26 November 1982. For the hours of darkness, 14 UT until 05 UT, observations of thermospheric winds were made by means of a ground-based Fabry-Perot interferometer (FPI) operated at Kiruna Geophysical Institute (21° E, 68° N). During this period, the radar observations describe well the ebbing and flowing of regions of strong convective ion flow associated with the auroral oval. As individual geomagnetic disturbances occur, the overall ion flow pattern intensifies and moves equatorward. The zonal thermospheric wind observed by the FPI responds rapidly to surges of the local ionospheric convection, while the meridional wind response is slower and apparently to much larger-scale features of the geomagnetic input to the high latitude thermosphere. From the data base, periods of strong heating of the ionospheric ions and of the thermospheric gas can be identified, which can be compared with Joule and particle heating rates deduced from the observations of ionospheric drifts, neutral winds, electron densities and auroral emission rates. A three-dimensional, time-dependent global thermospheric model is used to distinguish local and global features of the thermospheric wind field. Meridional and zonal wind components at 312 km may be theoretically derived from the EISCAT data using an appropriate model (MSIS) for neutral temperature. The EISCAT-derived meridional wind is within about 50 m s⁻¹ of the FPI observations throughout the period of joint observations. The EISCAT-derived zonal wind is systematically larger (by about 50%) than the FPI measurement, but the two independent measurements follow closely the same fluctuations in response to geophysical events until 03 UT, when the EISCAT solution is driven away from the FPI measurement by a sharp increase in both neutral and ion temperatures. Between 03 and 05 UT the EISCAT-derived zonal wind is 200–400 m s⁻¹ westward. Allowance for the neutral temperature rise would reduce the EISCAT values towards the very small zonal winds shown by the FPI during this period. We describe the relatively straightforward analysis required to derive the meridional wind from the radar data and the limitations inherent in the derivation of zonal wind, using the ion energy equation, due to the lack of precise knowledge of the background neutral temperature from the EISCAT data alone. For analysis of EISCAT ion drift observations at 312 km, the ground-based FPI temperature measurements do not improve the accuracy of the analysis, since the median altitude of the FPI measurement is probably in the range 180–240 km throughout the observation period. This median altitude and the temperature gradient both fluctuate in response to local geomagnetic events, while the temperature gradient may be considerably greater than that predicted by standard atmospheric models. When the neutral temperature is well known, or when there is a large enhancement of the ion temperature, the EISCAT-derived zonal wind exceeds the FPI measurement, but the consistency with which they correlate and follow ion-drag accelerations suggests that the differences are purely due to the considerable altitude gradients which are predicted by theoretical models.     

EISCAT measurements of ion temperatures which indicate non-isotropic ion velocity distributions

Substantial increases of the ion temperature can be observed at high latitudes as a consequence of strong convection electric fields. We have measured, with EISCAT, three independent components of the ion velocity vector and temperature in the same scattering volume, at about 300 km. During periods of strong variations in ion velocity (consequently of the E-field), the ion temperatures derived at the 3 sites are different. This difference, which appears to be systematic for the two experiments studied, can be interpreted in terms of different ion temperature perpendicular and parallel to the magnetic field, i.e. T_i⊥ greater than T_i∥. Assuming that a bi-Maxwellian distribution is present for convection electric field strengths as large as 50 mV m⁻¹, one obtains an anisotropy factor of approximately 1.5. It also appears that resonant charge exchange is the dominant collision process. During the evening sector events studied, the electron density was decreasing, whereas the electron temperature was generally increasing. Such events are strongly related to variations in the magnetic H component detected on the ground.     

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.     

Evaluation of electron and ion densities in the middle atmosphere from rocket-borne blunt probes

Detailed consideration has been given to the determination of electron number densities from conductivity data gathered by rocket-borne blunt probes in the middle atmosphere, and the intercomparison of these electron densities with those derived from other diagnostics. A definition of the difficulty of electron density determination from rocket-borne probes is presented. Also, the procedures for the determination of ion densities from blunt probe data in the middle atmosphere are critically evaluated. General aspects of particle collection by supersonic probes are compared with those of subsonic probes. It is noted that strong (× 10) compression regions will form in front of supersonic probes at altitudes up to 100 km, and the altered electron attachment rates could significantly affect indicated electron and negative ion concentrations. A summary of new analysis for determining electron densities from negative conductivities taken with a subsonic blunt probe is presented and the analysis is applied to data on several days where intercomparisons are possible. Blunt probe data from 31 January 1972 and 5 December 1972 (WI),² and 2 October 1975 and 29 September 1977 (WSMR)³ are reduced to predict electron density profiles. In the region of intercomparison, there is general agreement in the electron density predictions. The indications of electron density at altitudes below 70 km are new, and predict a region of moderately enhanced densities down to 45 km.     

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.     

Range ambiguity effects in Barker-coded multipulse experiments with incoherent scatter radars

A Barker-coded multipulse is the best of the classical modulations when high spatial resolution is needed in incoherent scatter measurements. Unfortunately, this type of modulation generates range ambiguities which reduce the quality of data. In this paper, range ambiguity functions for an EISCAT experiment, ESLA-T4, are displayed. A method of eliminating the ambiguities in data analysis is developed and its applicability is demonstrated. It is found that the method can remove the effects of the range ambiguities in the lower part of the E-region, but above 120 km altitude the correction is not sufficient. The residual of the fit is reduced by an order of magnitude. The changes in the temperatures are a few Kelvin at the lowest altitudes and a few tens of Kelvin between 110 and 120 km.     

Positive and negative ion composition measurements in the D- and E-regions during the 26 February 1979 solar eclipse

Two rockets bearing quadrupole mass spectrometers capable of measuring both positive and negative ion composition were launched from Red Lake, Canada, during the solar eclipse. Both instruments had liquid helium cryopumps and shock-attaching conical samplers. The payloads also contained two Gerdien condensers to measure total positive and negative ion concentrations and ion mobilities. Attitude control systems aligned the payloads with the velocity vector throughout ascent and descent. The first rocket was launched so that the D-region was in darkness 35 ± 8 s on the upleg and about 150 ± 15 s on the downleg for the study of ionospheric decay processes. The second rocket was fired after totality into 75% solar illumination for the study of ionospheric recovery. The positive ion composition above 105 km exhibited a strongly increasing NO⁺/O₂⁺ ratio with time after second contact due to O₂⁺ charge transfer with NO and a sharply diminished ionization rate. However, in both nights, the ionization below 105 km was created mainly by energetic particle deposition as exemplified by the increased ion concentrations and the composition signatures of a particle event: asignificant enhancement of O₂⁺ below 105 km and large amounts of H₅O₂⁺ ions in the D-region which result from the O₂⁺ clustering scheme. H₅O₂ was the major ion in the upper D-region while H₇O⁺₃, H₉O₄⁺ and H₅O₂⁺ were dominant ions at lower altitudes. Numerous minor species were also detected. The negative ion distributions in both flights exhibited a distinct shelf at 83 ± 2 km, decreasing by more than an order of magnitude by 90 km and with minima near 75 km. In the 75–90 km range, a significant percentage of the negative ions had masses exceeding 160 a.m.u. Comparisons are made with prior negative ion measurements during similar daytime auroral zone absorption (AZA) events. Two striking characteristics of the precipitating particles were apparent from these and past observations in daytime AZA events: there is a near absence of low energy electrons capable of ionizing above about 105 km and there is'a significant spatial and/or temporal variability in the electron flux. This paper is devoted principally to a presentation of the ion composition measurements and associated uncertainties.     

The EISCAT mesospheric measurements during the CAMP campaign

A brief outline is given of the experimental technique used during the Cold Arctic Mesopause Project to record the first D-region ion line spectra with the EISCAT incoherent scatter radar. The data analysis shows that echoes from mesospheric heights between about 70 km and 90 km can be detected during disturbed periods of enhanced electron density during particle precipitation events. Electron density profiles were determined which show a fairly high density, up to 5 × 10¹⁰ m⁻³ in the upper D-region. The measured meridional winds were lower than 10 m s⁻¹. A fit of the measured height profile of spectral width to temperature and neutral density models yielded a measured temperature profile in good agreement with simultaneous rocket data. The mesopause temperature was determined to be as low as 130 K. This detailed analysis of the spectral width profile indicates that below about 77–80 km the ratio of negative ions to electrons exceeded unity. Finally, some discussions are added on the limitations and significance of these first mesosphere observations.     

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance (n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km⁻¹ during the day and 0.5–0.7° km⁻¹ during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines.The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.     

Is the ion composition of the outer ionosphere related to seismic activity?

In analyzing the ion composition data from the low-altitude Intercosmos 24 satellite, significant longitudinal deviations in the latitudinal density variation of light ions, H⁺ and He⁺, were observed in the inner plasmasphere at altitudes of 2000–2500 km. These deviations display surprising correlation with the seismic activity in the Earth's crust. H⁺ and He⁺ concentrations significantly enhanced with respect to values observed above seismically inactive regions were found above the epicenters of future earthquakes for up to two days before shocks. These findings indicate that, at least under some conditions, processes of the coupling between the solid Earth and the ionosphere may be indicated by in situ measurements of plasma parameters in the Earth environment.     

Stratospheric negative ion composition measurements,ion abundances and related trace gas detection

Negative ion spectra obtained during four flights with a balloon-borne quadrupole mass spectrometer are reported and critically investigated. Ion abundances for NO₃⁻ and HSO₄⁻ core ions are reported and concentrations of HNO₃ and H₂SO₄ at altitudes between 32 and 35 km are deduced. The detection of minor mass peaks in negative ion spectra obtained at an altitude of 32 km is discussed. Major mass peaks observed at lower altitudes (from 20 to 28 km) are mainly NO₃⁻ due to core ions.     

Seasonal mean structure of the night-time F2 region over Arecibo

Seasonal mean night-time variations of ion and electron temperatures, electron density, ion drift velocity, and light ion composition of the F2 region are derived from incoherent scatter observations at Arecibo based on 19 nights of observation over the latest sunspot minimum years 1974–1976. It is shown that the downward flux of ionization is sufficient to maintain the nocturnal F2 region against recombination at low latitudes. The difference in the electron density decay rate from summer to winter is consistent with the seasonal variation in magnitude of the ionization flux. The mean eastward electric field, which is responsible for any vertical component perpendicular to B, is very small throughout the night. However, the southward electric field, i.e. east-west ion drifts, shows a substantial systematic variation during the night, being southward (eastward ion drifts) before midnight and northward after midnight, with a mean amplitude of 1–2 mVm⁻¹. The H⁺ ion concentration shows a marked seasonal variation. The mean relative concentration of H⁺ ion to electron density at 500 km sometimes exceeds 50% before sunrise in winter. A strong anti-correlation of H⁺ ion concentration with magnetic activity is observed. The observed ion temperatures average about 20–30 K higher than the prediction of the Jacchia (1971) neutral model for the observed range of the 10.7 cm solar flux.     

Scattered power from non-thermal,F-region plasma observed by EISCAT—evidence for coherent echoes?

Three rapid, poleward bursts of plasma flow, observed by the U.K.-POLAR EISCAT experiment, are studied in detail. In all three cases the large ion velocities (> 1 kms⁻¹) are shown to drive the ion velocity distribution into a non-Maxwellian form, identified by the characteristic shape of the observed spectra and the fact that analysis of the spectra with the assumption of a Maxwellian distribution leads to excessive rises in apparent ion temperature, and an anticorrelation of apparent electron and ion temperatures. For all three periods the total scattered power is shown to rise with apparent ion temperature by up to 6 dB more than is expected for an isotropic Maxwellian plasma of constant density and by an even larger factor than that expected for non-thermal plasma. The anomalous increases in power are only observed at the lower altitudes (< 300 km). At greater altitudes the rise in power is roughly consistent with that simulated numerically for homogeneous, anisotropic, non-Maxwellian plasma of constant density, viewed using the U.K.-POLAR aspect angle. The spectra at times of anomalously high power are found to be asymmetric, showing an enhancement near the downward Doppler-shifted ion-acoustic frequency. Although it is not possible to eliminate completely rapid plasma density fluctuations as a cause of these power increases, such effects cannot explain the observed spectra and the correlation of power and apparent ion temperature without an unlikely set of coincidences. The observations are made along a beam direction which is as much as 16.5° from orthogonality with the geomagnetic field. Nevertheless, some form of coherent-like echo contamination of the incoherent scatter spectrum is the most satisfactory explanation of these data.     

Positive ion composition in the lower ionosphere at high latitudes during MAP/WINE

Positive ion composition, total ion and electron density and ion production by energetic electrons were measured by rocket-borne experiments above Andøya (69.3°N, 16.0°E) in northern Norway. Observed altitudes of transition from molecular ions to proton hydrates and from electrons to negative ions are compared to results from an ion-chemical model. Nitric oxide and water vapour densities are inferred from the ion composition.     

Early incoherent scatter observations at Jicamarca

We describe some of the highlights of the incoherent scatter observations made at Jicamarca in the 1960s. Some of the observations were then and are still now unique and worthy of further study. Of particular note are (1) a long series of electron density measurements extending to altitudes as high as 10,000 km on a few occasions, (2) density and temperature observations during a total solar eclipse, (3) temperature and composition measurements extending past the O⁺-H⁺ transition region, and (4) observations of the ion gyro resonance effect for protons.     

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.