Rather than observing purely natural phenomena, space physicists have in recent years planned and conducted ‘controlled’ experiments in the ionospheric and magnetospheric plasmas. In such experiments, the medium in the vicinity of the space vehicle is deliberately perturbed by the injection of a signal that has known characteristics; the signal may be in the form of energetic charged particles. One example is when an electron gun injects electrons with a particular energy (≲keV), power, and pulse duration. Alternatively, the signal may be in the form of electromagnetic waves, one example being when the transmitter at Siple Station, Antarctica, injects VLF waves of known frequency, amplitude, and duration. Such signals then propagate away; they can be received later, enabling the medium remote from the space vehicle to be investigated.