Magnetic-field fluctuations in the magnetosheath observed by Pioneers 7 and 8

Fluctuations of the magnetic field at frequencies less than 0.05 Hz have been studied in the magnetosheath from data obtained by Pioneers 7 and 8. During the first two weeks of their flights, the probes were in similar trajectories along approximately radial directions from the earth to 250 RE at an earth-sun-probe angle of 30°. Magnetic fluctuations inside the magnetosheath are transverse to the field vector; the largest fluctuations are perpendicular to the ecliptical plane. The entire spectrum is excited, the differential spectral power being proportional to f−a, where 1<α<2, there being no spectral lines at any particular frequency. The magnetic energy associated with the fluctuations is found to exponentially decay with increasing distance transverse to the sun-earth line. These results suggest that the fluctuations represent an enhanced microstructure produced by the amplification of small-amplitude fluctuations occurring at the bow shock, as McKenzie and Westphal proposed.