On board the geostationary GEOS 2, a relaxation sounding (RS) experiment using a pulsed active wave technique has been operated in the frequency range 0.3–77 kHz. In situ measurements of the equatorial magnetospheric electron density at L ≃ 6, obtained in the range from 0.1 to ∼100 cm−3 through on line data processing, are presented. The results from 4 months of routine operations (November 20, 1978–March 28, 1979) are statistically studied. Many features of the density distributions at 6.6 RE displayed from GEOS 2 RS data corroborate the morphological characteristics evidenced in past experimental data sets of magnetospheric density, like those from ground‐based whistler observation and from embarked particle experiments. In providing an improved accuracy—especially for the cold plasma density—and unique coverage in space and time, the GEOS 2 RS data set brings some extension of results. The average properties of different magnetospheric regions that can be sampled at 6.6 RE with respect to LT are focused: the dayside plasma trough, with a few cm−3 typically, the plasmaspheric bulge (a few tens of cm−3) and the associated plasmapause crossings (variations in cm−3 per minute), and the geosynchronous nightside where the density fluctuates in the lower range available (0.1–1 cm−3). The important contribution of cold plasma to the dayside magnetospheric density is reported. There are striking variabilities of the density distributions found from day to day, especially as far as the plasmaspheric bulge morphology is concerned. Relationships between geomagnetic activity and both the LT position and the LT width of the bulge are found. They confirm the role played by timing effects on the large scale convection patterns, which experimental and model studies performed earlier on the plasmapause dynamics have focused on.

One hundred eighty‐six days of electric field data from the GEOS 2 electron beam experiment have been used to study magnetospheric fluctuations at geostationary orbit with periods between 150 and 600 s. While fluctuations are nearly always present in the electric field data from the dayside magnetosphere with typical amplitudes between 0.2 and 0.5 mV/m, it is often hard to find well‐defined concurrent pulsations in the GEOS 2 magnetic field data. Most events occur near noon and have the same characteristics: They are toroidal and nearly linearly polarized, the sense of polarization and the orientation angles of the polarization ellipses change sign near noon, the instantaneous frequency of the fluctuations is correlated with the instantaneous electron density, and in a given sector of the magnetosphere the sense of polarization depends on the frequency. There is strong evidence that these fluctuations are fundamental mode eigenoscillations of field lines in the vicinity of the spacecraft which are generated in the inhomogeneous plasma of the magnetosphere by some kind of solar wind‐driven surface waves at the magnetopause or at the low‐latitude boundary layer.

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The envelope and the wave form of the resonance signals obtained by a rocket‐borne RF sounder experiment have been recorded on the ground. The processing of the data concerning the third harmonic of the electron gyrofrequency (3fH) has led to the conclusion that it results from two waves having slightly different frequencies (typically, 1 kHz); the amplitudes of these waves depend on the antenna orientation. Beating between the two waves causes a modulation of the signal amplitude similar to the modulation already observed with the topside sounders, which can be explained in the same way.

The waveforms and envelopes of resonance signals observed in the ionosphere at 2, 3, 4, and 5fH by rocket experiment are compared with the analytical results provided by a spatial ray‐tracing procedure which takes the gradient of Earth's magnetic field into account. An approximate calculation yields the algebraic expression of the wave damping from whence the decrease of the received signals is deduced versus time as well as their frequency variations.