Introduction

The previous chapter contains contributions to the history of science in the field of space plasma physics. It explains how the plasmapause, this peculiar and unexpected magnetospheric frontier, was discovered independently in the late 1950s and early 1960s by two scientists from the two leading countries involved in space exploration. The discoveries were made by using two totally different technical methods of measurement: in situ spacecraft observations and electromagnetic sounding of the magnetosphere. These techniques were both in their infancy at the time.

The main results of electromagnetic sounding of the plasmasphere, from the ground and from satellites, will now be described. In situ satellite particle observations will be outlined in Chapter 3.

In both this chapter and the next the most relevant results will be presented without emphasis upon technical aspects of the experiments. Such aspects are well described in the specialized literature, examples in the case of the whistler method being works by Smith (1961a); Carpenter and Smith (1964); Helliwell (1965); Carpenter and Park (1973); Rycroft (1974a); Y. Corcuff (1975); Tarcsai (1975); P. Corcuff (1977); P. Corcuff, Y. Corcuff and Tarcsai (1977); Park and Carpenter (1978); Bernhardt (1979); Daniell (1986) and Rycroft (1987). An extensive review of the use of whistlers for magnetospheric diagnostics is given by Sazhin, Hayakawa and Bullough (1992).

Initial results

As noted above, Storey (1953) used whistlers for the initial identification of the dense plasmasphere, and Carpenter (1962b) used evidence of unusually low whistler travel times to infer the occurrence of deep, factor-of-∼ 10 depressions in electron density during the severe magnetic storms of the IGY.