Electrodynamic effects of metal ions in the noon equatorial E-region

The observations of metal ions in significant concentrations and in layer formation in the lower E-region are briefly reviewed. It is expected that a metal ion layer may alter the electro-conductivity of the ionosphere and modify the distribution of ionospheric dynamo current. The variation in the electroconductivity is theoretically calculated, and it is shown that the Pedersen conductivity is reduced by less than 14%. In the equatorial ionosphere, field line integration of metal ion effects is likely to be large. A metal ion layer, similar to those observed, is numerically modelled in the equatorial region and its electrodynamic effects on the equatorial electrojet (EEJ) are numerically examined using our self-consistent model of the ionospheric dynamo. The effects are found to be significant on the amplitude of the EEJ, but not effectively large on its peak altitude.     

Mechanisms for vertical separation of ions in sporadic-E layers

A model of the ionospheric E-region between 90 and 130 km altitude is constructed with normal molecular ions and two species of metal ion with different masses. This paper investigates whether the vertical structure observed in sporadic-E layers can be accounted for by separation of different ion species according to their mass. The result of the investigation is substantially negative. Another mechanism for range spread sporadic-E has signatures that may be sought in observational data.     

Balloon measurements of stratospheric ion conductivities over the tropics

Conductivity measurements of negative and positive ions were made from about 20 to 35 km by two identical balloon-borne spherical probes at Hyderabad (17.5°N, 78.6°E), India on 22 April 1989 and 22 December 1990. One balloon was launched at 0158 h IST (Indian Standard Time) which reached its ceiling around 0330 h IST. After that time, it floated for about 3 h, 1.5 h before sunrise and 1.5 h after sunrise. Thus it gave data for both day- and night-time conditions at float altitude. The other balloon was launched at 0535 h IST. It gave data for daytime only. Several interesting results have been obtained at the float altitudes. During the night, in the flight of 22 April 1989 the conductivity values of positive ions were found to be about 1.5 times those of negative ions at the float altitude. During the day, in the flight of 22 April 1989, the positive ion conductivity values were found to increase with the increase of solar elevation angle at around 37.5 km altitude. The negative ion conductivity values, however, did not show any day-night variation. In the flight of 22 December 1990, these features were not seen. Instead, a pocket was found where conductivity values were very high (of the order of 10⁻¹¹ mho m⁻¹) at an altitude of about 32.5 km. Also in this flight, the positive ion conductivity was always found to be approximately equal to that of the negative ion conductivity.     

Positive and negative ion densities and mobilities in the middle atmosphere over India—rocket measurements

This paper discusses the results from four rocket experiments conducted from Thumba, India, during the Indian Middle Atmosphere programme (IMAP). These rockets carried instrumented Gerdien Condenser payloads to measure ion densities and their mobilities. In the first two flights only positive ion measurements were attempted while the other two measured both positive and negative ion values. The results show that the positive ion density profiles go through a minimum around 62 km, as expected from the ion production models for this region. The ion density distribution is a function of solar zenith angle. An asymmetry with respect to noon is seen in these measurements, which is not expected theoretically. The positive ion mobilities indicate the ions to be water clusters, of the type H⁺ (H₂O)_n with n = 2 or 3, similar to the earlier reported ones. The negative ion density profile exhibits a maximum around 85 km, which is not predicted by the currently available ion density models and theories of D-region ionisation processes. The negative ion mobility measurements show the ions to have a mass range of 30–60 amu, which is within the range of mass spectrometric measurements.     

Effect of the large-scale convection electric field structure on the formation of thin ionization layers at high latitudes

A three-dimensional simulation of the high-latitude ionosphere was applied to investigate the geographical distribution of E-region thin ionization layers which may be formed by the action of the convection electric field. The simulation model computes the ion densities (O⁺, O⁺₂, N⁺, N⁺₂, NO⁺, Fe⁺), and temperatures as a function of altitude, latitude, and longitude. The stationary state momentum and continuity equations are solved for each ion species, then the energy equation is solved for electrons, neutrals, and a generic ion having the mean ion mass and velocity. The various electric field patterns of the Heppner and Maynard [(1987) J. geophys. Res.92, 4467–4489] convection electric field model were applied and the ionization density pattern was examined after a time sufficient for the formation of thin layers (≈2000 s). It was found that large areas of thin ionization layers were formed for each of the electric field patterns examined. Southward IMF B_z conditions resulted in thin layers forming in the pre-midnight sector in the latitude range north of about 70° to about 80°, and after midnight between 60 and 70°. For northward B_z conditions, the layers were mainly in the pre-midnight sector and covered a latitude range from about 60 to 80°.     

Comparison of plasma densities in the daytime polar F-region measured by the AEROS-B and S3-1 satellites

A comparative study of ionospheric measurements obtained by different sensors on two satellites has shown excellent agreement between the principal techniques used to measure the plasma density and ion composition. Results from an ion mass spectrometer on the S3-1 satellite and from an impedance probe and a retarding potential analyzer on the AEROS-B satellite have been compared for five cases that represent the closest coincidence of measurements that occurred during the satellite lifetimes. The crossings occurred at high polar latitudes and the studies have indicated that invariant latitude is more important than altitude, geodetic latitude, or geomagnetic latitude in systematizing the data. The ratios of the mass spectrometer ion densities to the impedance probe electron densities resulted in an average value of 1.00 with a standard deviation of 6%. The composition percentage for the molecular and atomic ions from the retarding potential analyzer agreed generally within about 20% of the mass spectrometer measurements.     

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.     

Altitude and latitude dependence of the equatorial electrojet

The effects of day-to-day or seasonal variation of altitude and latitude profiles of the Elayer plasma density in the equatorial ionosphere on equatorial electrojet (EEJ) structure are examined numerically using a self-consistent and high resolution dynamo model. It is found that variations in the E-layer peak altitude and amplitude and its gradient below significantly affect EEJ structure. For any realistic shape, the EEJ peak appears at or below the E-layer peak altitude. Distinct double peaks appear in the EEJ structure, such as revealed by rocket measurements, if the E-layer peak is above 105 km or the gradient is large, as when sporadic-E is present. The influence of the latitudinal variation of ionospheric field line integrated conductivities upon the amplitude and altitude of the EEJ peak is demonstrated.     

The quiet midlatitude D-region: a comparison between modeling efforts and experimental measurements

Two long-standing problems in the atmospheric sciences have been the correct modeling of the ion chemistry in the earth's atmosphere and the proper determination of the ion species and densities through in situ measurements. Comparison between experimental data and simulations of the data by computer modeling of atmospheric chemistry is a means of validating the model as well as indicating which processes are in need of further study. The DAIRCHEM computer code is used here to simulate data taken in the midlatitude D-region during quiet conditions. On the one hand, comparison between the total positive ion density profile derived from rocket measurements and the one computed by the code shows very good agreement in the 30–90 km range, with the exception that the simulated ion profile is somewhat smaller than the experimental one in the 60–75 km region. Such discrepancy is only partially explained by the inherent uncertainties in the NO density profile or the total ionization rate profile. On the other hand, comparison between the measured and the computed electron density profiles shows that the measured profile is consistently smaller than the computed profile in the 65–85 km range. We interpret this discrepancy as a deficiency in the modeling of the negative ion chemistry. Also, this deficiency is probably the main cause of the disparity between the total positive ion density profiles in the corresponding altitude range. It is felt that the positive ion chemistry of the D-region is reasonably well understood. However, the negative ion chemistry is in need of further study. Specifically, alternate electron attachment/detachment processes should be considered, as well as an as yet undetermined, possibly very massive, negative species which may affect the ion recombination rates.     

Plasma density discontinuity and spread F

It is shown that the generation of the equatorial spread F irregularity in the post-sunset period need not he always at the base of the F region. The irregularities are generated at the E region, provided there existed a retardation type E2 layer or a uniform blanketing sporadic E layer before the sunset. The irregularities are generated even at the region close to the altitude of peak ionization density, provided there is a discontinuity in the plasma density variation with height (a kink or a G layer). It is suggested that if proper electric fields are present at the sunset period, then the spread F irregularities are generated at any altitude having plasma density discontinuity.     

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.     

Parachute measurements of positive ion density of the middle atmosphere over the dip equator by spherical probe

A parachute-borne gridded spherical probe has been used to measure the total positive ion density. Two launches were made, using Soviet M-100 rockets, on 22 and 29 April 1987, at 1200 UT, from an equatorial station, Thumba (8°N, 76°E) India. Data were obtained for the altitude region 10 to 80 km. A broad maximum around 15 km and a broad minimum around 60 km have been noticed in the ion current profiles obtained in both flights. The theory of the operation of the probe has been given. A detailed discussion of the results obtained has also been included.     

Mesospheric temperatures and the OH layer height as derived from ground-based lidar and OH1 spectrometry

Night-time mesospheric temperatures were simultaneously determined from the Doppler broadening of the D₂ resonance line of atmospheric sodium excited by a laser and from the rotational distribution of the P₁(1), P₁(3) and P₁(4) lines of the OH(3,1) band by an i.r. spectrometer. Both instruments were located at the Andøya Rocket Range (69°N, 16°E). The mesospheric temperature gradient permits determination of the altitude of the OH¹ emitting layer from a comparison of the equivalent layer temperatures calculated from the height-resolved Na Doppler temperatures with the observed OH¹ rotational temperatures. The altitude of the OH¹ layer maximum is determined with an accuracy of ±4 km. For 3 nights in January 1986 the OH¹ emission layer is found near an altitude of 86 km.     

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.     

A spectral approach for studying middle and upper atmospheric phenomena

We report on the application of a newly developed spectral code to the study of the middle/upper atmosphere. The spectral approach offers conceptual and practical convenience for analyzing the generation and interaction of different components of atmospheric activity through the decomposition of the dynamical fields into components with different zonal wave numbers (m). As examples and tests, we obtain solutions for the m = 0 (the mean circulation) and 1 (the diurnal tides) components separately (no mutual interactions), as well as the m = 1 component under the influence of the mean circulation. By simulating gravity wave effects with Rayleigh friction and eddy diffusion peaking near 90 km altitude, the mean circulation thus generated can reproduce the observed mesospheric temperature anomaly under solstice conditions. The computed diurnal tides are in good agreement with results obtained earlier by other authors. The large temperature gradient (associated with the m = 0 component) set up by the mesospheric temperature anomaly under solstice conditions creates a condition favorable for the development of baroclinic instabilities in the mesopause layer, especially near the summer pole. In our time dependent calculation, waves with approximately 4-day period are generated in the m = 1 component, superimposing with the 1-day period tides.     

D-region electron densities observed by incoherent-scatter radar during auroral-absorption spike events

Electron density profiles in the night-time auroral ionosphere were obtained with the incoherent-scatter radar at Chatanika, Alaska, during short duration precipitation events characterized by riometer data as spike events. The measurements show exceptionally large electron densities in the D-region during spike events, the electron density typically exceeding 10⁶ cm³ at 90 km altitude for a short time. The existence of a steep horizontal gradient, particularly on the poleward edge of the event, is inferred. The altitude and thickness of the absorbing layer are deduced. It is shown that 20–40 keV electrons make the greatest contribution to an absorption spike and that the spectrum of electrons producing such an event is probably softer than that producing a more slowly varying absorption peak. These absorption layers are too high for their altitudes to be measured by the technique of multi-frequency riometry.     

EISCAT results during the ROSE campaign and comparison with STARE measurements

EISCAT measurements were performed during the four ROSE rocket launches. The results are presented. It is shown that the upper altitude limit of instabilities observed by in-situ measurements agrees with calculations using EISCAT results of drift and ion sound speed and assuming the two-stream-instability mechanism. The EISCAT results together with the STARE observations were used to calculate the ion velocity and the ψ-values from the dispersion relation of two-stream-instabilities. A comparison of EISCAT, STARE and in-situ measurements is discussed.     

Fine scale structure and turbulence parameters in the equatorial middle atmosphere

Two rockets carrying identical spherical probe payloads were launched from Thumba to measure positive ion density of the mesosphere and lower thermosphere over an equatorial location. Data obtained show the presence of strong irregularities in the ion density. From the measured positive ion current, the spectra of the spatial density fluctuation, turbulent velocity, energy dissipation rate and eddy diffusion coefficients have been derived in the altitude range of 70–100 km. The results are found to be different from those at middle and high latitudes.     

Boundary populations in the polar caps

The morphology of precipitating particles, measured at low altitude in the polar regions, varies systematically with the strength and direction of IMF B_z and with solar wind speed V_sw. We use particle data taken onboard the DMSP satellites to determine these variations. Both individual satellite passes during the storm/quieting period of 26 and 27 August 1990, and statistical maps compiled from a data base over 4.5 yr are presented. We focus attention on those magnetospheric populations that have magnetosheath characteristics, the boundary populations. We show that the precipitating ion boundary population, whose down-coming spectra can be fitted to streaming Maxwellians, expands from a region confined near the dayside cusp for southward IMF, to a thick, annular region, including the dayside cusp, for northward IMF. The expansion in local time is inhibited by increasing solar wind speed. Boundary electrons behave somewhat differently. They have easier access to the polar regions and their variations have shorter spatial/temporal scale lengths than the boundary ions. For strongly northward IMF, intense, agitated boundary electrons can be found over all or part of the polar cap. Broad regions (up to ~ 100 km) of strongly accelerated electrons (several keV) that produce visible arcs are embedded in this population. Two features of the ion boundary population help identify its source. (1) The spectra of the boundary ions expanding into the polar cap exhibit field-aligned streaming, which, downtail, is toward the Earth. (2) The region into which the boundary ions expand best maps magnetically to a dawn-dusk cut across the neutral sheet, rather than to the low-latitude boundary layer. Therefore, we conclude that the immediate source for boundary ions in the polar regions during northward IMF is the plasma sheet boundary layer. These ions reach tail lobe field lines by convection whose direction when mapped to the ionosphere is sunward. Significant change in the topology of the magnetospheric magnetic field, and, in particular, the closing of high-latitude field lines, is not required to explain the data.     

Applicability of the two-layer model for simulation of non-linear evolution of a large scale plasma cloud in the ionospheric F-region

The adequacy of the two-layer model of Lloyd and Haerendel for describing the behaviour of an ionospheric irregularity is verified by numerical simulation of large plasma cloud dynamics. The background ionosphere is approximated by a set of conductive layers with ion mobilities and concentrations corresponding to the real ionospheric conditions. Polarization electric field produces positive and negative image clouds, i.e. plasma density enhancements and depletions in each layer. Their intensity, form and orientation turn out to change with height depending on the local conditions. However, the drift and deformation of the released cloud slightly differ from the case when the ionosphere is characterized by constant, height averaged parameters, at least if altitude dependent neutral wind and photochemical processes are ignored.