The distant geomagnetic field: 2. Modulation of a spinning coil EMF by magnetic signals

The modulation of a spinning search coil signal by timevariant magnetic fields is discussed. For typical spacecraft usage, dipolar inhomogeneity produces a trivial signal. The primary contributions to the signal arise from the coil spin and field time variance. Variations in the scalar value of the field produce both amplitude and phase modulation. In a base field like the earth's with superimposed fluctuations, amplitude modulation from Alfven waves is usually trivial. The amplitude power spectrum depends on both the spin rate of the spacecraft and the Fourier decomposed frequencies of the fluctuations. Where frequencies near the spin are encountered, spectral inversion can result from what becomes effectively the mixing of two carrier signals. The results of the previous paper of this series are discussed under these conditions, the latter of which is essentially removal of the restriction ( partial delta H/ partial delta t << omega H). (auth)