Stanley D. Shawhan

The use of multiple receivers to measure the wave characteristics of very-low-frequency noise in space

Until now most very-low-frequency (VLF) radio noise experiments in the ionosphere, magnetosphere and solar wind have been able to provide only the amplitude and spectral characteristics of VLF phenomena. Experiments using multiple receivers to measure the amplitudes and relative phases of the magnetic and electric wave components, however, can give the wave characteristics in addition. Knowledge of both the spectral and the wave characteristics are desirable in making deductions about the noise source location and mechanism and about the properties of the propagation path. Expressions are derived for obtaining the electromagnetic wave characteristics — wave normal vector, Poynting vector and wave polarization — and the electrostatic wave characteristics — wave normal direction and field magnitude — from the amplitudes and relative phases of the wave components. The antenna systems capable of measuring the necessary wave components on payloads which are not spinning, spinning, or spinning and precessing are described. Consideration is given to the experimental technique of reducing payload interference, of transferring the required data to the ground and of obtaining the desired spatial, frequency, amplitude and phase resolution.The data obtained with such an experiment may represent the superposition of signals from multiple sources and multiple paths and from interference signals. Interpretation of these results is discussed and the use of the results of obtaining information on the source location and mechanism and on the propagation path properties is described. Recently several sounding rocket and satellite experiments capable of measuring some of the wave characteristics have been flown. The results concerning the wave normal directions for several different types of VLF noise phenomena are summarized.

Whistlers-Use for Determination of Composition and Temperature

A review is given of the theory of the ‘Eckersley’ whistler, the nose whistler, and the ion cyclotron whistler. The technique for deriving the magnetospheric electron concentration from nose whistlers and the results concerning the ‘knee’ in the density profile and the plasmapause are discussed. From ion cyclotron whistlers the ion concentration, the electron concentration, the ion gyro-frequencies, and the ion temperatures in the ionosphere can be obtained. The method is described and representative results are presented. The use of other whistler mode noise such as the lower hybrid resonance noise for the determination of composition and temperature is also discussed.

Observations of Ionospheric Very-Low-Frequency Radio Noise

A summary is presented of the AC electric and magnetic fields observed in the ionosphere by the Javelin 8.45 UI sounding rocket launched from Wallops Island, Virginia, on 21 September 1967. The electric dipole and magnetic loop antenna systems and the VLF (30 Hz–70 kHz) receivers are described. Data on the spectral characteristics, the noise amplitude, and the wave field geometry for two types of noise are summarized. A high frequency electromagnetic noiseband occurred between 7 kHz and 30 kHz throughout the flight. Marked spectral changes with altitude, a large ratio of electric to magnetic field amplitude and a field geometry which indicates that the noise is propagating perpendicular to the geomagnetic field were observed. It is argued that the lower cutoff frequency may be the lower hybrid resonance frequency. Precession modulated intense noise bursts were observed on the electric antennas only. This electrostatic noise occurred below 1 kHz in frequency and below 500 km in altitude both upgoing and downgoing. From the spectral shape, the phase of the noise amplitude with respect to the component of velocity perpendicular to the geomagnetic field and the relative electric field amplitudes, it appears that this noise may be caused by a turbulent wake of the payload.