Siple Station VLF two-frequency squelch experiments
References Brittain, R., P. M. Kintner, M. C. Kelley, J. C. Siren, and D. L. Carpenter. 1983. Standing wave patterns in VLF hiss. Journal of Geophysical Research, 88, 7059-7064. Helliwell, R. A. 1965. Whistlers and related ionospheric phenomena. Stan-
Siple Station VLF two-frequency squelch experiments ROBERT A. HELLIWELL Starlab Stanford University Stanford, California 94305
A surprising property of the Earth's magnetosphere is its ability selectively to amplify coherent, as opposed to noisy, very-low-frequency (VLF) whistler-mode signals by approximately 30 decibels (10 decibel equals one order of magnitude in power) and to trigger narrowband VLF emissions (Helliwell and Katsufrakis 1974, 1978). We shall refer to this interaction as the "coherent wave instability" (cwi). Although no complete theory of the cwi is yet available, the required energy is believed to come from radiation belt electrons through the mechanism of a Doppler-shifted cyclotron resonance with circularly polarized waves (Helliwell and man 1982). Theoretical work on this problem is difficult because of the highly nonlinear (e.g., generation of new frequencies) character of the observed interactions. Experiments conducted from Siple Station have therefore continued to play a key role in this field. Because coherence of the driving signal plays a critical role in amplification and triggering, new types of modulation experiments are desirable. To meet this need a new transmitter (called "Jupiter") with AM and FM capability was placed in operation at Siple Station in 1979 to replace the smaller, less flexible Zeus transmitter. The purpose of this article is to describe a new Jupiter experiment in which two equal-amplitude signals at frequencies 11 and 12 were observed to squelch one another when 12 - 11 equals approximately 20 hertz. These new experiments extend earlier results obtained using the limited FM capability of the Zeus transmitter (Chang, Helliwell, and Bell 1980). Examples of the transmitted and received signal spectra are shown in figure 1, along with the average amplitude of the received signal; 12 is varied up and down in steps of 10 hertz between f I + 10 hertz and f + 270 hertz. The total signal output is a maximum for A f equals 0 and then drops by more than 20 decibels at A f equals approximately 20 hertz. With further increase in A f the signal recovers first at 12 and then at f . The two frequencies are virtually uncoupled for A I is greater than 100-200 hertz. As the amplitude increases, triggering rate and emission intensity both increase. AtA f equals 20-30 deci1983 REVIEW
ford, Calif.: Stanford University Press. Kintner, P. M., R. Brittain, and M. C. Kelley. 1981. Whistler mode waves above the Siple Station VLF transmitter. Antarctic Journal of the U.S., 16(5), 205. Matthews, D. L. 1981. Siple Station magnetospheric physics campaign. Antarctic Journal of the U.S., 16(5), 202.
bels both signals are suppressed roughly equally. Near this region of mutual suppression there are sidebands at multiples of A f, with the upper sidebands usually being stronger. On many occasions half-harmonic sidebands are seen, usually between 12 and 12 + A f . An example is shown in figure 2, where z f equals 40 hertz, with a half-harmonic appearing at 4,730 hertz. This effect may be connected with the self-excited sidebands, or
kHz
::
R
II OCT 82
1633UT
ff
d:
05 15 25 35 45 55s kHz i
3.4 IM
dB 20
Figure 1. Two-frequency variable separation experiment. Upper three panels show transmitted format, received spectrum, and received amplitude (300-hertz filter bandwidth), respectively. Lower three panels show the same formats but with expanded frequency and time scales.
277
RO 25 MAY 82 1210:31 UT 4710!
Af =211z Af 2 Hz
RO 25 MAY 82 1253 1 IJT Hz_ 4710 467 0 1s Af : 2 Hz
Figure 2. Half-harmonic generation by two carriers (marked by arrows on left) spaced 40 hertz apart. The two carriers and their firstorder sidebands are shown by lines connecting the dynamic spectrum on left with the average amplitude spectrum on right. The upper arrow of the right-hand spectrum marks the half-harmonic. The effective bandwidth of the analyzing filter is 2 hertz.
pulsations, recently reported in association with a single-frequency, nontriggering Siple signal (Park 1981), in which comparable modulation periods were observed. An explanation of both the sidebands and the half-harmonic can be found in two previously demonstrated effects. In one, called "gap triggering" (Chang and Helliwell 1979), a 10-millisecond gap in a single-frequency wave train launches a risingfrequency emission which is often entrained by the driving signal when it is switched back on. The result is a transient fluctuation in amplitude and frequency. A similar effect may occur at each null of the beat between 11 and 1, giving rise to a non-sinusoidal modulation of both amplitude and frequency at and 12 f . The resulting harmonics of A f are then added to f' giving rise to (and the resufling) sideband components, spaced
278
f apart, which are amplified just like the driven signals to produce the observed spectrum. The other effect is the suppression of a periodic emission by a similar preceeding emission (Brice 1965). Such suppression decreases as the time between events increases. Thus an instability may occur in which a transient increase in one emission tends to reduce the next and that in turn allows the next emission to be larger. The result is the generation of a periodic wave train at double the beat period. This idea was first used to explain the prevalence of three-phase over two-phase periodic emissions. In the present instance (see figure 2) such alternation in growth introduces a modulation component at A f/2. An understanding of these coherence effects is necessary to the development of a comprehensive model of the cwi. This work is expected to lead to new techniques for the measurement and control of plasmas. In particular, it may be possible to employ wave injection from the ground to measure electron fluxes in the radiation belts without the use of satellites. This work was supported in part by National Science Foundation grant DPP 80-22282 and DPP 80-22540. The manuscript was prepared by K. Dean. The author acknowledges the contribution of Siple Station winter engineers Tom Wolfe (November 1981 to January 1983), John Green (November 1982 to present), and Dave Shafer (November 1982 to present).
References Brice, N. M. 1965. Multiphase periodic very-low-frequency emissions. Radio science, 69D, 157, Chang, D. C. D., and R. A. Helliwell. 1979. Emission triggering in the magnetosphere by controlled interruption of coherent VLF signals. Journal of Geophysical Research, 84, 7170. Chang, D. C. D., R. A. Helliwell, and T. F. Bell. 1980. Side-band mutual interactions in the magnetosphere. Journal of Geophysical Research, 85, 1703. Helliwell, R. A., and U. S. man. 1982. VLF wave growth and discrete emission triggering in the magnetosphere: A feedback model. Journal of Geophysical Research, 87, 3537. Helliwell, R. A., and J. P. Katsufrakis. 1974. VLF wave injection into the magnetosphere from Siple Station, Antarctica. Journal of Geophysical Research, 79, 2511. Helliwell, R. A., and J . P. Katsufrakis. 1978. Controlled wave-particle interaction experiments. In L. J . Lanzerotti and C. C. Park (Eds.), Upper Atmosphere Research in Antarctica, (Antarctic Research Series, Paper 5, Vol. 29). Washington, D.C.: American Geophysical Union. Park, C. C. 1981. Generation of whistler-mode sidebands in the magnetosphere. Journal of Geophysical Research, 86, 2286.
ANTARCTIC JOURNAL
Siple Station VLF two-frequency squelch experiments ROBERT A. HELLIWELL Starlab Stanford University Stanford, California 94305
A surprising property of the Earth's magnetosphere is its ability selectively to amplify coherent, as opposed to noisy, very-low-frequency (VLF) whistler-mode signals by approximately 30 decibels (10 decibel equals one order of magnitude in power) and to trigger narrowband VLF emissions (Helliwell and Katsufrakis 1974, 1978). We shall refer to this interaction as the "coherent wave instability" (cwi). Although no complete theory of the cwi is yet available, the required energy is believed to come from radiation belt electrons through the mechanism of a Doppler-shifted cyclotron resonance with circularly polarized waves (Helliwell and man 1982). Theoretical work on this problem is difficult because of the highly nonlinear (e.g., generation of new frequencies) character of the observed interactions. Experiments conducted from Siple Station have therefore continued to play a key role in this field. Because coherence of the driving signal plays a critical role in amplification and triggering, new types of modulation experiments are desirable. To meet this need a new transmitter (called "Jupiter") with AM and FM capability was placed in operation at Siple Station in 1979 to replace the smaller, less flexible Zeus transmitter. The purpose of this article is to describe a new Jupiter experiment in which two equal-amplitude signals at frequencies 11 and 12 were observed to squelch one another when 12 - 11 equals approximately 20 hertz. These new experiments extend earlier results obtained using the limited FM capability of the Zeus transmitter (Chang, Helliwell, and Bell 1980). Examples of the transmitted and received signal spectra are shown in figure 1, along with the average amplitude of the received signal; 12 is varied up and down in steps of 10 hertz between f I + 10 hertz and f + 270 hertz. The total signal output is a maximum for A f equals 0 and then drops by more than 20 decibels at A f equals approximately 20 hertz. With further increase in A f the signal recovers first at 12 and then at f . The two frequencies are virtually uncoupled for A I is greater than 100-200 hertz. As the amplitude increases, triggering rate and emission intensity both increase. AtA f equals 20-30 deci1983 REVIEW
ford, Calif.: Stanford University Press. Kintner, P. M., R. Brittain, and M. C. Kelley. 1981. Whistler mode waves above the Siple Station VLF transmitter. Antarctic Journal of the U.S., 16(5), 205. Matthews, D. L. 1981. Siple Station magnetospheric physics campaign. Antarctic Journal of the U.S., 16(5), 202.
bels both signals are suppressed roughly equally. Near this region of mutual suppression there are sidebands at multiples of A f, with the upper sidebands usually being stronger. On many occasions half-harmonic sidebands are seen, usually between 12 and 12 + A f . An example is shown in figure 2, where z f equals 40 hertz, with a half-harmonic appearing at 4,730 hertz. This effect may be connected with the self-excited sidebands, or
kHz
::
R
II OCT 82
1633UT
ff
d:
05 15 25 35 45 55s kHz i
3.4 IM
dB 20
Figure 1. Two-frequency variable separation experiment. Upper three panels show transmitted format, received spectrum, and received amplitude (300-hertz filter bandwidth), respectively. Lower three panels show the same formats but with expanded frequency and time scales.
277
RO 25 MAY 82 1210:31 UT 4710!
Af =211z Af 2 Hz
RO 25 MAY 82 1253 1 IJT Hz_ 4710 467 0 1s Af : 2 Hz
Figure 2. Half-harmonic generation by two carriers (marked by arrows on left) spaced 40 hertz apart. The two carriers and their firstorder sidebands are shown by lines connecting the dynamic spectrum on left with the average amplitude spectrum on right. The upper arrow of the right-hand spectrum marks the half-harmonic. The effective bandwidth of the analyzing filter is 2 hertz.
pulsations, recently reported in association with a single-frequency, nontriggering Siple signal (Park 1981), in which comparable modulation periods were observed. An explanation of both the sidebands and the half-harmonic can be found in two previously demonstrated effects. In one, called "gap triggering" (Chang and Helliwell 1979), a 10-millisecond gap in a single-frequency wave train launches a risingfrequency emission which is often entrained by the driving signal when it is switched back on. The result is a transient fluctuation in amplitude and frequency. A similar effect may occur at each null of the beat between 11 and 1, giving rise to a non-sinusoidal modulation of both amplitude and frequency at and 12 f . The resulting harmonics of A f are then added to f' giving rise to (and the resufling) sideband components, spaced
278
f apart, which are amplified just like the driven signals to produce the observed spectrum. The other effect is the suppression of a periodic emission by a similar preceeding emission (Brice 1965). Such suppression decreases as the time between events increases. Thus an instability may occur in which a transient increase in one emission tends to reduce the next and that in turn allows the next emission to be larger. The result is the generation of a periodic wave train at double the beat period. This idea was first used to explain the prevalence of three-phase over two-phase periodic emissions. In the present instance (see figure 2) such alternation in growth introduces a modulation component at A f/2. An understanding of these coherence effects is necessary to the development of a comprehensive model of the cwi. This work is expected to lead to new techniques for the measurement and control of plasmas. In particular, it may be possible to employ wave injection from the ground to measure electron fluxes in the radiation belts without the use of satellites. This work was supported in part by National Science Foundation grant DPP 80-22282 and DPP 80-22540. The manuscript was prepared by K. Dean. The author acknowledges the contribution of Siple Station winter engineers Tom Wolfe (November 1981 to January 1983), John Green (November 1982 to present), and Dave Shafer (November 1982 to present).
References Brice, N. M. 1965. Multiphase periodic very-low-frequency emissions. Radio science, 69D, 157, Chang, D. C. D., and R. A. Helliwell. 1979. Emission triggering in the magnetosphere by controlled interruption of coherent VLF signals. Journal of Geophysical Research, 84, 7170. Chang, D. C. D., R. A. Helliwell, and T. F. Bell. 1980. Side-band mutual interactions in the magnetosphere. Journal of Geophysical Research, 85, 1703. Helliwell, R. A., and U. S. man. 1982. VLF wave growth and discrete emission triggering in the magnetosphere: A feedback model. Journal of Geophysical Research, 87, 3537. Helliwell, R. A., and J. P. Katsufrakis. 1974. VLF wave injection into the magnetosphere from Siple Station, Antarctica. Journal of Geophysical Research, 79, 2511. Helliwell, R. A., and J . P. Katsufrakis. 1978. Controlled wave-particle interaction experiments. In L. J . Lanzerotti and C. C. Park (Eds.), Upper Atmosphere Research in Antarctica, (Antarctic Research Series, Paper 5, Vol. 29). Washington, D.C.: American Geophysical Union. Park, C. C. 1981. Generation of whistler-mode sidebands in the magnetosphere. Journal of Geophysical Research, 86, 2286.
ANTARCTIC JOURNAL
