Comparison of several probe techniques for ionospheric electron concentration measurements

Daytime and nighttime rocket flights up to 150-km altitudes have been used to compare several types of probes for ionospheric studies. Absolute measurement of electron concentration was achieved by means of a resonance relaxation probe that detected resonant plasma oscillations excited by a radio-frequency pulse. By comparison with this measurement it was demonstrated that the susceptance curve of an 11.4-cm sphere shows a sudden change in slope at the electron plasma frequency. The mutual admittance between two separate probes proved to be more dramatic, showing distinctive minimums at the several plasma resonance frequencies. On the other hand, the most obvious feature of both the self and mutual admittance curves is a resonance that occurs at a frequency between the electron gyrofrequency and the upper hybrid frequency; the resonance rectification effect occurs at the same frequency. This frequency is determined in large part by the ion sheath and is not particularly suitable for electron concentration determinations. The high-frequency capacitance of a spherical probe is also strongly influenced by the sheath, but in a manner that can be taken into account empirically; this capacitance can therefore be used as the basis for a practical probe with good sensitivity and resolution.