Wave Injection Experiments from Spacecraft

Wave Injection Experiments from Spacecraft Bo Thid´e Uppsala Ionospheric Observatory, S-755 90 Uppsala, Sweden

Presented at the Workshop on Solar Terrestrial Physics on Columbus Rutherford Appleton Laboratory, Chilton, Didcot, UK, 14-15 October 1986

I. BACKGROUND Injection of waves from spacecraft is a very convenient way to perturb the plasma environment for the purpose of studying fundamentally important phenomena regarding the excitation, propagation, and interaction of waves under semi-controlled conditions. The first experiments of this kind were the topside soundings in the early 1960’s. In addition to provide ionograms from above the F2 -layer, these soundings excited resonances at the electron Langmuir frequency, the upper hybrid frequency, the electron gyro frequency and its harmonics. Even for the modest output powers used by the sounder it was found that instabilities and non-linear effects are easily excited by a space-borne transmitter [Benson, 1977, 1982]. Non-linear effects were also observed in the early 1970’s in experiments on a motherdaughter ionospheric rocket with a transmitter output power of only a few watts [Folkestad and Trøim, 1974; Folkestad et al., 1975]. Because of the low power used, these effects are likely to occur in the plasma near the transmitting antenna. For higher transmitter powers similar and other non-linear wave-wave and wave-particle phenomena will take place in a larger plasma volume around the spacecraft. In fact, more recent topside sounding experiments have been interpreted in terms of caviton/soliton formation [Pulinets and Selegey, 1986]. Hence, these wave injection experiments resemble the HF heating experiments performed with high-power ground-based transmitters; [see, e.g., the special issue of Journal of Atmospheric and Terrestrial Physics on Active Experiments in Space Plasmas, December, 1985]. II. FUTURE EXPERIMENTS Drawing inferences from space and ground experiments and theoretical predictions, we can expect a large variety of wave phenomena to be excited by a satellite transmitter both for frequencies below and above the ambient plasma frequency. The expected phenomena include parametric decay instabilities, stimulated scattering, caviton and soliton formation, and others. The in situ excitation and observation of such processes from spacecraft will yield unique possibilities for a more detailed understanding of the underlying fundamental, but complex, physics. This is one of the objectives for the WISP (Waves In Space Plasmas) project to be carried out by utilising the NASA STS and free-flyers [Fredricks, 1983]. Similar, but also in many respects unique, experiments can be carried out with the ESA GEM (Global Electrodynamics Monitor) as proposed by the French group [Bauer et al., 1986] and/or the ESA Turbulence as proposed by British scientists [Bryant and Hall, 1986]. Of the anticipated non-linear wave-wave interactions the most probable ones are those where the injected “mother” wave (ω0 , ⃗k0 ) decays into two “daughter” waves (ω1 , ⃗k1 ) and 1

(ω2 , ⃗k2 ). If the non-linearity is not too strong the frequencies and wave vectors must fulfill the approximate matching conditions ω0 ≈ ω1 + ω2 and ⃗k0 ≈ ⃗k1 + ⃗k2 It is important to notice that the excited waves may have frequencies, wavelengths, polarisation, phase and group velocities that are largely different from that of the injected wave. Furthermore, the “daughter” modes may be either electromagnetic (transverse) or electrostatic (longitudinal) in nature, and may, in fact, have overlapping frequency ranges. Finally, the different modes may have growth times that are vastly different, ranging from milliseconds to seconds and have amplitudes that vary by many tens of dB. III. CONCLUSIONS From a space plasma physics point of view the proposed GEM and Turbulence equipped with powerful on-board HF/VHF transmitters could be utilised for carrying out interesting, comprehensive, and in many respects unique wave injection experiments. As far as we know today maximum information on linear and non-linear space plasma wave interactions can be obtained only if the following design criteria are fulfilled: • The sensors, antennas, receivers, spectrum analysers and associated hardware must be broad-band and have wide dynamic range. The telemetry will have to accommodate 10–100 MBPS. • There must be a way of discriminating between electrostatic and electromagnetic waves. The state of polarisation of the latter must be determined. • It must be possible to change transmitter and receiver frequencies independently and within a very short time. This may have implications on the design of the antennas. REFERENCES Bauer, P., M. Blanc, C. Hanuise, W. Kofman, D. Alcayd´e, J. C. Cerisier, M. Garnier, and J. P. Villain, “Global Electrodynamics Monitor (GEM)”, these proceedings, 1986. Benson, R. F., “Stimulated plasma waves in the ionosphere”, Radio Sci., 12, 861–878, 1977. Benson, R. F., “Stimulated plasma instability and nonlinear phenomena in the ionosphere”, Radio Sci., 17, 1637–1659, 1982. Bryant, D. A., and D. S. Hall, “Turbulence within Columbus”, these proceedings, 1986. Folkestad, K., and J. Trøim, “A resonance phenomenon observed in a swept frequency experiments on a Mother-Daughter ionospheric rocket”, J. Atmos. Terr. Phys, 36, 667–685, 1974. Folkestad, K., J. Trøim and J. Bording, “Interpretation of signals detected on a mother-daughter rocket in the polar F-region”, J. Atmos. Terr. Phys, 38, 335–350, 1976. Fredricks, R. W., “Waves in Space Plasmas (WISP): A Space Plasma Lab Active Experiment”, ESA SP-195, Proceedings of the International Symposium on Active Experiments in Space, Alpbach, Austria, 24-28 May, 1983, p. 369–375. Pulinets, S. A., and V. V. Selegey, “Ionospheric plasma modification in the vicinity of a spacecraft by powerful radio pulses in topside sounding”, J. Atmos. Terr. Phys, 48, 149–157, 1986.

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