H.Ranta (Geomagnetic observatory Sodankula, Finland)

The digital geomagnetic and riometer one minute sampling data at several Finnish stations have been used for analysis. It was found, that there are four types of Pc5 range (1-5 mHz) pulsations:

1) Geomagnetic pulsations do not accompanied by riometer ones,

2) Riometer pulsations do not accompanied by geomagnetic ones,

3) Simultaneous geomagnetic and riometer pulsations with similar spectra and coincided wave packets,

4) Simultaneous geomagnetic and riometer pulsations with different spectra and do not coincided wave packets.

Both types of simultaneous pulsations are observed out of phase and mostly in the late morning and near local noon or in the late evening. Very often the wave packets of geomagnetic pulsations at Sodankula look like the wave packets of riometer pulsations not at Sodankula, but at Ivalo, that is at higher latitudes. The geomagnetic pulsations without riometer ones are usually observed during main phase of the early morning (2-4 UT) substorms, simultaneous pulsations appear in recovery phase of its. The riometer pulsations without geomagnetic ones are often observed in the after noon. The interpretation of these facts is based on the different types of Pc5 geomagnetic pulsations in the magnetosphere - with large and small azimuthal wave number.

N.G.Kleimenova, O.V.Kozyreva (Institute of the Earth Physics RAN, Moscow, Russia)

J.Bitterly, J.J.Schott (Institute of the Earth Physics, Strasbourg, France)

V.A.Mullayarov, V.I.Kozlov (Institute of Cosmophysical Research & Aeronomy, Yakutsk, Russia)

The results of our experiments to measure the exit points from ionosphere of VLF emissions near Yakutsk in February-April 1994 are presented. The measurements are made at frequencies 1,2-3,0 kHz in 0,2 kHz with period duration of 20 s. It is obtained the incident angles of exit points, as a rule, are less than 60. Exit regions with small stable sizes are also detected - about 50 km at a height of low ionosphere. It points to ducted propagation of signals in the magnetosphere. Amplitude distribution of the signals has maximum in centre of exit region. Depending on the wave disturbance level the different positions of the exit region are possible, but region centres are usually near a zenith of the station. In some time intervals the ordered circular movement from point to point was observed. For example, the exit points during two minuts on February 24, 1994 are sequently displaced clockwise. There are the cases, in which the exit points form the arcs of > 180 deg. sizes although they are inconsistent. The prolation of the polarization ellipse depends on time of day and frequency. On a average, the prolation of the ellipse increases as frequency decreases. At frequences more then 2 kHz the polarization is close to the circular one in day-time.

The season-dependent relationship of monthly average values of the natural VLF emission noise component intensity at 8,7 kHz to variations in the sunspot number is established. It is obtained that a behaviour of these parameters is in antiphase. The higher anticorrelation coefficient is obtained for summer time (more then - 0.8). According to the nature of regular noise VLF-emissions, this means that the local thunderstorms in East Siberia and thunderstorms in the African world center, are in the antiphase with solar activity.

V.D.Tereshchenko (Polar Geophysical Institute, Murmansk, Russia)

The uniform density and temperature gradients across the steady magnetic field in the polar ionosphere have been able to generate unstable waves propagating close the direction of the drift of the ambient plasma. The aim of this report is to present the linear kinetic theory of electrostatic instabilities in such plasma with an enhanced electric field. Similar results for a simpler ionosphere model (without the electric field) have been presented in the literature.

The dispersion relation is derived in the electrostatic approximation using BGK collision term. The propagation of plasma waves is in the plane defined by drift velocities and the geomagnetic field. For frequencies near the cyclotron harmonics the dispersion relation has an analytic solution for the drift instability onset.

It is shown that the threshold of instability may vary with the magnitude of the temperature rations, the magnitude and direction of the gradients, the magnitude of the ambient electric field and gas composition of the ionosphere. The temperature gradient antiparallel to the density gradient and the steady electric field lowers the threshold of ion cyclotron drift instability. In the absence of an electric field onset of instability occurs when the diamagnetic drift speed is from 0,02 to 0,03 the ion sounded speed. For the electric field is greater than 50 mV/m the decrease factor is about 2 - 3. Such changes must lead to the generation of the instability on higher the cyclotron harmonics.

B.V.Kozelov, I.A.Kornilov (Polar Geophysical Institute, Apatity, Russia)

J.Manninen, T.Turunen (Geophysical Observatory, Sodankyla, Finland)

M.Vallinkoski (Sodankyla Geophys. Observ., Finland)

J.Kangas, P.Pollary (Oulu University, Finland)

T.Virdi, P.J.Williams (Univ. of Aberystwyth, U.K.)

E.Nielsen (Max-Plank Aeronomie Institute, Germany)

Ionospheric plasma parameters measured by the EISCAT within the regions of plasma turbulence detected by the STARE radars are analysed for near-threshold radar echo conditions. It is shown that at the altitudes around 110 km in the echo-layer of the Finnish STARE radar(F-radar) dependence of the threshold E-field of the echo appearance on the flow angle is essentially anysotropic (drift-aligned) with minimum E-field observed from the direction along the Hall electron drift and maximum in the perpendicular direction. Because of such property the threshold echo E-fields are interpreted as a real measure of the threshold E-fields of the generation of the Farley-Buneman (FB) plasma waves responsible for the F-radar echo.

The values of the threshold E-field of the FB-waves generation are studied as depending on the ratio of ion and electron temperatures(Te/Ti). It is shown that the properties of the dependence are essentially different for small and large flow angles, i.e. for primary and secondary unstable plasma waves, respectively. Namely, in course of the growth of the ratio when it is not less than about 1 the threshold E-field has a tendency to decrease for the primary FB-waves (become essentially smaller than usual FB-threshold electric field) but to increase for the secondary plasma waves.

The rate of the threshold E-field decrease found for the primary FB-waves is in a reasonable quantitative agreement with the theoretical estimates predicted by Gurevich and Karashtin (1984) due to a development of the termo-diffusion instability.

A.Kozlovsky, W.Lyatsky (Polar Geophysical Institute, Apatity, Russia)

Violation of the frozen-in condition for the plasmasheet ions when the spatial scale of disturbance is of order of the Larmour radius leads to the magnetospheric convection instability. This instability results in the separation of the convection plasma flow into jets with the spatial scale of order of few kilometers at the ionosphere level. The instability develops in these regions of the plasma sheet where the containing of the warm ions in the unite magnetic flux tube varies with the distance from the Earth, that take place at the inner or outer boundaries of the plasma sheet. The instability has high growth rate (the characteristic time of the growth is less then one minute). In the westward electrojet region the developing structures is stretched approximately along the auroral oval.

G.F.Remenets (Institute of Radiophysics, University of St.-Petersburg, Russia)

Without declaring the uniqueness of the solutions of the inverse VLF problems from the point of quantitative details of the effective electron concentration profiles gotten we state the uniqueness of the qualitative results relative to the unmonotony of electron concentration profiles in the following concrete cases of the VLF disturbances: 15 September 1982, 12.55-13.50 UT; 29 September 1982, 13.20-13.25 UT; 29 September 1989, 04.00-10.00 UT; 22 October 1989, 08.08-09.15 UT; 2 November 1989, 10.12-12.23 UT. Any other geophysical disturbances at the pointed time intervals were absent. Our analysis has confirmed the statement about relativistic electron precipitations along Aldra-Apatity radio trace (with the energies more then 300 KeV), which caused the anomalous ionization lower 55-50 km without disturbing the higher layer of the ionosphere.

A main result of the analysis is a statement that the almost similar behaviour in time (with minimums) of the amplitudes and phases for all radio signals in the range 10-14 kHz and for the middle zone of a radio source (600-1200 km) is a specific feature of the relativistic electron precipitations. The relativistic electrons generate the sporadic layer of ionization below ionospheric D-region. In accordance with these pointed signs the following 7 VLF-disturbances have taken place: 20 October 1985, 10.50-11.04-12.10 UT; 23 October 1985, 08.48-09.10-11.30 UT; 25 March 1986, 09.30-10.40-17.00 UT; 27 March 1986, 12.35-13.50-19.30 UT; 22 April 1986, 12.15-17.00-22.00 UT; 23 April 1986, 18.20-19.10-20.20 UT; 5 May 1986, 07.30-08.22-09.40 UT. The indicated intermediate time moments correspond to the maximums of the VLF disturbances. The initial results of this disturbances analysis are proposed.

V.A.Kornienko, N.F.Blagoveshchenskaya

M.Rietveld (EISCAT, Tromsoe, Norway)

A.Brekke, B.Holmslet (Auroral Observatory, University of Tromsoe, Tromsoe, Norway)

The influence of HF Tromsoe heater acting to an appearance of characteristic changes of geomagnetic field variations' polarisation has been detected. It has been found, that this is a local phenomena, according to data sets obtained at nearest geomagnetic observatories. It has been established that substorm bulge position controls the appearance and features of heating signatures in geomagnetic and HF Doppler data. It's been accepted as an evidence to possibility of local ionospheric substorm currents modification by HF powerful heating. Geomagnetic variations data and HF Doppler aspect reflection data show that there are local oscillations in PC 4 range during heating. The role of the HF heating modificated ionosphere to Alfven waves generation conditions is discussed. As a main reason of this acting we took the influence of HF heating generated Es - layer field-aligned striations movement transversal to geomagnetic field to high localized FAC's presented at the western edge of substorm bulge.