Spin modulation of high-latitude hiss measured by an electric dipole


  • H. G. James


Natural radio noise received by the ISIS satellites in the polar and auroral regions often exhibits spin modulation. For noise in the whistler-mode frequency domain observed on seven high-latitude passes, the data indicate that temporal-spatial variations in the level of the noise source are not important on a time scale of one spin period, and thus the dependence of the signal on the attitude of the receiving dipole antenna can be studied. Relative variations in level during one half of a spin period have been synthesized by simply convolving the attitude dependence of the antenna impedance with a spatial distribution of electric vectors. The resonance cone angle, corresponding to the infinity in the refractive index, has been predicted to an accuracy of 3°, while the theoretical amplitude and shape of the modulation agree well with what is observed. It is found that longitudinal electric waves yield better agreement than transverse electromagnetic waves. In some cases, preferred horizontal propagation directions can be roughly identified. The computations confirm the relevance of established theory for the antenna impedance and the ion sheath.