Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway

Geophysical phenomena during an ionospheric modi®cation experimentat Tromsù, Norway

N. F. Blagoveshchenskaya1, V. A. Kornienko1, A. V. Petlenko1, A. Brekke2, M. T. Rietveld3

1Arctic and Antarctic Research Institute (AARI), 199397, St. Petersburg, Russia2Auroral Observatory, University of Tromsù, N-9037, Tromsù, and The University Courses of Svalbard, Svalbard, Norway3 EISCAT, N-9027 Ramfjùrdbotn, Norway

Received: 13 October 1997 / Revised: 11 May 1998 /Accepted: 26 May 1998

Abstract. We present an analysis of phenomena ob-served by HF distance-diagnostic tools located in St.Petersburg combined with multi-instrument observationat Tromsù in the HF modi®ed ionosphere during amagnetospheric substorm. The observed phenomenathat occurred during the Tromsù heating experiment inthe nightside auroral Es region of the ionosphere dependon the phase of substorm. The heating excited small-scale ®eld-aligned irregularities in the E region respon-sible for ®eld-aligned scattering of diagnostic HF waves.The equipment used in the experiment was sensitive toelectron density irregularities with wavelengths 12±15 macross the geomagnetic ®eld lines. Analysis of theDoppler measurement data shows the appearance ofquasiperiodic variations with a Doppler frequency shift,fd and periods about 100±120 s during the heating cyclecoinciding in time with the ®rst substorm activation andinitiation of the upward ®eld-aligned currents. A rela-tionship between wave variations in fd and magneticpulsations in the Y-component of the geomagnetic ®eldat Tromsù was detected. The analysis of the magnetic®eld variations from the IMAGE magnetometer stationsshows that ULF waves occurred, not only at Tromsù,but in the adjacent area bounded by geographicallatitudes from 70.5° to 68° and longitudes from 16° to27°. It is suggested that the ULF observed can resultfrom superposition of the natural and heater-inducedULF waves. During the substorm expansion a strongstimulated electromagnetic emission (SEE) at the thirdharmonic of the downshifted maximum frequency wasfound. It is believed that SEE is accompanied byexcitation of the VLF waves penetrating into magneto-sphere and stimulating the precipitation of the energeticelectrons (10±40 keV) of about 1-min duration. This isdue to a cyclotron resonant interaction of naturalprecipitating electrons (1±10 keV) with heater-inducedwhistler waves in the magnetosphere. It is reasonable to

suppose that a new substorm activation, exactly aboveTromsù, was closely connected with the heater-inducedprecipitation of energetic electrons.

Key words. Ionosphere (active experiments;ionosphere ± magnetosphere interactions). Radioscience (nonlinear phenomena).

1 Introduction

Between 1980±1997 a large number of ionosphericmodi®cation experiments have been performed usingthe HF heating facility located at Ramfjùrdmoen nearTromsù, Norway (Rietveld et al., 1993; Stubbe, 1996).Many e�orts have been made to study experimentallyand theoretically the ionospheric modi®cation producedby powerful HF radio waves. These studies wereconcerned with di�erent topics such as large- andsmall-scale ®eld-aligned ionospheric irregularities (Djuthet al., 1985; Noble et al., 1987), stimulated electromag-netic emissions (Thide et al., 1982; Stubbe et al., 1984;Leyser et al., 1989; Leyser et al., 1990; Stubbe andKopka, 1990), HF wave anomalous absorption partic-ularly for heater frequencies near harmonics of theelectron gyrofrequency (Stocker et al., 1993; Robinsonet al., 1996), and secondary electromagnetic waves atvery, extremely and ultra low frequencies by amplitude-modulated heating (Rietveld et al., 1993; Stubbe, 1996).

It should be noted that in the heating experimentsperformed at Tromsù to date the plasma physics aspecthas clearly dominated. The geophysical aspect has notbeen adequately explored. In this connection the studyof Yeoman et al. (1997) should be noted. It shows thatthe combination of the latest ionospheric radars and theTromsù HF heater provide a powerful new tool foractive geophysical researches.

Correspondence to: N. F. BlagoveshchenskayaE-mail: [email protected]

Ann. Geophysicae 16, 1212±1225 (1998) Ó EGS ± Springer-Verlag 1998

Nonetheless, taking into account the distinctivebehavior and features of the auroral ionosphere suchas the ionospheric convection, ®eld-aligned currents,intense electrojets, precipitating particles one wouldexpect that the heating of this medium can lead to thegeneration of modi®cation e�ects not encountered indayside undisturbed ionosphere.

Here we report on the geophysical phenomenaobserved in the Tromsù heating experiment in thenightside auroral ionosphere during a magnetosphericsubstorm on 17 February 1996. We used HF long-distance diagnostic tools located near St. Petersburgcombined with multi-instrument observations at Trom-sù.

2 Experimental techniques and observations

The experiment was conducted at the Tromsù heatingfacility (69.6°N, 19.2°E, L � 6.2) from 20.00 UT on 17February, 1996 in the nightside auroral ionosphere. Themain technical parameters of the Tromsù heater duringthe experiment were the following: heater frequencyf � 4040 kHz, O-mode polarization, e�ective radiatedpower ERP � 150 MW, transmission scheme 4 min on/ 6 min o�. The ionospheric modi®cation was producedin the near ®eld-aligned direction, as the antenna beamwas tilted 6° to the south with respect to the zenith. Themagnetic ®eld inclination angle at Tromsù is close to78°.

To study phenomena initiated by the action ofpowerful HF radio waves in the nightside auroralionosphere during substorm we used the distance-diagnostic HF tools located in St. Petersburg at adistance about 1200 km from the Tromsù heater.Doppler measurements of HF diagnostic signals werecarried out on the London ± Tromsù ± St. Petersburgpath using ®xed operational frequency of 12095 kHz.The diagnostic transmitter was located in London. Thereception of HF diagnostic waves scattered of ®eld-aligned arti®cial irregularities produced by the Tromsùheater was made by Doppler spectrum method at St.Petersburg. Spectral processing was carried out by theuse of the fast Fourier transform (FFT) method. Theanalysis bandwidth used was 33 Hz, a speci®ed numberof 1024 FFT coe�cients allowed a frequency resolutionof 0.032 Hz. The receiving antenna was directed towardTromsù.

Doppler measurements were supplemented by datafrom the Tromsù dynasonde (Sedgemore et al., 1998 andreferences therein), IMAGE magnetometer network(LuÈ hr, 1994) and airglow at Skibotn.

Figure 1 shows the general view of the experimentgeometry (Fig. 1a) and locations of the IMAGE mag-netometers (Fig. 1b).

Figure 2 presents the dynamic Doppler spectra(sonogram) of the HF diagnostic signals at 12095 kHzon the London ± Tromsù ± St. Petersburg path in thecourse of the heating experiment on 17 February, 1996.The ®eld-aligned scattered signals appear after theheater is turned on and disappear after it is turned o�.

They are registered as additional tracks on the negativepart of Doppler sonogram shifted by about )2.5 Hzfrom that of the direct signal propagating from thetransmitter to the receiver along a great-circle path andcorresponding to the 0 Hz Doppler frequency. FromFig. 2 it is clear that scattered signals are observedduring all heating cycles from 20.00 to 20.46 UT.

Analysis of the Tromsù dynasonde ionograms, aswell as EISCAT UHF radar data, during the heatingexperiment on 17 February, 1996 shows the presence ofsporadic Es ± layer at altitudes between 100 and 130 kmsometimes with 0- and X-mode traces on the dynasondeionograms and maximal observed frequencies from 4.1to 4.5 MHz. We may conclude that the Es region of theauroral ionosphere was actually the region which washeated. The heating leads to the generation of intense

Fig. 1a, b. General view of the experiment geometry: a the radio pathwhere the experimental observations of HF distance diagnostic signalsscattered from arti®cial ®eld-aligned irregularities were made;b locations of the IMAGE magnetometer stations used in this study

N. F. Blagoveshchenskaya et al.: Geophysical phenomena during an ionospheric modi®cation experiment at Tromsù, Norway 1213

arti®cial ®eld-aligned irregularities (striations) in the E-region of ionosphere responsible for ®eld-aligned scat-tering of diagnostic HF signals on the London ±Tromsù ± St. Petersburg path.

Figure 3a±c shows the behavior of the magnetic ®eldvariations from IMAGE magnetic stations in the X, Yand Z components, respectively, on 17 February, 1996.The ®rst substorm activation started at 20.00 UT withan increase of negative magnetic perturbations (Fig. 3a)accompanied by large positive bays in the Y-compo-nents (Fig. 3b). The IMAGE X and Z componentfeatures (Fig. 3a, c) show that a maximum westwardelectrojet appeared poleward from Tromsù betweenSOR and BJN magnetic stations. It is well known(Rostoker, 1996; Kamide and Baumjohan, 1993) thatpositive Y component bays are detected near thewestern edge of the surge form produced by ionosphericHall currents surrounding the region of the upward®eld-aligned currents (FACs) in the head of the surge.Note that FACs were turned on (or greatly intensi®ed)at 20.00 UT and turned o� at 20.33 UT as can be seenfrom the appearance and disappearance of the positiveY component bays at the IMAGE magnetic stations(Fig. 3b).

The second substorm activation started at 20.33 UTas a large negative spike in the X component and peakedat 20.34 UT from SOR to PEL magnetic stations(Fig. 3a). An inspection of peculiarities in the X and Zcomponents at the IMAGE network (Fig. 3a, c) indi-cates that a new westward electrojet appeared exactlyabove Tromsù (maximum amplitude of negative spike inthe X component and dZ � 0). Thereafter a much largersubstorm occurred north of Tromsù at about 20.36 UTand peaked at 20.38 UT in the HOR magnetic station(Fig. 3a).

3 Results and discussion

An examination of dynamic Doppler spectra (Fig. 2) incourse of the heating experiment on 17 February showsthe following distinctive heating signatures during theauroral substorm.

Firstly, the occurrence of wave variations in theDoppler frequency shift, fd, with periods about 100±120 s was observed during the ®rst heating cycle (20.00±20.04 UT), coinciding in time with the onset of thesubstorm and initiation of the ®eld-aligned currents.

Secondly, the occurrence of the electromagneticemissions observed everywhere over the analyzed spec-tral bandwidth in the two heating cycles from 20.30 to20.34 UT and 20.40±20.44 UT can be clearly seen.

Thirdly, the appearance of additional very intenseshort-lived tracks with a lifetime of order 60 s (apartfrom the main Doppler heating track on sonogram)

Fig. 2. The dynamic Doppler spectra (sonogram) of the HF distancediagnostic signals at operational frequency 12095 kHz on theLondon ± Tromsù ± St. Petersburg path in the course of the heatingexperiment on 17 February, 1996. The direct signals propagating fromthe transmitter to the receiver along a great circle path correspond tothe 0 Hz Doppler frequency. Additional tracks on the negative part ofDoppler sonogram shifted by about )2.5 Hz from that of the directsignals correspond to the ®eld-aligned scattered signals. The intervals,when the Tromsù heating facility was turned on, are marked on thetime axis

1214 N. F. Blagoveshchenskaya et al.: Geophysical phenomena during an ionospheric modi®cation experiment at Tromsù, Norway

associated with the second substorm activation weredetected in the same heating cycles where emissions wereobserved.

To study the geophysical phenomena during aheating experiment it is advantageous to use thevariations of X and Y magnetic components from thequiet level. Figure 4 presents these variations obtainedfrom IMAGE magnetometers located along the latitudefrom AND to KEV (Fig. 4a) as well as along thelongitude from BJN to OUJ (Fig. 4b).

3.1 Wave processes in the onset of the substorm

The interesting peculiarity of the ®eld-aligned scatteredHF signals on London ± Tromsù ± St. Petersburg pathis the appearance of the wave variations in Dopplershift, fd, with periods about 100±120 s during the ®rstheating cycle from 20.00 to 20.04 UT (Fig. 2). The

amplitude of this ULF wave determined as the maxi-mum ¯uctuation of fd during the heating cycle (fdmax ±fdmin) was about 0.8 Hz. In the next heating cycles thevariations of fd on the scattered signal are absent. Thedirect HF signal from London to St. Petersburg doesnot contain such variations during the whole intervalanalyzed.

It is of interest to compare the observed ionosphericsmall-scale wave process and magnetic variation data inTromsù. There is evidence of the appearance of well-de®ned magnetic pulsations in the Y component of thegeomagnetic ®eld (Fig. 4a) during the ®rst heating cycle(20.00±20.04 UT). They have periods about 100±120 sand a large amplitude, of order 15±20 nT. Detailedcomparison of the wave variations in Doppler shift, fd,and magnetic (in the Y component) pulsations atTromsù during the ®rst heating cycle is shown inFig. 5 where close correlation between ionospheric andmagnetic pulsations can be seen.

Fig. 3a±c. The behavior of the magnetic ®eld variations from the IMAGE magnetometer stations: a X component; b Y component; c Zcomponent


It is appropriate to estimate the parameters of a ULFwave from Doppler measurement data. The relationshipof ionospheric (fd) and magnetic ®eld (Y component)¯uctuations corresponds to correlation between varia-tions in the meridional, north-south component of theelectric ®eld (Ex) and zonal, east-west component of themagnetic ®eld (By) in the arti®cially modi®ed iono-spheric Es region produced by Tromsù heating facilityduring substorm onset. In this case on the assumptionthat electron drift in the E region is a pure E ´ B drift,from Doppler measurement data the vertical componentof the Doppler velocity, the values of the verticaldisplacements of the arti®cially disturbed region of theionosphere, and electric ®eld of a ULF wave can beestimated.

The Doppler velocity Vd is given by

Vd � ÿ fd max � c2f � cos h ;

where fdmax is the maximum Doppler frequency shift, fis the operational frequency, h is the incidence angle ofthe diagnostic wave on the arti®cially disturbed region,and c is the velocity of light.

Knowing the vertical component of the Dopplervelocity Vd and periods T of oscillations of the Dopplerfrequency shift, one can estimate the values of thevertical displacement M of the heated region of theionosphere as

M � T � Vd :

The electric ®eld (north-south component) would beexpressible as

Ex � Vd � B0

cos I;

where I is the angle of the magnetic declination andB0 is the Earth's magnetic ®eld.

Rough estimations using the valuesfdmax � �0.4 Hz, f � 12095 kHz, cos h � 1,T � 115 s, B0 � 50 lT, I � 78° give Vd � �5.2 m/s, M � �600 m and Ex � �1.3 mV/m.

It should be pointed out that the distinctive feature ofthe experiment is the use of continuous heating insteadof amplitude-modulated heating of the ionosphere. Thismeans that the duration of the heating cycle wassigni®cantly larger than the periods of the ionospheric(in fd) and magnetic pulsations observed in the describedexperiment. Consequently, the origin of the ionosphericand magnetic pulsations could not be explained bymodulation of the conductivity and, therefore, thecurrent distribution ¯owing due to amplitude-modulat-ed heating of the E-region. Thus, one can conclude thationospheric and magnetic pulsations are possibly theresult of modi®cation of the speci®c nightside auroralionosphere, by action of powerful HF radio waves onthe Es region or they are natural pulsations duringauroral substorm onset.

It would be expected that the substorm activation isaccompanied by the generation of Pi2 magnetic pul-sations of 40±150 s. In this case the ionospheric andmagnetic pulsations observed during the ®rst heatingcycle could be signatures of natural pulsations. Itshould be noted that Yeoman et al. (1997) studiedwave activity in the ULF band with CUTLASS radarand IMAGE magnetic stations, during the Tromsùexperiment, of F2 region heating in the early eveninghours, and attributed these to naturally occurring ULFwaves.

To study this point in our heating experiment themagnetic ®eld variations in X and Y components wereconsidered from all IMAGE magnetometer stationsduring the onset of the substorm activation. The ULFwaves in magnetic ®eld measurements are shown inmore detail in Fig. 6, which displays X and Y compo-nents from eight IMAGE magnetometers, coveringgeographical latitudes from 74.5°±64.5° and geograph-ical longitudes from 16°±27°, bandpass ®ltered between120 and 80 s. ULF waves can be seen both in theun®ltered (Fig. 4) and ®ltered (Fig. 6) magnetic data.An ULF wave is most pronounced in the ®rst heatingFig. 3c.


cycle from 20.00±20.04 UT in the area boundedby geographical latitudes from 70.5° (SOR) to 68.02°(MUO) and geographical longitudes from 16.03°(AND) to 27.01° (KEV). This area can be identi®edwith a region covered by the upward ®eld-alignedcurrents. After the Tromsù heater was turned on at20.00 UT the abrupt increase of the oscillation ampli-tude in the X as well as in the Y component started.Note that irregular pulsations in the Pi2 range appearedonly slightly in ®ltered magnetic data (Fig. 6) over thewhole time interval analyzed. During the ®rst heatingcycle the period and phase of the ULF wave arerelatively constant with latitude from SOR to the southin both magnetic components, that is the characteristicsignature of a cavity resonance (McDiarmid and Allan,1990). Maximum amplitude (peak-to-peak) from un®l-

tered magnetic data up to 30 nT in X and 20 nT in Ycomponents can be seen from TRO, SOR and MASmagnetic data but they were heavily attenuated withlatitude, particularly in the X component. It should benoted that the ratio between X and Y componentamplitudes is more than one, X/Y > 1, only in a narrowlatitudinal region including magnetometer stationsAND, TRO, SOR, MAS and KEV that does notcontrast with the ®eld-line resonance (Yeoman et al.,1991). The resonance width was 1° latitude (110 km). Tothe south, from this narrow latitudinal region, the ratiois less than one X/Y < 1. It is of interest to estimate therelations between Y-component amplitudes within thewidth of a ®eld-line resonance. The ratios of SOR,MAS, AND and KEV Y component amplitudes to TROY component amplitude for the ®rst and second

Fig. 4a, b. Perturbations of the X and Y magnetic ®eld componentsfrom the quiet level during heating experiment on 17 February 1996for IMAGE magnetometers, a along the latitude from AND to KEV

and b along the longitude from BJN to OUJ (see Fig. 1b). Theintervals when the Tromsù heater was turned on are marked on thetime axis


maximums accordingly during the heater on periodfrom 20.00±20.04 UT are the following: YSOR/YTRO � 1.0; 0.7, YMAS/YTRO � 1.0; 0.8, YAND/YTRO � 0.4; 0.4, YKEV/YTRO � 0.6; 0.6.

There is good reason to consider the polarizationparameters (X-Y plane) from eight IMAGE magnetom-eter stations for the time interval containing the ®rstheating cycle shown in Fig. 7. One can recognize thatpolarizations are clockwise (CW) to the south andanticlockwise (AW) to the north of the TRO latitude.Further examination shows the change of the polariza-tion sense from CW to AW in the course of the ®rstheater on period observed in the restricted longitudinalregion at the TRO latitude, including TRO, MAS, SORand KEV magnetometers. It should be noted that thenearest AND station, located to the west from TRO, did

not show a change of the polarization sense. In addition,the most drastic change of polarization was ®rstobserved at TRO station and thereafter the polarizationsense changes occurred at MAS, SOR and KEVmagnetometer stations.

Let us consider the possible generation mechanismsof the ULF observed during substorm onset. Thepulsation behavior from IMAGE magnetometer sta-tions shows that it is not the typical signature of naturalirregular Pi2 magnetic pulsations. On one hand therelation X/Y > 1 was observed in a narrow latitudinalregion around Tromsù that is not contrary to thedevelopment of the ®eld line resonance (Yeoman et al.,1991). On the other hand the period and phase arerelatively constant with latitude, as expected for a cavityresonance (McDiarmid and Allan, 1990). Moreover the

Fig. 4b.


abrupt changes of the ULF characteristics, such asamplitude and the change of the polarization sense areclearly visible during the ®rst heater-on period. It shouldbe noted that substorm onset and generation of theupward ®eld-aligned currents, and, correspondingly, theestablishment of the Birkeland current wedge wereobserved in temporal coincidence with the ®rst heatingcycle.

In our opinion the observed pulsations can resultfrom superposition of natural and heater-induced ULFwaves. The generation mechanism of the heater-inducedULF wave may be the following. The powerful HFradio wave, in the lower ionosphere, locally modi®es theionospheric conductivity. In our experiment it would beexpected that the strong modi®cation of the ionosphericconductivity resulted because the pump wave wasre¯ected from Es layer. The region of the enhancedionospheric conductivity is polarized in the backgroundelectric ®eld, and the polarization electric ®eld propa-gates into the magnetosphere along the magnetic ®eldlines as the ®eld of the outgoing AlfveÂ n wave (Kan andSun, 1985; Lysak, 1990; Borisov et al., 1996). Theproblem of the AlfveÂ n wave generation over a circle-likeinhomogeneity of the disturbed conductivity, where thebackground ®eld-aligned current is upward, has beenstudied by Kozlovsky and Lyatsky (1997). From theseresults it may be deduced that an outgoing AlfveÂ n waveis generated from a heater-induced inhomogeneity of theenhanced conductivity. The ®eld-aligned current of thiswave is directed as background upward current due to

the total ®eld-aligned current increases. The ®eld-aligned current of the AlfveÂ n wave, re¯ected from themagnetosphere, would provide the downward ®eld-aligned current that reduces the background upwardFAC. As a result the additional ®eld-aligned currentsassociated with the heater-induced AlfveÂ n wave wouldmodulate the background upward FAC in the surge siteand lead to the appearance of the magnetic andionospheric pulsations. The period of these pulsationsis determined by the AlfveÂ n wave travel time betweenthe ionosphere and magnetosphere. It is exactly thatobserved in our heating experiment for L � 6.2 (from100±120 s). Note that the ULF wave was observed notonly at Tromsù but at other magnetometer stationslocated in the area of the background upward FAC aswell. The e�ect of the background FAC modulation fallso� with distance from Tromsù. It can be clearly seenfrom the relations between SOR, MAS, KEV and ANDY component amplitudes and TRO Y componentamplitude (peak-to-peak) during heater-on period. Weemphasize that the geophysical conditions (substormonset, ®eld-aligned currents, the presence of Es-layer)during the heating experiment signi®cantly controlthe occurrence of magnetic and ionospheric pulsa-tions.

Thus, we conclude that the observed ULF wave inthe Pi2 range can result from modi®cation of naturalULF wave by the heater-induced AlfveÂ n wave duringsubstorm onset and initiation stage of the backgroundupward ®eld-aligned currents.

3.2 Phenomena during the second activation

From Fig. 2 it can be seen that the heater switching onat 20.30 UT leads to the appearance of ®eld-alignedscattered HF signals as seen in the previous heatingcycles. They are displaced from the direct HF signals byabout )2.5 Hz. Then after about 60 s very strongemissions were observed everywhere over the analyzedspectral bandwidth. Thereafter, at 20.33 UT, theadditional very intense short-lived track with an averageexistence of order 60 s appears apart from the mainheating track on the sonogram. Its Doppler frequencyshift is )0.8 Hz from the main ®eld-aligned track. Theheater switching o�, at 20.34 UT, is accompanied by thedisappearance of the additional short-lived track but theemission was still supported for 1 min. In the nextheating cycle both the emission and additional short-lived track are also observed. But, in this case, theemission existed only 1 min, and the lifetime of addi-tional track was about 40 s, and its fd is )0.4 Hz fromthe main ®eld-aligned track. The main ®eld-alignedDoppler track was observed throughout the heatingcycle from 20.40 to 20.44 UT.

It should be remembered that the second substormactivation started at 20.33 UT. Further examination ofthe IMAGE magnetometer data showed that a newwestward electrojet appeared exactly above Tromsù.The reversal of the Z component sign from positive(SOR) to negative (MAS) values, dZ � 0, as well as the

Fig. 5. The wave variations in Doppler frequency shift on theLondon-Tromsù-St. Petersburg path (top) and in Y component atTromsù (bottom) during the ®rst heating cycle (20.00±20.04 UT) on17 February, 1996


maximum amplitude of the negative X spikedX � ÿ150 nT are observed in TRO magnetic data(Fig. 3a, c). Therefore, the occurrence of the additionalshort-lived track on the Doppler sonogram, and theonset of the second substorm activation are related toeach other. In the heating cycle, from 20.40 to 20.44 UT,some intensi®cation of the electrojet was also observed.

The heating of the auroral Es region has excited thesmall-scale ®eld-aligned irregularities (striations) in theionospheric E region responsible for ®eld-aligned scat-tering of diagnostic HF waves. The equipment used inthe experiment on 17 February was sensitive to electrondensity irregularities with wavelengths of 12±15 macross the geomagnetic ®eld lines. The properties ofthese Es region striations can be understood within theframe of the thermal resonance instability, taking placeat the upper hybrid level (Vaskov and Gurevich, 1977;Inhester, 1982). Note, that E-region striations have beenpreviously observed only with the 1 m STARE radar

(Noble et al., 1987) and with a mobile 3 m radar (Djuthet al., 1985; Noble et al., 1987).

The other unusual phenomenon observed from HFDoppler measurements at operating frequency12095 kHz was the generation of the well-de®nedstimulated electromagnetic emission (SEE) producedby the Es region heating. It is known (ThideÂ , 1990), thatSEE results from reemission of HF induced plasmawaves as electromagnetic waves due to a wave-waveinteraction, as well as from their scattering by striations.If the pump frequency fH is not extremely close to anelectron gyroharmonic, there are strong SEE signalsdownshifted from fH with a strongly pronouncedmaximum downshifted by the lower hybrid frequencyfLH. Such downshifted maximum, fDM � fH ) fLH, inthe SEE spectra is well known (ThideÂ , 1983). It iscommon knowledge (Leyser et al., 1989) that the lowerhybrid frequency is approximately 8 kHz in the iono-sphere above Tromsù. Moreover, from SEE observa-

Fig. 6a, b. Magnetic ®eld variations from eightIMAGE magnetometer stations, bandpass ®lteredbetween 120 and 80 s for a the X component andb the Y component


tions around the second harmonic of the Tromsù heaterfrequency it was shown (Blagoveshchenskaya et al.,1998), that wide-band emissions with a pronouncedmaximum at the second harmonic of the downshiftedmaximum, 2fDM � 2(fH ) fLH), appeared. They wereregistered 1200 km from the Tromsù heater whereasmost previous SEE measurements have been madewithin 100 km (Leyser et al., 1989; 1990; Stubbe et al.,1984; 1990; ThideÂ et al., 1982; 1983). Therefore, from theresult obtained it may be concluded that the SEE at theharmonics of the downshifted maximum may occur inthe heating experiments. In our experiment opera-tional frequency (f � 12095 Hz), used for Dopplermeasurements of HF diagnostic signals, was equal tothe third harmonic of the downshifted maxi-mum 3fDM (f � 3fDM, 3fDM � 3(fH ) fLH), wherefH � 4040 kHz, fH � 8 kHz). Because of this onecan suggest that wide-band emissions on the Dopplersonogram is due to the 3DM component of the SEEspectra.

The next phenomenon, closely connected with stim-ulated electromagnetic emissions at the frequency nearthe third harmonic of the downshifted maximum, wasthe appearance of very intense short-lived tracks withdurations of 40±60 s apart from the main ®eld-alignedtracks on the Doppler sonogram. The observed phe-nomenon can be qualitatively interpreted by thefollowing. Ionospheric parameters obtained by theTromsù dynasonde and EISCAT UHF radar showthe presence of a sporadic Es layer during the secondsubstorm activation. In the same time the auroral arcwas observed from airglow data. It is a matter ofgeneral experience that these structures formed by thetypical electron energies from 1 to 10 keV (BoÈ singeret al., 1996). On the other side, the strong HF heater-induced plasma waves can accelerate thermal electrons.Moreover, the generation of the strong SEE wasobserved. Perhaps one can concede that SEE at thefrequency 3fDM is accompanied by the excitation ofVLF waves. It is known (James et al., 1990; Kimura

Fig. 6b.


et al., 1994) that VLF waves in the whistler mode areable to propagate into the upper ionosphere andmagnetosphere. In this case it would be expected thatthe appearance of the stimulated precipitation ofenergetic electrons with energies between 10 and40 keV would occur due to a cyclotron resonantinteraction of natural precipitating electrons (1±10 keV) with heater-induced whistler waves in themagnetosphere. It was shown by Trefall et al., (1975)that a cyclotron generation mechanism is operating in a

narrow region of a magnetic ¯ux tube. Therefore, twoelectron populations involving natural soft electrons (1±10 keV) and HF heater-induced hard electrons (10±40 keV) could take place during the heating experimentin the substorm maximum. There are indications thatonly a hard electron component took part in the shortperiod electron precipitation as observed in a study of apulsating auroral arc by BoÈ singer et al., (1996). Thus,the analysis of experimental data shows the possibilityof the excitation of the stimulated electron precipitation

Fig. 7a, b. Hodographs presenting the time evolution of the Y-Xpolarization surfaces on 17 February, 1996 for the time interval from19.57 to 20.06 UT containing the ®rst heating cycle (from 20.00±20.04UT) from IMAGEmagnetometers, a along the latitude from AND to

KEV and b along the longitude from BJN to OUJ (see Fig. 4a, b).The Tromsù heater turning on and o� are marked by circles andrectangles correspondingly


in the heating modi®cation of the auroral Es region.From the result obtained it may be concluded that anew substorm activation exactly above Tromsù wasintimately associated with the heater-induced precipi-tation of energetic electrons. The critical requirementsof a similar stimulated precipitation (auroral trigger)are the presence of the ®eld-aligned arti®cial irregular-ities in the E-region, stimulated electromagnetic emis-sions, possibly closely connected with heater-inducedVLF waves and natural precipitation of the softelectrons.

4 Conclusions

We have studied phenomena initiated by the action ofpowerful HF radio waves at Tromsù in the nightsideauroral Es region during the magnetospheric substormon 17 February, 1996. The analysis is based onexperimental data from distance-diagnostic HF toolslocated in St. Petersburg. Doppler measurements of HFdiagnostic signals were carried out on the London ±Tromsù ± St. Petersburg path using a ®xed operatingfrequency of 12095 kHz. Furthermore, multi-instrumentdata from Tromsù dynasonde, IMAGE magnetometernetwork, EISCAT incoherent scatter radar, were used.

Fig. 7b.


The heating of the auroral ES region excited the small-scale ®eld-aligned irregularities in the ionospheric Eregion with wavelength of the order 12±15 m across thegeomagnetic ®eld lines. These irregularities then act asan arti®cially produced target for the diagnostic HFwaves on the London ± Tromsù ± St. Petersburg pathand are responsible for ®eld-aligned scattered diagnosticsignals.

Magnetic data from IMAGE network show that the®rst substorm activation started at 20.00 UT andcoincided exactly with the Tromsù heater turning on.The substorm onset was accompanied by developmentof the upward ®eld-aligned currents (FACs) at thewestern edge of the westward electrojet. The secondsubstorm activation started at 20.33 UT as a largenegative spike in the X component and peaked at 20.34UT with a maximum westward electrojet exactly aboveTromsù. Thereafter, a much larger substorm occurrednorth of Tromsù at about 20.36 UT.

The interesting peculiarity of the ®eld-aligned scat-tered HF signals is the appearance of wave variationsin Doppler frequency shift, fd, with periods about 100±120 s and amplitude about 0.8 Hz during the substormexpansion onset. These ionospheric pulsations closelycorrelated with magnetic pulsations in the Y compo-nent of the geomagnetic ®eld at Tromsù, falling in thePi2 range and with a large amplitude of the order 15±20 nT. The relationship of the ionospheric (fd) andmagnetic ®eld (Y component) ¯uctuations correspondsto correlation between variations in the meridional,north-south component of the electric ®eld (Ex) andzonal, east-west component of the magnetic ®eld (By)in the arti®cially modi®ed ionospheric Es region.Parameters of the observed ULF wave were estimatedfrom Doppler measurement data. They are the follow-ing: the vertical displacements of the heated Es regionM � �600 m and the north-south component ofelectric ®eld Ex � �1.3 mV/m. The analysis of themagnetic ®eld variations from the IMAGE stationsshows that ULF waves occurred not only at Tromsùbut in the adjacent area bounded by geographicallatitudes from 70.5° to 68° and longitudes from 16° to27°. This area can be identi®ed with region covered bythe upward ®eld-aligned currents. Moreover it wasfound the change of the polarization sense (X ) Yplane) from clockwise to anticlockwise in the course ofthe ®rst heater-on period observed in a narrowlatitudinal region around Tromsù (110 km). It doesnot contrary to the development of the ®eld lineresonance. We conclude that the observed ULF wavein the Pi2 range can be result from strong modi®cationof natural ULF wave by the heater-induced AlfveÂ nwave. It may be deduced that AlfveÂ n wave is generatedfrom a heater-induced inhomogeneity of the enhancedconductivity during substorm onset and initiation stageof the background upward ®eld-aligned currents(FACs). As a result the additional ®eld-aligned cur-rents of this AlfveÂ n wave would modulate the back-ground upward FAC in the surge site. The e�ect of thebackground FAC modulation falls o� with distancefrom Tromsù.

During substorm expansion we found the well-de®ned stimulated electromagnetic emissions (SEE) atthe third harmonic of the downshifted maximumfrequency (3fDM). The next phenomenon, closely con-nected with this SEE, was the appearance of very intenseshort-lived tracks (40±60 s) on Doppler sonogram apartof the main ®eld-aligned tracks caused by the arti®cial®eld-aligned small-scale irregularities (striations) in theES region during heater-on periods. It was suggested,that the SEE at the frequency of 3fDM is accompaniedby the excitation of VLF waves, which in whistler modeare able to penetrate into the magnetosphere. Then, acyclotron resonant interaction of natural precipitatingelectrons (1±10 keV) with heater-induced whistler wavesin the magnetosphere leads to stimulated precipitationof more hard electrons with energies 10±40 keV, re-sponsible for the appearance of additional short-livedtracks on Doppler sonogram. We conclude that thesecond substorm activation exactly above Tromsù wasintimately associated with the heater-induced precipita-tion of energetic electrons.

Results of the Tromsù heating experiment in thenightside auroral ES region clearly show the evidence onthe modi®cation of the ionosphere-magnetosphere cou-pling during magnetospheric substorm, produced bypowerful HF radio waves. The distinctive behavior ofthe auroral ionosphere as well as ®eld-aligned currents,precipitating particles, ionospheric convection, andsubstorm current wedge system signi®cantly controlthe type and properties of observed phenomena. It mustbe underscored that the heating of the auroral iono-sphere, essentially during magnetospheric substorms,leads to the generation of the new phenomena includingtrigger e�ects not encountered in the dayside, undis-turbed ionosphere.

Acknowledgements. We would like to thank the Director and Sta�of the EISCAT Scienti®c Association. EISCAT is an InternationalAssociation supported by Finland (SA), France (CNRS), theFederal Republic of Germany (MPG), Japan (NIPR), Norway(NFR), Sweden (NFR) and the United Kingdom (PPARC). Thiswork was supported by grants from the Norwegian ResearchCouncil and the Ministry of foreign a�airs of Norway (The BarentsProgramme). Russian authors are also grateful to Russian Foun-dation of Fundamental Researches, grant 97-05-65443. Ari Vil-janen of the Finnish Meteorological Institute (FMI) kindlysupplied the IMAGE magnetometer data. The authors thank M.Kosch for airglow data.Topical Editor D. AlcaydeÂ thanks A. Stocker and another

referee for their help in evaluating this paper.

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Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway