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.     

Simultaneous ground-based observations of the plasmapause and the F-region mid-latitude trough

The plasmapause and the mid-latitude ionospheric trough have been observed simultaneously from two Antarctic stations, Halley and Faraday, during five winter nights covering a range of geomagnetic disturbance conditions. The equatorial radius of the plasmapause was measured using whistlers recorded at Halley, whilst the poleward edge of the trough was located from ionospheric soundings at one or other of the stations.Before midnight the trough was well poleward of the plasmapause (by 1–2 L) when first observed (typically at ~21 LT), but then moved rapidly equatorwards. After local magnetic midnight the two features were roughly coincident, and in general moved slowly to lower L-shells with increasing local time. At no time were there simultaneous and identical movements of the two features, suggesting a lack of coupling between them. Agreement of the observations with statistical studies and models was fair, given the considerable variability among the five cases studied. For the geomagnetically quieter nights the trough data fit the Spiro model predictions, whereas in the most disturbed case, agreement is better with the Quegan et al. model. The latter model predicts a difference in L between the two features which would fit the data better if shifted 1–2 h later in local time.     

Atomic oxygen concentrations from rocket airglow observations in the equatorial region

A rocket payload designed to measure mesospheric sodium, hydroxyl and oxygen nightglow emissions, in addition to electron density and temperature, was launched from the Alcantara Launch Center (2°S, 44°W), Brazil, at 23:52 LST on 31 May 1992. The height profiles of the atomic oxygen OI557.7 nm and molecular oxygen Atmospheric (0-0) band emissions showed maxima at 100±3 km and 98±3 km, respectively. The emission data are used to calculate the atomic oxygen concentration profiles. The results show the validity for the equatorial region of the empirical parameters proposed by McDade et al. (1986).     

Seasonal variations of tropical F-region nightglow emissions and correlative ionospheric studies

Simultaneous zenith measurements of the forbidden OI 630 nm and permitted 777.4 nm nightglow emissions have been carried out at Cachoeira Paulista (22.7°S, 45.0°W; geomag. 11.9° S), Brazil, during the period February 1983-May 1984, a period of medium solar activity. This first long series of simultaneous observations has been analysed to study the mean seasonal-nocturnal variations of these emissions in conjunction with simultaneous ionospheric data, obtained at the same location. Salient features of these observations are presented and discussed. The OI 630 nm emission mean seasonalnocturnal variations show the presence of pre- and post-midnight enhancements, with intensity levels slightly lower in the winter season. The OI 777.4 nm emission mean seasonal-nocturnal variations show a monotonie intensity decrease in time, with very low intensity levels during the winter season. A comparison has been made between the F-region peak electron densities, and heights determined from the optical and ionosonde remote sensing methods. In general, a good correlation is found between the measured and the nightglow inferred ionospheric parameters. The observed airglow intensity variations are also compared with those predicted by a semi-empirical low latitude ionospheric model.     

First Dynasonde observations of F-region plasma flow at Halley,Antarctica

Plasma flow vectors have been derived from data recorded by the Advanced Ionospheric Sounder (operating as a Dynasonde) at Halley, Antarctica (76°, 27°W). Single bulk flow vectors derived from the motion of echo reflection surfaces in the overhead F-region ionosphere are consistent both with plasma flow vectors, poleward of Halley, observed simultaneously by the PACE HF radar and also, for various levels of geomagnetic activity, with published mean plasma flow at the same invariant geomagnetic latitude (62°). The results demonstrate application of the method and lend support to existing evidence that the velocity measured by this kind of technique, at least for moderate to active geomagnetic activity at high latitudes, represents ionospheric plasma flow.     

Field aligned distribution of plasma mantle and ionospheric plasmas

The density and bulk velocity distributions of warm magnetosheath particles and cold ionospheric O⁺ and H⁺ ions are calculated along a polar cusp (cleft) magnetic field line. The warm collisionless plasma is injected at 12 earth radii, and interacts with the cold polar wind electrons and ions of ionospheric origin. After magnetic reflections in the exosphere (above 1400km altitude) most of the ions move upwards along tail magnetic field lines and feed the plasma mantle flow. The bulk velocity of these warm protons is approximately 200km s⁻² and happens to be proportional to the average thermal speed of the magnetosheath ions at their injection point in the magnetosphere. The density in the plasma mantle is found to be about half that in the entry layer. The electric potential distribution is deduced from the quasi-neutrality condition, A large parallel electric field corresponding to an electrostatic double layer is obtained for the assumed boundary conditions.     

Meridional electric currents in the equatorial F-region

A study of the boundary conditions for the equatorial thermospheric transport equations by the authors has led to the theoretical prediction of the vertical electric field at the base of the F-region. Earlier, this result was applied to the calculation of the zonal wind field in the equatorial F-region. In this work, the aforementioned model is applied to the calculation of the F-region electric current field in the meridional plane as a function of time and the east-west magnetic field generated by these currents. In particular, the field at sunset is compared with the observations made by Magsat.     

Small scale stratification of the mid-latitude F-region

lonograms recorded over limited frequency and height ranges have been used to reveal systematic structures on both main and range spread traces for a mid-latitude region. This fine structure has apparently not been reported previously. When disturbances are present ionogram traces are often made up of quasi-horizontal segments, suggesting that the F2-layer consists of stratified regions spaced a few kilometres apart vertically. It is proposed that if in fact stratifications are present, they may be produced by instabilities created by the presence of internal gravity waves. Interference effects on ionogram traces are also reported. These are shown to result from the presence of overlapping traces. The existence of these quasi-horizontal trace segments and interference effects is shown to contribute to the diffuseness of a range spread mid-latitude ionogram. Thus a mid-latitude range spread ionogram often appears diffuse even though the spread may consist primarily of a number of duplicate traces produced by specular reflections from a range of off-vertical angles.     

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.     

F-region drift velocities in the dusk sector mid-latitude trough

Analysis of 6 months of ground-based ionosonde data from mid/high-latitude Digisonde stations at Millstone Hill, Argentina and Goose Bay, shows the relation between the formation of the mid-latitude trough in the dusk sector and the measured F-region drift velocities. The observed westward drift velocities in the trough are comparable in magnitude with the velocity of the Earth's rotation as required by the stagnation theory of trough formation. Using the Digisonde database of 15 min samples of electron density profiles and F-region drifts, a new trough detection algorithm automatically identifies the occurrence of the trough at any of the three stations. Correlating trough occurrence with the measured drift velocities indicates that troughs develop due to an increase in the horizontal westward velocity component. The extent of the trough formation relates to the magnitude of the horizontal velocity.     

Horizontal velocity dispersion of medium-scale travelling ionospheric disturbances in the F-region

Daytime observations of the horizontal velocity dispersion of medium-scale travelling ionospheric disturbances (TID) near the F2 peak height have been carried out using an array of HF Doppler sounders in central Japan. Cross-correlation analysis of sample records has shown that the horizontal trace velocity is a decreasing function of the period of fluctuations in the range 13.3–40 min. The theoretical dispersion of the atmospheric gravity waves is also calculated using Klostermeyer's (1974) method. Comparison between the observed and the calculated results suggests the possibility that the components of the lower period of the observed velocity dispersion may be a remnant of the quasi-evanescent mode pertinent to lower-height levels.     

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.     

Spread-Es structure producing apparent small scale structure in the F-region

When transmitting on 5.8 MHz the Bribie Island HF radar array synthesizes a beam that is 2.5 wide. The beam can be steered rapidly across the sky or left to dwell in any direction to observe the fading rates of echoes within a small cone of angles. With the beam held stationary, the time scale associated with deep fading of F-region echoes is usually more than 5 min. This is consistent with the focusing and defocusing effects caused by the passage of ever-present medium-scale travelling ionospheric disturbances (TIDs). On occasion the time scale for deep fading is much shorter, of the order of tens of seconds or less, and this is thought to be due to the interference of many echoes from within the beam of the radar. It is shown that the echoes are not due to scatter from fine structure in the F-region, but rather due to the creation of multiple F-region paths with differing phase lengths by small, refracting irregularities in underlying, transparent spread sporadic-E, (Spread-E_s). The natural drift of the Spread-E_s causes the phase paths of the different echoes to change in different ways causing the interference.Two methods are used to investigate the rapidly fading F-region signals. Doppler sorting of the refracted F-region signal does not resolve echoes in angle of arrival suggesting that many echoes exist within a Fresnel zone [Whitehead and Monro (1975), J. atmos. terr. Phys. 37, 1427]. Statistical analysis of F-region amplitude data indicates that when the range spread in E_s is severe on ionograms, then a modified Rayleigh distribution caused by the combination of 10 or so echoes is most appropriate. Using knowledge of the refracting process the scale of E_s structure is deduced from these results. Both methods find a Spread-E_s irregularity size of the order of 1 km or less. It is proposed that the Rayleigh type F-region signals seen by Jacobsonet al. [(1991b), J. atmos. terr. Phys. 53, 63] are F-region signals refracted by spread-E_s.     

The effect of vertical drift on the equatorial F-region stability

Owing to the high conductivity along magnetic field lines, the stability of the night-time equatorial F-region is determined by magnetic field line integrated quantities. However, slow vertical diffusion near the magnetic equator plus the rapid increase in ion chemistry rates at lower altitude combine to give a very small positive scale height for the electron concentration on the bottomside of the region. As a result, the field line averaged quantities are reasonably approximated by their equatorial values, provided that the E-region does not contribute significantly. The time-dependent behavior of the growth rate for the Rayleigh-Taylor gravitational instability on the F-region bottomside is examined here as a function of the vertical E × B drift velocity using reasonable chemistry to obtain approximate equatorial vertical profiles of ionospheric parameters. It is found that the growth rate exceeds the chemical recombination rate over most of the bottomside F-layer even without vertical drift, but that a realistic E × B drift can result, after about 1 h, in an increase of this growth rate by an order of magnitude. The absolute growth rate is so small (< 10⁻³ s⁻¹) with zero vertical drift that a seeding mechanism would probably be required for the formation of bubbles. The rapid appearance of bubbles shortly after sunset appears likely only after a period of upward drift, as is observed.     

Weak nonlinear theory of the ionospheric response to atmospheric gravity waves in the F-region

The nonlinear ionospheric response to atmospheric gravity waves is studied in an approximate fashion using a new approach. The concept of nonlinear travelling ionospheric disturbances (TIDs) is outlined, and the nonlinear behaviour of atmospheric gravity waves is calculated. A principal result is that harmonics are generated which cause the wave velocity perturbation to deform. The ionospheric response is investigated by solving the continuity equation for ionization in the F-region. The distortion of the TIDs waveform produced by the nonlinear interactions is depicted. The nonlinear TIDs depart seriously from a cosinusoidal wave described by previous linear TID theory. The distorted TIDs appear as ‘sharp peak’ and ‘sawtooth’ waveform shapes. The ‘peaks’ can be upward or downward, and the ‘sawteeth’ forward or backward, depending on the wave parameters. The nonlinearly distorted TIDs show a good agreement with various observed ionospheric irregularities produced by atmospheric gravity waves.     

Rocket measurements of the equatorial airglow: MULTIFOT 92 database

The MULTIFOT airglow photometer payload was launched from Alcântara (2.5°S, 44.4°W) on a SONDA III rocket at 23:52 hrs local time on 31 May 1992. A total of ten photometers, six forward-looking and four side-looking, measured the height profiles of the airglow emissions O₂ Herzberg band system, 01557.7 run, NaD 589.3 nm, 01630.0 nm, OH(8,3) band R branch at 724.0 nm, O₂ Atmospheric (0,0) band at 762.0 nm and the sky background at 578 nm and 710 nm. At the time of launch, a ground-based airglow photometer observed the intensity variations of these emissions, together with the rotational temperature of the OH(9,4) band, and a sodium lidar measured atomic sodium concentration from 80 to 110 km.