VOL. 79, NO. 16 JOURNAL OF GEOPHYSICAL RESEARCH JUNE I, 1974 VLF Wave Injection Into the MagnetosphereFrom Siple Station, Antarctica R. A. HELLIWELL AND J.P. KATSUFRAKIS RadioscienceLaboratory, Stanford University,Stanford, California 94305 Radio signalsin the 1.5- to 16-kHz range transmittedfrom Siple Station, Antarctica (L = 4), are used to control wave-particleinteractions in the magnetosphere.Observations at the conjugatepoint show signalgrowth and triggeredemissions including risers, failers, and hooks. Growth ratesof the order of 100 dB/s and total gainsup to 30 dB are observed.Triggered emissions may be cut off or havetheir fre- quencyrate of changealtered by Siplepulses. Similar effectsoccur in associationwith harmonicsfrom the Canadianpower system. The observedtemporal growth is predictedby a recentlyproposed feedback model of cyclotroninteraction. Plasma instability theories predicting only spatialgrowth are not in ac- cord with these observations.Possible applications include study of nonlinear plasma instability phenomena,diagnostic measurements of energeticparticles trapped in the magnetosphere,modification of the ionosphere through control of precipitation, and VLF communication through the magnetosphere. This is a report of the first resultsof a new experimentto plasma instability. To obtain further experimental data, a control wave-particleinteractions in the magnetosphere,us- transmitter was neededthat could operate at lower frequen- ing VLF waves.Coherent whistlermode signalswere injected cies and whose power, pulse length, and frequency could into the magnetospherein the 1.5- to 16.0-kHz range from a easily be varied. 100-kW transmitter located at Siple Station, Antarctica. The The next step was constructionof a 33.6-km-long horizon- output signalswere observedat the Sipleconjugate point near tal dipole at Byrd Station (L = 7.25) in 1965 [Guy, 1966]. This Roberval, Quebec. The signalsfollowed field-aligned ducts antenna was shared by a VLF step frequency sounder near L = 4 (seesketch in Figure la). Among the new resultsis operated by the University of Washington and a VLF the observationthat the signal receivedat Roberval generally magnetospheresounder operated by StanfordUniversity. The showsexponential growth as a function of time, with growth dipole was laid on the ice to lower its Q and henceincrease its rates of the order of 100 dB/s. Total power gains of up to 3 bandwidth. Soundings of the D and E regions were orders of magnitude (30 dB) have been observed.Following successfullycarried out over the freq'uencyrange 3-30 kHz the exponentialgrowth phase,emissions at new frequencies [Helmset al., 1968]. Whistler modetransmissions in the range are usually triggered, in the form of narrow band rising or 6-9 kHz were observed by the Stanford University VLF falling tones.A by-productof theseinvestigations is the find- receiveron the Ogo 4 satelliteas it flew over the transmitter in ing that VLF radiation at harmonics of the commercial the upper part of the F region. However, the signalswere too power systemappears to be present in the magnetosphere, weak to excite detectablewhistler mode signalsin the con- modifying the spectrum of the triggered emissions. jugate hemisphere.One reason was the low efficiencyof the In the following sectionof this paper we review briefly the low Q antenna and another was its high latitude. This high previotisexperimental work on VLF waveinjection into the latitude meant that signals from the transmitter would first magnetosphere.In the next three sectionswe describethe have to travel about 1000 km in the 1ossyearth ionosphere sitingof Siple Station, the apparatus,and someof the first ex- wave guide to reach the field lines on which propagation to perimental results.In the last sectionwe discussthese results the conjugatehemisphere was commonly observed. and some possibleapplications. SIPLE STATION PREVIOUS WORK From the Byrd experimentsit was clear that the transmitter Man-made whistler mode signals were first detected at would haveto be movedto a lower geomagneticlatitude. The Cape Horn from Station NSS on 15.5 kHz, located at An- desired site characteristics were (1) proximity to the napolis, Maryland [Helliwell and Gehrels, 1958]. Later dis- plasmapause(L • 4), (2) a stable ice sheet at least 2000 m crete VLF emissionswere observedto be triggeredby Morse thick, and (3) a conjugatepoint readily accessibleto year- code transmissions from Stations NPG on 18.6 kHz and round manned operation. A site satisfyingthese criteria was NAA on 14.7kHz [Helliwell et al., 1964].Nearly all triggering found at 76øSand 84øW (L = 4.1) and was occupiedin the was causedby the Morse dashes.When the dots did trigger, austral summerof 1969. It was named Siple Station; its con- the resultingemissions were alwaysweak falling tones [Lasch, jugate point is near Roberval, Quebec(48øN, 73øW). 1969]. Emissiontriggering by the long (• 1 s) pulsesfrom a With the aid of impedancemeasurements made on a test relatively low power (• 100 W) Omega transmitter operating dipole erectedabove the snow during the 1969-1970 austral on 10.2 kHz was also observed[Kimura, 1968]. Observations summer a 21.2-km-long operational antenna was designed during a drifting duct event showed that triggering was [Raghuramet al., 1974]. It was constructedduring the austral enhancedwhen the transmitter frequencywas near one half summers of 1970-1971 and 1971-1972. The measured reso- the equatorialgyrofrequency [Carpenter, 1968]. Theseresults nant frequency is 5.1 kHz, close to half the minimum could not be interpreted in terms of existing theories of gyrofrequency(•6 kHz) on the field line through Siple Sta- tion. The bandwidthis about 2 kHz. The radiationefficiency Copyright ¸ 1974 by the American GeophysicalUnion. at 6 kHz is estimatedto be 4%. The averageelevation of the 2511 2512 HELLIWELLAND KATSUFRAKIS.' BRIEF REPORT (a) (b) WWV PROGRAMMER RECE I V ER AND •LOOP CLOCK il TAPE PREAMP RECORDER RECEIVER ß 2O (c) GEOMAGNETIC EQUATOR TRANSMITTER ELEVATED DI POLE 2O 17000krn 21.2 krn _ I I00kW COMPUTER TELETYPE 6O AMP'ß i KEYER FREQUENCY I ! STANDARD Fig.1. (a)Sketch ofmagnetospheric pathfrom Siple Station (SI), Antarctica, toRoberval, Quebec (RO); HB is Halley Bay.(b) RobervalVLF receiverblock diagram. (c) SipleVLF transmitterblock diagram. antennaabove the snowsurface was initially 6 m, with passiveexperiments were also installed,including whistler provisionfor periodicallyraising the antenna to compensateand VLF emissionrecordings (Stanford University), magnetic for an annual snow accumulation of 1-2 m. field intensity(Bell Laboratories),magnetic pulsations A year-roundmanned facility, including the 100-kWhigh- (Universityof Minnesotaand University of NewHampshire), efficiencysolid state VLF transmitterfrom Byrd Station, was and ionosphericabsorption (University of SanDiego). constructedduring the australsummer of 1972-1973.Various Operationof the passiveexperiments began in early1973. 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"'-:-•- ":•':•':,-:-.-. "':':"::':::':":'":"..... ..-.... ..•,'•..:.....'•.... ::,::•.•........... ::•"• .. :,':........ ß.......... -.:. :.'. .... :. ...................................L7•:'::..'..... .'. ::::..•..... ::•:":•...:':...:...' :: .--•::."-:'..:,:' ß ..':.':'.:'.:"•:5-'--... :'•..'..-•-..:..::.' ............::::. '..:....:• I .........t............... :• ................. i............ •............... i ............... i ':i" •:•............. :• '"'"I ............. i .....• ....... :• '• ! ..... i.... i i I ......! 0 -'10 •O 30 sec Fig.2. Variablepulse length sequence received atRoberval. Lower panel shows the spectrum, andupper panel shows theamplitude ina 130-Hz bandwidth centered on5.5 kHz. Pulse lengths vary from 50 to 400 ms in 50-ms steps asis in- dicatedbythe numbers between panels. Atwo-hop whistler, with echoes at•3 kHz,appears at8-10 sand originates inthe sfericat 4.2 s. A strongwell-defined two-hop whistler component extending upto 5.5 kHz is seen at about 8.2 s and cor- respondsto the one-hopdelay of the Siplepulses. HELLIWELL AND KATSUFRAKIS:BRIEF REPORT 2513 Fig. 3. Pulseenvelope of the 150-, 200-, and 250-mspulses in the second18-s sequence shown in Figure2. Solid bars representthe transmittedpulses corrected in time for the one-hoptravel time of 2.02 s. The VLF transmitter was placed in servicein April 1973 and found that the period around local sunriseis often active. A has since operated an averageof 6 hours a day on different wide variety of spectralforms hasbeen observed in-the signals frequencies,with various modulation patterns and power stimulatedby the