T. Nygrén

A new method of measuring the ion-neutral collision frequency using incoherent scatter radar

Abstract A new method of measuring the ion-neutral collision frequency using the Doppler shift instead of the shape of the incoherent scatter spectrum is introduced. The method is based on the fact that, in the absence of neutral wind, the relation between electric field and ion velocity is determined by collision frequency. If both the electric field and the ion velocity are measured, the collision frequency can be solved from the equation of ion motion. The method is tested using the EISCAT radar. The applied experiment gives profiles of vertical ion velocity in the E-region and electric fields in the lower F-region. Data from a period with a weak neutral wind and a sufficiently strong electric field are analysed and the results are compared with those obtained using the conventional method. It is found that, under favourable conditions, the new method allows the determination of the collision frequency up to 130 km altitude. This is a considerable improvement as compared to the conventional method, which usually gives the collision frequency profile no higher than to the 110 km level. At altitudes where both methods can be used, a good agreement is found. The main drawback in the new procedure is that it can be used only during periods of negligible neutral wind. It is suggested that this difficulty can be avoided by using an experiment with at least two transmitter beam directions.

Observation of a thin Es-layer by the eiscat radar

Abstract High resolution E-region measurements carried out on 16 November 1983 using the EISCAT incoherent scatter radar are presented. The experiment was monostatic with a vertical radar beam, and it was based on a Barker-coded four-pulse code on one frequency channel and Barker-coded single pulses on three channels. The basic integration time was 15 s and the spatial resolution 450 m. The results reveal a short-lived but intense thin sporadic E-layer at 18:00–18:06 U.T. at an altitude of about 106 km. Both before and during the event, downward ion velocities of the order of 100 m s−1 are observed above this height. A convergent null in the vertical ion speed is occasionally seen at the layer altitude. The layer occurrence is associated with auroral arcs drifting across the radar beam. Simultaneous observations of the STARE radar show an ionospheric electric field of 25–30 mV m−1. The field always has a westward component, which is in accordance with the observed downward plasma flow. Most of the time when the layer is intense, the field points into the NW-sector. Theoretically, this field direction should create convergent vertical plasma motion. Therefore it is suggested that the observed Es-layer is created by the action of the auroral electric field rather than by the wind shear mechanism.

Incoherent scatter studies of sporadic-E using 300 m resolution

Abstract A new EISCAT UHF experiment with 300m range resolution is introduced. It is based on Barker-coded five-pulse patterns and single pulses on 8 channels. The high resolution is obtained by using a bit length of 2 μs. A thin and intense sporadic- E -layer drifting vertically within the altitude region 102–105 km has been observed with this experiment. The drift speed of the layer is approximately equal to the measured vertical plasma velocity. Occasionally the layer remains stationary and is compressed by vertical convergent plasma flow. The decay of the layer can be followed and it is found that it is, at least partly, caused by wave motions in the neutral atmosphere. It is shown that the layer contains heavy ions, most probably Fe + . The composition fit for molecular and Fe + ions gives molecular ion concentrations which are much higher than those estimated on the basis of the electron density. Therefore, it is concluded that the layer must also contain light metal ions, perhaps Mg + . According to composition fits of Fe + and Mg + , the abundance of Fe + in the layer is 60–75%.

Sporadic-E as a tracer for atmospheric waves

Abstract During three days of quiet geomagnetic conditions in August 1988, both the EISCAT UHF radar and the NOAA digital ionospheric sounder (Dynasonde) were operated from 12:00 to 22:00 U.T. Sporadic-E layers were observed on all three days, controlled by the action of the semi-diurnal tide. Gravity wave activity was very evident in both data sets, and particularly visible in the changes in intensity and position of the layers. By combining the two sets of measurements it is possible to build up a spatial picture of the phase fronts passing over Tromso, with sporadic-E layers acting as tracers for them in the E-region. It is observed that structures in the electron density associated with gravity waves descend from the F-region through the E-region to merge with the increases in the layer intensity. The enhancements in the layers, which we suggest are produced by horizontal convergence, are ribbon-like and aligned along the gravity wave phase fronts in the E-W direction, travelling southwards at 60 km h−1 and about 50 km apart.

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

Abstract 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 Es-layer. If the height difference of the ion sheets is not too great as compared to their thicknesses, the Es-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 Es-profiles are caused by the presence of more than one monoatomic ion species.

The effect of electric field-induced vertical convection on the precipitation E-layer

Abstract The effects of perpendicular electric fields on the high-latitude nocturnal precipitation E -layer are studied in terms of model calculations. The deformations of the electron density profiles caused by vertical plasma movements are described assuming various incident electron energies and flux densities, as well as different field intensities and directions. Manifestations of the profile variations in vertical sounding observations and changes in the conductivity profiles are examined. The results show that field-induced vertical convection is important in plasma densities even higher than 10 11 m −3 . Erroneous results may be obtained if the effect of the electric field is neglected when determining the incident electron spectrum from a measured ionospheric density profile.

A simple method for obtaining reflection and transmission coefficients and fields for an electromagnetic wave in a horizontally stratified ionosphere

Abstract A simple method for obtaining the reflection and transmission coefficients and fields for an electromagnetic wave propagating vertically in a horizontally stratified ionosphere is introduced. The ionosphere is divided into a great number of thin layers, and the boundary conditions of the electric and magnetic fields are applied at all layer interfaces. A recursive formalism is derived, which gives the altitude dependence of the reflection and inverse transmission coefficient matrices starting from the top of the layer. It is then shown how these results can be used in calculating the height dependence of a wave corresponding to any incident polarization. Test results are also presented in order to demonstrate the applicability of the procedure.

A method for determining ion-neutral collision frequency using radar measurements of ion velocity in two directions

Abstract This paper describes a method for determining the ion-neutral collision frequency in the E region using incoherent scatter radar. The method is based on ion velocity due to electric field and neutral wind. Height profiles of two ion velocity components are measured sequentially with vertical and eastward tilted directions of the radar beam, and the electric field is determined from tristatic velocity observations. The collision frequency is then solved from the momentum equation. The method is applicable at higher altitudes than the conventional method using the incoherent scatter spectral shape. The theory is given and examples of collision frequency obtained with the EISCAT UHF radar in August 1985 are presented. The results have been compared with collision frequencies calculated with the MSIS 1986 model atmosphere. They are found to be consistently higher than the model values, as are results from the conventional incoherent scatter method below 110 km, which leads us to believe that the model values for August may be too low.

A method of full wave analysis with improved stability

Abstract A new version of a previously published method of full wave analysis is presented. The purpose of the procedure is to avoid the numerical instability which in some cases is encountered when the original version is used. In the old method the reflection and inverse transmission coefficient matrices were recursively calculated starting from the top of the layer. It is found that better stability is obtained, if the reflection coefficients are calculated as before, but the transmission coefficients are computed starting from the bottom of the layer and proceeding upwards in the direction of the incident wave.

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

Abstract 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.