On the current-carrying properties of mid-latitude type sporadic E-layers

The current-carrying properties of mid-latitude type sporadic E-layers are investigated in some simple cases involving the presence of either a neutral wind or an electric field. It is shown that, in the northern hemisphere, the layer current is directed towards magnetic south provided the height profile of the neutral wind is a left-handed screw with a sufficiently small pitch. If the pitch is high or the screw is right-handed, the current flows northwards. A northward current is also expected in layers created by electric fields alone. It is further pointed out that the current driven by a neutral wind is carried merely by ions, while that due to an electric field is carried by electrons. Finally, the conducting properties of the layers are investigated in terms of a 2 × 2 conductivity tensor.     

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.     

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.     

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.     

On the problem of a connection between ozone dynamics at high latitudes and magnetospheric processes

Two data sets of ozone density measurements over a wide latitudinal range in the Arctic during summer and winter seasons arc presented. It is shown that geophysical effects manifest themselves in the O₃ dynamics in the high latitude region under various circumstances. Therefore a type of total ozone content diurnal variation is a distinctive feature in the auroral oval as well as the polar cap and must be taken into account in any full model of ozone dynamics in the polar regions.     

Scintillation at high latitudes during winter magnetic storms

Scintillation observations are described which were made at Kiruna in northern Sweden during three magnetic storm periods in the winter of 1984–1985. The results were obtained using transmissions from the multisatellite NNSS system, so that it has been possible to chart the development of scintillation activity over some 20° of geomagnetic latitude as a function of time for several days throughout each storm. A region of strong scintillation at the highest latitudes near magnetic noon is a common feature on all but the quietest days. This feature, probably associated with soft particle precipitation into the cusp, shows an abrupt boundary which moves equatorwards as the disturbance develops. In the magnetic midnight sector two latitudinally separate zones of scintillation are found, patchy at high latitudes although more sustained in the auroral zone. An absence of auroral scintillations around midnight UT can be followed by prolonged intense scintillation activity at auroral latitudes during the early morning hours on some disturbed days.     

Geomagnetic bays and Pc5 pulsation substorms at high latitudes

Morphological features of Pc5 pulsation substorms and of geomagnetic bays are examined in the light of current theories using data from Fort Churchill—a high latitude Canadian station in the auroral zone. The bays are classified using actual magnetograms into four categories:

• 1.(1) sleeping bays,
• 2.(2) pulsating bays,
• 3.(3) loaded bays and
• 4.(4) transition bays. Characteristics of all these bays are detailed and special circumstances are described in which they tend to occur on the magnetograms. A comprehensive model of the magnetosphere which largely fits the observations has been detailed.

     

GOES satellite timekeeping for field stations at high Arctic latitudes

The feasibility of using the GOES satellite time signal is discussed for field stations at high Arctic latitudes. Results are presented for three ground stations on islands in the Canadian Archipelago. The stations range in latitude from 74 to 81° 30' N. At all locations, time code reception was found to be satisfactory and capable of providing accurate time reference for remote experiments. A simple design for a high gain helical antenna, used successfully at these latitudes for time signal reception, is also presented. The antenna, primarily intended for a small research field station, is portable, inexpensive and readily constructed.     

VLF propagation parameters derived from sferics observations at high southern latitudes

Sferics are electromagnetic pulses generated by lightning events. Their maximum spectral energy is in the frequency range below 15 kHz. These powerful natural VLF transmitters can be used to determine the propagation characteristics of the atmospheric wave guide between earth and ionospheric D layer along virtually every propagation path. A VLF-sferics-analyzer was operating at the German Antarctic von Neumayer Station from January to June 1983. This analyzer recorded sferics from distant lightning events in the frequency range between 5 and 9 kHz. The method of measurement is described. The data are evaluated, and the propagation characteristics of the atmospheric wave guide are determined as a function of azimuth and season. The result is compared with theoretical calculations. It is shown that the difference between west-to-east and east-to-west propagation is much smaller than theory predicts, indicating that the ionospheric D layer at high southern latitudes behaves less anisotropic with respect to VLF propagation than at mid-latitudes.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

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

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

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.     

Concentrations of H2O and NO in the mesosphere and the lower thermosphere at high latitudes

Water vapour and nitric oxide concentrations in the mesosphere and lower thermosphere were derived from infrared emission and positive ion composition measurements above northern Europe during the Energy Budget Campaign 1980. The experiments were performed at different levels of geomagnetic disturbance. Both water vapour and nitric oxide are highly variable. Water vapour mixing ratios between 0.2 ppm and 10 ppm were observed. The nitric oxide peak densities varied by more than a factor of ten. Maximum values of 2 × 10⁹cm⁻³ were obtained.     

Small scale structure and turbulence in the mesosphere and lower thermosphere at high latitudes in winter

The MAP/WINE campaign has yielded information on small scale structure and turbulence in the winter mesosphere and lower thermosphere by a number of very different remote and in situ techniques. We have assimilated the data from the various sources and thus attempted to present a coherent picture of the small scale dynamics of the atmosphere between 60 and 100 km. We review physical mechanisms which could be responsible for the observed effects, such as ion density fluctuations, radar echoes and wind corners. Evidence has been found for the existence of dynamic structures extending over distances of the order of 100 km; these may be turbulent or non-turbulent. The results indicate that gravity wave saturation is a plausible mechanism for the creation of turbulence and that laminar flows, sharply defined in height and widespread horizontally, may exist.     

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°.     

Structure in the seasonal variations of Es at southern latitudes

The diurnal variations of the seasonal characteristics of sporadic-E occurrence have been studied by analyzing a large data set of ionosonde parameters for two southern hemisphere stations. The seasonal patterns are found to display anomalous short-term variations apparently not associated with solar control or the effects of dynamic meteorology.     

The influence of sunspot number and magnetic activity on the diurnal variation of the geomagnetic field at mid to high latitudes

The variations of the diurnal range of the geomagnetic field with sunspot number and with magnetic activity was studied at mid and high latitude stations in the northern hemisphere at different seasons. The effect of increasing sunspot number is small at lower latitudes and increases with geomagnetic latitude, while the effect of increasing magnetic activity is to increase the range at all latitudes, very greatly at the higher geomagnetic latitudes.     

The trouble with thermospheric vertical winds: geomagnetic,seasonal and solar cycle dependence at high latitudes

The vertical wind component is frequently used to determine the zero-velocity baseline for measurements of thermospheric winds by Fabry-Perot and other interferometers. For many of the upper atmospheric emission lines from which Doppler shifts are determined, for example for the OI 630 nm emission, available laboratory sources are not convenient for long-term use at remote automatic observatories. Therefore, the assumption that the long-term average vertical wind is zero is frequently used to create a baseline from which the Doppler shifts corresponding with the line-of-sight wind from other observing directions can then be calculated. A data base consisting of 1242 nights of thermospheric wind measurements from Kiruna (68°N, 20°E), a high-latitude site, has been analysed. There are many interesting short-term fluctuations of the vertical wind which will be discussed in future papers. However, the mean vertical wind at Kiruna also has a systematic variation dependent on geomagnetic activity, season and solar cycle. This means that the assumption that the average value of the vertical wind is zero over the observing period cannot be used in isolation to determine the instrument reference or baseline. Despite this note of caution, even within the auroral oval, the assumption of a zero mean vertical wind can be used to derive a baseline which is probably valid within 5 ms⁻¹ during periods of quiet geomagnetic activity (K_p < 2), near winter solstice. During other seasons, and during periods of elevated geomagnetic activity, a systematic error in excess of 10 ms⁻¹ may occur.     

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.