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.     

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.     

Field-perpendicular and field-aligned plasma flows observed by EISCAT during a prolonged period of northward IMF

The effect of a prolonged period of strongly northward Interplanetary Magnetic Field (IMF) on the high-latitude F-region is studied using data from the EISCAT Common Programme Zero mode of operation on 11–12 August 1982. The analysis of the raw autocorrelation functions is kept to the directly derived parameters N_e, T_e, T_i and velocity, and limits are defined for the errors introduced by assumptions about ion composition and by changes in the transmitted power and system constant. Simple data-cleaning criteria are employed to eliminate problems due to coherent signals and large background noise levels. The observed variations in plasma densities, temperatures and velocities are interpreted in terms of supporting data from ISEE-3 and local riometers and magnetometers. Both field-aligned and field-perpendicular plasma flows at Tromsø showed effects of the northward IMF: convection was slow and irregular and field-aligned flow profiles were characteristic of steady-state polar wind outflow with flux of order 10¹² m⁻² s⁻¹. This period followed a strongly southward IMF which had triggered a substorm. The substorm gave enhanced convection, with a swing to equatorward flow and large (5 × 10¹² m⁻² s⁻¹), steady-state field-aligned fluxes, leading to the possibility of O⁺ escape into the magnetosphere. The apparent influence of the IMF over both field-perpendicular and field-aligned flows is explained in terms of the cross-cap potential difference and the location of the auroral oval.     

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.     

Observation and theoretical modelling of a region of downward field-aligned flow of O+ in the winter dayside polar cap

Combined optical and radar measurements of ion drift at high latitudes near the terminator show that large downward field-aligned ion flows occur below the F-peak. At an invariant latitude of 72° and in the local time period from 1100 to 1500, downward velocities of 400 m s ⁻¹ have been observed. At the same time, the poleward component of field-perpendicular ion velocity was only 100 m s ⁻¹. The high latitude ionospheric model of Queganet al. (1982), as modified by Allenet al. (1984), predicts downward field-aligned velocities with the same sign morphology as the observations, but with only one fifth of the magnitude. However, the existence of downward neutral winds might lead to non-linear amplification of the downward ion motion. Using the vertical wind measurements of Reeset al. (1984), a possible explanation of the fast ion flow is suggested.     

Ionospheric sources of Pi(c) pulsations

Data collected at Macquarie Island (invariant latitude 64.5°S) shows average delays of 0.6 s and 0.2 s between D and H component Pi(c) pulsations, respectively, and the N₂⁺1NG (0,1) optical band pulsations. The dominant phase of the cross-correlations between the H component pulsations and the optical pulsations is consistent with the H component fluctuations being generated by increases in a westward electrojet. The D component fluctuations are consistent with either an increase in an equatorward Pedersen current or an increase in a field aligned current. The polarization of the Pi(c) pulsations may be explained in terms of the altitude of the respective currents.     

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.     

Spread-F-like irregularities observed by the Jicamarca radar during the day-time

Until now the presence of F-region irregularities responsible for spread-F (sp-F) traces in ionograms has been considered as a purely night-time phenomenon extending sporadically to the early morning hours. We herein report that, on two occasions (26 March 1974 and 1 February 1984) similar irregularities were observed between 1400 and 1600 hours local time with the Jicamarca radar. These irregularities caused enhancements in the power of the radar echo of as much as two orders of magnitude, were found over a region of a few hundred kilometers on the topside of the F-region extending from around 600 to 1000 km altitude, and persisted for 1–2 h. The irregularities were aspect sensitive (aligned with the magnetic field) and produced echoes with a fading rate of the order of one to a few seconds. The background zonal electric field, inferred from the vertical drift velocity, was fairly constant in altitude, with values smaller than 0.1 mV m⁻¹. During the duration of the events, zonal components of both signs occurred, with the component passing through zero several times. We have no information on the vertical component of E. These irregularities could not be observed with ground-based ionosondes, since they are on the topside of the F-region. They may be related to fossil bubbles that are responsible for HF ducting observed by satellites.     

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.     

Temperature anisotropy of drifting ions in the auroral F-region,observed by EISCAT

During relative drifts between the ions and the neutrals perpendicular to the geomagnetic field, the ion temperature in the auroral F-region becomes anisotropic with a higher temperature perpendicular than parallel to the magnetic field (T_⊥ >T_∥). It has been shown that for a gyrotropic ion velocity distribution the ion temperatures T_⊥ and T_∥ can be expressed as a function of the neutral temperature and of the squared normalized relative ion-neutral drift, with parameters β_⊥ and β_∥ describing the anisotropy and the collision process.In this paper, five increases of the F-layer ion temperature and ion drift velocity, found in EISCAT-CP1F data, were analyzed to obtain information about the anisotropy and the collision process. In the CP1F experiment, the angles between the magnetic field line ending in Tromsø and the antenna directions remain small, and the ion drift velocities of the investigated events in general were below 1500 m/s. Thus the ion velocity distributions were approximated by a bi-Maxwellian, and NO⁺ was assumed to remain a minor constituent at the F-layer maximum. For a quantitative analysis, generalized theoretical β-values for a bi-Maxwellian ion velocity distribution drifting through a mixture of different neutral components and for arbitrary observation directions were calculated. With these expressions it was possible to compare the drift dependence of the measured ion temperature for every antenna position directly with the theory. A statistical analysis of the heating events showed a good correlation between the ion temperatures of Tromsø, Kiruna and Sodankylä and the squared normalized ion drift, and values β_T, β_K, β_S could be calculated by linear regression. The fitted curves corresponded well with theoretical curves for a bi-Maxwellian velocity distribution of O⁺ ions drifting through a neutral atmosphere consisting of O and N₂.     

Refilling process in the plasmasphere and its relation to magnetic activity

When geomagnetic activity is moderate, the geosynchronous orbit crosses the plasmasphere bulge region in which the variations of plasma density from day to day can therefore be detected by geosynchronous satellites. The plasma density was measured by the Relaxation Sounder onboard ESA's GEOS-2 satellite. Variations of plasma density reflect the combined effects of refilling of particles from the ionosphere and loss of plasma by convection. The saturation level of the electron density at the geo-synchronous orbit and the refilling rate under different conditions of geomagnetic activity have been obtained and are found to be 70.5 cm⁻³ and 7–25 cm⁻³ day⁻¹, respectively. In this paper the refilling morphology and the relationship between the refilling process and magnetic activity (Dst index) are analysed. The refilling rate or refilling time constant inferred from the data, either directly on fairly well-defined refilling events, or indirectly through a simple model, are found to compare reasonably well with the refilling time constant expected by theory. The observed correlation of refilling rate with Dst index is interpreted as resulting from the modification of the composition of the topside ionosphere occurring after intense storms.     

Thermospheric wind measurements with EISCAT

Thermospheric wind measurements with the EISCAT UHF radar around the evening Harang discontinuity are presented both in the E- and F-layers. Within the E-layer auroral oval the Lorentz and Coriolis force are shown to be more or less in balance. The neutral velocity is a factor of the order of two smaller than the ion velocity and is on average advanced 90° in a clockwise direction compared to the ion velocity. In the low electron density region just before the Harang discontinuity and outside the auroral oval a large (~250 m s⁻¹), thermally dominated neutral wind is closely followed by the ion wind in the antisolar direction. There is also a large downward flow present just before the Harang discontinuity. In the F-layer the neutral wind approximately follows the ion convection pattern, except for a couple of hours after the sudden change in the ion convection just after the passage of the evening Harang discontinuity. The close resemblance between the equilibrium ion and neutral flow when the neutral-ion collision frequency is close to twice the Earth's angular velocity may be connected to back pressures created by Joule heating in the case of an appreciable ion-neutral velocity difference.     

The role of electric field and neutral wind direction in the formation of sporadic E-layers

The effect of an electric field and a homogeneous neutral wind on the vertical ion motion in the ionospheric E-region is investigated. An electric field pointing, in the northern hemisphere, in the quadrant between geomagnetic north and west is found to he capable of driving plasma towards a certain height from both above and below. A homogeneous neutral wind blowing in a direction between east and north has a similar effect. Unlike in the wind shear model, the resulting plasma sheet may be created within a quite limited height interval only. It seems possible that the midnight occurrence maximum of mid-latitude type E_s-layers, observed at high latitudes, is caused by electric fields in the Harang discontinuity region. It is also suggested that the flat type E_s-layers often observed before a substorm onset are caused by electric fields. The wind shear theory is investigated using a screw-like neutral wind profile. The effects of right- and left-handed wind screws are compared and rules are derived which define the conditions leading to convergent and divergent nulls in the vertical ion velocity. In the northern hemisphere, a right-handed screw is found to be more effective than a left-handed one with equal pitch in compressing plasma into thin sheets.     

The morphology of dayside Birkeland currents

Intense (10⁵ A) electric currents flow into and from the Earth's two polar ionospheres near magnetic noon. These currents, called Birkeland or magnetic field-aligned currents, are the agent by which momentum couples from the flowing solar wind plasma to drive plasma motions in the high latitude ionosphere. Coupling is strongest when the interplanetary magnetic field (IMF) has a southward component and when this occurs there exist two principal regions of Birkeland current near magnetic noon called the region 1 and the cusp systems. We present a simple model bringing theoretical order to the many patterns proposed previously for the morphology of these dayside Birkeland currents as observed by orbiting satellites in the topside polar ionosphere. Specifically we show that the cusp Birkeland current system is not a latitudinally separate region but is instead the extension in longitude of the region 1 Birkeland current from either dawn or dusk; which particular one depends on the sign of the east-west (Y) component of the IMF. The presence of an IMF Y-component therefore leads to two region 1 current systems near magnetic noon, with the poleward one being that previously called the ‘cusp’ system.     

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.     

Simulations of the seasonal variations of the thermosphere and ionosphere using a coupled,three-dimensional,global model,including variations of the interplanetary magnetic field

The University College London Thermospheric Model and the Sheffield University Ionospheric Convection Model have been integrated and improved to produce a self-consistent coupled global thermospheric/high latitude ionospheric model. The neutral thermospheric equations for wind velocity, composition, density and energy are solved, including their full interactions with the evolution of high latitude ion drift and plasma density, as these respond to convection, precipitation, solar photoionisation and changes of the thermosphere, particularly composition and wind velocity. Four 24 h Universal Time (UT) simulations have been performed. These correspond to positive and negative values of the IMF BY component at high solar activity, for a level of moderate geomagnetic activity, for each of the June and December solstices. In this paper we will describe the seasonal and IMF reponses of the coupled ionosphere/thermosphere system, as depicted by these simulations. In the winter polar region the diurnal migration of the polar convection pattern into and out of sunlight, together with ion transport, plays a major role in the plasma density structure at F-region altitudes. In the summer polar region an increase in the proportion of molecular to atomic species, created by the global seasonal thermospheric circulation and augmented by the geomagnetic forcing, controls the plasma densities at all Universal Times. The increased destruction of F-region ions in the summer polar region reduces the mean level of ionization to similar mean levels seen in winter, despite the increased level of solar insolation. In the upper thermosphere in winter for BY negative, a tongue of plasma is transported anti-sunward over the dusk side of the polar cap. To effect this transport, co-rotation and plasma convection work in the same sense. For IMF BY positive, plasma convection and co-rotation tend to oppose so that, despite similar cross-polar cap electric fields, a smaller polar cap plasma tongue is produced, distributed more centrally across the polar cap. In the summer polar cap, the enhanced plasma destruction due to enhancement of neutral molecular species and thus a changed ionospheric composition, causes F-region plasma minima at the same locations where the polar cap plasma maxima are produced in winter.     

Measurements of gradients in ionospheric parameters with a new nine-position experiment at Millstone Hill

A new nine-position experiment is now routinely carried out with the Millstone Hill incoherent scatter radars which allows estimation of spatial gradients in the measured ionospheric scalar parameters N_e, T_e, and T_i, and in the components of the ion velocity vector v_i. Use of this technique results in improved estimates of basic and derived parameters from incoherent scatter data at times of significant gradients. We detail the data analysis method and present the first results from this new experiment. The gradients in N_e and in the components of vi are used to compute the motion term in the ionospheric F region continuity equation ▿ · (Nv), which is then combined with ∂N/∂t to estimate the O⁺ recombination rate β at night. Meridional neutral winds U_mer are computed from the field-aligned ion velocity v_∥ and a calculation of the O⁺ diffusion velocity v_d, and it is found that horizontal gradients in the ion velocity field at times significantly affect the calculation of the neutral winds.     

Solar activity effects on equatorial plasma bubble zonal velocity and its latitude gradient as measured by airglow scanning photometers

Using a new mode of scanning 630-nm photometer operation the zonal velocities of ionospheric plasma depletions were measured over Cachoeira Paulista in Brasil in two east-west planes tilted 30°N and 30° S with respect to zenith. The measurements cover a time period of approximately 2 years, from January 1988 to January 1990, a period marked by significant increase in solar activity of the ongoing cycle. The results have permitted a rather detailed evaluation of the local time and latitude variations in the zonal plasma bubble velocity as a function of solar activity. Although the mean trend in the velocity local time variation is a decrease from early evening to post-midnight hours, a strong tendency for velocity peaks is observed near 21 LT and midnight. The velocities as well as their height (latitude) gradients show perceivable increases with solar activity represented as sunspot numbers. The present results are compared with the ambient plasma velocities measured using the Jicamarca radar by Fejer el al. (1985), J. Geophys. Res. 90, 12249, with that measured on board the DE 2 satellite on the equatorial latitudes by Coley and Heelis (1989), J. geophys. Res. 94, 6751, and with various theoretical calculations, in an attempt to bring out the salient features of the plasma dynamics of the equatorial ionosphere.     

A comparison of three methods of measuring tidal oscillations in the lower thermosphere using EISCAT common programmes

The EISCAT Common Programme can be used in three ways to monitor tidal oscillations in the lower thermosphere. In Common Programme One (CPI) tristatic observations provide measurements of the ion-velocity vector at several heights in the E-region and one height in the F-region. In Common Programme Two (CP2) monostatic measurements give profiles of ion velocity in the E-region while tristatic measurements give continuous measurements of ion velocity in the F-region. From the ion velocities and the ion-neutral collision frequency, the vector of the E-region neutral wind can be determined and both east-west and north-south components of the diurnal, semi-diurnal and ter-diurnal oscillations can be identified. CP1 and CP2 also provide profiles of the field-aligned ion velocity, and these can be used to calculate the north-south component of the neutral wind without knowing the ion-neutral collision frequency, but the result is affected by any vertical component of neutral velocity. The three methods are compared and the advantages of CP2 demonstrated.     

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index (S_10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread-F (ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.