V. Y. Trakhtengerts, A. G. Demekhov (Institute of Applied Physics, Russian Academy of Sciences)

Y. Hobara, M. Hayakawa (Department of Electronic Engineering, The University of Electro-Communications 1-5-1, Chofugaoka, Chofu, 182, Tokyo, Japan)

Recent analytical and numerical results concerning the nature of discrete emissions in the magnetosphere are reviewed. The theory is based on analysis of whistler cyclotron interactions with energetic electrons having sharp (step-like or beam-like) distribution functions evolving in space and time. These beams are formed under the action of quasi-monochromatic whistler wave packets, which come from ground-based VLF transmitters or arise naturally as a result of the backward wave generation regime of the magnetospheric cyclotron maser.

Features of electron beam formation by the interaction of a VLF wave packet from a ground-based transmitter are considered. The beams are formed due to trapping of electrons at the forward front of the wave packet, their acceleration inside the packet, escape of the accelerated electrons from the moving backward front of the packet, and their following free motion in an inhomogeneous magnetic field. It will be shown that spatial and temporal gradients of the parallel velocity of the beams formed can be opposite to the case of a pure adiabatic motion. Such a behavior together with the phase modulation of the beam at the exit from the wave packet determines characteristics of the secondary emissions.

The analytical self-consistent theory of second-order cyclotron resonance effects as related to generation of discrete VLF emissions will be presented based on cyclotron instability of the hydrodynamic type, taking place in the presence of nonstationary electron beams with sharp gradients in velocity space. Under these conditions, the conventional hydrodynamic instability of such beams is drastically modified and second order resonance effects become important. It is shown that the optimal conditions for the instability occur for a nonstationary quasi-monochromatic wavelets, the frequency of which changes in time. The wave amplification is evaluated and spatio-temporal characteristics of these wavelets are determined.

Numerical calculations based on the developed theoretical approach will be presented. They demonstrate the capability of the theory to provide some clarification for some important aspects of the nature of discrete ELF/VLF emissions in the magnetosphere such as chorus and triggered emissions. Conditions for generation of different forms of dynamical spectrum (fallers, risers or hooks) are analyzed.

A. G. Demekhov (Institute of Applied Physics, Nizhny Novgorod, Russia)

P. P. Belyaev, S. V. Isaev (Radiophysical Research Institute, Nizhny Novgorod, Russia)

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

We calculate the frequency spectrum of the electromagnetic background noise in frequency range 0.1-5 Hz, based on the model attributing its formation to the ionospheric resonant filtration of the radiation from lightning discharges (Belyaev et al., 1989). Characteristics of the resonance spectral structure (RSS) formed due to the ionospheric Alfven resonator are obtained and their dependence on ionospheric parameters is considered. RSS variation during a day and during a solar cycle is discussed. The calculations are compared to the ULF ground based observations in Sodankyla and Nizhny Novgorod and mutual consistency of experiment and theory is demonstrated. Opportunities of use of such data to determine some ionospheric parameters are discussed.

V. Y. Trakhtengerts, A. G. Demekhov (Institute of Applied Physics, Nizhny Novgorod, Russia)

S. V. Isaev, S. V. Polyakov, P. P. Belyaev, V. O. Rapoport (Radiophysical Research Institute, Nizhny Novgorod, Russia)

A self-consistent model for the generation of Pc~1 pearl emissions based on the nonlinear magnetosphere/ionosphere coupling is considered. Formation of pearls is attributed to the pulsating regime of the Alfven sweep maser (ASM) with nonlinear selective mirrors. Such mirrors are formed by the conjugate ionospheres: their reflection coefficient has an oscillatory frequency dependence due to eigenmodes of the ionospheric Alfven resonator; nonlinear magnetosphere/ionosphere feedback is provided by the dependence of the value and frequency of the reflection maxima on the flux of energetic protons precipitated into the ionospheres in the course of Alfven wave generation in the magnetosphere. A nonlinear soliton-like solution of the ASM model is found, which corresponds to a single wave packet having the positive frequency drift and oscillating between the conjugate ionospheres. Properties of this solution are shown to explain many observational characteristics of Pc 1 pearls, such as their morningside predominance, correlation with low magnetic activity, spatio-temporal and spectral patterns. Results of numerical simulations of the ASM equations are presented.

N.Yagova, V. Martines-Bedenko (Institute of the Earth Physics, Moscow)

The dynamic autospectral, polarization and cross-spectral analysis were applied to Pc3-5 geomagnetic pulsations detected by 3 geostationary satellites. With the use of the numerical model of the magnetospheric Alfven resonator the frequencies of the spectral maxima were interpreted as the first 6 harmonics of Alfven field line oscillations. The set of frequencies obtained made it possible to reconstruct the field-aligned plasma density distribution.

The formation of the field-aligned plasma density distribution with a steep density gradient has been suggested. The parameters of this distribution are estimated on the basis of the one-dimensional non-stationary plasma convection model in a field flux tube in the local morning hours.

Azimuthal propagation characteristics of ULF waves at geostationary orbits are analysed for the pulsation event with the multi-harmonic resonant structure. Azimuthal velocity dispersion has been estimated and compared with the predictions of the MHD waveguide model.

T.V. Romantsova, M.M. Mogilevsky (Space Research Institute R.A.S, Moscow, Russia)

J. Hanazs (Space Research Center, P.A.S., Torun, Poland)

Radio emissions observed onboard Electron-2 and on Ogo 1 (Benediktov et al.,1968; Dunckel et al.,1970) have discovered that intense radio emissions in the several hundred kHz frequency range were closely associated with magnetic disturbances in the high-latitude auroral regions of the earth. Later studies by Gurnett (1974) showed that these radio emissions are closely associated with the occurrence of auroral arcs on the nightside of the earth. The radiation is presumably produced by the same intense fluxes of several keV electrons that give rise to the aurora currents responsible for the magnetic disturbances. Observations of Auroral Kilometric Radiation by satellites have shown that AKR is associated with the auroral electrojet index AE during substorms (Gurnett, 1974; Kurth et al.,1975; Voots et al.,1977). In the last publication W.S.Kurth and D.A.Gurnett (1998) have proposed to use the integrated intensity of Auroral Kilometric Radiation (obtained by Polar satellite) as a proxy for the auroral electrojet index AE in support of studies the magnetosphere relating to response to variations of the magnetic activity. In this paper we present the results of comparative analysis of AKR intensity, obtained from POLRAD onboard the INTERBALL-2 (Auroral Probe) satellite, and magnetic field measurements, made by ground based networks IMAGE and CANOPUS. We also used auroral UV image from POLAR satellite. It was found that existance of correlation between measurement of AKR and magnetic field variation depends strongly on the correlation distance between spacecraft and ground station.

1. Benedictov E.A., Getmansev N.A. et al. Relation between geomagnetic activity and the sporadic radio emission recorded by the Electron satellitees. Cosmic. Res., 6, 791(1968).

2. Dunckel N., Ficklin B., Rorden L. And Helliwell. Low-frequency noise observed in the distant magnetosphere with Ogo 1. J.Geophis.Res.,75, 1854, 1970.

3. Green J., Gurnett D.A. and Shawhan. The angular distribution of auroral kilometric radiation. J. Geophys. Res.,1977.

4. Gurnett D.A. The earth as a radio source: Terrestrial kilometric radiation, J. Geophys. Res. 79, 4227, 1974

5. Kurt W.S., Baumback and Gurnet D.A Direction-finding measurements of auroral kilometric radiation. J. Geophys. Res., 80., 2764,1975.

6. Voots G.R., Gurnett D.A. and Akasofu S.-I. Auroral kilometric radiation as an indicator of auroral magnetic disturbances. J.Geophys.Res.,82,2259,1977.

7. Hanasz J., et al. Polrad-description of experiment, Interball-Mission and Payload., ed.CNES, P.313-1995.

8. Kurth W.S. and Gurnett D.A. Auroral kilometric radiation integrated power flux as a proxy for AE. Adv.Space Res., 22, 1, p.73-77,1998

D.L. Pasmanik, V.Y. Trakhtengerts (Institute of Applied Physics RAS, Nizhny Novgorod, Russia)

The cyclotron interactions of energetic electrons and whistler waves in the case of ducted waves propagation are studied. Such situation may occur when the waveguide for whistler waves is formed by plasmapause (whispering gallery propagation) or by ducts of enhanced background plasma density.

The spatial structure of eigenmodes of waveguide in the case of cylindrically symmetry duct is analyzed. The development of cyclotron instability due to interaction of energetic electrons injected into this waveguide and whistler waves with eigenmodes structure taken into account is studied. The efficiency of excitation of a different eigenmodes and spatial structure of energetic electrons precipitation is considered.

E.E. Titova, A.G. Yahnin (Polar Geophysical Institute, Apatity, Russia)

F. Jiricek, J. Smilauer (Institute of Atmospheric Physics, Bosni 11, 14131, Prague 4, Czech Republic)

M.M. Mogilevsky, T.V. Romantsova, A.A. Rusanov (Institute of Space Research, Moscow, Russia)

R. Smith (Geophysical Institute, University of Alaska, Fairbanks, Alaska, USA)

J.L. Rauch (LPCE/CNRS, Orleans, France)

The Interball-2 satellite auroral hiss observations performed during November-December 1996 were compared with simultaneous observations of auroras. Interball-2 had the apogee above the Northern Hemisphere at high latitudes at geocentric distance 2,5-4 R_e. In the considered events the satellite crossed the auroral zone in the evening sector, went to the polar cap, and again crossed the auroral zone in the post-midnight sector. Auroral data were obtained from the ground-based observations in Loparskaya and Lovozero (Kola Peninsula, north-west of Russia) and in Longyearbyen, Svalbard. Auroral dynamics was used as indicator of the temporal and spatial behavior of the acceleration region.

Main attention is paid to strong variations of auroral hiss intensity and frequency at latitudes poleward of the auroral oval, and their relation to auroral dynamics. The temporal intensifications of the hiss, which were observed well poleward from the auroral oval, correlate with both substorm onset and week intensification of a single aurora arc. The auroral hiss cutoff frequency likely depends on distance between auroras and satellite. The cutoff frequency of auroral hiss decreases when the distance between satellite footprint and auroras decreases and vice versa. Thus the variations of intensity and cutoff frequency of auroral hiss observed by the satellite poleward of auroral oval are close related with both temporal and spatial dynamics of auroral electron acceleration region. Behavior of cutoff frequency can be explained by propagation in the whistler mode at wave normal angles near the resonance cone from a spatially localized moving source. Applicability of auroral hiss characteristics for diagnostic of acceleration region is discussed.

N.V. Semenova (Kola Branch of the Petrozavodsk University and Polar Geophysical Institute, Apatity, Russia)

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

A.G. Yahnin, E.E. Titova (Polar Geophysical Institute, Apatity, Russia)

Sensitive search coil magnetometer in SGO provides the continuous digital registration with sampling rate of 0.05 seconds since 1995. To visualize the observational results the daily spectrograms (dynamic spectra) of magnetic variations in the range 0-4 Hz are routinely calculated and plotted. An interesting phenomenon called as Spectral Resonance Structure in the noise background intensity /1/ is often clearly seen in the spectrograms. Till present time, most of the SRS related investigations have been carried out on the basis of mid-latitude observations. In this study we statistically investigate the morphological features of the SRS in the auroral zone on the basis of the SGO data set. It has been found that probability of the auroral SRS observations decreases with the increase of the geomagnetic activity. In general, it is higher in the winter season and diminished in summer. The probability is maximal in the premidnight sector and minimal in the morning, but there is also some dependence of the local time of the probability maximum and minimum on season. Frequency of the SRS varies during the day; it is maximal near midnight and minimal at noon, that is, it anticorrelates with the ionospheric F-region density. We conclude that our findings are in agreement with the theory /2/ describing the SRS as a result of existence of the so-called Ionospheric Alfven Resonator.

1. P.P. Belyaev, S.V. Polyakov, V.O. Rapoport, and V.Y. Trakhtengerts, Discovery of the resonance spectrum structure of atmospheric electromagnetic noise background in the range of short-period geomagnetic pulsations, Doklady Akad. Nauk SSSR, 297(4), 840-843, 1987 (in Russian).

2. P.P. Belyaev, S.V. Polyakov, V.O. Rapoport, and V.Y. Trakhtengerts, Theory of formation of the resonance spectral structure of atmospheric electromagnetic noise background in the range of short-period geomagnetic pulsations, Izv. Vuzov - Radiofizika, 32(7), 802-810, 1989 (in Russian).

N.V.Koropalova (Kola Branch of the Petrozavodsk University and Polar Geophysical Institute, Apatity, Russia)

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

A.G.Yahnin (Polar Geophysical Institute, Apatity, Russia)

Sensitive search coil magnetometer in SGO provides the continuous digital registration with sampling rate 0.05 seconds since 1995. To visualize the observational results the daily spectrograms (dynamic spectra) of magnetic variations in the range 0-4 Hz are routinely calculated and plotted. We examined these plots to investigate the morphological features of different types of geomagnetic pulsations, such as Pc1, PiB, IPDP. Considering the occurrence frequency dependence of the pulsations on MLT we found that pulsations Pc1 exhibit the maximum of occurrence in the day sector, while PiBs show the maximum in the night sector. The IPDP are seen in a wide range of MLT in the evening-night sector with nearly the same probability. In general, these findings agree with the results of some previous studies, but some new features are also obtained. In particular, we found that occurrence of Pc1 does not depend significantly on geomagnetic activity in a range of Kp-index of 0-4, but decreases when the activity grows (Kp>4). Another interesting finding is that the occurrence of the PiB-like pulsations become to be significant in the day sector when global geomagnetic activity is high. Seasonal dependence of the pulsation occurrence as well as the dependence on interplanetary magnetic field and solar wind parameters is also discussed.

V. Pilipenko, N. Yagova (Institute of the Earth Physics, Moscow)

Geomagnetic pulsations are widely used for the monitoring of magnetosphere and solar wind parameters. However, quasi-monochromatic signals are not regularly observed. The most common manifestation of magnetospheric ULF activity is a colored-noise. Its parameters may make a useful instrument for the analysis of magnetospheric plasma. We analyzed the statistical parameters of ULF noises in the inner magnetosphere. The integral noise parameters along the meridional profile of stations at middle and low latitudes under various geophysical conditions are calculated.

For the detection of transient signals or variations in noise properties the statistical adaptive algorithm is suggested. It is demonstrated that such algorithm can be effectively used to reveal the fine structure of a substorm onset or to retreive man-made effects from magnetic recordings.

It is shown that the ULF noise distributions at the ground have rather complicated forms. Their features are controlled mainly by the spatial and temporal scales of irregularities of the propagation media. The application of the ULF noise to the monitoring of natural hazards is impossible without the detailed analysis of its background variations. The results obtained enable to include the geomagnetic noises into the traditional MHD diagnostics of the magnetosphere with ULF waves.

T. Bosinger, A. Kero, P. Pollari (University of Oulu, Oulu, Finland)

A. Pashin (Polar Geophysical Institute, Apatity, Russia)

P. Belyaev (Radiophysical Research Institute, Nizhny Novgorod, Russia)

M. Rietveld (EISCAT Scientific Association, Ramfjordmoen, Norway)

E.V. Pchelkina, A.B. Pashin, E.G. Belova, W.B. Lyatsky (Polar Geophysical Institute, Apatity, Russia)

During the experiment on November 28, 1995 step-like increase of pump wave Effective Radiated Power (ERP) up to value of 170 MW has been used. The heating HF radio wave with frequency 4.04 MHz was consequently modulated with frequencies in VLF range: 925 Hz, 1375 Hz and 2375 Hz. During the experiment the ionospheric parameters were measured by EISCAT VHF radar. We used the electron density data in the altitude range from 70 km to 105 km with 1 minute resolution. The artificial low-frequency emissions were detected by VLF receiver at Porojarvi at the distance of 96 km SE from the transmitter. The VLF receiving system, used in this study consists of two magnetic loop antennae oriented in magnetic N-S and E-W direction. The digital data obtained with time resolution equal to 25 ms were processed with the standard FFT algorithm. We derived from these data the amplitudes and phases of the horizontal components of the artificial signal. The amplitude and polarisation characteristics of the magnetic disturbances on the ground were calculated from experimental data. The dependencies of amplitude of the signal on the values of ERP are masked by the more fast and significant time dependence of the disturbance amplitude. We believe that this time dependence is related with wave-like electron density variations in the altitude range 70-85 km.