The Helios search coil experiment provides accurate low background noise measurements of interplanetary magnetic fluctuation spectra from about 4 Hz to 2.2 kHz adjacent to the frequency band from 0 to 4 Hz of the Technical University of Braunschweig flux-gate magnetometer. Apart from a slowly varying fluctuation component ranging up to 100 Hz near 1 A U and beyond 500 Hz near 0.3 A U the following superposed 'events' can be discerned in the fluctuation spectra which also have a distinct signature in the slowly varying magnetic field: (1) directional discontinuities acting as wave guide boundaries, (2) directional discontinuities producing whistler wave fields because of instability, (3) reversible magnetic field variations, mostly dips of about 1 min duration associated with whistler wave fields, (4) interplanetary shocks, where, for example, the oblique shock of January 8, 1975, has a thickness of about I proton gyroradius and produces an increase in whistler wave fields by more than 2 orders o' magnitude in power spectral density leading to a power spectrum of 1 3,2/Hz f-a.o4 in the wake region.

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During the Galileo probe's descent through Jupiter's atmosphere, under the ionosphere, the lightning and radio emission detector measured radio frequency signals at levels significantly above the probe's electromagnetic noise. The signal strengths at 3 and 15 kilohertz were relatively large at the beginning of the descent, decreased with depth to a pressure level of about 5 bars, and then increased slowly until the end of the mission. The 15-kilohertz signals show arrival direction anisotropies. Measurements of radio frequency wave forms show that the probe passed through an atmospheric region that did not support lightning within at least 100 kilometers and more likely a few thousand kilometers of the descent trajectory. The apparent opacity of the jovian atmosphere increases sharply at pressures greater than about 4 bars.

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