“So far seven different experiments have been carried out with sounding rockets in the Sahara and in Sardinia.”
Active experiments in the magnetosphere include some intended to be primarily diagnostic (barium release, electron guns) and others designed to perturb the system to generate instabilities (VLF or ULF wave injection from ground stations, ionospheric heating). Recently, a scheme has been proposed which would modify the magnetosphere by injecting plasma near the equator beyond the plasmapause and initiating wave‐particle instabilities. The expected effects have been examined theoretically. Injection of plasma into this region is also a naturally occurring phenomenon produced by the cross‐tail electric fields which are associated with geomagnetic activity. For further investigation of magnetospheric instabilities, the advantages of examining artificially injected plasma (control of time and location of injection and of the volume of plasma injected) contrast with the advantages of studying natural enhancements (no extra payload, frequent occurrence). Thus, the two types of experiments are complementary. In preliminary studies of natural plasma enhancements both ULF and ELF emissions have been observed. The ELF noise is consistent with generation by the electron cyclotron instability.
These experiments include the release of heavy ion plasma clouds and the injection of accelerated charged particles, as well as the injection of electro. magnetic wave energy at frequencies ranging from ULF (0.001 to 10 Hz) to HF (3 to 30 MHz).
Barium clouds. Plasma clouds of heavy ions, usually barium, have been released both in the ionosphere and deep in the magnetosphere [Foppl et al., 1965; Haerendel et al., 1967; Haerendel and Liist, 1970; Brence et al., 1972]. At low altitudes, the drift motion has been used to infer electric fields [Haerendel, 1971a; Schutz et al., 1971] and to relate the observed electric fields to the dynamics of substorms [Haerendel, 1971a]. The development of striations in the barium clouds [Haerendel, 1971b] has demonstrated the importance of nonlinear process. 
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