On the role of auroral electric fields in the formation of low altitude sporadic-E and sudden sodium layers

The characteristics of metallic and molecular ion sporadic-E (E_s) layers, formed by the action of strong electric fields at auroral latitudes, are examined using computer simulations. It is found that, for electric fields directed between northward and westward (northern hemisphere), thin metallic ion layers (<2 km thick) can be formed above about 105 km altitude. For electric fields directed from westward, through southward, to south-eastward, slightly thicker (4–6 km thick) metallic ion layers can form between 90 and 105 km altitudes. Thin layers of molecular ions can be formed by electric fields directed between north and west if the ion density is low. Examples of E_s layers observed by the EISCAT radar, together with simultaneous observations of electric fields and ion drifts are presented which show good agreement with the simulations. The relationship between the lower-altitude E_s layers and sudden sodium layers (SSLs) is discussed leading to an explanation of some of the characteristics of SSLs at high latitude. A possible involvement of smoke particles in the formation of both E_s layers and SSLs is proposed.     

Retrieval of east-west wind in the equatorial electrojet from the local wind-generated electric field

The paper presents a method to retrieve the height-varying east-west wind U(z) in the equatorial electrojet from the local wind generated electric field E_W(z), or from the radar-measured phase velocity V_p^II(z) of the type II plasma waves. The method is found to be satisfactory when E_w ⪢ E_p, where E_p is the vertical polarization electric field generated by the global scale east-west electric field, EY, and E_y < 0.2 mV m⁻¹. Measurements of V_p^II by a VHF backscatter radar can be inverted to obtain the causative wind profile by this method. The method is tested using a simulation study in which E_w(z) and V_p^II(z) as generated by two different wind models are used. The retrieved winds are compared with the original wind profiles and it is found that the error in the retrieved winds is mostly under 5%, for the case of no errors in the model Pedersen conductivity (σ₁) profile and the E_w(z) or V_p^II(z)(z) profiles used in the inversion. Even with a ±20% error in the above profiles, the errors in the retrieved winds are found to be less than 20% over 75% of the altitude range and 20–30% for the remaining 25% of the altitude range, on the average.     

Three solar cycles of day-time southern hemisphere Es activity

The hourly values of the ionosonde parameters f_oE_s and f_bE_s for times near mid-day have been examined over three solar cycles 1947–82. The daily parameters were studied for two southern hemisphere stations: Christchurch (temperate zone) (−43°.6′ geographic, −48°.1′ geomagnetic) and Rarotonga (subtropical) (−21°.2′ geographic, −20°.7′ geomagnetic). The data represent the longest such analyzed sporadi-E record in the literature. The seasonal variations of per cent occurrence of f₀E_s ⩾ 5 MHz and f_bE_s ⩾ 4 MHz for both stations show no sunspot dependence, strong activity in local summer and, in the case of Christchurch, minor enhancements in local winter. Long-term f_bE_s occurrence is closely associated with Zurich sunspot number R_z (correlation coefficients 0.93 for Christchurch and 0.76 for Rarotonga) with all seasons, showing the same in-phase R_z control for both stations. Long-term f_oE_s occurrence for both stations and for all seasons exhibits no dependence on R_z. There is evidence of a 6 year period in seasonal f_oE_s activity for Rarotonga. The data show a large and unexplained systematic decrease in f_oE_s occurrence for both stations and for all seasons by a factor of about 3 from 1947 to 1982.     

Modelling the penetration of thundercloud electric fields into the ionosphere

In a previous paper, we considered the penetration of DC thundercloud electric fields E into the ionosphere and also into the region between the ionosphere and the ground (Velinov and Tonev, 1994). In the present paper, we extend the analysis by making a more precise approximation of the electric conductivity profiles by 5–10 piecewise exponential functions of altitude instead of the two functions used up to now. This allows a much more realistic representation of the atmospheric conductivity profile. Besides, Maxwell's equations are solved for more general boundary conditions, taking into account that the electrosphere is not a perfect conductor. This leads to the appearance not only of the transverse E_r (as had been assumed until now), but also of the geomagnetic field-aligned E_z component of the penetrating thundercloud electric fields. The computations show that both E_r and E_z cause significant variations of the electron density profiles N(z) in the ionosphere.     

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.     

Density profiles of sporadic E-layers containing two metal ion species

Ionospheric plasma containing two types of metal ions is investigated under the action of the wind shear mechanism or, alternatively, an electric field causing convergent vertical plasma flow. It is shown that the different ion species are separately collected into thin sheets with a height difference ranging from some hundreds of meters to several kilometers. Theoretical density profiles for Mg⁺ and Fe⁺ ions are calculated assuming a screw-like wind structure or a strong auroral electric field. It is found that the two ion layers usually partially merge forming a single E_s-layer. If the height difference of the ion sheets is not too great as compared to their thicknesses, the E_s-profile is single peaked and approximately symmetric. With increasing layer separation the two sheets will gradually be discerned, until finally a double peaked profile is created. It is suggested that some of the observed complexities in E_s-profiles are caused by the presence of more than one monoatomic ion species.     

HF observations of the vertical structure of mid-latitude spread-Es

The Bribie Island HF radar array (27°S, 153° E) can be set up to make angle of arrival and Doppler shift measurements throughout the range of spread-E_s, layers. Results of this experiment show that the range spread seen on ionograms is not due to multiple reflection with varying obliquity, but rather a genuine height spread exists. Where velocity measurements can be reliably made, reflector velocity appears to be a slowly varying function of height. Spread-E_s, can be blanketing or non-blanketing, sequential or non-sequential and at first impression it seems that the chief difference between spread-E_s, and normal E_s, is a small scale, partially transparent structure in lower regions that allows higher regions to be observed. It is suggested that on occasion spread-E_s, irregularities are further modulated by the passage of gravity waves.     

Sudden sodium layers in polar latitudes

We discuss in this paper sudden sodium layers (SSLs), which we observe with a sodium lidar instrument at Andenes, Norway (69°N). We speak of a SSL if, in a narrow altitude range (typically less than 2km), the Na density increases over the normal Na density by a factor of at least 2 within 5 min. Between December 1985 and November 1987, we have observed 42 such layers in 378 h of lidar measurements. This number increases to 75 if we only require an increase of a factor of 1.5 within 8 min. At our observation site, SSLs have the following properties: (a) they develop between 90 and 110 km altitude, (b) they develop between 20 and 02 LT, (c) their appearance shows a strong, positive correlation with that of ƒ-type E_s layers, and (d) their appearance does not show a strong correlation with either riometer absorption or meteor showers. We discuss a number of potential processes for SSL formation. SSLs above 100km can be formed in ƒ-type E_s layers by the conversion of Na⁺ ions into neutral Na. The development of SSLs below 95 km requires the presence of an additional reservoir of Na, such as Na-bearing molecules, ions, and/or ‘smoke’ particles. We also evaluate the proposal that SSLs are the outcome of single meteoroids entering the upper atmosphere, a proposition for which we find little observational support.     

Electrodynamic structural features of the auroral zone as deduced using data from a meridional chain of ionospheric and magnetic stations

This study has used ionospheric and magnetic observational data obtained at a meridional chain of stations during the high latitude geophysical experiment ‘Taimir-82’ in the winter of 1982–1983. Mean statistical latitude-time distributions of the occurrence probability of various types of E_s, their blanketing frequency and of the amplitude of geomagnetic field H-variations have been constructed. Based on these distributions and taking the E_s properties into account, an analysis is made of the mutual correspondence of large-scale structures of the auroral ionosphere and ionospheric currents.Ionospheric currents flow mainly in the region of high E-layer ionization. With increasing magnetic activity, the zone of currents and the zone of ionization expand simultaneously toward lower latitudes. The evening eastward electrojet and the morning westward electrojet are localized inside the zone of diffuse auroral precipitation which is responsible for the formation of E_s type r. The equatorial part of the midnight westward electrojet is also located in the zone of diffuse precipitation which coincides also with the region of maximum ionization of the E-layer. The polar part of this electrojet, which extends far into the dusk sector, is located in the zone of discrete auroral precipitation (a type E_s). Whereas there exists in the meridional cross-section quite a definite relationship between the Harang discontinuity and ionospheric parameters, such a relationship is not manifested in the zonal cross-section of the Harang discontinuity.     

Sudden neutral sodium layers: a strong link to sporadic E layers

A sodium LIDAR instrument located at Andenes, Norway (69°N; 16°E) observed several sudden developments of narrow sodium layers in the 90–100 km altitude region. These layers grow with typical time constants of 5 min and have a width of 1 km in altitude. We present the temporal and spatial properties for a number of these events. In a first step towards identifying the processes which create these layers we study the correlation of the growth phase of sudden sodium layers and of sporadic E layers. The latter have been recorded by an ionosonde located 129 km east of the LIDAR site. Within the mutual altitude and time resolution available in our common records a strong correlation of simultaneous occurrence of sudden sodium layers and E_s−l layers is observed, which establishes a strong link between the formation of the two types of layers. We further discuss processes which potentially could give rise to the formation of sudden sodium layers.     

Internal structure of the equatorial ionospheric dynamo

Using the Sq current profiles measured with rocket born magnetometer, off the coast of Peru, by Daviset al. (J. geophys. Res. 72, 1845) comparison is made between measured and calculated profiles near noon on geomagnetic dip equator, and a mismatch is pointed out in the height pattern of Sq current. Theory is worked out to determine the eastward electric field (E_y) with which computed j_y (eastward current density) coincides with the observed one. It is found that the neutral wind plays very important rôle in keeping E_y height-independent. E_y is found to be about 0.6 mVm⁻¹, ranging from 0.5 to 0.7 mVm⁻¹ in some cases. Flow of meridional current is obtained and its effect on jet current construction is discussed.     

Analysis of Es traces from a calibrated oblique ionosonde

An analysis is performed on ionosonde data produced during five years of operation of an oblique sounder transmitting on a path from Darwin (12.4°S, 130.9°E) to Alice Springs (23.5°S, 133.7°E). It is found that the occurrence of sporadic-E (E_s) shows a relatively mild diurnal dependence, with a significant amount of E_s occurring in the early evening before midnight. It appears that, on average, nighttime E_s produces weaker reflections than daytime E_s.The power of the E_s reflections as a function of frequency is collated for all ionograms. The resulting power curve exhibits total and partial reflection sections. In trying to reproduce the partial reflection section of the curve it is shown that a layer without horizontal structure is required to be only 100 m thick. A second model involving a layer consisting of horizontally localised clouds of scatterers, with scale sizes ranging from 100 to 1000 metres, reproduces the partial reflection section of the curve quite well. The size, intensity and distribution of the clouds affects the curve shape on individual ionograms, resulting in the suggestion that nighttime layers are more irregular than daytime layers.     

Small-scale fluctuations of magnetic and electric components of the ELF and VLF wave fields in the sub-auroral topside ionosphere : stochastic characteristics of the wave field

When the Interkosmos-14 and Interkosmos-19 satellites crossed the region of spatially varying electron concentration in the topside ionosphere adjacent to the high-latitude boundary of the main ionospheric trough, it was discovered that there were simultaneous fluctuations of plasma density, temperature and the amplitudes (H_x and E_y) of the ELF and VLF radio/plasma emissions. The probability characteristics of the naturally perpendicular H_x and E_y fluctuations are analysed. The correlation coefficient R(_H, E_y) turned out to be less than 0.6 at frequencies of F ⩽ 4.65 kHz, while at higher frequencies R increases, up to 0.9 at 15 kHz. The following interpretations are proposed:

1. 1.1. While measuring noise emissions, as a rule a mixture of numerous elementary waves is recorded.
2. 2.2. At frequencies exceeding the local lower hybrid resonance frequency (in our case f_LHR ≈ 5 kHz), a mixture of electromagnetic waves experiencing the influence of the inhomogeneous electron concentration N_e is registered.
3. 3.3. At frequencies which are lower than the local value f_LHR the mixture mainly consists of ELF waves. The wave field has a complicated structure, and the dynamical coherence between electric and magnetic field components is not as simple as at VLF frequencies (f ≈ 15 kHz).
4. 4.4. It is shown that the wave components for a mixture of electromagnetic and electrostatic waves (for instance a mixture of VLF and lower hybrid frequency waves) have a lower correlation coefficient because the electrostatic waves are unrelated to the electromagnetic waves.
5. 5.5. The correlation analysis offers an opportunity to detect the presence of waves of various types in the wave mixture.

     

Characteristics of field-aligned E-region irregularities over lioka (36°N), Japan—I

Measurements with a 25 MHz radar over Iioka, Japan show that field-aligned E-region irregularities occur mainly at night in association with E_s-layers at an altitude range of about 100–110 km and drift predominantly westward with speeds of the order of 60 m s⁻¹. These observed characteristics of the irregularities are shown to be in reasonable agreement with quantitative predictions of the gradient drift instability theory. The predictions are based on appropriate models for neutral air densities and temperatures, ionic composition and ionospheric electric fields and on available observations of electron density profiles of E- and sE_s-layers.     

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.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—II. Characteristics of electric fields on quiet and disturbed days

Using the Doppler spectra of VHF radar signals, the height profiles of the phase velocity (V_p) of 2.7 m irregularities in the equatorial electrojet (EEJ) over Thumba (dip: 56′S) are obtained. The day-time east-west electric fields (E_y) are deduced by matching experimentally observed V_p profiles with theoretically deduced ones for a number of quiet and disturbed days. The experimental E_y values show: (i) a large day-to-day variability; (ii) a large decrease in the afternoon hours on some days (quiet and disturbed); (iii) the frequent presence of short period fluctuations with amplitudes of 30–50% of the background value and with typical time scales of 30–60 min, on moderately disturbed days (9 ⩽ A_p ⩽ 30); (iv) a significant decrease of the average E_y on disturbed days compared to that on quiet days during 0900–1200 h L.T.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—I. A method of estimating electric fields

Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V_p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E_y in the EEJ can be deduced from the experimental height profiles of V_p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E_y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated.It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E-region.     

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.     

Lunar modulations of the equatorial electrojet

An attempt has been made to reproduce the counter electrojet (CE) in the equatorial dynamo by considering neutral winds with solar (1,–2), (2, 4), (2, 2) and lunar (2, 2) tidal modes as well as a constant electrostatic field (E_y). The daily variation of conductivity (σ) is assumed to consist of steady (average), diurnal and semi-diurnal components. An equation governing the relationship between j_y (jetcurrent), E_y, σ and wind is given, and this equation is then used to describe diurnal, semi- and ter-diurnal variations of j_y separately. It is found that: (1) the lunar tide is relatively powerful in affecting semi- and ter-diurnal components of j_y; (2) such a possibility is a maximum for the afternoon CE near new and full moon and (3) the morning CE is likely to occur at lunar age between the new and full moons. From this theory, the seasonal characteristics and the solar activity dependence of CE are demonstrated to be predictable.     

Observations of the reflection coefficient of the sporadic E-layer at high latitudes

A method to measure the reflection coefficient of the sporadic E-layer using a conventional ionosonde is described, and some results obtained at Sodankylä, Finland (Φ=63.8°, φ= 120.0°), are presented. The observations are often found to be in agreement with the theoretical frequency dependence of the reflection coefficient of a thin horizontal layer in the presence of mode coupling. Some of the results can be interpreted as indicative of scattering from small-scale irregularities or reflections from larger inhomogeneities in the E_s layer.