A. Huuskonen

Interference of tidal and gravity waves in the ionosphere and an associated sporadic E-layer

Abstract Observations made on 10 July 1987 with the EISCAT UHF radar are presented. The F -region measurements of both electron density and field-aligned ion velocity show that an upward propagating gravity wave with a period of about 1 h is present. The origin of the gravity wave is probably auroral. The E -region ion velocities show a tidal wave and both upward and downward propagating gravity waves. The gravity waves have three dominant periods with a possible harmonic relationship and similar vertical wavelengths. These waves are either reflected at a single reflection level, ducted between two levels, or they are generated in a non-linear interaction between gravity and tidal waves. The E -region electron density is dominated by particle precipitation. After a short burst of more intense precipitation, a sporadic E - layer forms at 105km and then disappears 40min later. Within this time, the layer rises and falls by a few kilometres, following closely the motion of a convergent null in the velocity profile. We suggest that the formation and destruction of this layer is controlled by both the precipitation, which indirectly provides a source of metal ions through charge exchange, and the superposition of gravity waves and the tidal wave.

High resolution observations of the collision frequency and temperatures with the Eiscat UHF Radar

Abstract Ion-neutral collision frequencies and ion and electron temperatures have been deduced from high resolution (600 m) EISCAT UHF radar measurements. The results are based on 80 h of data measured in February, July and August 1984 and August 1985. They are compared with other EISCAT observations and with collision frequencies and neutral temperatures based on the MSIS-86 model neutral atmosphere. Theoretical error estimates for the plasma parameters are presented and they are used to study the possibility of deducing the collision frequency and/or the electron-ion temperature ratio from the data. Collision frequency results have been obtained between 93 and 110 km altitudes using the assumption of equal ion and electron temperatures. These observations agree with the other published collision frequency observations by EISCAT. The observations indicate that the MSIS-86 model gives correct collision frequencies in July but underestimates them in February and August. The temperature observations from each of the three months show an event-to-event variation which exceeds that predicted by the MSIS-86 model. It is proposed that, although Joule and particle heating can explain the variation partly, the observations indicate larger changes in the neutral temperature than those given by the model. This is especially evident below 100 km, where the heating mechanisms are inefficient, but large changes in the ion temperature are nevertheless observed. The ion temperature exceeds the electron temperature above 110 km but near that height the temperatures are closely equal. This fact justifies the assumption of equal ion and electron temperatures used in the analysis below 110 km.

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.

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%.

On the dependence of the Farley-Buneman turbulence level on ionospheric electric field

The mean electron density fluctuation amplitude of the Farley-Buneman E-region wave turbulence has been determined using coherent radar backscatter intensity and drift velocity measurements made with the STARE system, together with ionospheric electric field and electron density measurements obtained with the EISCAT incoherent scatter radar facility. The observations made with the two radar systems were obtained in nearly a common volume in the E-region. It is shown that the turbulence level increases from 1.5 to 2.7% as the electric field intensity increases from 20 to 35 mV m−1. It stays approximately constant as E > 35 mVm−1. This increase and saturation of the wave turbulence level is discussed in the frame of existing theories.

Range ambiguity effects in Barker-coded multipulse experiments with incoherent scatter radars

Abstract A Barker-coded multipulse is the best of the classical modulations when high spatial resolution is needed in incoherent scatter measurements. Unfortunately, this type of modulation generates range ambiguities which reduce the quality of data. In this paper, range ambiguity functions for an EISCAT experiment, ESLA-T4, are displayed. A method of eliminating the ambiguities in data analysis is developed and its applicability is demonstrated. It is found that the method can remove the effects of the range ambiguities in the lower part of the E-region, but above 120 km altitude the correction is not sufficient. The residual of the fit is reduced by an order of magnitude. The changes in the temperatures are a few Kelvin at the lowest altitudes and a few tens of Kelvin between 110 and 120 km.

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.

Determination of E region effective recombination coefficient using impulsive precipitation events

A set of sudden increases followed by slow decay has been observed in E region electron density on December 17, 1990 using the EISCAT UHF-radar. Simultaneous photometer observations indicate that the density enhancements are caused by impulsive particle precipitation with short duration (4 ± 1 s). Effective recombination coefficient profiles in the E region are calculated from decaying electron densities using the continuity equation, and the results are compared with previous observations. It is pointed out that short-lived particle precipitation affects the ion composition in the E region and therefore the effective recombination coefficient in this situation is different from the steady-state value.

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.

Range ambiguity effects in a phase coded D-region incoherent scatter radar experiment

Abstract For lower ionosphere studies with EISCAT incoherent scatter radars there exists a sophisticated Barker-coded pulse-to-pulse correlation experiment GEN-11. It has been used widely since 1985, but so far all the effects arising from the complexity of the experiment have not been treated properly. In this paper we present a detailed description of the modulation and how it produces ambiguities to the raw data. This leads into an addition to the standard analysis procedure which removes these ambiguities. The analysis works well at altitudes from 70 km to 100 km. During a precipitation event, the obtained correlation times above 90 km are about 20% smaller than without the correction. Below 80 km the corrected correlation times can be larger by a factor of two, which has a significant effect to the estimated negative ion to electron density ratio.