References

Orville, R.E., R.W. Henderson, and L.F. Bosart. 1983. An east coastlightning detection network. Bulletin of the American MeteorologicalSociety, 64(9), 1029-1037.

Park, CC., and D.L. Carpenter. 1978. Very low frequency radio waves

in the magnetosphere. In L.J. Lanzerotti and C.G. Park (Eds.), Upperatmosphere research in antarctica. (Antarctic Research Series, Vol. 29.)Washington, D.C.: American Geophysical Union.

Smith, A.J., and D.L. Carpenter. 1982. Echoing mixed-path whistlersnear the dawn plasmapause, observed by direction-finding receiversat two antarctic stations. Journal of Atmospheric and Terrestrial Physics,44(11), 973-984.

ISIS-11 satellite observations duringSiple Station very-low-frequency wave-

injection experiments

T.F. BELL and J.P. KATSUFRAKIS

Space Telecomin u nications and Radio Science LaboratoryStanford University

Stanford, California 94305

One of the critical scientific objectives of space plasma physicsis to understand the processes that couple distinct parts of theEarth's plasma environment, such as the solar wind, magne-tosphere, ionosphere, and upper atmosphere. An importantsource of coupling between the magnetosphere, ionosphere,and upper atmosphere is the flux of energetic particles whichare precipitated from the Earth's radiation belts through interac-tions with both natural and manmade very-low-frequency (VLF)

waves.One of the main goals of the Siple Station VLF wave-injection

experiments is to obtain an understanding of wave-particleinteractions by performing controlled studies in which VLF

waves are injected into the magnetosphere and radiation beltsand in which measurements of the wave and particle propertiesduring the interactions are carried out both on the ground andin space (Helliwell and Katsufrakis 1974). An important compo-nent of these experiments has been the support provided byvarious satellites, such as Explorer 45 (United States), Exos-B(Japan), ISEE-I (United States), isis-i (Canada), isis-il (Canada),and DE-i (United States). Correlative data from these satelliteshave been used to determine the characteristics of the injectedwaves and energetic particles in the ionosphere and magne-tosphere, and to establish the importance of coherent whistler-mode waves in magnetospheric wave-particle interactions (Bell,man, and Helliwell 1981; Bell et al. 1983a, 1983b; Bell, Kat-sufrakis, and James 1985; Kimura et al. 1983; Rastani, man, andHelliwell 1985; Sonwalkar and Inan 1986).

In the coming austral summer (November 1987 to February1988), isis-li spacecraft observations will be carried out duringSiple Station wave-injection experiments as part of a joint inter-national research effort involving workers from the StanfordUniversity STAR Laboratory, the Canadian Communication Re-search Center, the Japanese Research Institute of At-mospherics, Radio Research Laboratory, and National Instituteof Polar Research.

One of the goals of this study is to understand a newlydiscovered phenomenon in which high-amplitude electroststic

waves are stimulated by electromagnetic VLF whistler-modewaves propagating at low altitudes (less than 8,000 kilometers)(Bell and Ngo in press a). This phenomenon is very common atall latitudes, and theoretical models (Bell and Ngo in press b)indicate that the electrostatic waves are stimulated when theinput electromagnetic waves scatter from small scale (less than100 meters) magnetic-field-aligned plasma density irreg-ularities. It is believed that the stimulated electrostatic wavesproduce enhanced pitch angle scattering of energetic radiationbelt particles, resulting in enhanced particle precipitation. Theprecipitated flux produces plasma density enhancements in theionosphere, and upward diffusion of thermal plasma from theregions of enhanced ionospheric plasma density creates addi-tional magnetic-field-aligned plasma density irregularities inthe magnetosphere. Thus, a feedback system is establishedwhich tends to maintain plasma density irregularities andwhich provides coupling between the magnetosphere,ionosphere, and upper atmosphere.

An example of the electrostatic wave stimulation phe-nomenon is shown in the figure. This data was acquired on theisis-il spacecraft at a time when the subsatellite point was within1,000 kilometers of Siple Station. The upper spectrogram ofpanel a shows six swept-frequency transmissions from the Si-plc Station transmitter as observed on the spacecraft. The lowerpart of a shows the signals as transmitted atSiple. Because of thepresence of stimulated electrostatic waves, the bandwidth of thereceived signals lies in the range 200-500 hertz, approximatelytwo orders of magnitude larger than the nominal bandwidth ofthe transmitted signals.

The upper part of panel b shows six swept-frequency trans-missions from Siple at a slightly later time when no electrostaticwaves were stimulated. The lower part of b shows the signals astransmitted at Siple. It is clear that the bandwidths of the trans-mitted and received signals are comparable. Theory (Bell andNgo in press b) indicates that the electrostatic waves can bestimulated only when the frequency of the input wave is greaterthan the local lower-hybrid-resonance (LHR) frequency(Laaspere, Johnson, and Semprebon 1971). This requirement issatisfied for the data shown in the upper spectrogram of panel a,where the measured local LHR frequency is less than 3 kilohertz.However, the requirement is not satisfied for the data shown inthe upper spectrogram of panel b, where the measured localLHR frequency is higher than the frequency of the Siple trans-mitter pulses.

Wave injection experiment planned for the 1987-1988 australsummer will improve our understanding of the electrostaticwave stimulation phenomenon and the magnetospheric-ionospheric upper atmospheric coupling which results from it.

This research was supported in part by National Aeronauticand Space Administration grant NGL 05-020-008, and in part byNational Science Foundation grant DPP 86-13783.

286 ANTARCTIC JOURNAL

0935 UT8-

LHR

kHz

ISIS - It

ISIS - It 9 JULY 1982

kHz

kHz

>m 57.47°5 a L 3.69, c/ g = 78.4 0 W, h: 1403 km

0933 UT ISIS-It

3 '-*rY •r;

I I I

I SI(a)

''

1 ['8 - - - f - - - -

3

3-

SI(b)8—

kHz

10

15

20sec

An example of the electrostatic wave stimulation phenomenon as observed on the isis-ii satellite, a. The upper panel shows signals from theSiple Station transmitter and stimulated electrostatic waves as observed on the spacecraft. All signal frequencies are above the local LHR

frequency. The lower panel shows the signals as transmitted at Siple Station. b. The upper panel shows normal signals from the Sipletransmitter as received on the spacecraft. All signal frequencies are below the local LHR frequency. The lower panel shows the signals astransmitted at Siple Station.

References

Bell, T.F., U.S. man, and R.A. Helliwell. 1981. Nonducted coherent VLF

waves and associated triggered emissions observed on the 1SEE-1

satellite. Journal of Geophysical Research, 86-4649.Bell, IF., U.S. man, I. Kimura, H. Matsumoto, T. Mukai, and K.

Hashimoto. 1983a. EXOS-B/Siple Station VLF wave-particle interac-tion experiments: 2. Transmitter signals and associated emissions.Journal of Geophysical Research, 88, 295.

Bell, T.F., H.G. James, U.S. Inan, and J.P. Katsufrakis. 1983b. Theapparent spectral broadening of vi.F transmitter signals during tran-sionospheric propagation. Journal of Geophysical Research, 88, 4813.

Bell, T.E, J.P. Katsufrakis, and H.G. James. 1985. A new type of VLF

emission triggered at low altitude in the subauroral region by Siple

Station viE transmitter signals. Journal of Geophysical Research, 90,12183.

Bell, T.F., and H.D. Ngo. In press a. Electrostatic sideband wavesstimulated by coherent vu signals propagating in and near the innerradiation belt. Journal of Geophysical Research.

Bell, IF., and H.D. Ngo. In press b. The excitation of electrostatic wavesby electromagnetic whistler mode waves scattering from magnetic-field-aligned plasma density irregularities. Journal of GeophysicalResearch.

Helliwell, R.A., and J.P. Katsufrakis. 1974. VLF wave injection into themagnetosphere from Siple Station, Antarctica. Journal of GeophysicalResearch, 79, 2511.

Kimura, I., H. Matsumoto, T. Mukai, K. Hashimoto, T.F. Bell, U.S. Inan,R.A. Helliwell, and J.P. Katsufrakis. 1983. EXOS-B/Siple Station vi.iwave-particle interaction experiments: 1. General description andwave-particle correlations. Journal of Geophysical Research, 88, 282.

1987 REVIEW 287

Laaspere, T., W.C. Johnson, and L.C. Semprebon. 1971. Observationsof aurora! hiss, LHR noise and other phenomena in the frequencyrange 20 Hz-540 kHz on OGO-6. Journal of Geophysical Research, 76,4477.

Rastani, K., U.S. man, and R.A. Helliwell. 1985. DE-1 observations of

Siple transmitter signals and associated sidebands. Journal ofGeophysical Research, 90, 4128.

Sonwalkar, V.S., and U.S. man. 1986. Measurements of Siple transmit-ter signals on the DE-1 satellite: Wave normal direction and antennaeffective length. Journal of Geophysical Research, 91, 154.

A possible identification of very-low-frequency wave-induced precipitation

in high-frequency sounding radarmeasurements

F.T. BERKEY

Center for Atmospheric and Space SciencesUtah State University

Logan, Utah 84332-4405

M.J. JARVIS

British Antarctic SurveyNERC, Madingley Road

Cambridge, CB3 OETUnited Kingdom

Active experimentation on very-low-frequency (VLF) wave-particle interactions (Helliwell and Katsufrakis 1979) has beenthe primary purpose of research at Siple Station and manyimportant new VLF wave phenomena have been discovered as aresult of that research. While ionospheric precipitation effectsdue to triggered emissions and whistlers have been observed,precipitation effects directly attributable to the Siple VLFtransmitter have not. The range of particle energies expected tobe precipitated by the Siple transmitter extends from approx-imately 300 electronvolts to 20 electronvolts (Helliwell 1983).The high-frequency sounding radar installed at Siple Stationcan effectively monitor the whole ionosphere from approx-imately 80 to 500 kilometers, therefore offering the potential fordetecting wave-induced precipitation effects over the samerange of energies.

The mechanism causing particle precipitation in the lowerionosphere is a gyroresonant interaction between radiation beltparticles and electromagnetic waves, which results in pitchangle scattering of the energetic particles (Kennel and Petschek1966). A variety of sources for the electromagnetic waves exists,such as whistlers, triggered VLF emissions, or signals from suchmanmade sources as VLF transmitters or the 50-60 hertz-fre-quencies radiated by electric power grids. Energetic electronsprecipitated by this mechanism can cause secondary ionization,optical emissions, X-ray bursts, and heating in the ionosphereover the altitude range 80-200 kilometers [see e.g., Rosenberget al. (1971, 1981), Helliwell et al. (1973, 1980), Doolittle (1980),Doolittle et al. (1978), Doolittle and Carpenter (1983), Carpenterand man (1987), and man and Carpenter (1987)].

Doolittle (1982) has shown that wave-induced precipitationwill cause a signature in the ionosphere which can be detected

using a phase coherent high-frequency sounding radar. One ofthe results of his work shows that a perturbation in the phase ofa totally reflected high-frequency signal can be expected, ifionization due to wave-induced precipitation is produced alongthe reflected signal ray path. This signature can be detected bymeasuring the rate-of-change of phase of a reflected echo at afixed sounding frequency over a suitable interval of time.

As a consequence of the density dependence of the refractiveindex of the ionospheric plasma, changes in the phase of anordinary mode echo resulting from changes in the local densityalong the path will be in the opposite sense from the densitychange. For an extraordinary mode echo, the situation is some-what more complex; however, the phase will also change in theopposite sense from the density change when the probingsignal frequency is less than the electron gyrofrequency of themedium (Doolittle 1982). The change of phase will last as long asthe density along the path is being modified.

Until recently, no evidence for wave-induced precipitationhad been discovered in the Siple Station sounding radar dataalthough one whistler wave-associated event had previouslybeen found in high-frequency data from Halley, Antarctica (Jar-vis unpublished data). Several instances of a large decrease ofrates-of-change of phase were discovered within a 4.2 mega-hertz fixed-frequency sounding which began at 1151:36 univer-sal time on 13 November 1982. The observations were recordedduring the recovery phase of a magnetospheric substorm,which is consistent with the conditions during which all pre-vious correlations have occurred (Doolittle 1982). On 13November, the auroral electrojet index (A1) exceeded 1,000nanoteslas at 0950 universal time and total signal absorption(blackout) occurred at Siple Station between 1005 and 1120 uni-versal time.

Six events characterized by a large negative excursion of rate-of-change of phase and followed by a slow increase to a positivevalue were found and five of those events occurred during a 10-minute interval of Siple Station VLF transmissions. Note that thechange in rate-of-change of phase began at the same time theSiple transmissions started. For five of these events, the meandecrease of rate-of-change of phase was - 250° per secondwhereas the largest observed coherent decrease of rate-of-change of phase in data recorded prior to or after the events inquestion was a factor of three smaller. In figure 1, the rate-of-change of phase of 4.2 megahertz echoes and the frequencyformat of the Siple VLF transmissions has been displayed as afunction of time from the start of the sounding (1151:36 univer-sal time). Although not shown here, whistlers were recorded atPalmer Station (L equals approximately 2.4) during the Siplekey-down periods at t = 146, 163, 174, 208, and 225 seconds(Carpenter personal communication). Four clear examples of arate-of-change of phase decrease are evident in the data andthese four plus a less definitive fifth event (at t 225) have beendenoted in figure 1. Note that each value of the rate-of-change ofphase is a three-point running mean, except between events.

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