-100 -50 0 +50

LIJ0ZwcD000

Dst(Y)

at the plasmapause. As a next step, the above re-sults will be compared with Si pie very low frequency(VLF) determinations of equatorial density profiles.

Long-lasting Pc-i events have been found in theSiple data. A spectrogram of an interesting longevent is shown in figure 1. The event has a sud-den beginning and then divides into two frequencybands, later coalescing back into one. An analysisof the dispersion of the two frequency bands sug-gests very different paths of propagation for thetwo branches. Long events often consist of pulsat-ing signals such as those seen in the upper branchof the one in figure 1. Since the Pc-i peaks theoreti-cally are amplified by a resonant interaction withenergetic protons, and because the pulsations seenin the long events are comparable to proton driftperiods, an attempt is under way to correlate Ex-plorer-45 proton measurements with such events.In the few cases studied, trapped proton pitchangle distributions after the commencement of anevent show a marked change from near isotropyto rounded, peaking at 900 • This correlation sug-gests that the particles are indeed supplying energyto the waves in resonant interaction.

About 35 long events were found in the 1973data. A statistical study of these events in correla-tion with Dst is shown in figure 2. The long dura-tion events clearly do not occur during times ofring current inflation; in fact, they appear to favortimes of +Dst, when the solar wind is apparentlycompressing the magnetosphere.

This research was supported by National ScienceFoundation grant GV-35174.

References

Dowden, R. L. 1965. Micropulsation mode propagation in themagnetosphere. Planetary and Space Science, 13: 761-772.

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Dowden, R. L., and M. W. Emery. 1965. The use of micro-pulsation "whistlers" in the study of the outer magnetosphere.Planetary and Space Science, 13: 773-779.

Liemohn, H. B., J . F. Kenney, and H. B. Knaflich. 1967. Protondensities in the magnetosphere from pearl dispersionmeasurements. Earth and Planetary Science Letters, 2: 360-366.

Taylor, W. L., B. K. Parady, P. B. Lewis, R. L. Arnoldy, andL. J . Cahill, Jr. In press. Initial results from the search coilmagnetometer at Siple, Antarctica. Journal of Geophysical Re-search.

Watanabe, T. 1965. Determination of the electron distributionin the magnetosphere using hydromagnetic whisders.Journalot Geophysical Research, 70: 5839-5848.

F-layer duct propagation ofhydromagnetic waves in the

polar cap ionosphere

R. R. HEAcOCKGeophysical InstituteUniversity of Alaska

Fairbanks, Alaska 99701

Operation of the three-component inductionmagnetometer and associated data recording sys-tems at Vostok Station (Soviet Union) continuedin cooperation with Soviet scientists of the Arcticand Antarctic Scientific Research Institute, Lenin-grad.

It is well established that some type Pc-i hydro-magnetic wave events propagate horizontally in theF-layer duct (e.g., Manchester and Fraser, 1970).Some theoretical features of duct propagation havebeen determined (Greifinger and Greifinger, 1968;P. Greifinger, 1972; Greifinger and Greifinger,1973). Fraser and Summers (1972) find that Pc-isignals propagating in the duct are approximatelylinearly polarized in the horizontal plane in thedirection of wave propagation, consistent with theprediction of Greifinger and Greifinger (1968).

Greifinger and Greifinger(1968) also predict theexistence of a low-frequency cutoff in the neighbor-hood of 0.1 to 0.4 hertz for waves propagating inthe duct. It is difficult to test that prediction byusing Pc-I data because the source mechanism, theproton cyclotron resonance instability, imposes anatural low frequency cutoff that is generally inthe range 0.2 to 2 hertz, depending on the L valueand plasma parameters at the source location.However, to this end the broadband magneticnoise designated type Pi may be used. Our archivesof pulsation data recorded at College, Alaska,clearly indicate that when College is on the Pi source

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lines, as evidenced by large amplitudes and corre-lated particle precipitation effects, the Pi fre-que ncies extend well below 0.1 hertz with moreintegrated power below 0.1 hertz than above.

Pulsation recordings taken at the polar sitesVostok and Thule disclosed many Pi events withan apparent low frequency cutoff, generally 0.1to 0.3 hertz (e.g., figure 1). To identify this fre-quency as a true low frequency cutoff for ductpropagation of Pi waves is difficult because we can-not confirm that the observed event was propa-gated to the polar sites from simultaneous Piactivity observed at auroral oval sites. In structuredPc-i events, similar features of structure permitone to identify Pc-i signals that propagate from acommon source region to two or more observingsites. Despite lack of similar verification for Pievents, we feel that the evidence strongly supportsexistence of a low frequency cutoff for events likethe one in the figure: that is, under some ionos-pheric conditions Pi signals originating at auroraloval latitudes propagate to the poles and suffer alow frequency cutoff in agreement with the predic-tion of Greifinger and Greifinger (1968). Prelimi-nary evidence suggests that poleward propagationof Pi signals is more common than equatorwardpropagation. The gradients associated with wallsof the main ionospheric trough probably are asignificant factor in inhibiting equatorward propa-gation.

The banded continous missions of Tepley andAmundsen (1965) were probably instances of Pienergy propagating in the horizontal duct to lowlatitudes. Some band-limited Pi-1 events ofMcPherron et al. (1968) may have been cases whereshort-distance propagation was involved, which re-sulted in some attenuation of frequencies below0.1 hertz.

Wave polarizations in the H-D plane for eventslike that shown in the figure were examined. Alarge scatter in the alinements of the major axesfor any given event greater than 45° was observed,though approximately linear polarization withconstant alinement was observed for some wavepackets. If this scatter indicates scatter in arrivaldirection for the Pi-1 waves (Fraser and Summers,1972), then the observed Pi-I signals probably ori-ginated over a significant local time sector of theauroral oval field lines.

This research was supported by National ScienceFoundation grant GV-41 157.

References

Fraser, B. J ., and W. R. Summers. 1972 Simultaneous ob-servations of Pc- I micropulsat ion polarization at four lowlatitude sites. Annales de Gophyoque, 28: 697.

THULE H

0.5-

Hz 03-

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Example of type Pi-1 activity with a low frequency cutoff near0.1 to 0.15 hertz. The low-frequency cutoff decreases withtime in this example. The presence of the cutoff may indi-

cate propagation of the P1-1 noise in the F-layer duct.

Greilinger, C., and P. Grei6nger. 1968. Theory of hydromag-netic propagation in the ionospheric waveguide. Journal ofGeophysical Research, 73: 7473.

Greifinger, P. 1972. Ionospheric propagation of oblique hydro-magnetic plane waves at micropulsation frequencies.Journalof Geophysical Research, 77: 2377.

Greifinger, C., and P. Greiflnger. 1973. Wave guide propaga-tion of micropulsations out of the plane of the geomagneticmeridian. Journal of Geophysical Research, 78: 1973.

Manchester, R. N., and B.J. Fraser. 1970. Occurrence of hydro-magnetic emissions at two southern hemisphere sites. Plane-tary and Space Science, 18: 291.

McPherron, R. L., G. K. Parks, F. V. Coroniti, and S. H. Ward.1968. Studies of the magnetospheric substorm, 2. Journal ofGeophysical Research, 73: 1697.

Tepley, L. R., and K. D. Amundsen. 1965. Observations ofcontinuous sub-ELF emissions in the frequency range 0.2 to1.0 cycles per second.Journal of Geophysical Research, 70: 234.

Whistler-mode VLFpropagation measurements

at Siple Station

G. J . BURTTPhysics and Engineering Laboratory

N. Z. Department of Scientfic and Industrial ResearchLower Hutt, New Zealand

For about 10 years the Physics and EngineeringLaboratory, N.Z. Department of Scientific and In-dustrial Research, has been carrying out a study ofvery low frequency (VLF) radio signal propagation

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